Compression test

[powpounder]

just wondering what the correct way to do a compression test? warm engine or cold? open throttle or closed? any input would be great, thanks

 

[krazy ken]

Warm engine doesn't need to be hot and throttle wide open.

 

[Summitric]

just wondering what the correct way to do a compression test? Warm engine or cold? Open throttle or closed? Any input would be great, thanks

cold or warm engine doesn't really matter.... Reading will be a little higher when engine is warm due to expansion of hotter parts. Spark plugs out and wires grounded or ignition coil disconnected. Park brake applied. Throttle wide open, and compression test one side at a time..... Reading should be 15psi maximum difference between cylinders..... Average compression ratio varies depending on type of engine, but should be somewhere between 120 - 160psi ...... Pull engine over 4 or 5 full pulls. If somewhere between those readings and cylinders are within 15psi of each other, then you should be good to go..................

 

[powpounder]

thanks for the input guys

 

[Scotty]

cold or warm engine doesn't really matter.... Reading will be a little higher when engine is warm due to expansion of hotter parts. Spark plugs out and wires grounded or ignition coil disconnected. Park brake applied. Throttle wide open, and compression test one side at a time..... Reading should be 15psi maximum difference between cylinders..... Average compression ratio varies depending on type of engine, but should be somewhere between 120 - 160psi ...... Pull engine over 4 or 5 full pulls. If somewhere between those readings and cylinders are within 15psi of each other, then you should be good to go..................

Well said Summitric... Hotter parts? you mean the piston as it is the quickest part to expand. The cylinder expands at a much slower rate as it has a larger mass, so compression stays pretty much the same on a cold engine as compared to a hot engine. Dissplacement stays the same. Thats what i have always believed. Pretty much what you are saying. Just thought I'd expound a little on this

However, I think that 15 psi difference between cylinders is a bit much? I would be pulling things apart at 15? Scoring or rings maybe? Rideable but not the most efficient on one side at least.

 

[Mr.Runner]

i've always went with 14 - 15 psi differnce on a automotive engine. but a sled i think you'd want to keep that gap tighter, likt 5- 8 psi diffrence

 

[ZRrrr]

10% difference, so on a 120PSI test that's 12 pounds. Take it from there.

I have always done cold and warm. With the cold I do it dry then add a couple spoon fulls of oil in the cylinders. Get some intersting results but always looking for consistency.

 

[Haggis]

How do you ground the wires with the spark plugs out??