Inliners International Forums General Discussion Engines Compression Readings on 250 Chevy Race Engine

I purchased a race engine for dirt oval track racing. I did a compression test wiht a compression tester. Each cylinder was at about 240lbs. Is this good bad? What would the compression ration be? Another guy in the club I am in told me he was happy when he got a reading of 180 on each cylinder. Make me wonder if the readings were off. I used a compression guage with the rubber end. Thoughts? 250 inline Chevy

Unless its compression tester is missed, for 240 lbs cylinder pressure, you have 14.98 to 1 compression ratio, nearly 15 to 1...wow !!!Hope this helps,
MALDONADO
Brasil - South America

Dear Mr. Cong;

Try testing with another guage. If the same check valve adjustment.

Also try "NOVAMAN" He is expert on those.

Good luck, John M., #3370

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Maldonado- how did you figure that? It sounds like the program you used does not compensate for cam over lap.

My motor with 13.2 compression has 255 psi.

Chong- It seems like that motor depending on the cam might have 11.0-12.0 somewhere in that area.

Steven

Somebody needs to check out their thermdynamics textbooks. I dont think you can ratio the psi readings compared to atmospheric pressure to arrive at compression ratio. Because the pressure rise hapens fast in a spinning engine the compressed gas heats up - there is not enough time elapsed for the heat energy to dissipate through the cylinder walls. If I remember correctly this is called 'adiabatic compression', resulting in much higher pressure numbers you are seeing.

PV=nRT

I'm no expert you can check CCs to come up with compression and that gives you what I beleive is static compression? and again this is a ball park figure. But like French said Heat also plays a big park in your Finale compression Or None as True Compression. You will also find that if you leave The plugs in the motor when you try and pull a compression test you readings will be different then if the plugs out of the motor The Tork of the starter, battery
amps all can show different compression numbers. Just like cam over lap and all that other good stuff that play there part in compression. So i don't know if this only made things a little more confuseing Or not,But i hope it all helps. }[oooooo]


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NovaMan/Twisted6/Larry

As far as I know, there's two types of compression ratio,...statics and effective compression ratio, so the one that I calculated being the static compression ratio. The effective compression ratio shows the numbers much higher than that. However I know that it's a theoretical number, but my intention was to give him an idea of that. I hope to clear this misunderstood....subject!
Thank you,
MALDONADO
Brasil - S. America

Thanks for the info. I have to admit that I'm further out in the weeds than when I started the topic. In General though, with 240 PSI - where would I be at as far as compression ratio? 12, 13, 14, 15. Would 12 be a safe guess.

Also - in reference to an earlier post, I did take all of the plugs out to do the test. However, I only warmed the engine up for 2-3 minutes prior to the test as the thing has zoomies and is louder than hell and my neighbors are not active inliners. Could this have caused any problems with the readings? Thanks for the help! Great knowledge out there.

Yes, the warmup did cause the readings to be higher than necessary. When I responded to your other post in Drivelines I said:

"As long as the engine has not been running, so as to excessively heat the air being drawn in, it's a close enough calculation."

Everything Frenchtown said about thermodynamics is absolutely on the mark. I just didn't say anything about it in the other post because I thought it unnecessary at the time. You WILL heat the air when you do the test, even if the engine is cold. But a cold test will be a LOT closer to reality than anything else.

I used to have a stock GTO that was 10.25:1 compression. The cold readings were about 140 psi and the hot readings were about 185 to 190. When I went to UTI here in Phoenix, they always told us to do the tests both ways in order to find out if there was unusual variation between the cylinders. I don't remember them explaining in depth why the readings were so much different. Some of us assumed that it was because the rings and valves sealed better hot than cold (which is true). But temperature has a lot more to do with the pressure change than you think.

Redo the test cold and let us know what you get

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David
newbie #4153

Dear Mr. Cong;

Listen to the "nova man" the expert (although modest) and CC the **** thing. This will tell you what you want to know: The compression ratio.

Compression pressure tests are relative. They are only used to determin 'general' engine condition(s),ie; are the cylinders equal etc.

Good luck, John M., #3370.

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