# In general..whats the compression ratio of most 2 stroke engines?



## slipknot (May 10, 2011)

Hey folks....Does anyone know what the compression ratio is on most 2 strokes. If you know specifics about a make and model post that....or if you are a porter... how high are you running? I sure would like to find out what some of my saws are running if there was a way to figure that....if there is i dont know how...so im here to learn....


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## HILLBILLYREDNEC (May 10, 2011)

*Compression*

In most two strokes 120 + is good, tho there are some weird motors that run less.


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## mtngun (May 10, 2011)

slipknot said:


> Hey folks....Does anyone know what the compression ratio is on most 2 strokes. If you know specifics about a make and model post that....or if you are a porter... how high are you running? I sure would like to find out what some of my saws are running if there was a way to figure that....if there is i dont know how...so im here to learn....


6.5 - 7.5 to one is pretty typical swept compression (only counting above the exhaust port).

8.5 - 10.5 would be typical uncorrected ratios. 

I'm at work now, but could post formulas and measurement techniques tonight.

Bikes and carts run higher ratios, but they have centrally located spark plugs, longer strokes, and better cooling.


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## slipknot (May 10, 2011)

Can it be compared to say how a chevy 396 might have 11.5 to 1 compression ratio? Is most 2strokes something like 12:1?? thats what im looking for...that number in general. Thanks for the replies though fellas


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## Stihl Crazy (May 10, 2011)

2 stroke will run lower than the Chevy. 6.5-8.5 range seems right.


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## slipknot (May 10, 2011)

Stihl Crazy said:


> 2 stroke will run lower than the Chevy. 6.5-8.5 range seems right.


 
Wow..i thought they were higher compression engines and thats why we have to run 93 octane in them. Also SC...i wasnt being a smart azz about that efco case...if you need it or just want it....id give it to you for sending me that carb tuning manual while back..just pay the shipping.....Pm if you want it....I Know i know...its yellow and you would'nt want a mcdonald's edition efco 152 (red and yellow) But if ya did:msp_wink:


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## Nitroman (May 10, 2011)

Do the engines have expansion chambers/pipes? If so the numbers will be lower. My Polaris XCR-800 is running ~140psi static, but with the supercharging effect of the pipes, this increases when the engine is running.

Some on here with mildly modded motors run 170psi static. Check with TW and Brad for some of their numbers.


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## SkippyKtm (May 10, 2011)

*I found some good info:*

*http://www.sacoriver.net/~red/uccr.html*


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## tdi-rick (May 10, 2011)

mtngun's already mentioned it but here's a fuller explanation between corrected and uncorrected compression ratios.

Uncorrected compression ratios are where you measure the pistons swept volume divided by the measured chamber volume at TDC, as you would in a four stroke.
When I raced karts in another lifetime, most classes had a stipulated chamber volume which my engine builder would measure with oil and a burrette, exactly the same as when building a four stroke. 

Corrected C/R's are the swept volume _above_ the exhaust port divided by the chamber volume and is related to what you and everyone on AS measure with your compression gauge's, ie. the higher the exhaust port, the lower the pressure you'll measure with a comp gauge and vice versa. 

You will have a lower corrected c/r on a piped race engine (higher exhaust port) as the pipe will pack the charge back in while still having a higher uncorrected c/r (smaller chamber) than a stock saw, even though the stock saw will show a higher comp gauge pressure.
A compression gauge only tells part of the story.

As mtngun mentioned it's a bit harder to measure the chamber volume accurately on a saw as the sparkplug is offset. Karts and bikes generally have a central plug which makes it easy and accurate to do.


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## wyk (May 10, 2011)

This has intrigued me some lately. When you test the PSI on a saw, do you plug both the intake and exhaust? And when you do, what's to keep the PSI from going up and up and up until it explodes, assuming enough arm strength? Where is the leak? When does it stop? "How does it know?" Also, how do you compare a saw's PSI to a 4 stroke engine, etc. Is 170 psi about what an 11:1 engine would get ya? etc etc. I'm curious. I know it can't be as easy as taking the PSI and dividing it by Bars(14.5PSI) to get the compression ratio...


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## heimannm (May 10, 2011)

They're not modern, but many of the old McCulloch saws were 5:1 to 6:1 or just a touch higher.

Certainly compression ratio and compression are related, but sealing and volumetric efficiency greatly influence the actual compression you can achieve. 

Mark


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## ttyR2 (May 10, 2011)

Things like leakdown affect the measured pressure. At some point, the pressure in the combustion chamber is higher than the rings can seal, so the piston can't pack in any more air, and the compression gauge doesn't read any higher.


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## mtngun (May 10, 2011)

Formula for uncorrected compression ratio (UCCR).






For swept compression ratio, instead of using the stroke, use how far the piston travels above the exhaust port.

I use a 10cc syringe and rainwater. You'll need the piston with a ring in the top groove. Carbon must be cleaned from the head of the jug, otherwise carbon will throw off the measurements.





Put the piston in the jug and push it all the way to the top, snug against the squish band. 





Put the jug in the vise with the spark plug hole pointing straight up. Piston still snug against the squish band. 

Syringe has been filled with exactly 10cc water. No air bubbles allowed. 





Dribble water into the plug hole until the water floods the bottom thread. 

The reason I flood the bottom thread, rather than stop as soon as water is up to the plug hole, is to MAKE SURE the chamber is really full of water.





To be sure, I dump the water, blow out the jug with compressed air, and repeat the measurement 3 times. 

This particular 066 jug & pop-up piston had a 9cc zero squish chamber volume (ZSCV).


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## mtngun (May 10, 2011)

We need to add the squish volume (SV) to the ZSCV.

SV = (squish x D x D) / 50.13 where squish is in thousandths, D is in mm, and SV is in cc.

For a 54mm jug with 0.020" squish:

SV = (0.020 x 54 x 54) / 50.13 = 1.16 cc 

Total chamber volume (CV) = 9cc + 1.16cc = 10.16cc

UCCR = (91cc + 10.16cc) / 10.16cc =* 9.96*

To calculate the swept compression ratio, we use the volume swept above the exhaust port, instead of the engine displacement.

Measure the distance between the exhaust port and the squish band.





25.5mm on this 066. Subtract the squish (0.020" or 0.5mm) and that leaves 25 mm swept, which works out to 56.8 cc swept volume for this 066.

Swept compression ratio = (56.8 + 10.16) / 10.16 = *6.59*

Ya wanna know what it'll blow ? There's a formula, but it seems to give very optimistic results :

Pgage = (Patm x (CR raised to the 1.4 power)) - Patm

At sea level, Patm = 14.7 psi

Pgage = (14.7 x (6.59 raised to the 1.4 power)) - 14.7 = 190 psi at sea level.

In reality, you will always get less than the theoretical pressure. This 066 top end was supposed to blow 162 psi at my elevation, but the bore had some light damage so it didn't seal well and actually blew only about 130 psi, if I remember right.


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## Nitroman (May 11, 2011)

wyk said:


> This has intrigued me some lately. When you test the PSI on a saw, do you plug both the intake and exhaust? .



No, you hold the throttle wide open. If you pull 4-5 times the compression will peak. If you keep pulling the reading on your gauge won't increase.

Try it cold and hot.


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## slipknot (May 11, 2011)

Wow!! Thats complicated stuff!:msp_scared:


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