Compression Test Results

[kutdawg]

Hey guys, Im new to the forum, but ive had my z for about 12 yrs.
1995 300zx twin turbo, 139k miles. manual 5 speed trans. the only mods ive done at this point
is a Borla catback exhaust system. There is a valve cover leak, and it will need the 120k service, which will cost about $2k. I am deciding whether or not to repair or sell, so i
decided to do a compression test to give me a baseline about engine health/life. the results
are:

Cylinder1
1. 160 psi
2. 160 psi
Cylinder 2
1. 155-158 psi
2. 160 psi
Cylinder 3
1. 155 psi
2. 155-158 psi
Cylinder 4
1. 160 psi
2. 158 psi
Cylinder 5
1. 155-158 psi
2. 155-158 psi
Cylinder 6
1. 140-143 psi
2. 140-143 psi

So i am concerned about cylinder 6. From what i have read, results should not vary by more than 10% and cylinder 6 varies by exactly 10%. Is this bad news or satisfactory for the engine? engine rebuild is not an option at this point. Compression test was a dry test, engine was warmed, but i forgot to hold the throttle wide open when testing. not sure how much that affects results. Any help would be greatly appreciated!!

[NolimitZ32]

Fact that it was a dry test will have some impact on the results. 144 is not horrible, its drivable however it is a bit low.

[t.mcginley.jr]

Agreed, 144 isn't horrible but it's toward the low end of being "healthy". I would do a leak down test also just to get a "second opinion". It's easier to do than a compression test IMO. All you do is put the cylinder you're measuring at TDC (with spark plugs out), screw in the adapter and hook it into the gauge. The other side of the gauge is hooked up to your air compressor. Then there's a chart that tells your % leakage is based on how much the pressure drops. For example, if the tank pressure is at 70psi and the cylinder drops to 66 psi, then your % leakage is 5% (there's a table for this)

http://www.gress.org/home/cars/tracktal ... %20DIY.htm

Anyway, the reason #5 and #6 are lower than the rest is because they wear somewhat faster due to a design flaw in the block preventing adequate cooling. The numbers overall are pretty low for only having 139k miles though.

[kutdawg]

A perfect engine, twin turbo that is, should test out at 171psi according to my research. So my numbers are considered low?

[NolimitZ32]

Standard: 186
Minimum: 142
Max ▲: 14

Your compression is above the threshold but the ▲ is out of spec. Your saving grace may be that the adjacent cylinders are close to spec so you shouldn't have any blow-through problems.

[kutdawg]

Thanks for the info Nolimit. I think i will invest in the repairs and go from there. Thanks to you too T-mac!!