Calculating Cylinder Pressure Reading

Nebr_Rex wrote: The intake valve does not close until it is past bottom dead center and on the compression stroke.
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Yeah, that's true. There is valve overlap at TDC so it's not so simple.

Motor Head wrote: Also Altitude will directly effect compression readings.

Because atmospheric pressure changes.

Garn wrote: 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!

I would guess 100 is probably not so good, just a guess.

bountyhunter wrote:

Nebr_Rex wrote: The intake valve does not close until it is past bottom dead center and on the compression stroke.
.

Yeah, that's true. There is valve overlap at TDC so it's not so simple.

I know it's not so simple. I was keeping to the basics as not overwhelm the original poster.
That way they would have an easier time understanding the concept. I wasn't trying to be
the ultimate authority and poster. Just trying to help out.


.

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

Depending on valve timing/overlap, and speed of the crank during the test, the compression test results can be wildly different.

Motor Head wrote: 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.

+1

Especially "When in doubt, refer to the service manual for the service limit for compression testing."

That is the best source for what the pressure typically will be. It can't really be calculated with any precision without a million different equations and parameters.


Also, make sure the check valve on the gauge doesn't leak. It should hold the reading indefinitely.

Also, make sure the check valve is right at the tip. Some gauges have a check valve near the gauge, then a long, open hose to to connect to the cylinder. That is a useless design, and appears whoever designed the tool doesn't understand what it is for.

Loudhvx, I have, from the above posts, come to understand that it near impossible to calculate with so many variables. Nevertheless, i would now expect that an order within say 10%, could be nominated by an experienced person, that has worked with engines of this size and of this specified compression ratio.

The compression reading of 100psi (with a good approved gauge), in my mind does seem a little low and worth a looksee with other tests, whereas, I would be probably quite comfortable with 120psi.

As mentioned, I have not seen any specific service limit for compression testing this four stroke TX500.

RegardZ.

Don't know which year you got there. But reading over at the Yammie forum, I see 150psi. Also that these early heads cracked pretty easy. I'd do a leak down test, dry and wet. If a cylinder leaks much, then you know its got a problem. Of course you can have only 4% leakage, and have the cam timing off, and it will have low compression like you measured. This is one reason for a leak down test, it can point you in the correct direction for further checks or just disassembly, once you find a burned valve or other loss.

Yes, I understand what you're saying, and I agree, 100psi seems low. But someone with direct XT500 experience would have a much better answer than us, 2-valve guys, guessing at it. I just don' want to see you tear the motor apart until you get confirmation that 100 is too low.

I like Motorhead's suggestion in the mean time. If you do a wet test, and compression comes way up, then we can assume the rings are not sealing.

I had a 1975 TX500 (XS500)about 2 years ago. It was a nice clean bike with only 7000 miles.
The compression on my bike was 135-150 and it ran good. Parts are getting tough for that bike.
Try finding a link for the timing chain! Tear down requires breaking the chain and linking it back together for assembly, the links used to be common and cheap, can't find em anymore.

Garn wrote: Loudhvx, I have, from the above posts, come to understand that it near impossible to calculate with so many variables. Nevertheless, i would now expect that an order within say 10%, could be nominated by an experienced person, that has worked with engines of this size and of this specified compression ratio.

The compression reading of 100psi (with a good approved gauge), in my mind does seem a little low and worth a looksee with other tests, whereas, I would be probably quite comfortable with 120psi.

As mentioned, I have not seen any specific service limit for compression testing this four stroke TX500.

RegardZ.

RoadFlyer, you must be the experienced person that nominated the approx pressure reading....
I've just come home from a motorcycle show at Windsor, Sydney, Australia.
I met a friend there, who said he had a Service Data Sheet for the TX500A.
Would you believe it says ...
COMPRESSION RATIO/ NOMINAL PRESSURE ~ 9:1 / 145psi.
Motor head , you were pretty close too!
Thanks again to all! I've learnt a bit from this forum!.
After retracing my compression test procedure, I have found that I did it with a cold motor, so there is hope that it will rise to the occasion, when I repeat the the test again.

RegardZ

Just some feed-back re compression.... 100psi when completely cold.
Both cylinders checked when engine hot..... 136psi.
Makes me happy!
RegardZ.