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Discussion Starter · #181 ·
Thanks xlcc.

I found your write up in General Discussion inspirational as well. Taking your Tundra 300 into those areas is over the top.

I think upping the compression is a great idea given the elevation you run at.

The original head that Aaen machined had .040 off the head and .020 off the squish at 16 degrees. The dome volume is 24.7cc give or take and the squish is .075" at the cylinder wall. With stock timing, I ran with this head a full season, including long pulls at WOT without issue. I had the Main Jet down to 190 for some of that time and was comfortable with exhaust temps running up to 1250f and rpms around 7600.

"The short easy answer", keeping in mind I have absolutely zero experience with anything other than this motor:

I'd be comfortable taking .020" off and calling it good. Stock head volume is 27.2cc and taking off .020 will bring that down to around 25.2 which is what I'm running now. With stock timing, that should be fine. Squish would be somewhere around .075"

"More work" answer:

I would say the head I'm using now would work better but it's a fair amount more work. In order to get the squish down, I had to machine the head flush with the squish band, another words, the step is gone. I machined the squish at about 10.5 degrees so the squish opens up just a touch. That gives a squish of .060. So it would be hard to get tighter squish without other mods. I don't remember for sure, but I think I took .040-.060" off so I had to take quite a bit out of the dome to get the volume back up to a safe level.

I'm pretty sure I will machine another head. One thing I will do is change the squish angle from 10.5 degrees to 12ish degrees. This will ensure the squish opens up a touch more, increase the size of the squish band, and also will remove a touch more metal toward the dome. I want to tighten the squish a touch more too but I'm not sure what route I will take. I have 3 options.

  1. Take .010 or so off the bottom of the jug and raise the exhaust port back up.
  2. Take .010 or so off the top of the jug and machine the oring groove back to proper depth
  3. Machine a reverse? Step in the head so the squish sits down in the bore a touch.

No matter which way I go, I will need to take a little more out of the head to keep the volume at 25cc. On the gen 2 head I exposed the spark plug a little more than I like, though it didn't seem to hurt any.

More piston stuff. The local mechanic had a SPI piston .010 over so I took it home to compare with my OEM piston. The dome (from the pin) is .010 shorter at the edge which is the wrong direction as I want less squish not more, and the center of the dome is .010 higher than OEM. I have a 10 over OEM piston coming from the local dealer that I will look at and see what my next move is.
 

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Thanks stevetundra,knocking .020 off the head to get more compression is easy to do.Seems the squish is so great I wonder if it would be noticeable weather it is .075 or .060"?I kinda doubt it but it is nice to know that you are trying for the optimum.

The sledding around here is really good right now.It is possible to get into some areas that needs Spring snow to do it.I will likely wait 'till the season is done before doing any engine work.

Yes, that winter camping is fun.
 

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xlcc

That's actually what I would do in your shoes. Hind sight, I would have stayed stock exhaust and did the same thing. With the porting and tuned pipe, I gained some top end, but lost a little torque in the process. With this sleds intended use, I think the torque is more important. This may be a little too basic and maybe wrong, but with CVT I look at it like hp gets you the rpm (speed) but torque is what holds it there. I repaired the stock pipe enough to try it with the Aaen head and it had more torque or grunt, but didn't have the top end. The pipe also ran hotter with that head and porting. I had to richen the needle and MJ. I also swapped heads with a stock one with my current exhaust and found the shaved head had noticeably more power.

Now that I'm already on this path, I'm enjoying the challenge-it's good for my brain cells. In a nut shell, I'm trying to eliminate the issues with the piston and ring while not losing any performance and if I'm lucky, bring back a little bit of that torque.

Here's a couple of reads on squish. In the first article, the tundra's stock timing is already conservative,

DTRSQUISH (dynotechresearch.com)

https://www.motorcycle.com/how-to/print/wrenching-with-robchemical-soup-the-mystery-of-detonation-3420.htm
 

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Interesting articles,I always thought that the tighter squish clearances around .030 were optimum.Thinking about the rapid burn process of a tight or minimal squish clearance compared to say a tapered or or wider clearance when the burning process starts and as the pressure rises it is pushing outwards in all directions so it will be trying to force the unburned fuel back into the squish area where it is cooler and not going to help much to produce more power.

Those articles were very clear explaining this.Good information here.

The shape of the combustion chamber and piston crown will also have an effect.

Thank you for enlightening me.
 

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FYI, you can talk about power (Hp) without torque. The reference to torque is normally used as a means to show higher power in the lower end of the rpm range, yet power is measured in Hp. In other words you can refer to having more power in the lower rpm range, which will amount to the same as more torque.

