Proper procedure to measure compression

[kgb007stb]

I need to know that the proper procedure to measure compression is? Does the engine have to be hot, warm, or cold? Does the throttle have to be wide open or closed? I have never measured compression before and I do know that these factors will affect my reading, but by how much I have no idea so all of your opinions are greatly appreciated.

Is there also a web site that shows what the proper compression numbers for different engines should be? I have a 1991 Mazda 2.2l four cylinder engine, maybe my car manual will have some specs on what the proper compression should be.

 

[Eddie]

Engine should be up to tempeture. Disconnect ignition coil and remove all spark plugs. Open throttle wide open. Pressure should be max out at about 3 compression strokes.

 

[tom slick]

actual numbers don't mean as much as consistant numbers. you want all 4 cyls to be about the same compression.

a much better way to check engine condition is a leakdown test. it eliminates many variables that that make a compression test inaccurate. down side is you need an air compressor and a $70 leakdown testor.

 

[Boomer58]

Just an air compressor and a way to regulate.

 

[mechtech]

Pull the fuel pump fuse, as well.
Note how the gauge goes up on each compression cycle.

 

[1 FMF]

you shouldn't see too much difference between hot engine vs cold engine. Ideally, do compression test cold then run the engine to warm it up and do another test. Depending on the engine, it might not be practical and you might be stuck doing it on a cold engine or risk burning your arms off. Otherwise, warm engine is usually better if you had to choose between one or the other.

procedure is same for any engine, looks like everything was covered: remove all spark plugs, disable ignition & fuel, open throttle blade.
record compression # at same stroke for all cylinders. All cylinders compression #'s should be close to one another, something like 10% i think.

Compression ratios can give a hint to what cylinder pressure should be from a compression test. No gasoline engine will see cylinder pressures much above 180-200psi. Higher than that results in detonation for pump gas. So most engines, with CR's around 10:1 which is the max for most street engines, will see 160-170psi. Lower CR engines around 8:1 will see down around 140psi. Anything lower than 110psi would be cause for concern on any engine.

 

[bulwnkl]

Are compression pressures tied that closely to compression ratios in the real world? I mean, in theory, you can't possibly guess the compression from the ratio, because it depends upon the CC volume. But maybe real life engine construction has practical parameters(?)

 

[guss]

9.1 comp is 165 pounds.8.4.1 is 155 in my book.thats all i found

 

[XS650]

quote:

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Originally posted by bulwnkl:
Are compression pressures tied that closely to compression ratios in the real world?

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It also depends a lot on cam timing. Higher compression engines -tend- to have longer duration cams, so at cranking speeds you don't usually get the extra compression pressure that the CR would indicate.