[RotaxFlyer]

His response back to Unit Cost. Now is this 2159 Piston he talks about the same as original 340 TNT or is it just a 335?

======================================================================================

We do have price breaks at 12, 24, 50, 100 and 250. Machining features and any coatings gives the variance in the pricing.

12 parts - $125-140 ea. which includes rings, wrist pin and circlips

24 @$105-115

50@ $80-85

Thanks,

Al

=====================================================================

Allen:

So what is the unit Cost for each Piston if I buy 12? Is there a Price break for say 24, 48, etc?

Rich

 

[sightation]

His response back to Unit Cost. Now is this 2159 Piston he talks about the same as original 340 TNT or is it just a 335?

======================================================================================

Neither - 2159 is for the 73-74 340 TNT TWIN

335/ 340 single was low dome part number 2142P8

Never made a high dome TNT piston - you would have to send them a OEM sample for them to copy

 

[sightation]

I first used the old Electric Dremel Tool at first doing a set of Chevy 327 2.02In 1.60 Ex Heads that took forever, but I soon went to Air Grinders and Chevy Big Blocks, 396/402, 427, then 454's. Air Die Grinders $20 to $100, come in different Shapes, Lengths and Angles with Carbide Bits. For these light Intake Machining cuts you can use a cheap Drill Press, and they make a Cheap small X & Y Milling Vice for them. Today, you have many Cheap Mini Mills and Mini Lathes that can handle these jobs also, some are even CNC. You also have a lot of fancy used Manual/CNC equipment to pick from today that start out at $600+ if you have the room for them, but most use 220 3 phase. I can't get that at my house last time I checked. I do have a 3 Phase Industrial Variable Speed Drill Press that use's a Phase converter to run it on 220 1 Phase. I don't have the room to use a Big CNC Mill like I want. My Grizzly G1007 Mill with Power Feed I bought New almost 20 years ago for $1400 and my Grizzly 12" x 37" Lathe New was like $2800. They haven't went up much in Price. With Hand Tools, it's hard to keep it straight and level, whereas a Mill makes it straight. With CNC you could Program it to follow a Contour. With the right Fixture this would be an easy task. You spend more Time setting up the Machine than actually Maching it. With Free CAD/CAM Software today and Cheap CNC Machines, you open a lot of things that could be done right at home that you can't do by hand. Even these Small Cheap CNC Routers out today can do light Machining.

My Mill being converted to CNC. You can buy all the parts off eBay today, about $1,500 each to CNC the Mill & Lathe, if you shop around. You may have to Design your Motor Brackets depending on what Model you choose.

Things you could make, your own Billet Heads, Case's, Machine your own Stock Heads for different CR's. Machine your Case's for Bigger Cylinders, Bore your Cylinders, turn your own Cranks, make Head Inserts, etc.

You said
"Every body has different Incomes and Talents on what they can do, Welding, Painting, Machining, Fabricating, etc. I try to show what can be done for as little of money as possible."

For 99.9% of the people in the world riding sleds - investing in a mill set-up is complete overkill. So is investing in a lathe etc.

Aaen and others will port a cylinder for $250 or less.

For the average Joe who is working on a $700 sled who wants to port a cylinder - hand held grinders are the most likely tools of choice and will cost less than $100 for a basic set-up

Most will try a Dremel style tool first because they are cheap and available. Not an issue for porting one or two cylinders, just takes a little more time. Get a flexible drive too. I still use one like the kit below for a lot of quick jobs including filing ring end gaps. $40 total investment to port a cylinder.

If you plan on doing more cylinders - Get a heavy duty foot operated hanging motor, flex drive model along the lines of of the Foredom Units. For $ 100 bucks total the one I showed will get you in business plus the leftover $$ will allow you to buy a bunch of other bits etc.

Air die grinder - Been there done that. Hard to use on sled cylinders because they are BIG and typically are On/Off no real speed control. Don't forget they use a lot of air. Typical small compressors like most folk have nowadays - have enough pressure but take a long time to recycle - Good for air nailers, impact wrenches set. But for use on an air grinder with constant flow, a lot of start/stopping required while grinding a port. Figure the price of a new compressor too if you don't have one big enough.

And count on buying a pencil grinder also for the fine detail work needed. Just be sure to get one with a throttle control - they spin mega-fast wide open.

 

[sightation]

A Shorter Intake Tract would help some. Aaen, said a Good Flat Side Carb was as close to Fuel Injection as you can get. There are cheap New 34mm Flat Sides on eBay. It would have been nice to Dyno one of these 335's with the Stock 32mm and a 34mm Carbs with Stock Intake Port and then make the Mod to the TNT Intake Port Spec's, since they felt the difference in the seat of there pants just going from a 32mm to 34mm Carb. Yes, changing the bottom end, Piston Skirt is on the list of things that I want to discuss on what can be done, 1st Intake Port, 2nd Exhaust Port, 3rd Transfers, 4th Piston, then 5th discuss a Tuned Pipe once all these Mods that can be done to Improve it. I personally 6th, would like to even do an EFI Mod.

34mm Flat Side $30 on eBay.

If you are sticking with the 34mm carb there is very little to do port wise.

At current port duration 32mm will flow about 26 CFM. 34MM will flow around 29CFM
Increase the intake duration to TNT specs and its 32mm@28CFM and 34mm @31CFM
26CFM to 31CFM is about 19% more air so yeah butt dyno might feel it.

You will probably want LONGER intake tract not shorter.

Your priorities are about backward of what I would recommend for a 1st timer porting an an engine. Given the 34mm restraint I would recommend.

1 Put on the 34mm carb with proper adaptor. Run the engine compare to stock - Like it and you're done.
2 Exhaust. Replace with less restrictive can -TNT, Donaldson, Etc.
3 Modify piston - use TNT duration specs where possible. Intake, exhaust, maybe transfers
Run the engine for comparison to stock. Like it - you're done.
Worst case - you trash a $35 piston and start over.
4 Modify intake for bigger carb 36-38(HD adaptor)
5 Modify exhaust port for less restrictive can muffler.
6 Modify transfers (risky business)
7 Design expansion chamber

But will follow any order you like.

BTW - have you ever asked those flat slide guys about different Jets - you will need a lot of options to jet correctly. If they are not Std Mikuni or something you will be buying a lot of spares to drill out to correct sizes you need.

 

[RotaxFlyer]

Neither - 2159 is for the 73-74 340 TNT TWIN

335/ 340 single was low dome part number 2142P8

Never made a high dome TNT piston - you would have to send them a OEM sample for them to copy

Ok Thanks. They did lower their Price and minimum Quantity you had to Order for a Custom Piston. I think years ago when I was looking for that 583/617 79.4mm Big Bore Piston you had to order then 25 Pistons minimum and they were like $185 each.

Looks like it would be easier to just use the 335 cheap pistons on ebay for a Trail Sled if your going to Mill the Head for maybe more CR anyway.

There is (1) 2142P8 says .080" Over Size Piston on eBay $106.

