greenslade

just wondering if any has ever checked this out,i have read here that normal compression is around 180 psi,as the ratio goes up you would expect the compression to also go up.So I was wondering what psi you could expect at different ratios.--I'better get back into the garage seems I have too much time on my hands.lol

jaunty75

If you do have all this time on your hands, go get your high school chemistry text and re-read it!


It's simply PV = nRT. Assuming the temperature (T) and amount of gas (n) remain constant, then the right side of this equation stays constant. Therefore the quantity PV must be constant. So if V is lowered by forcing the gas in the combustion chamber into a smaller volume, P must raised by the same factor.

Cut V in half, and P doubles. So if you have a compression ratio of 2 to 1, the volume is halved and the pressure is doubled, from 15 psi (roughly) to 30 psi. So a 10 to 1 compression ratio should give you an increase in pressure by a factor of 10, from 15 psi to 150 psi. 11 to 1 gives you 165 psi. Etc. You can take it from there!

compedgemarine

unfortunatly that would only work if all the engines used the same camshaft. changes in the duration and overlap, lobe center, etc all have an affect on compression. a 9:1 motor could by theory have the same cylinder compression as an 11:1 motor by using different cams.

jensenracing77

X2, a good example of this is in the ATV world. many of the new 450 sport/race quads have at least 12:1 and run on pump gas. the lobe separation on the cam has the cylinder pressure lowered

InfoJunkie

They do indeed have the cylinder pressure by the CSM 1970 Chebby 307 & LS6. In theory, if you want more compression don't waste money on pistons ,machine work or smaller chamber heads. Have a cam cut with stock lift and duration but less overlap. Presto more pressure. It may not work the greatest but it has more pressure. Consult with a good engine builder about what you want from your engine and do as they say. Save your money by doing it right the first time.

jaunty75

You cannot overturn the laws of physics. If one motor has a claimed 9:1 compression ratio and the other 11:1, but they both have the same cylinder pressure, then they both have the same compression ratio, period, regardless of what's stamped on the side of the engine or what the manufacturer claims.

It's a simple fact that has nothing to do with camshafts, lobe centers, or anything else. The best way to determine the actual compression ratio for an engine is to measure the cylinder pressure. If it reads, say, 180 psi, which is a common value, then the compression ratio is 12.2 to 1, which is 180 divided by atmospheric pressure, 14.7 psi. It's that simple.

jaunty75

Well then it's very simple. It runs on pump gas because the compression ratio is not 12:1. If some change in the lobe separation has affected the cylinder pressure (lowered it), then the compression ratio has been lowered and is no longer 12:1. Doesn't matter if the piston travel hasn't changed. Something HAS changed to lower the pressure, and that lowered the ratio.

Rickman48

Jaunty75 - what you're saying is correct, if you don't take the camshaft into consideration.
If you were to remove the rockers on a cyl., and test for compression, it'd be way higher, on any motor.
The cam timing has the effect on pressures - bigger cam, less pressure.
Check it out on one of the cam manufactures websights.
Big difference between true compression and static compression! And static is what you get when the cam is operational!

jaunty75

I've done some more reading, and I think I see what you guys are all saying. My simple calculations are based on the cylinder being sealed when the piston is at the bottom of the stroke and as it begins the compression cycle, and the gas in there at that time is what's compressed. But if the intake valve closes slightly after the piston reaches the bottom of the stroke, which is controlled by cam timing, then some of the gas drawn in during the intake stroke is forced out before the valve closes, so the quantity of gas in there is not at 1 atm pressure but is slightly below that. That will lower the "dynamic" compression ratio, as I understand it.

Oldwolf

[QUOTE then they both have the same compression ratio, period, regardless of what's stamped on the side of the engine or what the manufacturer claims.
[/QUOTE]

Yes, the same "dynamic" compression ratio, an d at least the same cylinder pressure, at a sertain rpm range, but not the same static compression ratio.

MDchanic

In addition to variations due to cam design, don't forget practical issues like loss through the ring gaps when cranking at low RPMs. At high RPMs, those few thousandths of an inch of leakage don't have enough time to affect the compression, but at the specified cranking RPMs, the ring gap can significantly influence the measured pressure.

- Eric

BlackGold

Ding ding ding ding!!!
And even the mildest cam closes the intake valve more than "slightly" after the piston reaches bottom dead center. For example, the lowly 2-bbl 350 in 1970 used a cam which closed the intake valve 54 deg ABDC. Think that's a lot? The big W-30 cam closed the intake valve a whopping 92 deg ABDC -- more than half of the way back to the top!

greenslade

high school text went about 10 years before the disco records,lol
So,in theory, if an engine would not run on pump gas you could change the cam to reduce cyl pressure and not have to tear the engine down to replace the pistons.
PS;jaunty I used the spell check just for you.

jensenracing77

yes but if i am thinking right, that would take a tighter lobe separation which will make it want to load up a little at idle. well i guess it would be a matter of how much you had to go. as always, it would be best to plan the proper engine build before hand to avoid that.

greenslade

This is not something I plan to do,just kicking around thoughts.My eng is a bare block sitting on the eng stand,been looking at pistons,will be a very mild build,looking for about 9:1 ratio.