sancho

I just went out and re-did a compression test on my rebuilt L98 engine in my '89 iroc; my old results were a bit skewed.

The new results...

DRIVER'S SIDE
1. 193 PSI
3. 192 PSI
5. 189 PSI
7. 202 PSI

PASSENGER'S SIDE
2. 200 PSI
4. 200 PSI
6. 200 PSI
8. 212 PSI

One thing that I noticed this time was that, on cylinders 4, 6, and 8, I heard a popping sound comming out of the head once for each compression stroke (the intake manifold was off for the test). I have no idea as to what to make of that, except that it makes me wonder if it has something to do with the valve timing (no expert on valve timing, but it almost sounds like compressed air was leaving a valve that was opening early or something).

Do these results tell you anything? Also, do I need to consider removing a head or something?

I'd like to know something by tonight as to what I need to do next, as tomorrow is the only other day of the week that I will have to work on this car...

Thanks!

ede

test results look very good, why do you think you have a problem? or do you have a problem? what is it?

sancho

The problem is that, ever since we've finished rebuilding this engine, it overheats in less than fifteen minutes (temp gague redlines and radiator boils over). Also, the timing must be set high for the car to idle--and even then, the idle is high at 1000+ RPM. Plus, if you try to put it into drive at idle, the engine just dies unless I really play with the gas (if it's in gear and idling, I have to keep my foot on the gas to keep it from dying).

Consequently, this car has not left my driveway even once for nearly eight months.

My guess is that, if I can find out why it's overheating, that will be what's causing all of the problems. The compression test is just the latest in a long series of test to figure out just what could be wrong.

If you want a more detailed picture of what I've been through with this car, see my old post here (there's a link in that post to an even older post, where it all started).

jcb999

your going to at lest need a high capacity rad and plenty of airflow.With 200psi.. that engine is making quite a bit of heat over stock.

RB83L69

Your compression test results are rather high... they look like you have a really, really weenie cam in there. They're typical of a LG4 305 (8½:1), with the tiniest cam Chevrolet ever screwed car owners with. But there's nothing "wrong" with your motor as such, according to the compression readings. Your problem lies elsewhere.

Your description of your overheating problems are typical of severely retarded timing, and/or very lean mixture.

Forget what your timing light says, adjust the timing until it's as far advanced as it will go without pinging, and see if it runs any better like that. That may be too far advanced to just leave it like that, but it will give you some idea of what's really going on. The timing mark is wrong on almost every used balancer, because it's not on the part of the balancer that's actually connected to the motor, it's on the part that's just flying around out at the outside edge, only loosely coupled to the motor by a blob of spooge.

sancho

Does the fact that my numbers seem a little higher suggest that I might have higher-than-stock compression now (which equals more horsepower)? Or do my numbers have more to do with the cam than engine compression?

The cam that I got from the machine shop is supposed to be stock--or as close to stock as possible.

On another note, I was getting ready to pull these heads tomorrow (the passgner's side, at least). However, if I can keep from having to do that, I definately want to. Would there be any reason to go through taking the heads off at this point?

I'm just trying to figure out what my next plan-of-attack should be (I've tried pretty much everything that I can think of)...

MIG-29

dear sancho
i've read in chevy high performance just that, about "cranking compression" and they were CLEAR about not going over 195, it seems that your stock cam is the wrong one for your heads, if i'm correct (please correct me if i'm wrong) what you need is a longer duration intake cam, or something like that, to bring it down a little, also they said that for a high compression engine, a cam that will make around 170/185 will be "da bes".
i'll look again and will be back to you with the real thing.
good luck.
Fernando

sancho

Fernando,

If you could let me know the issue of that Chevy High Performance that you're talking about, I'd be interested...

Also, that part where you said "a cam that will make around 145"... do you mean 145 PSI max compression?

AlkyIROC

Typical stock cams will give close to 150 cranking pressure. The amount of cranking pressure achieved is usually made by the cam profile. If the valve timing has less overlap then more pressure can be built in the cylinders since it's not being pushed out open valves.