 

[RotaxFlyer]

You said
"Every body has different Incomes and Talents on what they can do, Welding, Painting, Machining, Fabricating, etc. I try to show what can be done for as little of money as possible."

For 99.9% of the people in the world riding sleds - investing in a mill set-up is complete overkill. So is investing in a lathe etc.

===> Yes, if your just doing (1-2) projects. A Tool, any Tool, usually Pays for itself almost the first Job you use it for, even expensive Tools like a Mill and Lathe which can be used to make almost anything you can dream up. <====

Aaen and others will port a cylinder for $250 or less.

====> Yep, but at just $250 each to Port them. Let me see now, I have alone one 247(1 Cylinder), three 277(3 Cylinders), one 299(1 Cylinder), four 335(4 Cylinders), one 340 TNT(1 Cylinder), probably 3 spare Cylinders alone, that's 13 Cylinders @ $250 each, equals $3,250 alone for just these Old Singles. Then through in all the Twin Cylinder Engines and probably some spare Cylinders, probably another 16-22 Cylinders. With the right Lathe Jig, that lathe can Bore your own Cylinders @$65 each today just these Cylinders say 30 Cylinders could cost you $65x30 = $1950. Factor in Head Machining Average $100 each, that's another $2000+, Case Machining, and then Custom Billet Parts you could make also, so even those expensive Tools Pay for themselves pretty quick. There's used small/large CNC mills on Craigslist for every Budget that would be ok on these Aluminium Sled Engine parts. A used Mill similar to mine only CNCed was on there last week for $1400 about 4hrs away one way. Just like restoring/fixing up Sleds there is cheap Manual/CNC Tools out there that can be cleaned, rebuilt, repainted, even upgraded fairly cheap if you look around. With New Billet Race Heads averaging $400 each, Cranks $1500+, Case's $2000+, that machine could pay for itself fairly quick. What would a Billet 246/292/340 Blizzard Free Air or 292/340 TNT Billet Head, Case, Crank sell for today? A piece of 6061 off ebay to make a 583/670 head was around $25, a CNC Machinist guy I talked to once said he could make one complete 670 Head on his oldest, slowest 1st generation CNC Mill & Lathe in about an 1:30, thats Head Shell and Domes. With Machining Time today around $80hr. So $120 Labor +$25 Material. Once setup, the more you make the cheaper they get.

$800/$400 (2) Billet Heads

https://cedarrapids.craigslist.org/tls/d/dyna-myte-2400-cnc/6401050772.html

$1850/$400 (5) Billet Heads (Most of these older CNC Mills/Lathes can be updated to use a cheap PC with Mach3/4 Software and a Flat Screen.

https://stlouis.craigslist.org/tls/d/1999-victor-cnc-bed-mill/6362212288.html

$2500/$400 (7) Billet Heads

https://stlouis.craigslist.org/bfs/d/kitamura-mycenter-1-cnc-mill/6387126189.html

$5000/$400 (13) Billet Heads

https://kansascity.craigslist.org/tls/d/cnc-mill/6378146068.html

<======

For the average Joe who is working on a $700 sled who wants to port a cylinder - hand held grinders are the most likely tools of choice and will cost less than $100 for a basic set-up

=====> Yep, I agree for a 1-2 off Porting job, but like Time is Money in a Business, Time doing your own Porting is Time away from Family & Friends & Riding that Sled. A Professional Boat Racer I talked to years ago, said his Race Heads took on average 30+hrs to Port & Polish them. One of his friends bought one of them New fancy CNC Porting Machines and it took less than 2hrs. Now the Home guy isn't going to buy one them big machines for doing Big Engines, but if you look, today they have small 3, 4, 5 axis machine attachments that would work for doing these small Sled Engine parts. Once you make the Jigs to hold the parts to Index them and have them programed right, your setup time gets reduced.

$455 attachment

Rotary axis. 4 axis , 5th axis A axis for cnc router / cnc engraving machine

<=====

Most will try a Dremel style tool first because they are cheap and available. Not an issue for porting one or two cylinders, just takes a little more time. Get a flexible drive too. I still use one like the kit below for a lot of quick jobs including filing ring end gaps. $40 total investment to port a cylinder.

======> Yes, You can keep it simple. But like me, I have multiple hobbies, so I use my tools maybe on a broader scope then most people to justify the cost. Well, maybe not if you talk to the ole lady! I shut her up once, when I made her a Custom attachment for her Vacuum Cleaner. <=====

If you plan on doing more cylinders - Get a heavy duty foot operated hanging motor, flex drive model along the lines of of the Foredom Units. For $ 100 bucks total the one I showed will get you in business plus the leftover $$ will allow you to buy a bunch of other bits etc.

====> I already got Air Tools to do most Porting & Polishing jobs, I do like that mini Blue Die Grinder you show. <======

Air die grinder - Been there done that. Hard to use on sled cylinders because they are BIG and typically are On/Off no real speed control. Don't forget they use a lot of air. Typical small compressors like most folk have nowadays - have enough pressure but take a long time to recycle - Good for air nailers, impact wrenches set. But for use on an air grinder with constant flow, a lot of start/stopping required while grinding a port. Figure the price of a new compressor too if you don't have one big enough.

=====> I have a Industrial Large Air Compressor that handles anything I use, Glass Bead Cabinet, Sand Blaster, Soda Blaster, Air Tools. Yes, I agree they are big, but they have smaller ones out today, also longer Carbide Bits. I have been thinking of maybe trying my Mill with one of my Mini USB Cams, one has a LED Light on the end mounted on it to see down inside and use those long Carbide Bit's. With just a DRO on a Mill you could take accurate Straight cuts and go slow. <=====

And count on buying a pencil grinder also for the fine detail work needed. Just be sure to get one with a throttle control - they spin mega-fast wide open.

I just got a email notice a box is being delivered, hope it's my 335 Cylinder & Piston I ordered. Got the whole house re carpeted yesterday to keep Honey Doo happy, so I'm wore out from moving stuff. I'll try later today to get it cleaned up and take some Measurements & Photo's. Supposed to be a Piston with it also that looked good in the Photo's.

 

[RotaxFlyer]

If you are sticking with the 34mm carb there is very little to do port wise.

At current port duration 32mm will flow about 26 CFM. 34MM will flow around 29CFM
Increase the intake duration to TNT specs and its 32mm@28CFM and 34mm @31CFM
26CFM to 31CFM is about 19% more air so yeah butt dyno might feel it.

You will probably want LONGER intake tract not shorter.

Your priorities are about backward of what I would recommend for a 1st timer porting an an engine. Given the 34mm restraint I would recommend.

1 Put on the 34mm carb with proper adaptor. Run the engine compare to stock - Like it and you're done.
2 Exhaust. Replace with less restrictive can -TNT, Donaldson, Etc.
3 Modify piston - use TNT duration specs where possible. Intake, exhaust, maybe transfers
Run the engine for comparison to stock. Like it - you're done.
Worst case - you trash a $35 piston and start over.
4 Modify intake for bigger carb 36-38(HD adaptor)
5 Modify exhaust port for less restrictive can muffler.
6 Modify transfers (risky business)
7 Design expansion chamber

But will follow any order you like.