Take a low rpm for a snowmobile @ 4000-5000 rpm, or a truck @ 1000-2000 rpm. The ones with more torque also have more power.

I think the reason we refer to torque is to counter the effect of comparing peak Hp which doesn't do well to show the full picture. For example I may boast a truck having 200 Hp, but it doesn't do so well against the truck which has 150 Hp but more power down in the low rpm to climb a hill without having to play so much with going through the gears to keep the engine in its peak Hp.
 

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Discussion Starter · #186 ·
Hi Daag

I'm not sure I agree or disagree as we might be talking about slightly different things. I want to follow up, but I want to be careful we don't go to far off tangent.

From what I understand, torque is the expression of rotational force (tightening a bolt or opening a jar of pickles) or the ability to perform work and power is defined as the rate at which the work is done. This video kind of explains how I see torque and hp.

Horsepower vs Torque - A Simple Explanation - YouTube

Peak power is higher then the peak torque curve. I see peak torque as that rpm where you feel the hardest acceleration. As you get past the peak torque rpm, that pull you feel decreases, but you are still accelerating and making more hp. I don't have Aaen's clutch book handy and I'm not sure I have this right, but he said something to the effect that under load, the rpms resist dropping down because they are dropping down into the peak torque curve so torque is increasing. In essence, balancing peak torque and peak hp helps keep the motor spinning a tuned clutch at a given rpm. A higher strung motor doesn't have as well of a defined peak torque or perhaps it's lower, so it is harder to hold rpms at peak power.
 

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After reading your reply and doing a little editing, I came to the conclusion that you have the right idea with torque being an appropriate measure of power and in many cases a better one. I am so used to looking at Hp power curves from snowmobiles that I have inadvertently removed Torque out of the equation. It stems mostly from conversations about small to medium size engines for karts and work equipment like a chipper. What I keep hearing is a Honda will produce as much power than a cheap brand showing higher Hp. I am not informed with how they measure Hp for these types of engines nor the accuracy, but with understanding the snowmobile segment I can imagine how the Hp ratings can be deceiving. On a wood chipper I need the torque down low to keep it from stalling when it slows down. On a chainsaw I can regulate the speed of the engine with how hard I press down on the saw. So if I fall out of the peak power, then I can ease up and let it reach its peak power. It does get a little tricky when I am cutting a limb 70 feet up and the small arborist saw jams at the worst time. I have used a cordless saw at those heights in the hardest of wood, and I did not enjoy having so little torque :)
 

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I read through a couple of threads that talked about porting on this motor which got me looking closer at mine. It appears that Olav opened up the Intake as well as the exhaust port. I also found this tuning guide which (I think) seems to be pretty good.

Two_Stroke_Performance_Tuning.pdf (vintagesleds.com)

I measured the ports to get duration and sizes. What I need is to verify what I have and get a little more guidence on what it means.
The book above suggests that with this style of piston and its L shaped ring that port opening and closing should be measured from the ring and not the piston top edge.
I used this calculator with 66mm stroke, 120mm conrod, and 2mm crown to top of barrel (top of ring).

Port timing calculators - Lambretta Images Archive (lambretta-images.com)

Exhaust Port: 30x42mm (44mm wide now)
Top 35.5mm
Bottom 65.5mm
Duration 194

Transfer Ports:
Top 51.5mm
Bottom 68mm
Duration 135

Blowdown?? 29.5 Not sure what this is, but there was a calculation for it..

Intake Port: 30x43
Top 5mm (Piston ring measured with bottom of piston at top of port)
Bottom 35mm* (piston ring measured with Bottom of piston at bottom of port)
Piston skirt 67.5mm (Piston height inlet side from top ring)
Used 67.5 + 35mm = 102.5 for Inlet Floor depth
Duration 164

Looking at the Intake port, it appears that it is open 2-3mm lower then stock, which may explain my problem with it bogging when I hit the throttle from idle.

After some discussion with the local mechanic, I opened up the exhaust port sides near the top about 2 mm total. I didn't want to do too much and I was careful to leave the top rounded for the ring.

Also, I hadn't realized the intake was opened up. So along with the exhaust being opened as well, I'm also now wondering about boring out the carb a little bit.

The only place I noticed carbon build up beside a little on the piston top was in the exhaust socket. There was almost a ring of carbon about 1 ¼" or so out from the gasket.
 

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I don't like the new shape drawn for the exhaust port. Too much ring movement for my liking. Blowdown is the amount of exhaust port duration before the transfer ports open to the fresh charge.

