Compression test #'s. What do they mean?


New Member
Sep 24, 2003
Hope, RI
Hi: I have an '86 GT motor with Edelbrock Performer heads (1.90" intake) o-ringed head gaskets, ARP head studs, Performer intake, F303 cam, Comp cams roller rockers, etc. Bottom end has approximately 80k miles on it. Top end has about 10k. I did a compression test and all cylinders were between 170-175 lbs. I also did a leak-down test and the largest differance was about 5%, the smallest was about 1%.
My questions are:
1. Are the compression test numbers good? The variance from one cylinder to the other is very close so that is good, correct? Is 175 lbs good?
2. Are the leak-down test numbers good? While testing every cylinder, the sound of leaking air seemed to be in the crankcase. I base this on the fact that playing with the oil fill cap changed the tone of the air noise.

I want to super charge this motor (leaning towards Kenne Bell) and was wondering if it needs any attention (rings, honing etc) before adding more power?

FYI: Motor is installed in an '88 Turbo Coupe

Thanks for any input,
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Thanks for the input everybody. you think I can safely add 8-10 pounds of boost?
Does anybody have a picture of a Fox with a KB blower installed? I want to eliminate any clearance issues before buying the blower. My T/C has a large master cylinder and I'm also concerned about hood clearance.
Thanks again,
I did have the stock pistons notched.
See pic of engine installed.



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Boost really depends on what your compression ratio is. Typically a 9:1 compression ratio should be good for 8-10lbs of boost. Some people run a higher compression but run the risk of detonation. A lot can depend on the tune that you get but higher the compression the smaller of a window you have to tune it to be reliable. FYI, if it is the stock shortblock the reliable limit before it splits is about 450-500hp. Again some get more but are either on borrowed time or have some lucky cast block. Very nice setup BTW.
Thanks for the compliments.
I'm looking to make 400-425 hp, not much more. I don't race, it's more of a bragging rights/ego (guy) thing I guess.
Please bear with my lack of knowledge, but how do I determine my actual compression ratio? When I do get the blower on, I intend on getting a safe tune on a dyno. I typically run 93 octane, so I think I can have a stout street performer that isn't a ticking time bomb.
Speed shop that helped me select my parts knew my long range plans for forced induction and said I should be good for 8-10 lbs. He was pushing me towards a Vortech, but I like the looks and what I've read about KB (except their tech support). Still unsure on which way to go.
Someone can help you figure out your compression ratio by using your bore, piston info, and head info. Obviously stock piston's are stock 4" bore and then, I assume stock crank & rods, so that would mean stock stroke. Basically someone can probably figure your compression ratio with the info provided. Have the heads been milled any? Do you know which head gaskets were used (specifically the thickness)? Those two things are probably about all they'll need from you to figure your compression ratio.

If you're good with math, you can probably find the formula somewhere on the net. you may even run across a calculator if you search enough.

Have you thought about a custom cam for when you get the blower? Might help ya out a little.