BTW - have you ever asked those flat slide guys about different Jets - you will need a lot of options to jet correctly. If they are not Std Mikuni or something you will be buying a lot of spares to drill out to correct sizes you need.

34mm is not chiseled in stone yet, but I would be happy with 6500rpms and a 34mm Carb should supply enough Air for that, 19% more as you say, but I would consider 7000rpms also, which needs a bigger 36mm Carb. For me to Bore it bigger isn't going to be a big deal, but to some it may. Most Newer Sled/Bike Engines have the Carbs mounted closer, that's why I assumed closer would be better. These Old Singles have that Plastic thick Spacer, then the Tilly Carbs are Longer which would make that Gulping Effect you talk about, I would think. My UBR Billet 670 Intakes are short with just them Rubber Boots Bolted on, so Carbs mounted real Close also. The main thing is we Talk about what can be done in each of these Steps. Like if you want to turn 6500rpms you can get by with a 34mm Carb, but if you want higher say 7000-7500rpms you will need to Bore Cylinder Intake for a 36-38mm Carb.

Mikuni TM Series Flat Side Carburetor 36mm 1.038
LeClare Powersports - Same Day Shipping!!
Brand New

• $137.55

Mikuni TM Series Universal Flat Side Performance Carburetor 38mm TM38-85
Brand New

• $188.95

Every Body has different wants and needs and Budgets even for a Hot Trail Sled. I inquired once a while back if that guy selling them sold different jets, the guy said he didn't know. For the Cheap Cost it might be worth the try if your happy with 6500rpms. Some even come with that Billet Intake and Rubber Boot. It looks like a knock off Mikuni. Will probably never see these different mods ever Dynoed and Published to fully prove them out.

34mm Intake Carburetor 2 Stroke Racing Flat Side Part Carb For Dirt Bike ATV
Brand New

• $23.79 (China)

I sent Goose an e-mail the Can Mufflers are built to same Spec's as Skidoo. He says "The inner workings of a 335 Oly muffler are smaller than the 340 TNT muffler and you would be restricting the motor. I am now taking orders (slowly) on mufflers. My muffler are made exactly like the originals. So you are looking for 340 TNT muffler? The current cost on those is $145." It would be interesting to see both Cans Dynoed on a 335 and a 340 TNT to see what difference they make.

I have seen Pistons with (1) of them Angled Cuts, but not (3) what effect does that have?

 

[RotaxFlyer]

A Billet Intake with Pulse Port.

 

[RotaxFlyer]

Any Pros or Cons on these Type of different Case's? One has the Bearing Cover Plate. Not mine.

 

[sightation]

Most Newer Sled/Bike Engines have the Carbs mounted closer, that's why I assumed closer would be better. These Old Singles have that Plastic thick Spacer, then the Tilly Carbs are Longer which would make that Gulping Effect you talk about, I would think.

Gulping effect has NOTHING to do with intact tract resonance (long or short tube)
Gulping is the need to suck in a lot of air in a very short time.

You have always quoted carb sizes based on 100% efficient engines - Not a real world situation ...
Take your 100% efficient 335cc engine. No argument about the AMOUNT of air needed- 77CFM at 6500 rpm. Coincidentally the same as a 34mm Carb (76.9CFM)

The issue is the flow RATE How many cubic feet per minute you need to flow to get a total of 77 cubic feet of air.
Flow RATE of 77CFM x 1 minute = 77 Cubic Feet total

BUT ... the problem is the intake port - It's not open for 1 minute. Rotate your engine and look in the port - it is only open about 1/2 the time each revolution. So instead of 1 minute to flow 77 cubic feet you only have 30 seconds total OPEN time to flow what you need.

The engine can't suck in anything when the port is closed. It needs to "GULP" it in during the short time the port is open.

77 CFM x 30 seconds(1/2 minute) = 38.5 Cubic feet - HALF of what the engine needs to be 100% efficient. The intake is really only 50% efficient.

To get the AMOUNT it needs - 77 cubic feet - you have to Double the Flow RATE
154CFM x 30 seconds = 77 cubic feet of air. Exactly what you need to be 100% efficient.

A 44mm Carb at 130CFM is close - but would be useless because the rectangle shaped intake port size in the cylinder is only 1246 sq. mm. (26.5 x 47mm) About the equivalent area of a 39mm diam. carb.

Going any bigger than a 38mm would only make low end suffer and not give any increase in top end. Because the port window is too small, a really big carb never gets a strong enough pulse.

Using a 38mm carb = 96CFM x 0.5 minutes = 48 Cubic feet 48/77= 62%
The engine has now gone from about 50% intake efficiency with a 34mm carb to over 60% with a 38mm.

It is actually more complicated than this but not difficult to figure. If you understand this simplistic example, then you can start to understand how intake duration and port dimensions play a MAJOR part in carb size selection.

Without knowing them, you are just guessing about proper carb sizing.

 

[sightation]

I have seen Pistons with (1) of them Angled Cuts, but not (3) what effect does that have?

PORTING 101

Carving the piston is what you do to avoid porting the actual cylinder.

Is it as efficient as working the port ? ... No.
Is it a lot easier with less potential for permanent screw-up? ...Yes

Lowering Piston exhaust edge = Raising exhaust port = Longer Exhaust Duration

Lowering Piston side edges = Raising Transfer Ports = Longer Transfer Duration

Raising Piston Skirt Intake Side = Lowering Intake Port = Longer Intake Duration

I always recommend 1st timers work the piston as much as they can to get a feel for the tools, practicing their technique before touching the cylinder - The point of no return.

 

[RotaxFlyer]

Here is a good article I just found also.

Transfer Port Enhancement with Piston Ramp

On page 117 of "Two Stroke Tuners Handbook" Gordon Jennings wrote how one could grind transfer and exhaust port ramps into the piston crown to test porting ideas without grinding on the more expensive cylinder. He suggested it as a temporary trial for porting without mention of the aspect I would like to focus on here.

Flow Restiction in Ports
On page 113 Jennings wrote: "There is a very considerable contraction of flow through any sharp edged orifice, and such orifices may be made effectively larger by providing them with a rounded entry. Improvements in flow in the order of 30 percent could be had were it possible to give the port window edges a radius of, say, 1/4 inch." Here's his illustration:

His focus was on rounding the edges of the port whereas mine is on rounding the edges of the piston top where it is adjacent to the transfer ports as a means to reduce flow restriction due to the near-90-degree edges. What I did was I ground down the piston edges (adjacent to the ports) .8mm at a 40 degree angle to horizontal. Actually I made the transition curved somewhat at the beginning and end of each "ramp". The more the edges are "in line" with the desired flow, the more efficient and less restricted that flow will be. In other words, for the same area of opening you can have more flow as if the piston had already moved farther down and out of the way.