Following your progress with and explanations of what you are grasping from tuners like AGB, it sure makes a difference when I read through A Graham Bell's performance handbook today. Have you thought of reaching out to reach out to Kevin Cameron with a mention of what you have been doing on DooTalk? I can't see anyone better suited to understand what Jennings and Bell wrote about performance tuning, and how it applies to snowmobiles.
 

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xlcc

That's actually what I would do in your shoes. Hind sight, I would have stayed stock exhaust and did the same thing. With the porting and tuned pipe, I gained some top end, but lost a little torque in the process. With this sleds intended use, I think the torque is more important. This may be a little too basic and maybe wrong, but with CVT I look at it like hp gets you the rpm (speed) but torque is what holds it there. I repaired the stock pipe enough to try it with the Aaen head and it had more torque or grunt, but didn't have the top end. The pipe also ran hotter with that head and porting. I had to richen the needle and MJ. I also swapped heads with a stock one with my current exhaust and found the shaved head had noticeably more power.

Now that I'm already on this path, I'm enjoying the challenge-it's good for my brain cells. In a nut shell, I'm trying to eliminate the issues with the piston and ring while not losing any performance and if I'm lucky, bring back a little bit of that torque.

Here's a couple of reads on squish. In the first article, the tundra's stock timing is already conservative,

DTRSQUISH (dynotechresearch.com)

https://www.motorcycle.com/how-to/p...mical-soup-the-mystery-of-detonation-3420.htm
===============================

Torque and Hp go hand in hand with RPM. Sayyour engine makes [email protected] = 21.0 ft lbs of Torque. Peak hp and Peak Torque happen at different rpms. If Torque goes down Hp made goes down. Messing with the Port Timing is where your problem is. Squish Band has more to do with Detonation. Too Wide of Squish Band Lowers your Compression. I have seen anywhere from 0.024" to 0.065" Squish Band Clearance. With your 277F, 72mm Bore, Head Chamber is usually 1mm Over-Size, but measure it, so 73mm at 50% = 36.5mm/2 = 18.25mm is the Squish Band Width. For Squish Band Angles, I have seen anywhere from 7 degrees to 17 degrees. You should be able to put your Piston at TDC and then Measure the Angle from Case Deck to the Center of the Piston Dome. You have to also factor in Head Gasket Thickness. The Higher the CR you run the tighter the Squish Band should be. Piston Domes between Brands of Pistons can be +/- 0.010".

[email protected] = 21.8 ft-lbs of Torque.
[email protected] = 22.5 ft-lbs of Torque.
[email protected] = 23.3 ft-lbs of Torque.
[email protected] = 24.0 ft-lbs of Torque.
[email protected] = 24.8 ft-lbs of Torque.

HP to TORQUE CALC.

Used Stock 277UL 11.8cr Compression 150psi. What are you seeing? If your Piston is say 4mm High at the Center your Angle would be 6.31°. Remember, your Head Gasket used, also raises the Head X amount in thousandths of an inch. Your Chamber Depth starts X amount from the Bottom of the Head also.
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Quick clarification on squish area
Way back on page 2 of this thread, two pictures were posted that correctly depicted what a 50% squish AREA looks like.

Unfortunately still confusion out there thinking 50% diameter = 50% Area

For the stated 73mm chamber diameter. A 50% squish band area = about a 10.7mm wide squish band.

A 18.25mm Squish band width as calculated in the previous post = 75% squish area
Such a high squish area traps an excess amount of fuel that is not available to make power. It burns too late, mostly in the pipe after the exhaust port is open, reducing cylinder pressure/power


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Quick clarification on squish area
Way back on page 2 of this thread, two pictures were posted that correctly depicted what a 50% squish AREA looks like.

Unfortunately still confusion out there thinking 50% diameter = 50% Area

For the stated 73mm chamber diameter. A 50% squish band area = about a 10.7mm wide squish band.

A 18.25mm Squish band width as calculated in the previous post = 75% squish area
Such a high squish area traps an excess amount of fuel that is not available to make power. It burns too late, mostly in the pipe after the exhaust port is open, reducing cylinder pressure/power


View attachment 1975150 View attachment 1975151
I think you are understating the confusion. Below is an older link showing the squish area from different OEM engines having between 54% and 74%.

Squish Area
 

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I think you are understating the confusion. Below is an older link showing the squish area from different OEM engines having between 54% and 74%.
Interesting you showed the 74% Blaster head back then.
The old McDizzy two stroke site did a big write up of modifying a Blaster.

He spotted the issue with the head pretty quickly.
For the modification package he cut the squish area back from 74% to 55%.
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Interesting you showed the 74% Blaster head back then.
The old McDizzy two stroke site did a big write up of modifying a Blaster.

