Cams and comp check?

[jayro]

So I was wondering if aftermarket cams have an effect on what reading you would get on a compression check. I have a rebuilt 3.2 that I was going to check compression on after it is all broke in. I know that cams effect the actual cylinder compression during running because of the increased overlap. Would this give a lower psi reading on a compression check as well? Also, how long should I wait before testing in order to get the correct results? Thanks.

 

[musclestang_95]

So I was wondering if aftermarket cams have an effect on what reading you would get on a compression check. I have a rebuilt 3.2 that I was going to check compression on after it is all broke in. I know that cams effect the actual cylinder compression during running because of the increased overlap. Would this give a lower psi reading on a compression check as well? Also, how long should I wait before testing in order to get the correct results? Thanks.

In my experience yes, a camshaft with increased overlap will effect readings given during a compression check due to the low speeds the engine is spinning when performing the test. Since the engine is only moving at a few hundred rpm, the overlap allows air more time to escape and reduces compression readings. I've read a lot of debate on the topic, but in my actual experience there is no debating it. As far as how long to wait, i assume you mean how long after a recent rebuild. Give the engine time to break in, not much is worse for a fresh engine than endless low speed cranking trying to get compression readings on 6 cylinders. Personally id give it a few hundred miles.

 

[jayro]

Anyone?

 

[AREA 91]

Increased overlap with large cams will lower cranking compression.

FWIW, I have gone to the drags after a few hundred miles of break in.

 

[jayro]

Increased overlap with large cams will lower cranking compression.

FWIW, I have gone to the drags after a few hundred miles of break in.

Thanks. So I should expect slightly lower numbers during a compression check due to the cams......but all the cylinders should be the same. Kinda makes me wish I had gone with hi comp pistons during the build to counter act the loss in comp from the cams.....those things are pricey though. I do have a spare 3.0 short block now. Hmmmmm

 

[jayro]

In my experience yes, a camshaft with increased overlap will effect readings given during a compression check due to the low speeds the engine is spinning when performing the test. Since the engine is only moving at a few hundred rpm, the overlap allows air more time to escape and reduces compression readings. I've read a lot of debate on the topic, but in my actual experience there is no debating it. As far as how long to wait, i assume you mean how long after a recent rebuild. Give the engine time to break in, not much is worse for a fresh engine than endless low speed cranking trying to get compression readings on 6 cylinders. Personally id give it a few hundred miles.

Thanks. I will wait a few hundred miles. I am bedding the rings at high loads as instructed on here. I probably will just hit the dyno to check numbers. If all is good there then I really have no need to do a comp check except curiosity.

 

[Lorenr]

Leak Test It

In my opinion a standard compression check with a simple compression gauge will only tell you the differences among the cylinders. It takes practice to even get that right.

I used to run compression checks on motors and rarely got the same numbers even week to week. It was the leak down tester that finally made sense of it all.

Google it and look to make your own or buy one off of the internet. They really tell the truth. It tells you the condition of the valve seats, ring to cylinder wall seal and head gasket seal.

:wave: