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but a good rule to follow is to make the cylinder head's squish band about 50-percent of the cylinder bore area.
That is where the confusion comes from. People incorrectly assume that statement means 50% bore Diam
50% bore AREA is not 50% Bore DIAMETER

You Previously posted math for 72mm Bore 277
"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."

Equation for Area of a Circle - Area = Pi x R squared
73mm = 4185 square mm
36.5mm= 1046 square mm
4185-1046= 3139 = 75% squish band area NOT 50%

Correct math for 50% AREA on 73MM bore = approx.10.7mm band
NOT 18.25mm squish band as you previously calculated based on diameter only

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That is where the confusion comes from. People incorrectly assume that statement means 50% bore Diam
50% bore AREA is not 50% Bore DIAMETER

You Previously posted math for 72mm Bore 277
"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."

Equation for Area of a Circle - Area = Pi x R squared
73mm = 4185 square mm
36.5mm= 1046 square mm
4185-1046= 3139 = 75% squish band area NOT 50%

Correct math for 50% AREA on 73MM bore = approx.10.7mm band
NOT 18.5mm squish band as you previously calculated based on diameter only

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There you go getting all Technical. I notice you cut off part of what was said under photo 1!

"The Squish Bands should consitute "About HALF the Cylinder Bore Area." Clearance between Piston and Cylinder Head should be held to a Minimum to avoid effectively Losing about 5% of working Mixutre.

The Squish Band Actual Lenght will be different on every 2 Stroke Engine account they all use different SIZE COMBUSTION CHAMBERS and BORES.

The Squish Band starts at the outermost part of the Head Chamber. Like the Stock 277F Bore is 72mm, but most Stock Heads OD of the Chamber is 1mm = 0.03936996" Oversize or 0.5mm = 0.01968498" per side.
Your Piston Dome sticks up into that Chamber X Amount. While the Stock Chamber OD is usually 1mm Oversize from the Bore, the Squish Band also has a Height where it Starts set by the Angle. Piston Domes are not Straight lines, Squish Band Angles are. So the outermost part of the Squish Band might be 1.0 mm (0.03936996") to the Inner part 1.5 mm (0.05905494"). You need Clearance between the Piston and Squish Band account the Piston Exspans with Heat.

Part of the problem is the Stock 277 Head doesn't have a very wide Squish Band to start!

A very Good Read. "Squish Band/Squish Velocity info."


Quote #1 from Article. "The graph above is mine from the data. Head C's average power increase over a head w/o squish band from 5000 to 7000 is only 1.2%, a very minor amount, so I think this disproves the common concept that squish bands significantly increase engine power. It helps some, but probably mostly due to piston cooling. Below is a graph from the study that shows how minor the power differences are." You would probably be better off using the Ceramic Piston & Combustion Chamber Coatings to Cool the piston top!

Another Good Read. "Measuring Squish".

Quote #1 from Article. "Before the modification, the squish band was 1.9 mm(0.07480292") to 2.8 mm (0.1102359"). Compression 185 psi. After the modification, the squish band was 1.0 mm (0.03936996") to 1.5 mm (0.05905494"). Compression 195 psi." You have Gasket thickness that raises the Head. Then you have the Depth the Squish Band where it starts at the Outside. The Base Gasket thickness used can also affect the Squish Band, and the Base Gasket thickness also affects Port Timing.
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Stock 277 Head Chamber above gives you 11.8cc.

Like on this Chamber the outermost X Squish is 1.06mm = 0.04173216". Dome Height is 5.40 mm = 0.2125978". You also have Head Gasket Thickness used to consider.
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There you go getting all Technical. I notice you cut off part of what was said under photo 1!

"The Squish Bands should consitute "About HALF the Cylinder Bore Area." Clearance between Piston and Cylinder Head should be held to a Minimum to avoid effectively Losing about 5% of working Mixutre.

The Squish Band Actual Lenght will be different on every 2 Stroke Engine account they all use different SIZE COMBUSTION CHAMBERS and BORES.
If knowing the difference between Area and Diameter makes me too technical- I'll live with it.

Nothing was cut off in the photo -everything you quoted is there
From "About Half the Area" right up to "5% of working mixture." Perhaps you need a better computer monitor

You posted - "Like on this Chamber the outermost X Squish is 1.06mm = 0.04173216". Dome Height is 5.40 mm = 0.2125978".
When quoting numbers to 8 decimal accuracy - 75% Squish area is not "ABOUT HALF".
Not even with rounding.