He spotted the issue with the head pretty quickly.
For the modification package he cut the squish area back from 74% to 55%.
View attachment 1975193 View attachment 1975194
I added a link below to his build with the head. Interesting that he added a blend radius. Rick has good info on his site. Is the subscription/forum portion accessible for you? It sends me to some Mac Dizzy auto sport website in France.

1989 Blaster Engine Rebuild - Part 10
 

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Is the subscription/forum portion accessible for you? It sends me to some Mac Dizzy auto sport website in France.

1989 Blaster Engine Rebuild - Part 10
I used to be a member - along with a number of the other racers in the area. Funny we shared each others secrets.... But we mostly had different motors in different classes.

Had trouble with PayPal and let the subscription expire. Looks like the site is basically dead- copyrights 2011. Luckily some of the links still work. Rick archived a lot of what he did, and had it for free on the site.

But you are right - subscription stuff goes to France now.
Wonder what would happen if you emailed the old subscription address.
[email protected]
 

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I used to be a member - along with a number of the other racers in the area. Funny we shared each others secrets.... But we mostly had different motors in different classes.

Had trouble with PayPal and let the subscription expire. Looks like the site is basically dead- copyrights 2011. Luckily some of the links still work. Rick archived a lot of what he did, and had it for free on the site.

But you are right - subscription stuff goes to France now.
Wonder what would happen if you emailed the old subscription address.
[email protected]
The updates link as shown below says it hasn't been updated in 10 years, but it appears to be an automated answer using today's date to show the number of years. I hope not all is lost on that forum. I sent an email and awaiting a response.

MacDizzy's Updates 2011!
 

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Discussion Starter · #198 ·
Good catch on the squish band issue on that previous post.


It’s getting to be that time of year again when I start to get antsy so here’s a little update.

3rd generation head/Squish
Earlier this summer I was able to machine a head with a tighter squish and larger squish band. In order to do this, I had to machine a step in the head. I elected to do this instead of machining the top of the jug as a mistake on the jug is a little more costly.

I haven’t had it together yet to measure, but the Squish should be around .035” with a squish band of just about 10mm. the angle is 11 degrees. For reference, Squish on the 2nd gen head is .062” with a band of 8mm with an angle of 10.5 degrees. Volume is the same. Just for grins I have different thickness base gaskets coming so I can play a little bit with volume/compression. Standard is .010” and I have .015 and .020 coming. This way I can increase the volume by as much as 1 cc. I realize this will affect the port timing a little but I’m guessing it won’t be that noticeable.

Piston to wall clearance
The Jug and a new OEM .010 over piston is currently at the shop waiting for work. We are hoping to tighten up the clearance to roughly .004-.005” from .008-.009”. The mechanic doesn’t want to go any tighter because he’s seen (aftermarket) pistons cold seize at .004 on that engine.


So I’m hoping that between tightening up the piston to wall clearance and tightening up the squish, I can eliminate the piston ring failure issue. I don’t think it will take much as it was still running after a full season though you could clearly see damage. Along with hopefully reducing heat near the piston edge and reducing knock, tightening the squish should create more turbulence for a more complete burn. Also, perhaps shorten burn time which might change when spark is needed. Perhaps even to the point that using the RMS CDI with less aggressive timing might be beneficial. That’s a long shot, but it sure would be cool.

Here is a couple pix of the 3rd gen head. I might still clean it up a little.



Steve
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From 2 Stroke Tuners book. But if you want to use a true (measured from exhaust-closing) compression ratio much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a non-squish cylinder head will be necessary. Considerable variation is possible, but a good rule to follow is to make the cylinder head's squish band about 50-percent of the cylinder bore area. For example, in a 3-inch bore - which has a total area of 7.07-inches 2 the squish band would be wide enough to represent an area of just about 3.5 in2 . Assuming that you have centered the combustion chamber proper on the bore axis, then your squish band would be a ring having the same outer diameter as the bore, and an inner diameter of just over 2-inches. The combustion chamber itself, to meet the previously-stated minimum surface/volume requirement, would again be a spherical segment - with a radius that provides the total volume, added with that from the clearance space between piston and squish band, to give the desired compression ratio. The clearance space between piston and cylinder head must be enough to avoid contact at high engine speeds, yet close enough to keep the mixture held there cooled during the combustion process. This vertical clearance between squish band and piston should not be greater than 0.060-inch (1.5mm), and it is my opinion that the minimum Two-Stroke TUNER’S HANDBOOK 42 should be only barely enough to prevent contact - usually about 0.015-inch in small engines (with tight bearings and cylinder/rod combinations that do not grow, with heat, disproportionately) and up to about 0.045-inch in big engines. 2Stroke Tuners Book. Most Heads Chamber is 1mm wider than Bore.

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