did a compression test on my 928...

[mspiegle]

Well, i started pulling my engine bay apart for my SC project. I did a compression check to get a baseline of where my motor was at...
i don't remmeber the EXACT numbers, but it was something like...

1 - 170
2 - 130 **
3 - 165
4 - 170
5 - 165
6 - 170
7 - 170
8 - 165

suposedly, the motor used to have about 90 on cyl-1 and cyl-8, and about 150 on the rest, but was rebuilt. I guess by the looks of these numbers, the motor WAS rebuilt, but i'm troubled by cyl-2. While the compression guage was in, I cranked 7 times. On the first crank, cyl-2 hits about 95 just like all the otheres, but it doesn't max out at 165-170 like the others.

My garage buddy said he did a compression check on another 928, and his numbers were all over the place. The guy went to get a leakdown test, and the results were great.

So, i'm curious - what factors can go into differences between cylinders in a compression check? How accurate are they? Do you think I have a whole lot to worry about?

I'm gonna go ahead with the project, and get a leakdown test after its done just to make sure.

Thanks in advance!

[JRowe]

Was the engine warmed up? Were the other spark plugs pulled? Did you repeat the test to verify the results? If you can answer yes to all these questions, then your test wasn't the problem and it's something with the engine. Could be any one of several causes - piston rings, worn cylinders, worn valve seats/valve heads, head gasket or possibly even valve seals(not likely).

[mspiegle]

engine was warm. after checking cyl2, checked another known good cyl to make sure everything was, then checked cyl2 again. All plugs were off.

So it sounds like you believe that compression tests are a very accurate test (assuming it is done properly).

[grapedrink]

do a leak down. compression checks are not that accurate. spray some brake fluid in there.

[mspiegle]

thanks, i'm putting the leakdown test on my list of TODO

[RedBaron86325]

928's are infamous for giving bogus numbers during a compression test. Drive it for another day and do the test over again. Common problem is carbon build up on the valves. I don't know why but the build up problem is common on all (16v and 32v) 928's. I used to frequently get bad numbers on either cyl 3 or cyl 5. Reinstall the plug, fire it up for a few minutes while taking the throttle up to around 5000 a couple of times. Usually the carbon holding the valve open will come loose. I was told that frequent use of Techron was an absolute must with the 32v engine (S4, GT, and GTS).

[mspiegle]

hey, thanks for that info, very useful.

I did make sure to drive the car before testing, i drove it a little hard, but it took maybe 45-60 mins before we actually got to the compression test... damn pre S4 928s.. i have to remove part of the intake manifold to get to the spark plugs.... and my girlfriend stopped by wtih some food, so i ate right beforehand.

Anyways, i'm kind of assuming that everything is ok... why the hell would a motor with good compression numbers like that just goto complete crap on 1 cyl? I checked the spark plug and it looked fine... I will probably get the leakdown test anyways, but thanks for your response.

This may also explain why the other 928 s4 we tested had erradic compression numbers. We're actually adapting the blower to a 16v model, so i'll keep that in mind when we do its compression test.

[RedBaron86325]

ALL 928's are a bitch to work on. I always worried about denting the top of the quarter panels while trying to reach the engine. Padded fender covers are a must for working on a 928.

I forgot the name of a 928 guru in Orange County who sold 6.2L and 6.5L stroker kits for the early 928's. He also used to make intake manifolds for roots style super chargers. If I remember his name (or website) I'll let you know. It might take some digging since I haven't owned a 928 in a long long long time.

[mspiegle]

yeah, i've been careful to stay off the fenders... i'd be pretty sad if i dented one badly.

I know theres a shop somewhere here in CA called devek that does 6.4L rebuilds with some very impressive hp numbers... expensive as hell tho and take a long time.

The kit i'm running is a roots-style system my garage-buddy designed from the ground up. His car is running a whipple twin screw with about 8-9psi pushing 400rwhp... i'm gonna push 4psi for now and make sure my car can handle it. Will raise boost later when I get the SMT6 in and can control ignition timing better.

[Numbersix]

Yep. 928's, esp. higher mileage cars that have seen a lot of city driving, are known to give weird #'s on compression. I'd drive the crap out of the thing, and/or MotorVac it to try and remove some of the carbon deposits, then comp. test it. I'd also highly reccomend a leakdown test. I've seen a couple cars with decent compression #'s show 30% leakage on at least 1 cylinder.