To be safe this mod should only be done when there is at least 2mm between the crown edge and the top of the piston ring. (Mine has 2.8mm)

Transfer Ports Enhancement with Piston Ramps

What I did was I ground down the piston at a 40 degree angle to horizontal that gave the equivalent of .8mm higher at each port. And I made the transition curved somewhat at the beginning and end of the "ramp". For the same area of opening you can have more flow as if the piston had already moved farther down and out of the way. Jennings had wrote that differences and inconsistencies of the transfer ports tops mostly affect just the low and mid range power because at top rpms the time that those inconsistencies are significant is too short. So in regard to the transfers intake flow being less restrictive with this mod I would say Jennings has explained why the biggest effect is not in the high rpm range. The engine doesn't hardly recognize this mod at all above 7000 rpm. It really affects only the lower rpms which is how it broadens the powerband without changing top rpm porting. It also bleeds off the crankcase pressure at low rpms which allows the majority of the intake charge to enter the cylinder at a lower speed so that it doesn't loop around and exit the exhaust port as much. This allows more of the intake charge to stay in the cylinder for combustion which is how it increases power at lower rpms.

I have only done these mods on my 55cc reed-valved engine and 48cc piston port engine so can't speak of experience with bigger engines. But I would try it in a heartbeat on a bigger engine (up to 125cc) because of the difference it has made with my small engine. I expected to lose low rpm power but instead have actually gained some. I go up one hill at 23mph whereas I used to go up at 20mph. I really wouldn't of believed it without experiencing it.
When I changed my port timing from 106.2° ATDC exhaust, 129.5° transfers to 102° exh, 128°transfers I incorporated this mod. It maintained the same top speed but gained more low end power.
The following quote is a very important point which explains the low rpm power gain;
"these engines spend so much time in the low revs says that the mixture spends more time crossing these boundary layers that you have chamfered giving you the gains you've seen".

My final thoughts is that this .8mm ramp will have the greatest effects on engines w/o a high peak rpm. The higher rpm an engine is ported for, the less noticable will be this mod because the lower rpm range that this mod effects would be farther from the existing powerband. Someone with a 14,000 peak rpm engine tried this and reported that he got no benefit. My engine peaks at 7500 rpm. So any non-screamer engine with enough space on the piston to put a ramp on it can probably benefit from this mod. On my 55cc a .8mm piston ramp adds 6 degrees transfer port duration. For the same duration change a Honda 125 (with 54.5mm stroke) needs a 1.1mm ramp, and a Honda 250cc (with 72mm stroke) needs a 1.45mm ramp. Also, the longer the piston stroke (ie: larger engine), the faster the piston speed at the same rpm. I believe that the ramp effect is not realized at high rpms. So if my 55cc engine gets the most benefit at less than 3000rpm then a Honda 125 gets the same benefit below 2100rpm and a Honda 250 at below 1654rpm. These figures depend on the piston speed at transfer opening.

With four strokes taking over the motocross world I think the last remaining competing two strokes need every little trick in the book to be competitive again. 4 strokes main advantage is the wide powerband they have. This mod restores some width to the 2 strokes powerband.

Vito's superstock pistons have a bevel on the exhaust side that is 1.5mm down at the side and 2.5mm from the ring. One customer of theirs told me it increased his Blaster's "power everywhere" but Vito's don't make any claims other than to say it raises the powerband. [A Blaster has a 195cc single cylinder engine with 57mm stroke.] A person on www.bansheehq.com wrote: "I used them in my cousins Banshee a while ago. Put about 4-5 hard seasons on them. He claims he could notice a seat of the pants difference in power. He did pretty well in his heads up class. Only thing that beat him was a CRF450 dirt bike. Also used one in one of my Blasters. I could notice it being a lil more peppy. It also held up quite well. As far as I know it's still going strong and that was about 6-7yrs ago."

But I don't recommend a piston ramp for the exhaust since it would have the opposite effect in that it would benefit high rpm power and lessen low rpm power. At slow piston speeds the port (exhaust or transfer) would have a greater port duration. At the transfers that would benefit low rpm power by lessening the pressure at port opening. (The higher the transfers are, the less the crankcase compression ratio.) At the exhaust that would only benefit high rpm power at the expense of low rpm power.

The higher the crankcase compression ratio, the more of an effect this mod would have. To accurately determine your ratio click here.

Unfortunately the 48cc and 66cc have really good transfer port timing that would be thrown off too much by addition of transfer ramps. If you buy a taller piston or lathe off 2mm from the cylinder base then you can do the ramps. Somewhere in these pages I list where you can buy a 1mm taller piston for the 48cc. With it you can add 1.8mm ramps. The 55cc and 60cc cylinders have too low transfer ports that allow you to utilize 2mm transfer ramps. Also raise the 55cc transfers 1mm, and the 66cc transfers .5mm higher for a perfect port duration of 117 degrees. Then for perfect exhaust port duration of 157 degrees raise the 55cc to 25.45mm and the 60cc to 25.6mm from cylinder top.

HOME

 

[RotaxFlyer]

Finding the Crankcase Compression Ratio

1. Remove the cylinder head and mark the crank where the piston top is level with the transfer port top. Also, for a piston port engine mark it where the piston just closes the intake port.

2. Remove cylinder + piston + upper con-rod bearing and position the crank at TDC for a reed-valved engine, or at the degrees where the piston skirt closes the intake port of a PP engine.

3. Raise front or rear wheel until cylinder mating surface is level.

4. Pour oil or gasoline into the crankcase (at TDC for RV engines or at intake port closing for PP engines) up till its even with the cylinder mating surface.

5. Use aquarium tubing to siphon out the liquid in the crankcase into a measuring cup. This is the TDC lower crankcase volume but if some of the cylinder walls extend into this space then you'll have to calculate those walls volume and subtract them from the total.

6. Measure the con-rods width, thickness, and length with the crank at TDC position, measuring from a level equal to the cylinder mating surface to the top of the rod so you can calculate and guesstimate the volume of space it occupies.

7. Turn the cylinder upside down and insert the piston (with pin + bearing) into it and move the piston to its TDC position for a RV engine, or to where the piston skirt closes the intake port for a PP engine.

8. Pour the same liquid into the piston bottom and the transfers until its at the cylinder mating surface.

9. Pour that amount of liquid out into an empty measuring cup and note the volume.

10. Add the volume of steps 5 (crank) and 9 (piston/transfers), subtracting the volume of #6 (con-rod). This is the TDC crankcase volume.

11. Determine online (http://www.basic-mathematics.com/volume-of-a-cylinder-calculator.html) the volume of a cylinder with the length of your engines piston stroke till transfer opening as the "cylinder height" and the radius (1/2 diameter) of your cylinder bore. To be really accurate you'll need to subtract the volume the con-rod occupies for the same height. To do that you'll need to figure the equivalent cross sectional area of the rod as a cylinder by trial and error at http://math.about.com/library/blcirclecalculator.htm. Then use 1/2 that diameter (as the radius) and the piston stroke till transfer opening as the length of an equivalent cylinder and figure the volume. That will be the volume to subtract.