Yes the squish band will be different widths for different bores. Math fact - 50% area of a Big number is MORE than 50% of a little number.
377vs 670 = 62vs78mm = 9.1 vs 11.4mm Squish band width.

The general purpose of a squish band is to keep the edges of a piston cool. An improperly designed Squish band width/height can reduce power or worse.
A 75% band , can reduce power by trapping way too much fuel that can't produce power. OR That wide of a band with too little clearance can make the Maximum Squish Velocity (MSV) so high it causes detonation destroying the piston it was supposed to protect.

Good article by dragonfly75- especially the part where he says
"Squish Band Surface Area - It varies from 30% - 60% of Bore area. The more area, the higher the squish velocity. It is large for low RPM engines to compensate for the low RPM since MSV lessens with less RPM. 35 to 45% is typical for high reving engines."

Stevetundra's 300 at 7500-7800 rpm is not low RPM - ( A 5500rpm Elan is)
75% squish area is not in the range dragonfly gives for any option - even low RPM
 

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Discussion Starter · #205 ·
Got the cylinder back from the mechanic (Dockside Marine). The cylinder wall to piston clearance is set up at .005”. Jason didn’t want to go any tighter as he’s seen one of these motors seize at .004”.

Fortunately, I double checked my work on the head. Squish was good at .036 at the piston edge and .047 at the inner edge of the squish band, but when I measured the compression chamber volume, I came up with 22cc. That’s a little low considering I was aiming for 23.8-24cc…. I compared head volume to the other heads and it was good, then checked the chamber volume with the 2nd gen head and it was good also…. It was then that I realized that I didn’t compensate for machining the gasket area outside of the squish step when I was adjusting head volume. Wish I’d a saw that when I cut the head, but glad that I double checked before putting the motor back in. I can picture the10 or so seconds of euphoria I’d have felt the first time I pinned it… and then the sadness, sitting on the silent sled after it blew up. So back to the machine shop I went.

I got it put together and it seems to run fine. Compression was the same as with the 2nd gen head at 135 psi (short adaptor on the tester). We have about 2” of snow on the ground which was enough for me to rip down the drive a couple of times and that was about it. It definitely feels like there is more torque, but I haven’t been on it since March so I will reserve judgement. Need more snow so I can hit my test strip. Can’t wait to see what the exhaust temps say. Should be lower due to a quicker burn with more of the heat staying in the motor. Also want to try the less aggressive CDI for the same reason. If all goes well, should be able to bring the timing a couple of degrees closer to TDC and perhaps step away from the ragged edge a little bit...

Daag
I intended to open up the squish angle a little more, but still ended up at 11 degrees. Measurements do show it opening up a little bit so I’m thinking it should be good.

On a side note, was able to do a rough comparison to a stock jug when I picked up mine. Confirmed the exhaust port was 1 mm higher and roughly 1.2 mm wider (after I opened it up a little bit this spring). Also, the bottom of the intake appears to be about 1 mm lower and it’s maybe .7 mm wider. This has me wondering if I could go with a bigger carb or open up the 34mm carb that’s on it? I’m leaving that for now though.

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Also, the bottom of the intake appears to be about 1 mm lower and it’s maybe .7 mm wider. This has me wondering if I could go with a bigger carb or open up the 34mm carb that’s on it? I’m leaving that for now though.
Back when we were racing - I was able to get a little bit more on the dyno by going to a 36mm carb vs the original 34 BUT
The intake manifold and carb boot were restrictive and needed work to actually gain anything.

Ended up grinding the manifold and switching to a curved piece of radiator hose to mount the carb


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Discussion Starter · #207 ·
Back when we were racing - I was able to get a little bit more on the dyno by going to a 36mm carb vs the original 34 BUT
The intake manifold and carb boot were restrictive and needed work to actually gain anything.

Ended up grinding the manifold and switching to a curved piece of radiator hose to mount the carb


View attachment 1980244
Thanks Sightation!
I was wondering about adapting a 36mm carb. Probably not to bad to deal with, though I'd have to figure out if I could rig it up to my air box. I do have a little bit of concern about bottom end as it isn't all that great. Did going with the 36 effect that any?
I had also thought about just opening up the upper 2/3rds of my 34, but I'm guessing I'd hardly notice it. At any rate I need to see how she runs now and play with the CDIs and see if there is any jetting that needs doing.
 

