squish, compression ratio, and a few questions...

[rbreak]

Hello all, recently there are a few threads here with great info and pics on squish and compression, and port to piston timing (thanks F5 and motopsycho). I have my head off and cut .035" off to get a .045"~.047" squish. I measured compressed volume at 17cc which with a 72mm bore/72mm stroke gives approx. 18.2:1 static compression which sounds crazy high. I would need to cut another 7cc's out of the compression chamber to drop it down to approx. 13.2:1. But, with a 2 stroke that doesn't actually start compressing anything until the piston is above the transfer and exhaust ports, it looks like you should only calculate about half the stroke. How does one accurately measure true running compression ratio on a 2 stroke with all the ports and openings in the cylinder? (I'm probably missing something obvious here). Thanks for any response or info.

[(F5)]

What you say is true. . . At kick over speed. When it's running the pipe effect regains any loss. Just trust the known numbers.

[motopsycho87]

I agree. Otherwise that little bleed hole in your cylinder that aids kick starting would be detrimental to running at high revs also. You have to remember at higher revs there are charges coming from both the inlet and exhaust side too, technically compression the charge before the piston returns. Good luck trying to find the true compression ratio when dealing with expansion chambers and power valves etc,

Stick with the numbers that work

[Jakobi]

The UCCR is the magic number. The Gas Gas 2T is just like any other 2T, and the uncorrected comp is what most engine tuners use to set the engine up for your altitude/fuels.

The 250's seem to run more compression than the 300's stock, and are even easier to get crazy high trapped volumes like you have.

How did you measure your trapped volume? Same method F5 suggested using burette?? I can't seem to find my notes on the 300 chamber volumes.

[(F5)]

Originally Posted by rbreak

. . . . I would need to cut another 7cc's out of the compression chamber to drop it down to approx. 13.2:1. . . . .

On my thread bottom of page 1

http://www.gasgasrider.org/forum/showthread.php?t=16560

you can see how much I've cut out to effect a drop in comm down to 13:1. Cut fairly deep & remeasure. Took me ~5 cuts.

[Jakobi]

My first 250 head was machined from 17.5cc to 21cc and still sat at 14:1UCCR with 1.25 squish.

[rbreak]

Thanks everyone for all the responses. I actually googled calculating 2 stroke compression after I posted this and learned a lot (I guess I could have done that first). A few interesting finds were;
"Dynamic is something not really measurable unless someone is monitoring actual air flow into the engine from intake side and calculating VE - but that's not entirely accurate either - because it is possible (and actually common place) to manage to suck more air through the intake then you manage to TRAP. The two strokes usually all have about a 12-13:1 compression ratio measured statically. What is published is what it has with the exhaust port considered. I typically only used the static compression ratio as my guideline on a two stroke and go from there. The irony is once you get your two stroke to really run well, often you go with less compression because both the delivery ratio (amount of air you get through the intakes) and trapping ratio (amount you keep in the cylinder) go up considerably making the dynamic compression ratio go up."

On compression testers (I know F5 is not a fan) and fuel octane;
"For engines intended for use with 93 octane fuel, I tend to set 190 psi as a lower limit with a preferable 200 - 210 psi target. For every octane number less than 93, the compression pressure needs to be about 5 psi less to avoid detonation under normal circumstances."

Back to my original post, I cut .035 off my head because I used 2 base gaskets (approx. .040" total). I used 2 base gaskets because when the piston was at BDC, it was still blocking a small amount of the transfer ports. However, the piston was never blocking any of the exhaust port and now is even farther below the exhaust port at BDC. Which creates some new questions, how important is it for the piston to completely uncover the transfer ports, and what effect does having the piston lower than the exhaust port at BDC have? If it is more detrimental to have the piston below the exhaust port than to cover a small portion of the transfer ports, then I can go back to one base gasket, but then I would have to cut even more from the combustion chamber to lower the compression.
Thanks again for all the responses!

[(F5)]

In pure terms from what I have read recently it is better for piston cooling to have the transfers not obstructed at btc, worse to have them above piston for a while. Ex Port can have some advantage with piston going below. Dirt bike engines aren't on the thermal limit so I wouldn't waste any sleep at all about it.

None of this matters a damn compared to how long they are open. In isolation it is a consideration but not what you are aiming to optimise.

[rbreak]

Originally Posted by (F5)

In pure terms from what I have read recently it is better for piston cooling to have the transfers not obstructed at btc, worse to have them above piston for a while. Ex Port can have some advantage with piston going below.

So your saying it is better to raise the cylinder to keep the transfers unobstructed? What is btc? What did you mean by "worse to have them above the piston for a while"?

Sorry for all the questions, I didn't understand what your response meant.

[Harry]

Hi These are my notes from when I machined the head on my o2EC300.
Hope it helps