12. The volume of step 10 divided by the result of the step 11 volume subtracted from that of step 10 [ie: 10/(10-11)] gives the crankcase compression ratio.

The higher the number, the higher the peak crankcase pressure. (see chart below) A ratio of 1.5 means the expanded crank volume of "1.5" is compressed to "1". A ratio of 2 means a volume of "2" is compressed to "1". I have read that most MX engines have a ratio around 1.5 but that around 1.2 is better for low rpm power. Here's a graph showing the relationship between the crank ratio and the crank pressure:

My 48cc engine only has a ratio of 1.1 which gives about half the transfer pressure that a ratio of 1.2 gives. To change it to be 1.2 I need to reduce the crank volume by 80 square centimeters which you can come close to achieving by reducing the space by 60ml by filling the extra space to the left and right of the flywheel at the cases (use JBWeld). A 1.2 ratio yields 3psi transfer pressure while a 1.1 ratio yields 1.5psi.

More space can be occupied by glueing aluminum plates to the insides of the flywheels but the glue (JBWeld) would insulate the aluminum from the hot flywheels and thus insulate the fuel/air from the same heat. The intake charge needs to be heated up so that the fuel atomizes and becomes more readily combustible. (see this site)

On my crank I had also occupied 20ml space with JBWeld and aluminum plates on the inner flywheel sides for testing purposes. With my 48cc piston port top end it increased mid range and top rpm power but lost some low end power which I mostly regained by use of an intake extension (between intake manifold and carb). With my 55cc reed valved top end the ratio was increased from 1.2 to 1.4 (because reed valves cause a longer intake stroke than with a piston port top end). The results were a loss of power unfortunately but that may of been due to the transfer roofs angling upwards instead of being horizontal. The roof angle has to match the compression ratio. (With a reed valved engine how much space is between the intake port and the valve is of utmost importance. My reed valve is installed inside the intake tract and so there is little free space there which allows much more crankcase ratio than if the valve was located back some. Measuring the space is essential to really know the ratio.) The increased pressure also caused the left side crankshaft seal to wear out faster. It is a very skinny seal and one of the weak links to this engine. I think it is still OK if you only have a piston port intake and a 1.2 ratio. But higher ratios with a reed valved intake may spell more maintenance. Luckily the seal only costs $4 and can be pryed out with a miniature screwdriver and the new one just banged back in.

Here's how to stuff the crankcase space: Measure the inner diameter of the hollow circular area of the side of the crank (the fat weighty part). Place the crankcase halves on the floor so that their interior side is facing up. Cut a strip of aluminum (from a cheap bread pan) and make a circle of it of a diameter slightly less than that of the inner crank circle that was measured. Oil it so JBWeld won't stick too well to it. Place it around the crankcase area that is needed to be filled in. Mix up some JBWeld and fill in that area and let it set and dry for a day. Pull off the aluminum. Check to see if the crank touches any of the JBWeld and file/cut/dremel those spots down. Put it back together. (Get a crankcase gasket before starting and maybe also crank bearings and seals from pistonbikes.com. Get two of the skinny left seal since that seal wears out faster with increased crankcase pressure). Luckily it is easy to replace that seal in the future by removing the magneto and just prying it out with a jewelers screwdriver. Grease everything and just push the new one into place with your fingers.

In the research paper "Some Development Aspects of Two-Stroke Cycle Motorcycle Engines" it is written that "the engine speed at which the maximum delivery ratio is obtained moves to a higher value in proportion to the square root of the crankcase volume". So I figured that reducing the volume from 200ml to 140ml increases the peak rpm by 1500.

Primary compression

from http://www.lortim.demon.co.uk/vsih/crankcas.htm

Modern short stroke water cooled engines that employ large transfer port areas do not require a high primary compression ratio, in fact a high primary compression ratio substantially increases the pumping losses and decreases the power available. However, this does not apply to the long stroke of the Villiers unit that uses small transfer ports, even though pumping losses do increase there is a net increase in power. For road racing a minimum geometric primary compression ratio of 1.4:1 is required, 1.5:1 would be better but it is difficult to achieve without extreme measures. The higher ratio is used to squirt the charge through the narrow port. The speed of the charge entering the cylinder is further enhanced by the narrowing of the port, its inlet area should be at least 150% larger than its outlet. A good primary compression will improve tractability and performance whether for road or off-road riding, don't allow this pressure to be diluted through the use of a hollow gudgeon [piston] pin.

Modern engines should be considered as having transfers fulfilling a "store and forward" function for the fuel with the real control being exercised by the exhaust. The modern exhaust shape required easy access to a supply of fuel mixture, exactly what the large transfer system supplies. What a modern system cannot do is sustain a long deep draw of fuel from the crankcase through inadequate transfer passages.

In the long stroke small transfer engines, there is significant advantage to be gained from the truly explosive entry of the compressed gases from the crankcase into the cylinder, sweeping the remaining exhaust gases out into the exhaust port in a manner described by Schnurle many years ago. As the piston descends and before the transfer ports are uncovered, a hollow gudgeon pin will bleed off some of the primary pressure that you have fought to create. The hollow pins should be plugged with an alloy slug or slugs which should have no more than a 1 thou clearance fit and be coated with Loctite or similar. Interference fits will swell the gudgeon pin causing fitment problems. The lack of interference fit is required to allow the Locktite to key and bond, a zero or interference fit will not allow the Locktite to work properly. At racing speeds, the difference in power can be measured on the dynamometer.

The geometric primary compression ratio (GPCR) is useful in determining whether crankcase stuffers are necessary. Be under no illusion: good primary compression is important. If your GPCR is less than 1.2 then action is definitely required.

 

[RotaxFlyer]

Gulping effect has NOTHING to do with intact tract resonance (long or short tube)
Gulping is the need to suck in a lot of air in a very short time.

====> I would think a longer Intake tract would affect it more, it's like open your mouth and take a deep breath, short Intake, or Suck Air through a 4-5" Straw, long Carb Intake tract. <====

You have always quoted carb sizes based on 100% efficient engines - Not a real world situation ...
Take your 100% efficient 335cc engine. No argument about the AMOUNT of air needed- 77CFM at 6500 rpm. Coincidentally the same as a 34mm Carb (76.9CFM)

=====> Yep, that's Why I try to always say "IF" 100% efficient and "IF" not making this HP, you have to figure out WHY it's not making that HP. You have to small of Carbs, Small Intake/Exhaust Ports, Small Carb Boots, Small Transfers, Small Intakes, Muffler vs a Tuned Pipe, Low CR, other Restrictions like Air Filters, each Engine will have different problems to overcome. <=====

The issue is the flow RATE How many cubic feet per minute you need to flow to get a total of 77 cubic feet of air.
Flow RATE of 77CFM x 1 minute = 77 Cubic Feet total

=====> Yes, 335cc at 6500rpms, IF 100% efficient needs 77cfm. Like you said if only 50% efficent, we have to figure out what if anything can be done, will we get to 100% I doubt it, but 80-90% efficient I think is doable. Going from Stock 335 Intake Port (26.5mm x 47mm) = 1246 sq mm or 39+ mm circle as you said to the TNT Intake Port (29mm x 47mm) = 1363 sq mm or 41+ mm circle would Help, but would change your Duration. <====

BUT ... the problem is the intake port - It's not open for 1 minute. Rotate your engine and look in the port - it is only open about 1/2 the time each revolution. So instead of 1 minute to flow 77 cubic feet you only have 30 seconds total OPEN time to flow what you need.

