compression test readings?

[jt512]

i have a 86 k5 i recently bought, it has a swaped in 396 bb. i have checked the #'s on the block and heads, the block lists as a 69-72 396/402 and the heads are 69 396 truck heads. i am in the tear down process of the rebuild of the truck, i did a compression test on the motor and have two questions: 1) what is a ideal or acceptable reading for this motor? 2) i have one cylinder that checked out lower then the others, i tested it 3 times, same readings each time, and then squirted in some motor oil and rechecked with no difference. i have searched and can not find any sites that give any specs as far as readings go. any info will be greatly appreciated!!!!

 

[tacoma]

Factory TSM will have specs on that.

 

[toomany]

Generally you're not looking for real high numbers, but numbers that are within 10% of each other. From what I've seen/done a number around 120psi is acceptable.

 

[jt512]

most of the cylinders tested out between 135 and 145psi. two cylinders were oddballs though, both happened to be the rear cylinders, pass. side was approx 152psi and d side was the low one with 125psi? retested them both and got the same readings kind of puzzled about it?

 

[4X4HIGH]

Compression readings will vary depending on CR of the engine and also the overlap of the cam. In general though a bone stock engine with around 9:1 CR will show up around 150-160psi range. The numbers you came up with are good though. Pretty much you should only have about a 10% difference between the high and low reading.

 

[jt512]

thanks for the input already guys!!! as far as my low cylinder not changing even when adding oil in to it, any thoughts? i plan on at least doing a minor go through on the motor, would you personally worry about the readings or would you do a full rebuild ? this will not be a crazy hp motor, but i hate doing things twice, so what would be your recommendations as far as replacing components? thanks again!!!!!

 

[Jason4x4]

Usually if a cylinder comp. goes up when you squirt oil in it it is because you have bad rings and it is sealing them, if it doesn't go up it is usually in the Valve train.

 

[4X4HIGH]

Since the oil in the cylinder did not affect your reading that means that the rings are good. With good compression why do you want to take it apart? If anything just change the timing cover, oil pan, valve cover, and intake gaskets to assure that it doesn't leak and call it good. With the pan off you could always go the extra step and replace the rear main seal as well. Just be careful not to nick or scratch the crank while changing it.