Compression Test Question

[Monkton]

Okay, My Dad did a compression test on my car and he actually asked me to come here and ask you guys if you knew what the norm pounds of pressure should be for my 4cyl Quad4 in my 1991 Cutlass. Right now I think we're getting 175, 175, 175, and 200. Are these normal readings?

[freecanadian]

Hi,

 

As far as I know, the only correct way to compare one engine's compression readings to another is to compare the same kind of engine (e.g. two stock Quad 4s that have not had camshaft or cam timing changes, since that would alter the compression readings). The best use of compression testing is to compare several cylinders in the same engine to see if they are close (within 10% is my rule-of-thumb). If you had 175-185-175-110, then you would know that #4 cylinder had a machanical problem causing a loss of compression.

 

I have seen another tool for testing cylinder pressures called a leak-down tester. These cost about $60, and you have to apply compressed air through a hose to the guage set which is connected to each cylinder in sequence, just like a compression tester but you do not crank the engine, you just apply air pressure and read the guages to tell you how much aiir is escaping (leaking down) from the presurized cylinder. The one I saw showed the percentage leak-down as well as three color ranges to identify the condition of the cylinder and its sealing ability based on the leakage. This is the correct way to check cylinder condition in terms of normal wear, not by compression testing.

 

If anyone has a Quad 4, you could compare your compression readings with them, but I donn't think that will tell you much.

 

In terms of general compression numbers, my old 89 Nissan Micra (with 230,000 Km /138,000 Miles) measured in the 150-180 range when I tested compression on it last fall. My 72 Chevelle V-8 measures approximately 210-225. The numbers for different engines can vary a lot based upon engine design factors, such as compression ratio and valve timing (especially cam shaft specifications like opening and closing degrees and valve overlap).

 

I hope this helps.

 

Keith.