===> Yes, and that's part of the Problem, Intake Port ID, Intake Tract ID, Intake ID, Carb Boot ID Size used, Carb Size used, but you also have to address the other problem areas like I have said, will try to go 1 step at a time to address each of these problems which can be applied to almost any of these other Engines once People know what to look for. We have to address all of these different areas to see what can be done to maybe Improve them. We can show what can be done for the Stock 5500rpms, what you can do to make simple improvements, we can show what can be done for a little more Hot Trail 6500rpms, and even show for even more 7000rpms. The faster you turn them, you face different problems which may need addressed differently. You are limited on how much you can do with each Engine. The Standard 335 Intake can only be Bored so Big and will only support so much rpms. Once you change one thing it usually affects other things that need addressed also. Just going from a 32mm to a 36-38mm Carb would be a good Improvement. <====

The engine can't suck in anything when the port is closed. It needs to "GULP" it in during the short time the port is open.

77 CFM x 30 seconds(1/2 minute) = 38.5 Cubic feet - HALF of what the engine needs to be 100% efficient. The intake is really only 50% efficient.

To get the AMOUNT it needs - 77 cubic feet - you have to Double the Flow RATE
154CFM x 30 seconds = 77 cubic feet of air. Exactly what you need to be 100% efficient.

A 44mm Carb at 130CFM is close - but would be useless because the rectangle shaped intake port size in the cylinder is only 1246 sq. mm. (26.5 x 47mm) About the equivalent area of a 39mm diam. carb.

Going any bigger than a 38mm would only make low end suffer and not give any increase in top end. Because the port window is too small, a really big carb never gets a strong enough pulse.

Using a 38mm carb = 96CFM x 0.5 minutes = 48 Cubic feet 48/77= 62%
The engine has now gone from about 50% intake efficiency with a 34mm carb to over 60% with a 38mm.

It is actually more complicated than this but not difficult to figure. If you understand this simplistic example, then you can start to understand how intake duration and port dimensions play a MAJOR part in carb size selection.

Without knowing them, you are just guessing about proper carb sizing.

Yes, I basically understand what your saying and each Engine will have different pitfalls to overcome, that's Why I usually try to say, "IF 100% efficient" this is the Minimum Size Carb needed at this Specific Rpm, most Engines aren't 100% efficient for different reasons. As we have learned here, there is Cylinder Intake Port Sizes to consider 335 (26.5 x 47mm), Carb Boot & Intake ID Sizes to consider, Carb CFM Ratings to consider for your Specific Rpms used. So IF, you didn't modify the 335 Intake Port, 1246 sq. mm. (26.5 x 47mm) but just modified the Intake tract and Intake to handle a 36mm-38mm Carb would probably be best for a Hot Trail Sled.

335cc at 5500rpms making 22hp comes in at 50%, so at 20hp would be a little less.

30mm (59.8cfm x 0.5 minutes = 29.9 Cubic Feet) x 2 = 59.8cf (Dual Carbs)

32mm (68.1cfm x 0.5 minutes = 34.1 Cubic Feet) x 2 = 68.2cf (Dual Carbs)

34mm (76.9cfm x 0.5 minutes = 38.5 Cubic Feet) x 2 = 77.0cf (Dual Carbs)

36mm (86.2cfm x 0.5 minutes = 43.1 Cubic Feet) x 2 = 86.2cf (Dual Carbs)

38mm (96.0cfm x 0.5 minutes = 48.0 Cubic Feet) x 2 = 96.0cf (Dual Carbs)

40mm (106.4cfm x 0.5 minutes = 53.2 Cubic Feet) x 2 = 106.4cf (Dual Carbs)

42mm (117.4cfm x 0.5 minutes = 58.7 Cubic Feet) x 2 = 117.4cf (Dual Carbs)

44mm (128.7cfm x 0.5 minutes = 64.4 Cubic Feet) x 2 = 128.8cf (Dual Carbs)

I show 128.7cfm for a 44mm Carb, not 130cfm, but close enough.

 

[sightation]

I sent Goose an e-mail the Can Mufflers are built to same Spec's as Skidoo. He says "The inner workings of a 335 Oly muffler are smaller than the 340 TNT muffler and you would be restricting the motor. [/size]I am now taking orders (slowly) on mufflers. My muffler are made exactly like the originals. So you are looking for 340 TNT muffler? The current cost on those is $145.[/size]" It would be interesting to see both Cans Dynoed on a 335 and a 340 TNT to see what difference they make.

Yup TNT is less restrictive than the Oly muffler.

That is why I put it second on my list of modifications to make,
Way too many 340 TNT engines have been burned down by guys simply dropping them into Oly sleds. The muffler is to restrictive causing exhaust temps to go up and piston melt to follow. The more power a Mod 335 will put out - the worse it will get.

It won't make much difference to you since you don't actually have a sled to put it in yet. But for anyone reading this and planning on following some of the ideas this year- best to put that muffler swap high on the list.

1 Put on the 34mm carb with proper adaptor. Run the engine compare to stock - Like it and you're done.
2 Exhaust. Replace with less restrictive can -TNT, Donaldson, Etc.
3 Modify piston - use TNT duration specs where possible. Intake, exhaust, maybe transfers
Run the engine for comparison to stock. Like it - you're done.
Worst case - you trash a $35 piston and start over.
4 Modify intake for bigger carb 36-38(HD adaptor)
5 Modify exhaust port for less restrictive can muffler.
6 Modify transfers (risky business)
7 Design expansion chamber

 

[sightation]

Here is a good article I just found also.