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Thanks Sightation!
I was wondering about adapting a 36mm carb. Probably not to bad to deal with, though I'd have to figure out if I could rig it up to my air box. I do have a little bit of concern about bottom end as it isn't all that great. Did going with the 36 effect that any?
I had also thought about just opening up the upper 2/3rds of my 34, but I'm guessing I'd hardly notice it. At any rate I need to see how she runs now and play with the CDIs and see if there is any jetting that needs doing.
I wish I could tell you if it made a difference in low end - but oval racing we had the engagement set at around 4500rpm and geared for power off corners more than speed. Never spent much time at anything but full throttle

We ran without the air box. One thing you might try - D&D Racing Bullseye airbox relief holes. Nicer than gutting the box , These things let more air into the box - sometimes help sometimes not, depending on airbox restriction level. Need some jetting changes afterwards.
 

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Unless there are different Intake manifolds, and I thought the 277, 377, 440, 503 all used the Same Intake, mine sitting on my desk, the ID = 38mm! So the 277UL rated [email protected] uses a 36mm Bing 54, the Sled version was rated [email protected] and used a Mikuni 34mm Round Side.

Niko runs a 38mm Mikuni Flat Side on his Racing 277F. Turns it 9200rpm using a Modified Aermacchi Dual Plug motorcycle head running 14.0cr with a Modified 670 Tuned Pipe! He estimated making about 55hp. I bought one of them Dual Plugs Heads off eBay for like $40.
 

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I only know what the dyno told me -
Better flow = more power

Blended the area inside the manifold where it transitions from round to rectangle. Used a shape similar to ports used on the old singles.

Also knife edged (beveled) the manifold lip to cut down on turbulence as fuel entered manifold from rubber boot.

Ground the ridges out of the carb boot - but worried about rubber cracking afterward - switched to the curved radiator hose that was smooth to start.



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I only know what the dyno told me -
Better flow = more power

Blended the area inside the manifold where it transitions from round to rectangle. Used a shape similar to ports used on the old singles.

Also knife edged (beveled) the manifold lip to cut down on turbulence as fuel entered manifold from rubber boot.

Ground the ridges out of the carb boot - but worried about rubber cracking afterward - switched to the curved radiator hose that was smooth to start.



View attachment 1981708 View attachment 1981709
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Oh I agree with doing everything you can to Increase Air Flow in the Intake, Intake Ports, and Exhaust Ports and Case. I have never seen the Sled 277F Exhaust, or a Dyno Print from one. All we know is what the manual says it was rated, [email protected] with a 34mm Carb.

One version of the 277UL was rated [email protected] but actually Dynoed Max [email protected] with a 36mm Carb, so my thinking is that 277UL Muffler is too Restrictive. That's one of the reasons I started researching into How to Build my own Dyno. I'm mainly interested in the Old Skidoo Singles and the 277UL. But if your going to Build one, you just as well spend the extra $200-$300 and make it to do Bigger Engines. Never seen the 277UL rated [email protected] Dynoed. From the 277UL 26hp Dyno Sheet it was Averaging +2.8hp per +250rpm up to about 5500rpm.

JBM makes some of the Best Carb Boots and Seals.

I got my better Digital Caliber out and remeasured the Intake ID and it was 37.6mm not 38mm, next Circle is 38mm, then 40mm. OD was 44.03mm. +2mm = About 10% more Air = 10% more hp. I bought a 38mm Mikuni Flat Side off eBay to put on mine.


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RotaxFlyer ---
Choosing proper size carbs is more than just putting a Micrometer on the holes.
Too big a carb can kill low and mid range throttle response on a trail sled like Steves.

38mm on 583s turn 7700 rpm no problem - A 277 would need to turn over 8000 rpm to pump that much air. (Like your buddy Nico)

You already know a 36mm can support [email protected] on a 277.
You already know a 36mm carb can support [email protected] on a 503
You already know a 36mm carb can support [email protected] on a 582
A measly 34mm carb can support [email protected] rpm on a 550F

I sure hope you have all the porting tools you need to be able to port your 277 wild enough to efficiently use that 38 flat slide at high RPM.



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Discussion Starter · #213 ·
We got some snow, but not enough for grooming, so I haven't been able to do much testing. I'm pretty sure though that it runs at least as well.

After 25 miles of pretty hard running, I took the motor apart. I found that the issue with the pitting or detonation under the ring is still there and a slight hint of issues at the locating pin as well. The good thing is that taking it apart after a short run time, I was able to see without a doubt that the pitting starts at the exhaust end which I'm guessing is heat related?
At any rate, tightening up the squish and piston to cylinder wall didn't do what I had hoped.

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I'm pretty much resigned to the fact that I need to run less aggressive timing. I will put the other new piston in so I can pull it apart later and make sure the issue is gone. Once we get enough snow to test, I see how the new head performs and do a quick comparison between the 2 CDIs.

