I have a C4 I am looking at and I am having my mechanic run a compression test tomorrow morning on it. What PSI should a healthy C4 be at? This car has 78K miles.

 

The PSI reading will depend on test conditions and the accuracy of the gauge, what is more improtant is that all 8 cylinders are very close, within about 10 psi of each other.

 

By doing "dry" and"wet" tests, you'll learn about your rings and valves.
115 lbs is great, 70 lbs and ya need to do some work on the inerds.
Yes, you want the cylinders within 10 or so lbs.

 

I disagree that 115# would be great. I wouldn't be too happy with anything much under 150#. However, it is the difference in the readings from cylinder to cylinder that is more important, as stated. I wouldn't bother doing the wet test unless the dry test indicates problems and you are considering buying the car despite the troubling readings.

RACE ON!!!

 

O.K., so 115 isn't "GREAT" , but it's a heck of a lot better than 70.

 

A compression test at cranking speed can be misleading, if the cam has allot of overlap which would normally be considered a high horsepower cam then it will show lower than normal compression.

 

Oh ya,.... if you're goin' to do a comp test, you're wastin' your time if ya don't do bothdry and wet. I don't care what your initial readings are, get the whole story. ( especially if your lookin' to buy )

 

If I were having a compression test done in consideration of buying a car, I would pass on one with 115# for cranking compression pressure. At 70# it would likely have to be towed to the test shop. I have a 1984 with 165,000+ miles on it. It burns a quart of oil every 200 to 300 miles depending on how it is driven. It is just flat worn out. Thecranking compression is 160#, ±10#. As long as I make sure it has non-oil fouled plugs it it, it runs great. If it had 115# of compression,the enginewould be on the engine hoist by now.

RACE ON!!!

 

It is just possible that your 160 reading is due to oil in the cyl., helpin' out some worn rings. Ya think? Hmmmm,.....could be........

 

Just for the record, I ran a cold dry compression test on my supposed L-48, all jugs were in the 95 - 105 psi range, I thought, holly cow this thing is worn out. Motor doesn't burn oil, starts like a dream, runs strong. I don't really know what internals are in it, I kinda' assume it's been massaged a little.