There are a few other tests that you need to do.

Cranking pressure test is done in 2 parts. All the spark plugs should be out. First it's done dry. That's when you screw the compression tester into the spark plug hole and crank the engine over. Second is a wet test. Use an oil can and squirt some oil into the cylinder. Take another compression test of the cylinder. The compression should increase.

The last test to do is a leakdown test. You can make a leakdown tester but it's easier to just buy one. How it works is you use a regulator to regulate air pressure to 100 psi. It's then forced into the cylinder through the spark plug hole and the amount of air loss is recorded. If the pressure drops to 95 psi then there's a 5% leak. You can also listen for air leaks in different parts of the engine to determine if there's any internal problem. Leaking head gaskets or cracked heads will cause bubbles in the coolent etc. Remove the rockers so the valves will stay closed while testing.

You can also check the cylinder walls for cracks. Rotate the crank so the piston is at TDC. Filling the cylinder with compressed air should not force the cylinder down if it's directly at TDC. Record any leakdown. Now rotate the engine so the piston is at BDC and do another leakdown test. If there's a huge difference in pressure drop then something is wrong with the cylinder wall.

Don't always trust the timing marks on the balancer. I was constantly advancing my timing and was still making HP. When I reached 45* I was a little concerned. I indexed my cam and found my timing marks were off 8*. The 45* was actually only 38*. When I had it set at 36* the car was a dog to drive because the timing was actually only 28*.

Sitting Bull

This may sound funny but have you replaced your rad cap? Maybe even get one that will hold a pound or two more pressure.

Mine was boiling over and making me crazy. Turned out to be the rad cap had lost some of its ability to hold the pressure at 16 lbs/square inch. Replaced it for $4 and no more trouble

MIG-29

ok sancho man...
its the Chevy high performance, september 2002.
1- the proper way is on a cold engine and all cylinders done the same, disconnect distributor, wide open trottle, bla bla bla.
and in few words, an early closing intake valve cam, tends to maximize cylinder filling, therefore it will give you more cranking compression, thats only good on ridiculous compression ratios such as 8.7:1.
the opposite (late closing) applies to the higher compression engines, they even said that you can have a 11:1 compression ratio engine and still use 87 gas, if you choose the right cam for THAT.
to finish, they also said that between 170 and 185psi of cranking pressure, you're "right on the money honey".
if you cant find the magazine, tell me and i'll try to scan it and send it 2 ya via e-mail.
it'll be a pleassure to help a fellow third gener.
Fernando.

MIG-29

here i am....(rocking like a hurricane)
ok i just did scan the whole thing, so i'll e-mail it to you right to your adress, is a 3.3k. document on adobe acrobat reader.
if somebody wants it, just ask (i hope it is not against the rules. )
by the way, our good friend could be right with the cap idea, but sure with 200psi. you are asking for a HOT engine.
Fernando.

Vader

Sancho,

Your results are not abnormally high for a good engine, good valves, and cam with almost no overlap. The fact that they are all within 6% of 200PSIG is a good indication that you have a snug engine. The fact that you knew you already had oil in teh cylinders could be pushing the numbers up a bit. I wouldn't be too concerned with the readings.

I would be more interested to learn why there was so much oil in the cylinders.

As for a cylinder leakdown tester, you should be able to find a 1.0mm or #60 drill at any good hardware store. The term "good" leaves out the Home Depots, Lowes, and other chains in the home improvement supermarket game. You need to find a real hardware store - one that has tooling, hydraulics, a pipe threader, a locksmith, and no friggin' wallpaper or curtains. Lacking that, you can pick up one at a local industrial supplier, or order one at McMaster, MSC, or the like.

Once you get it together, perform the test with the pistons at TDC. As Steven said, the pressure difference can be different at Bdc, from either cylinder wall problems OR excessive taper wear.