static compression converted from PSI?

[250X_project]

Can you convert the PSI reading you get from compression testing to the static compression ratings? For example 9.4:1 of a 250X (and 8.9:1 of a 300EX) is between 170 and 199 PSI.

Thanks for the help, and sorry if I don't reply right away.

[Bigbadfords]

There is really no 100% accurate way to calculate your compression ratio from psi, but there is a formula that will get you fairly close. First take your psi reading and divide it by 14.7 (for example, psi reading is 170 divided by 14.7 = 11.56). Then take your engine size and multiply by .01 (for example 250cc x .01 = 2.5). Then add 1 to that number, so then you have 3.5. Last you will take the 11.56 that you got from dividing the psi reading (170psi) by 14.7 and subtract that 3.5 from the 11.56 ( so you have 11.56 - 3.5 = 8.06). And that is your approximate compression ratio (8.06:1). Like I said, there is no formula that will get you an exact reading just because it is virtually impossible, but that formula always gets you fairly close.

[Colby@C&DRacing]

A formula will get you close but it can't take into account for loss of PSI from ring blow by and cam duration.

[Bigbadfords]

That is exactly right. You will never be able to get an exact compression ratio but the formula will get you close enough so that you have a pretty good idea of what your ratio is.

[bwamos]

Or, reverse it for a ballpark of where your PSI should be at your compression?

Reversing that.. an 11:1 330ex should come in at, oh, about 224 psi? If it were a true 11:1 330.

Obviously cam overlap, timing, ring bypass, etc.. will all affect the true numbers.. but could give a decent ballpark to see if you're way off?.

I'm jsut having a hard time seeing how displacement is related to the psi.

If you compress 10,000 ml into 1000 ml or 100 ml into 10ml I would think it would have the same pressure result. 10:1 vs 10:1.

[wilkin250r]

I'm interested. Where did that formula come from?

[Bigbadfords]

I got that formula from a conversion tables textbook I have from when I was in college sutdying engines and hydraulics.

[exboomer]

Cranking compression and static compression ratio are two different things. You could have an engine with a 10 to 1 compression ratio and another engine with a 12.5 to 1 compression ratio and they could both have the same cranking compression. Unless you have varible cam timing the compression ratio will stay the same. Alot of factors will affect cranking compression (worn rings, tight valves, etc). Dynamic compression ratio accounts for cam timing (intake valve closure, etc). I wouldn't put alot of stock in any formula that calulates one from the other.

[wilkin250r]

Originally posted by bwamos
I'm just having a hard time seeing how displacement is related to the psi.

I agree, but it's kinda hard to argue with a textbook formula, they usually put a lot of research into those.

Do you think the blowby might be related to engine size? I think it's possible. For any given ring design, the blowby is going to be a constant volume (roughly). For example, let's say it's 1cc. If you drop 1cc from a combustion chamber of 20cc (a small engine), it's going to be a significant change. But if you drop that same 1cc from a much larger chamber, like 80cc, the change is much less severe.

The engine size in the equation might be to estimate the effects of those types of losses, losses that have less impact on larger-displacement engines.

[bwamos]

Originally posted by wilkin250r
I agree, but it's kinda hard to argue with a textbook formula, they usually put a lot of research into those.

Do you think the blowby might be related to engine size? I think it's possible. For any given ring design, the blowby is going to be a constant volume (roughly). For example, let's say it's 1cc. If you drop 1cc from a combustion chamber of 20cc (a small engine), it's going to be a significant change. But if you drop that same 1cc from a much larger chamber, like 80cc, the change is much less severe.

The engine size in the equation might be to estimate the effects of those types of losses, losses that have less impact on larger-displacement engines.

Yea.. I was doing some searching.. and I beleive that's what it's trying to account for.

There's a couple other formulas out there that were pretty good too.

I'll post a summary of one of the more logical reads below. Give me a few minutes to alter it to fit ATV cc's a CR's.

[250X_project]

Yeah, I'd like to see that. I tried that formula on a couple engines I know, and came up with a few barricades.

400EX- They have a low PSI rating because of a decompression?

My snowmobile (2-stroke) has a CR of about 5.0:1 with cold PSI, and 4.0:1 warm PSI. This brings up a few questions for me. Do you use warm or cold PSI? Also, how does this formula relate to 2-stroke engines, with out poppet valves and cam duration?