Some of you guys probably know that I had blew my motor quite a while back. I’ve been working on my LS1 project, but anyways I’ve decided to call it quits. So I found a 13Brew motor at a junk and was wandering if I could do a cold compression test on it.. The people that works there wont do it, but they say I am welcome to do it if I want to. What are the numbers I should be looking for when doing a cold compression tester on that motor..? I’ll be using a piston compression tester with that valve out.

 

where are you located? I may have something for you.

 

The numbers won't mean anything unless you can get it cranking to about the right RPM. Is it in a shell with a working battery and starter?

Without the ability to crank, just turn it over and listen for consistent whooshes.

Please correct me if I'm wrong and there is a way to get that data, but I'm pretty sure this is why buying a used motor is such a crapshoot.

Dave

 

I'm located in California.



The motor is out. I was thinking of doing the compression check using a starter and a healthy battery. I was there yesterday and had turn the flywheel. The flywheel does seem kind of soft, then hard.. then soft. Forgot to listhen to the consistent whooshes, but the motor does make that sound. I'm on my way to do the compression check right now. So I'll let you guys know how it goes. thanks

 

you're right Dave. You'll also need a way to mount the starter (usually via the bellhousing) to the engine. I got about 90lb/in^2 on a semi bridge with a normal compressio tester like you are talking about. you may want to bring a bottle of ATF to lube the seals before you start cranking at it. You are going to need to first jump the starter engagement seleniod and then jump the starter motor itself. You'll hear a clunk when the starter engages the flywheel.

Rob

 

You have to have a starter, trans, and flywheel, as well as battery and jumper cables to do the test. Even then, expect cranking rpm to be slightly lower than it should be, because the battery will likely be a bit discharged, and jumper cables do not make the most awesome connection possible.

Cold numbers, with all else being equal, are almost always higher than hot numbers. This is why borderline weak rotaries can have hot start problems yet start fine when cold...there is more compression when cold.

I usually do the cold test and take 5psi away, and that is fairly close to the hot numbers you should see. It is not too big a difference...the main thing is the cranking rpm, and having the throttle open, and having both front and rear plugs out when doing the test (of course the tester is in one of those open holes).

 

BTW, injecting oil or atf or anything else will artificially raise compression numbers. With a test of a running engine (with no excess fluid in chambers, but run recently enough to still have the light coating of oil which helps form compression) the best numbers you could hope for would be 120-130psi. On that same engine, if you pump atf or oil into it, that engine might show 150-160psi compression. But, it's obviously not that high, so the numbers are useless when obtained with atf. This is how some of the jspec engine sellers show compression tests in the 130+psi range.

The only bad thing is that this engine probably has not been run in quite a while, so the light coating of oil that is normally there on a running engine is probably gone. So the numbers might be a bit lower than they otherwise would be if the engine had run recently. I would do a test without atf, and one with. Subtract 30-40psi from the one with atf, and average that against the first, take away 5psi for a hot estimate, and that's about as close as you can get as it is.

 

Okay I just got back. It was really hard to read the numbers, because this piston compressor was just jumping really fast. Anyways the numbers were quite low. It jumbs from 70lbs to 69lbs. I then tried the second rotors and same results. So I just told them I didn’t want it, because it has low compression. However it was really consistent tho. A normal and good engine should be 90lbs or higher right..??

 

I feel ya, but by the same token it's a good idea to lube up the seals if the engine has been sitting around a while.

 

Are you positive that you had a good battery? You may want to try it using your vehicle as a jumpstarter if you can drive up close enough for a set of jumper cables to reach.

 

The battery is came from a working car, beside we even use their truck to do the compression test as well. I did notice it turns really slow at the beginning of the compression test on the motor. When I tap the Positive wire to the Postive battery, it'll make a quick turn, and jump to 30psi, then 0, 0, and die. I’ll have to tap the pos. battery once again to make it turn. But it really feels like it wont turn over at all.