Acceptable Comp Loss

[fhptom]

Background: motor originally built in 2001 and used 2 or 3 times a year for

track events only plus some run time while working on it

throughout the year.

 

Did a Compression check last year (210s)and just finished one tonight(190s).

 

Pretty much an equal 12-14% loss across all 6 cylinders.

 

Last dyno run had about 185 rwhp so it is not a high HP motor.

Although it does have high enough compression that I had to run 105 octane race gas to keep detonation away.

 

Is that an acceptable drop in compression?

Is it most likely leaking in the bottom or top end?

 

Thanks for any answers..

 

Tom

[JMortensen]

Try again and squirt a little oil in the cylinders. If the #'s come up then the loss is due to rings. If not then it's probably valves leaking. I think your numbers can also vary widely by the gauge, whether the engine is hot or cold when the test is done, etc. You can also do a compression leak down test, which is much more accurate than a normal compression test and would show a problem more than just showing a difference between the last time you tested it and this time.

[Daeron]

When was your last compression test? Month, even week would be best. It is Cold and DRY right now in Florida, and if it was muggy before then even despite the warm, less dense air I would hazard a guess that with the humidity on a more NORMAL Florida day, the water vapor in the atmosphere just might cause increased static compression figures relative to today.

 

Compression check numbers, even using the same motor, same testing procedure, same tool, same user, WILL vary based on atmospheric conditions. This is why a leak-down test is considered the more informative diagnostic check.

 

Compression check is really best suited for comparing each cylinder to the others, when all are tested following as strict controls as the situation allows.

 

Try a wet compression check to see if the numbers go up, but if they don't, do a leakdown check.

[fhptom]

I did it in December of last year but hard to remember if it was this cold.

I will try the oil to see if I detect any change and then maybe the leakdown check.

 

Thanks for the input.

 

Tom

[Daeron]

No; last December was unnaturally warm. VERY Cold November, warm December last year. I was talking about it with my dad tonight actually..... (Of course, you ARE about 250 miles north of me.. and the fronts seem to have been stalling right around the tampa/daytona line rather more often than not in the last two or three years.)

 

I'm kinda guessing on the humidity thing, the logic seems acceptable to me but I am not sure my facts are 100% right; but we all know water compresses less readily than air, so I was figuring it might offer more resistance to compression, hence higher PSI figures. BUT.... that was just a stab at explaining why exactly the compression check itself might be uncontrolled. The fact remains that two compressions checks performed 24 hours apart can be fairly different for no reason other than the atmospheric conditions at the time of the test.