His focus was on rounding the edges of the port whereas mine is on rounding the edges of the piston top where it is adjacent to the transfer ports as a means to reduce flow restriction due to the near-90-degree edges. What I did was I ground down the piston edges (adjacent to the ports) .8mm at a 40 degree angle to horizontal. Actually I made the transition curved somewhat at the beginning and end of each "ramp". The more the edges are "in line" with the desired flow, the more efficient and less restricted that flow will be. In other words, for the same area of opening you can have more flow as if the piston had already moved farther down and out of the way.
To be safe this mod should only be done when there is at least 2mm between the crown edge and the top of the piston ring. (Mine has 2.8mm)

Transfer Ports Enhancement with Piston Ramps
What I did was I ground down the piston at a 40 degree angle to horizontal that gave the equivalent of .8mm higher at each port. And I made the transition curved somewhat at the beginning and end of the "ramp". For the same area of opening you can have more flow as if the piston had already moved farther down and out of the way. Jennings had wrote that differences and inconsistencies of the transfer ports tops mostly affect just the low and mid range power because at top rpms the time that those inconsistencies are significant is too short. So in regard to the transfers intake flow being less restrictive with this mod I would say Jennings has explained why the biggest effect is not in the high rpm range. The engine doesn't hardly recognize this mod at all above 7000 rpm. It really affects only the lower rpms which is how it broadens the powerband without changing top rpm porting. It also bleeds off the crankcase pressure at low rpms

That's a lot of words. He starts out saying "ramping" the piston is all about flow. But quickly gets to the meat of the discussion, Port durations- and how carving pistons is effective in changing duration.

He is changing the transfer timing significantly with his piston modification.
In his 48 cc engine .8mm piston taper = 7 degrees and similar in others

Porting 101 - Effect of port timing changes
For Piston Port Motors ........

Raise Exhaust = More duration .... Results More Rpm - less torque. Low end power decreased.

Lower Intake = More Intake Duration = More RPM - harder starting, more carb spit-back.

Raise Transfer = More Transfer Duration = Potential Torque/RPM gains - UNLESS it makes exhaust blowdown time too short. (Rotax 277 has this problem) One of the risks of playing with transfers.

Take all that into Consideration - And you have PORTING RULE OF THUMB ONE
Better to go wider on ports if possible - Then go up (or down)

For a graphic illustration of this- Look at the result of the TNT Porting vs OLY ports.

The Oly makes MORE hp at 5500 rpm (20hp) than the TNT makes at 6000 rpm (17hp)
The TNT long duration ports don't kick in till over 6000 RPM

 

[sightation]

Yes, I basically understand what your saying and each Engine will have different pitfalls to overcome, that's Why I usually try to say, "IF 100% efficient" this is the Minimum Size Carb needed at this Specific Rpm, most Engines aren't 100% efficient for different reasons. As we have learned here, there is Cylinder Intake Port Sizes to consider 335 (26.5 x 47mm), Carb Boot & Intake ID Sizes to consider, Carb CFM Ratings to consider for your Specific Rpms used. So IF, you didn't modify the 335 Intake Port, 1246 sq. mm. (26.5 x 47mm) but just modified the Intake tract and Intake to handle a 36mm-38mm Carb would probably be best for a Hot Trail Sled.

Alright - You have passed the first test- you seem to have grasped that you have to look at intake duration to properly size a carb to a engine. You have graduated to

Port Theory 102 - Port timing

Ports are measured in Degrees - so lets look at it that way
Intake Duration
120 degrees = 120/360 = 0.333
130 degrees = 130/360 = 0.361
140 degrees = 140/360 = 0.389
150 degrees = 150/360 = 0.417
160 degrees = 160/360 = 0.444
170 degrees = 170/360 = 0.472 - Practical limit for a piston port trail sled.
180 degrees = 180/360 = 0.500 - Same as 30 second example
190 degrees = 190/360 = 0.528
200 degrees = 200/360 = 0.556
210 degrees = 210/360 = 0.583
220 degrees = 220/360 = 0.611

Basically - you can't get close to 1.00 with piston port or rotary valve two stokes - They Won't RUN!!
0.65 is about tops with a rotary valve engines

Even the famed 670HO rotary valve engine only had a 221degree intake duration. (0.614)
335cc per cylinder @8200RPM 100% efficient = 97CFM /0.614 = 158CFM needed. Even the 44's they came with aren't big enough. And it's why places like the Crank Shop offer 44's bored to 48mm

The 335 Oly has about a 135 intake duration - 135/360 = 0.375
The 340 TNT has about a 145 intake duration - 145/360 = 0.403

"So IF, you didn't modify the 335 Intake Port, 1246 sq. mm. (26.5 x 47mm) but just modified the Intake tract and Intake to handle a 36mm-38mm Carb would probably be best for a Hot Trail Sled."

Keeping the 335 at current duration and sq mm as you suggest
32mm (68.1cfm x 0.375 = 25.4 Cubic Feet) - Stock Oly HP = 20
34mm (76.9cfm x 0.375 = 28.8 Cubic Feet)
36mm (86.2cfm x 0.375 = 32.3 Cubic Feet)
38mm (96.0cfm x 0.375 = 36.0 Cubic Feet)

At the TNT 340 Duration - 145- degrees the numbers are
32mm (68.1cfm x 0.403 = 27.4 Cubic Feet)
34mm (76.9cfm x 0.403 = 31.0 Cubic Feet)
36mm (86.2cfm x 0.403 = 34.7 Cubic Feet)
38mm (96.0cfm x 0.403 = 38.7 Cubic Feet) - Stock TNT HP = 26

Using this method you can pick and choose how you want to get the 38.7CFM of the Stock TNT
Real porting for bigger carbs, real porting for duration, or some piston carving mods for duration.

Example - A 34mm bolt on carb at TNT durations - will flow almost as much as 36mm carb at 335 durations without all the port work.
31CFM vs 32.3CFM

Not saying which is best - Just saying there are options depending on what the guy doing the modifications feels comfortable with.

 

[RotaxFlyer]

Alright - You have passed the first test- you seem to have grasped that you have to look at intake duration to properly size a carb to a engine. You have graduated to

Port Theory 102 - Port timing

Ports are measured in Degrees - so lets look at it that way
Intake Duration
120 degrees = 120/360 = 0.333
130 degrees = 130/360 = 0.361
140 degrees = 140/360 = 0.389
150 degrees = 150/360 = 0.417
160 degrees = 160/360 = 0.444
170 degrees = 170/360 = 0.472 - Practical limit for a piston port trail sled.
180 degrees = 180/360 = 0.500 - Same as 30 second example
190 degrees = 190/360 = 0.528
200 degrees = 200/360 = 0.556
210 degrees = 210/360 = 0.583
220 degrees = 220/360 = 0.611

Basically - you can't get close to 1.00 with piston port or rotary valve two stokes - They Won't RUN!!
0.65 is about tops with a rotary valve engines

Even the famed 670HO rotary valve engine only had a 221degree intake duration. (0.614)
335cc per cylinder @8200RPM 100% efficient = 97CFM /0.614 = 158CFM needed. Even the 44's they came with aren't big enough. And it's why places like the Crank Shop offer 44's bored to 48mm

The 335 Oly has about a 135 intake duration - 135/360 = 0.375
The 340 TNT has about a 145 intake duration - 145/360 = 0.403

"So IF, you didn't modify the 335 Intake Port, 1246 sq. mm. (26.5 x 47mm) but just modified the Intake tract and Intake to handle a 36mm-38mm Carb would probably be best for a Hot Trail Sled."