I did do one plug chop with a used plug. About a minute WOT in deeper snow at 32f outside, hitting the kill switch then letting off the gas and hitting the brake. Roughly 7500rpms with a max exhaust temp of 1130f. I've never been very confident reading plugs, especially used ones. It looks rich to me and there is no timing indication on the center electrode
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After 25 miles of pretty hard running, I took the motor apart. I found that the issue with the pitting or detonation under the ring is still there and a slight hint of issues at the locating pin as well. The good thing is that taking it apart after a short run time, I was able to see without a doubt that the pitting starts at the exhaust end which I'm guessing is heat related?.
Not going to comment on the Plugs - my screen may be distorting colors. But given all the problems you have had- darker (rich) is better in my book.

What brand piston/rings are you using?
Some aftermarket pistons have a reputation for having soft rings that wear quickly. I won't mention specific brands - but if part number starts with
09-xxxx they likely came from a single Taiwanese supplier that uses that system for all their private label customers.

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Discussion Starter · #216 · (Edited)
Sightation
I've only used OEM piston and rings so far. Unfortunately, the ring gap comes already out of "new" spec but is still in the wear limit at .018". I cannot find OEM 10 over rings without buying the piston as well. I do have a Kimpex ring that looks like it will work but I'm hesitant to try it.
The issue with the ring has unfortunately been a distraction to the original intent of this thread. I put a new piston and rings in and switched over to the less aggressive CDI. (To recap, the ring issue didn't surface until I changed the timing and I've been stubbornly trying to correct the issue without losing the gains seen with the aggressive timing.) My plan is to run with the less aggressive CDI for a while (with the exception of a few runs testing the new Gen3 head to compare to runs I did with the Gen2 head last year.) and inspect the ring again. If it's still rough, I will probably put a thicker base gasket in it.

Daag
While the plug looks dark, I think it looks a little suspicious too and the EGT seem to be a little warm. That is the plug Iwas running at the end of the year though so the textured look might have already been there? However, I noticed that I had a 230 MJ in there and that's 1 step leaner than I found was optimal last year. I swapped that out for a 250mj.

We lost most of our snow this last week and there isn't any in the forecast so testing is still a ways out. The riding I did do showed at least no loss in performance with the new head. One thing I noticed is it is running a little hotter then I expected given that I've read the exhaust should be cooler with the mods I've done to the squish. Supposedly, with more of the fuel mixture being pushed to the center the burn should be faster and the piston edge cooler with little to no fuel there, both leading to less heat escaping the engine. I was running the MJ one step leaner so that might be part of it, though ambient temps were pretty warm.

Jetting
Has anyone who has modified the squish needed to change jetting at all? I've heard timing should be retarded to compensate for the quicker burn, but I haven't read anything about changing the jetting.
I did smooth out the transfer ports and opened up the exhaust port width just near the top about .040 and polished it. Could this effect jetting?
 

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RotaxFlyer ---
Choosing proper size carbs is more than just putting a Micrometer on the holes.
Too big a carb can kill low and mid range throttle response on a trail sled like Steves.

38mm on 583s turn 7700 rpm no problem - A 277 would need to turn over 8000 rpm to pump that much air. (Like your buddy Nico)

You already know a 36mm can support [email protected] on a 277.
You already know a 36mm carb can support [email protected] on a 503
You already know a 36mm carb can support [email protected] on a 582
A measly 34mm carb can support [email protected] rpm on a 550F

I sure hope you have all the porting tools you need to be able to port your 277 wild enough to efficiently use that 38 flat slide at high RPM.



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The 277UL, 268.8cc uses a 36mm Carb! You can see in the Dyno Print on the one rated [email protected] it started losing Power around 5500rpm and made max Power 25.4hp@6000rpm. So either under Carbed, or to Small of Porting, or Exhaust to Restrictive. Never seen the [email protected] version Dynoed. Before the 26hp version started losing Power about 5500rpm it was avg 2.8hp per +250rpm.

That's Twin 36mm Carbs on a 582 making 65hp/2= 32.5hp using a header muffler exhaust. Rotax Rick has a Tuned Pipe for the 582/583 that's making [email protected] I think. R&D Aero's 618UL Tuned Pipe on a 583 made around [email protected] At 580.9cc/2= 290.45cc. My 292 TNT Single is 291.69cc and has a 38mm Carb!

503 Dual Carb is 49.6hp Rotax rated it [email protected]
503 Single Carb is 45.6hp.
503 Free Airs were supposed to be +2hp, but I have never seen any Dyno Proof!

