Calculating Cylinder Pressure Reading

This is probably pretty basic to some of you guys.
I tested a four stroke 500 (498cc) twin the other day, (throttle wide open).
Reading was 100psi on both pots!
Now the Bore & stroke is 73mm & 59.6mm (2.874 in & 2.346in)
The compression ratio is 9.0 to 1.

I'm afraid I couldn't make a judgement.... except, i think its OK!
How can I work out what a good compression reading would be for this engine.

RegardZ

You would need to know the initial volume (bottom of stroke) and final volume (top of stroke).

Thanks for the reply.... What I'm thinking is the compression ratio would give me..... the ratio of the swept volume of the cylinder + the vol of the compression chamber, divided by the vol of the compression chamber and this equals 9
I still can't get how iI would get to PSI of cylinder!

Or should the max PSI for a compression of 9 to 1 be 14psi (normal air pressure) X 9 = approx 126psi.
Guessing!

RegardZ

bountyhunter wrote: You would need to know the initial volume (bottom of stroke) and final volume (top of stroke).

Actually, this will yield your compression ratio, which is already known.
Calculating cylinder pressure also involves the cam profile, and is not an exact science.

koolaid_kid wrote:

bountyhunter wrote: You would need to know the initial volume (bottom of stroke) and final volume (top of stroke).

Actually, this will yield your compression ratio, which is already known.
Calculating cylinder pressure also involves the cam profile, and is not an exact science.

+1, all depends on when the intake valve closes.

Thanks for replies thus far... Understanding as Koolaid-kid says "it not an exact science".

I usually check the compression of an engine, with all spark- plugs out, and carb off or throttle fully open.
With a compression gauge in the spark-plug hole. I then use the starter to turn the engine over, until I see the pressure on the gauge build up and level off. The engine usually turns over about six to ten times, giving me the maximum pressure in the cylinder being checked.

The maximum pressure would be with both valves closed. I cannot see how valve timing, cam profile would stop me measuring the maximum pressure. I do understand that there would be losses.... Valves not sealing or rings worn!

RegardZ
PS. I would expect to see a higher pressure in a cylinder with a 9:1 comp ratio then one with an 8:1 comp ratio, given the same bore & stroke... Wouldn't I?

The intake valve does not close until it is past bottom dead center and on the compression stroke.
The later in the cycle the valve closes the less air volume is trapped in the cylinder before the
piston gets to top dead center. The earlier the valve closes the more volume that is trapped. And it
all depends on camshaft duration and if timing is advanced or retarded.


.

When in doubt, refer to the service manual for the service limit for compression testing. Add oil to wet seal the cylinder, do a leak down test both dry and wet. You'll soon find that the cylinder is either good or bad.
An aspirated engine with 9:1 comp ratio should be like 150 psi or possibly more.

As I understand it, here are some of the issues with trying to calculate the PSI by using the difference between bottom dead center (BDC) and top dead center (TDC).

Doing that assumes the intake valve will be open right up to the instant the piston reaches BDC and then at that instant it will close completely and stay closed until the piston reaches TDC. However, it is unlikely that the valve timing set to make the intake valve snap shut at BDC, plus there is some minor delay associated with the time it takes for the intake valve to move from the open to closed position.

Aside from the intake valve issue, there is also the scavenging effect on the exhaust valve side which may affect the amount of air/fuel mix drawn into the cylinder when the intake valve opens. Plus there is the venturi effect where air/fuel mix rushing into the cylinder may affect the amount of mixture drawn into the cylinder. As a result, even if the intake valve was timed to open exactly at TDC and close exactly at BDC the amount of air/fuel mixture in the cylinder may be different than what it would be in the theoretical model where the assumption is that the volume of air/fuel mix exactly equals the volume of the cylinder.

Then there is the issue of the gasoline in the mixture. Since the air/fuel mix (stoichiometric mixture) is at least 14/1 the cylinder will contain at least 7% gasoline. Since only the air in the cylinder (not the gasoline) can be compressed the measured PSI will be different than what would be calculated using the entire volume of the cylinder. In effect, the gasoline in the mixture raises the measured PSI. This issue doesn't come into play if the carbs are not on the engine, so the actual PSI measurements will vary depending on whether the carbs are on or off the engine. Ed

Very good explanations, Nebr_Rex and 650ed. Even something as insignificant as valve clearances can have an effect.
As I stated previously, it is not an exact science.

Thanks Guys, I knew you would explain my problem....
So, I may be way down on compression... Funny I couldn't find that pressure reading in Operators or Wotkshop manual.
Incidently, the twin engine I'm concerned about is a low milage '73 XT500 (four valve/ pot)... a new addition.
RegardZ to all.

Also Altitude will directly effect compression readings. But a leak down test will Not be effected by altitude.
A second compression gauge to confirm readings is also a good idea.