Keeping the 335 at current duration and sq mm as you suggest
32mm (68.1cfm x 0.375 = 25.4 Cubic Feet) - Stock Oly HP = 20hp@5500rpms using 9.0cr.
34mm (76.9cfm x 0.375 = 28.8 Cubic Feet)
36mm (86.2cfm x 0.375 = 32.3 Cubic Feet)
38mm (96.0cfm x 0.375 = 36.0 Cubic Feet) (So a 38mm Carb is MAX Size on a Stock 335 Cylinder what it can be Bored too.)

At the TNT 340 Duration - 145- degrees the numbers are:
32mm (68.1cfm x 0.403 = 27.4 Cubic Feet)
34mm (76.9cfm x 0.403 = 31.0 Cubic Feet)
36mm (86.2cfm x 0.403 = 34.7 Cubic Feet)
38mm (96.0cfm x 0.403 = 38.7 Cubic Feet) Stock TNT 26hp@6500rpms using 10.5cr TNT Cylinder can handle a Bigger Carb or EFI. What is TNT Max?
40mm (106.4cfm x 0.403 = 42.9 Cubic Feet)

42mm (117.3cfm x 0.403 = 47.3 Cubic Feet)

44mm (128.7cfm x 0.403 = 51.9 Cubic Feet)

46mm ?

Using this method you can pick and choose how you want to get the 38.7CFM of the Stock TNT
Real porting for bigger carbs, real porting for duration, or some piston carving mods for duration.

Example - A 34mm bolt on carb at TNT durations - will flow almost as much as 36mm carb at 335 durations without all the port work.
31CFM vs 32.3CFM

Not saying which is best - Just saying there are options depending on what the guy doing the modifications feels comfortable with.

Some very good info. Thanks for your insight into all this, some of the best Info I have seen here in years. It gives people some Options on what they can do.

I got my 335 cylinder partially bead blasted, the plexiglass is getting so bad I need to replace it, can't hardly see what I'm doing. Going to try an replace that later today. What do you figure a 340 TNT Cylinder can handle for Max Carb Size? With the 335/340 TNT Intake Ports both 47mm Wide, could they be made a little Wider? Any Pro's/Con's on doing that?

Total window area ..... TNT - 29x47 = 1363 sq mm or 41+ mm circle

Total window area ..... 335 - 26.5x47 = 1246 sq mm or 39+ mm circle

Example 335:

Say Total window area ..... 335 - 26.5x48 = 1272 sq mm

Say Total window area ..... 335 - 26.5x49 = 1298.5 sq mm

Say Total window area ..... 335 - 26.5x50 = 1325 sq mm

Say Total window area ..... 335 - 26.5x51 = 1351.5 sq mm

 

[sightation]

What do you figure a 340 TNT Cylinder can handle for Max Carb Size? With the 335/340 TNT Intake Ports both 47mm Wide, could they be made a little Wider? Any Pro's/Con's on doing that?

You are starting to ask some good questions-

Second question first - can going wider help yes. How wide?

Well there is a rule of thumb intakes 65% of bore max. This has to do with the fact the piston pushes hard on the intake side of the cylinder wall -the bigger the intake hole - the less support piston
Stock - 47mm = 47/78 = 60.3% ... Mod - 51mm = 51/78 = 65.4%

Will racers go bigger - sure, but they are looking for every last drop of HP
I've seen TNT race cylinders with wider intakes - but we are talking trail sleds here. 50-51mm is reasonable for a 78mm bore trail sled. I'd personally 50 if it were my trail sled.

However this is a BIG THING to be careful of when widening intakes
Be sure the ring End gaps don't become exposed at BDC. Sometimes you have to shape the port so that the ring ends stay covered but the rest of the port is wider.

Maximum Carb size ??

This is a tough question - I'll need little more time to put together an explanation.

If you have ever heard the the phrase "time-area" of ports - you know that most books get very detailed and complex about how to figure it etc.

I'll try to put together a simple view

 

[RotaxFlyer]

You are starting to ask some good questions-

Second question first - can going wider help yes. How wide?

Well there is a rule of thumb intakes 65% of bore max. This has to do with the fact the piston pushes hard on the intake side of the cylinder wall -the bigger the intake hole - the less support piston
Stock - 47mm = 47/78 = 60.3% ... Mod - 51mm = 51/78 = 65.4%

Will racers go bigger - sure, but they are looking for every last drop of HP
I've seen TNT race cylinders with wider intakes - but we are talking trail sleds here. 50-51mm is reasonable for a 78mm bore trail sled. I'd personally 50 if it were my trail sled.

However this is a BIG THING to be careful of when widening intakes
Be sure the ring End gaps don't become exposed at BDC. Sometimes you have to shape the port so that the ring ends stay covered but the rest of the port is wider.

Maximum Carb size ??

This is a tough question - I'll need little more time to put together an explanation.

If you have ever heard the the phrase "time-area" of ports - you know that most books get very detailed and complex about how to figure it etc.

I'll try to put together a simple view

Since were really Talking Hot Trail Sleds here and not going for every last drop of hp as in Pro Racing, the "Rule of Thumb 65%" is a good Standard I think and since most people here probably have more Standard 335's anyway, which has more limits on what it can go to than the 340 TNT. But those People who have 340 TNT's can also see that the "Rule of Thumb 65%" is a good Standard for them also.

My Thinking was, a Standard 335 Cylinder with the 38mm Carb Mod and Intake Port Mod to maybe 26.5mm x 51mm Wide get's it real close to what the Stock 340 TNT makes, if CR and Muffler is changed to TNT Spec's. It would breathe now to at least 6500rpms.

To me, if your going to design it to say use 91 Octane Pump Gas and raise your CR with a Head mod, you ought to take full advantage either go 9.0cr to TNT 10.5cr or go Max for 91, say 11.5cr. If we use the 2hp per +1.0cr Rule.

Total window area ..... TNT - 29x47 = 1363 sq mm or 41+ mm circle

Total window area ..... 335 - 26.5x47 = 1246 sq mm or 39+ mm circle

Example 335:

Say Total window area ..... 335 - 26.5x48 = 1272 sq mm

Say Total window area ..... 335 - 26.5x49 = 1298.5 sq mm

Say Total window area ..... 335 - 26.5x50 = 1325 sq mm

Say Total window area ..... 335 - 26.5x51 = 1351.5 sq mm vs TNT 1363 sq mm

If I remember right, the Stock 340 TNT Intake Gasket was like 44.5mm ID, so probably Max 46-48mm Carb/EFI throttle Body.

To Widen the Intake to 51mm a 1/2" End Mill, 2mm = 0.07873992" each way. Or to widen for 50mm, 1.5mm = 0.05905494" each way.

To put this in USA, Inch Measurement 1/32" = 0.03125", 1/16" = 0.0625", 3/32" = 0.09375", 1/8" = 0.125" so your not widening it that much. So even hand Tools could be used.