Compression numbers and how to interpret them...

[cchase]

My truck has a 305ci engine which, according to the Haynes Service manual should have cylinder compression of 150psi.

I tested the compression on all 8 cylinders back a few months ago. All 8 of them registered between 165-175 psi. This engine has about 180k miles on it.

To the best of my knowledge, higher than normal compression on a stock engine might indicate a build-up of carbon on the piston rings. The engine has been run a strict diet of Mobil 1 10w30 and 5w30 all its life. The top of the engine is pretty spotless but, of course, I have no way of knowing the condition of anything else without tearing it down.

I'm not looking to "fix" anything because the engine runs beautifully, has plenty of power, gets excellent gas mileage, doesn't smoke, doesn't burn oil, and the plugs always look great.

Higher than normal compression would lead me to believe that I would be experiencing issues using regular 87-octane fuel, like pre-detonation or something along those lines, yet I don't have that problem.

So, what's the real story behind it all? Is the Service Manual just incorrect?

 

[bdcardinal]

What does a factory service manual state? I have seen enough problems in Haynes and Chilton manuals to not even consider them as a accurate source of information anymore.

 

[Trav]

There where low compression engines and ones with higher,no idea which years are involved.
Either these numbers are for a low compression motor or a maybe a minimum spec.
Your compression numbers look ok to me.

 

[bdcardinal]

O and in agreeing with Trav, the numbers look good. I;m usually not concerned with actual numbers as much as the overall range of compression numbers. As long as they are within 10% of eachother you're golden. Remember on some cars you have to do the test cold unless you want threads coming out with the spark plugs.

 

[Chris142]

There is a whole bunch of things that can effect the compression readings. You need to look for uniforminity and not total psi so much. All your cylinders are within 10psi of each other and thats good.

It's not carbon on the rings that raises the compression. It's carbon on top of the pistons and on the valves and head.

It's good..Run it.

 

[crazyoildude]

maybe the guage?

 

[Gary Allan]

I've see 180+/- as a much more common number than 150.. and even then, as everyone says, you're looking for variance to find for issues.

 

[Eddie]

Usually the manufactures give a Minimum pressure OR a lower & upper range and a maximum difference between cylinders. You listed only 150 psig.

 

[mechanicx]

This is a '93 TBI 305 right? I think 150psi would be more applicable to carbureted mid 80's 305. Most GM engines I've compression tested ideally seem to be in the 170+ to 180 psi range. I think the compression ratio on the late 80's and up models is in the 9.x:1 ratio, and 170-180 psi in all cylinders is usually a sign of a healthy engine.

 

[punisher]

Agree with above. As long as all of your numbers are close, thats all you can really hope for. Any number of things can cause higher than "normal" compression numbers on a smallblock. Cam timing, and especially worn cam lobes on smallblocks, can change numbers.

 

[cchase]

Thanks, this makes me feel a bit better, I've been perplexed about this for quite a while. Unfortunately I don't have a factory service manual. All I have is the "installation is the reverse of removal" Haynes book...

Thanks again for all the information and for confirming my suspicions.

I did mean to mention though that I checked the compression gauge against a known good gauge and it's right on.

 

[oldmaninsc]

I agree with everyone else as well. It all sounds good. I know you weren't trying to "fix" anything, but my Dad used to tell me: "If it isn't broke, don't fix it!"

 

[cchase]

Originally Posted By: oldmaninsc
I agree with everyone else as well. It all sounds good. I know you weren't trying to "fix" anything, but my Dad used to tell me: "If it isn't broke, don't fix it!"


Amen. Truck sits all winter though and I like to keep my mind busy thinking up projects. Not that I can think of a project that would involve this anyway. Just like to know that the engine is nice and clean inside and out.

Within the past few hours I've already concocted a slew of other tests to do on it to pass the time.

 

[mechtech2]

Those Haynes #s are rather general.
Don't count on them for NASA accuracy.
To get a good comp reading,first yank all the plugs, disable the fuel and ignition, open the throttle, and give each cylinder 4-5 compression strokes .. See how it comes up for each cylinder. You can't rely on a compression reading that is obtained by long crank times just to get higher readings.

 

[Gary Allan]

Originally Posted By: oldmaninsc
I agree with everyone else as well. It all sounds good. I know you weren't trying to "fix" anything, but my Dad used to tell me: "If it isn't broke, don't fix it!"


Your dad wasn't a member of BITOG. Here if it's not broke you fix it until it is. Jeep ownership is very similar.

 

[cchase]

Originally Posted By: Gary Allan
Originally Posted By: oldmaninsc
I agree with everyone else as well. It all sounds good. I know you weren't trying to "fix" anything, but my Dad used to tell me: "If it isn't broke, don't fix it!"


Your dad wasn't a member of BITOG. Here if it's not broke you fix it until it is. Jeep ownership is very similar.


Don't jinx me.

 

[oldmaninsc]

Originally Posted By: Gary Allan
Originally Posted By: oldmaninsc
I agree with everyone else as well. It all sounds good. I know you weren't trying to "fix" anything, but my Dad used to tell me: "If it isn't broke, don't fix it!"


Your dad wasn't a member of BITOG. Here if it's not broke you fix it until it is. Jeep ownership is very similar.