Compression Test.....

[boostingsx98]

So today i did a (dry) compression test on my 7 bolt motor. This is what the numbers look like....175,170,160,150. They were not even across the board and there seems to be a big difference between the first and last piston... Is this ok or is there something wrong?

 

[red97tib]

I would say that looks pretty good. You usually don't want more than about 10% difference between the highest and lowest numbers (don't quote me on that, it's an estimate). Being you are working with a motor that is 11 years old, I would say you are holding up pretty well. If you are that worried over it, you can do a leak down test and see if you are leaking out of cylinder 4, but I wouldn't fret over it.

 

[pboglio]

Could very likely be carbon deposits on the valves. Happened on my car; all cylinders at 170 psi, then one at 160 psi. Ran bottles of Seafoam thru the motor, still same thing. Then I realized the cleaner wasn't being distributed evenly thru the pcv port on the intake manifold, so I switched over to the fuel pressure regulator vacuum fitting on the intake manifold and sucked in the cleaner that way. I had about 12 years worth of carbon deposits on cylinder 1 or 2 if I recall correctly that finally came off. Brought that 160 cylinder up to 170 psi, and the leakdown from 11% leakage to around 5%. Give it a shot.

 

[boostingsx98]

ok thanks for the input!

 

[Fejery4491]

not bad numbers for a car of its age

 

[pboglio]

9 times out of 10 with the 2g's getting up there in age, it's carbon deposits screwing up the compression reading. The 175 reading is definitely carbon buildup on the piston top. The lower compression readings I'd bet will come right back up to spec with proper combustion chamber cleaning. A cylinder leakdown test will isolate the problem.

My heavily modified 97 GSX with 120,000 miles reads 170,170,170,170 on the dot. Leakdown average is about 4% among all cylinders.

Obviously, you want all compression numbers equalized to minimize knock and maximize power. 2-3 cans of "Deep Creep Sea Foam" run 1/2 thru the PCV fitting on the intake manifold while idleing, the other 1/2 thru the fuel pressure regulator vacuum port on the intake manifold will evenly distribute the combustion chamber cleaner thru cylinders 1,2,3,&4. After that, change the oil, then pull the spark plugs and clean them or replace. Car will smoke for a good few miles while driving. Should be good to go.