The 2004 550F Dynoed 70hp@7000rpm with a pair of 34mm carbs. Aaens Tuned Pipe made +8hp = 78hp. 550F = 553.6cc/7cc= 79.0hp!
Later 550F's were Detuned using a pair of 30mm carbs rated [email protected]! So -12hp going from 34mm to 30mm Carbs.

Hot Trail / Oval Race 277F Engine!
268.8cc = 16.40318ci x 7500 max rpm = 123,023.85.
Take 123,023.85 x 1.05 (105% VE) = 129,175.0425.
Then 129,175.0425 / 3456 = 37.377 cfm x 2 for a 2 Stroke = 74.754 cfm = 34mm Carb minimum needed.
==============================
----> If we use the 112% VE of the 380HO the 268.8cc makes 74.3cfm@7000rpm.

268.8cc = 16.40318ci x 7000 max rpm = 114,822.26.
Take 114,822.26 x 1.12 (112% VE) = 128,600.9312.
Then 128,600.9312 / 3456 = 37.210 cfm x 2 for a 2 Stroke = 74.421 cfm = 34mm Carb minimum needed.

An Ex Racer once said, a 2 stroke Gulps Air and you should Size your Carb about +20%
74.421cfm +20% = 89.306cfm which needs a 37mm = 91.3cfm. While you can Special Order Odd Number Carbs, most Shops only Stock Even Number Carbs. With a 36mm (86.5cfm) it would probably be starving for Air on the top end, using a 38mm would be most logical to use to make max hp. Intake ID 37.6mm, next Circle is 38mm. Difference .4mm = 0.01574798 in.

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As we discussed earlier, my Intake sitting on my Desk measured ID 37.6mm and could easily be Ported & Polished to probably 39mm.

Have you used one of the Dial A Jet products? Any Pro's and Con's?
 

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Daag
While the plug looks dark, I think it looks a little suspicious too and the EGT seem to be a little warm. That is the plug I was running at the end of the year though so the textured look might have already been there? However, I noticed that I had a 230 MJ in there and that's 1 step leaner than I found was optimal last year. I swapped that out for a 250mj.
I keep forgetting it is a single lol
 

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The 277UL, 268.8cc uses a 36mm Carb! You can see in the Dyno Print on the one rated [email protected] it started losing Power around 5500rpm and made max Power 25.4hp@6000rpm. So either under Carbed, or to Small of Porting, or Exhaust to Restrictive.

The 2004 550F Dynoed 70hp@7000rpm with a pair of 34mm carbs. Aaens Tuned Pipe made +8hp = 78hp. 550F = 553.6cc/7cc= 79.0hp!
Later 550F's were Detuned using a pair of 30mm carbs rated [email protected]! So -12hp going from 34mm to 30mm Carbs.
As I said - too big of a carb can hurt low end performance.
38mm flatside carb can be shown to be larger than needed for a 277 TRAIL sled.

Per YOUR NUMBERS
[email protected] 6800rpm - 30mm carb= 29hp per carb (58/2)
[email protected] - 34mm carbs= 39hp per carb (78/2)
Those numbers indicate:
The 28hp 277UL could have used a 30mm carb. (28hp less than 29hp)
34mm enough for 277F at 268.8cc/7 = 38.4hp. (38.4hp less than 39hp)
The 26hp 277UL is not under carbed - it is port / exhaust restricted.

Rotax used 34mm carb on the 277F because it was a big enough carb.
Rotax used 36mm carb on 277UL because of parts standardization. All ultralight engines used 36mm regardless of size.

Since you are an ultralight flyer - Take a look at the 447UL dyno sheet.
A SINGLE 36mm carb gives better performance than the twin 36mm carbs all the way to 5000 rpm where the twins finally catch up because the single manifold restricts flow. Just 2hp more at 6500rpm where they both drop off due to ports/exhaust

Again, too big of a carb hurts the low end performance needed on TRAIL snowmobiles like Stevetundras


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Discussion Starter · #220 ·
Daag
I think you are forgetting this is a single. ;)

Rotax
At the moment, I'm more interested in the effects the squish and porting work has on the carb that I'm currently using then a bigger carb.


Jetting
Has anyone who has modified the squish on a motor needed to change jetting at all? I've heard timing should be retarded to compensate for the quicker burn, but I haven't read anything about changing the jetting.
I did smooth out the transfer ports and opened up the exhaust port width just near the top about .040 and polished it and the exhaust manifold. Could this effect jetting? Perhaps increasing air flow and the MJ needs to be bigger to compensate?
 
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