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.
squish, compression ratio, and a few questions...
- Thread starter rbreak
- Start date
motopsycho87
New member
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
Stick with the numbers that work
Jakobi
Super Moderator
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.
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)
Gold Level Site Supporter
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.
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!
"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)
Gold Level Site Supporter
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.
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.
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.
(F5)
Gold Level Site Supporter
Well you quoted and took heed of the wrong part of the message
try this bit:
bdc, oops typo, I meant bdc bottom dead centre.
What I'm saying is that getting the ports to line up is like polishing the side of the axe head. yes it will help the head slide through the tree but sharp edge is where you should be concentrating your effort on.
in this instance my analogy is loose as in this case they are are linked. If you align port bottoms you raise or lower the port tops which in turn changes the time they are open for.
Now what I am not saying is that raising the barrel is bad, I am saying raise the barrel if you want to increase the port timings. Not because you wish to optimise alignment at the bottom.
try this bit:
bdc, oops typo, I meant bdc bottom dead centre.
What I'm saying is that getting the ports to line up is like polishing the side of the axe head. yes it will help the head slide through the tree but sharp edge is where you should be concentrating your effort on.
in this instance my analogy is loose as in this case they are are linked. If you align port bottoms you raise or lower the port tops which in turn changes the time they are open for.
Now what I am not saying is that raising the barrel is bad, I am saying raise the barrel if you want to increase the port timings. Not because you wish to optimise alignment at the bottom.
Ok I get what your saying now, thanks for the clarification. I still feel like the port should be unobstructed for best performance. Using the piston to block off a portion of the port will of course have the effect of creating a shorter duration port opening and alter the port timing, but it also creates a "step" (the piston edge) that the incoming fuel charge has to go over and I would think that would create some turbulence in the port flow. A larger partially obstructed port would not flow the same as a smaller port that provided a clear path for the incoming fuel charge. But then again, it may not make a difference in performance one could even feel, and therefore not worth worrying about either.
Hi These are my notes from when I machined the head on my o2EC300.
Hope it helps
Thanks for sharing your notes, it will be interesting to see how they compare when I get my work finished (if I ever get time to work on it, real job keeps getting in the way).
(F5)
Gold Level Site Supporter
Hate to tell you this but that piston keeps moving
Yeah, I would imagine covering a small portion of the transfer ports is not something that would make much of a difference in performance, but it just bugs me when it's not a clear smooth path. Part of my OCD I guess. Plus it just looks better when the ports line up cleanly (LoL).
Ok I will bite, does the piston somehow sink further than BDC?
Jakobi
Super Moderator
Ahuh! The fraction of time spent at bdc doesn't really account for that much in the grand scheme of things. Its very easy to get caught up with raising the ports and chasing max hp, however it very well could come at the expense of the more usable low to mid range curve. It all depends what you're hoping to achieve.
(F5)
Gold Level Site Supporter
Just to contradict myself (why- who knows?) the bottom is a useful part of the flow time as the first bit of the downstroke the pressure is still high enough that spent gas that has not made it out the pipe is forced down the transfers, so the first bit of flow is backwards, then it reverses & has to play catch up.
Of course this changes depending on revs & throttle as the amount of charge burnt affects the pressure & the amount of time for these flows to expell alters too.
But don't lose any sleep over it.
Of course this changes depending on revs & throttle as the amount of charge burnt affects the pressure & the amount of time for these flows to expell alters too.
But don't lose any sleep over it.
