924Board.org

CrashTest · Posted: Thu Sep 11, 2008 9:39 am Post subject: Compression test

So I finally bought a compression test gauge, but I have a problem.

The directions say to perform the test on a warm engine. As mine is not ready to run, I am wondering if a test on a cold engine is worth performing...?

I also wonder if a compression test can be performed without the use of a starter, by simply rotating the engine by hand? Is pressure likely to build in this way?

I ask because the engine is on a stand at the moment for exhaust manifold repair/replacement and clutch renewal. I suppose it's possible to install the flywheel and starter to turn the engine for a test...

TIA,
sam
_________________
'80 931 primer, brown interior, five lug In pieces now and parting out.

Smoothie · Posted: Thu Sep 11, 2008 10:13 am Post subject:

Yes it's normally done with a starter turning the engine, and with the throttle valve propped open. -But I was in the same situation about a year ago...I got a junkyard engine to rebuild for my Jeep and wanted to give it some kind of test before the return period elapsed.. I had it on an engine stand, all spark plugs removed, throttle propped open, and was able to get readings of approx. 125psi cranking it by hand with an offset wrench on the front crank bolt. You have to start with the crank turned to the start of the compression stroke for the cyl you're testing and with the end of the wrench positioned low, give it your bestest-fastest blast of energy toward-up-and-away from yourself (wearing gloves of course). (Also, I was a shot-putter and discus thrower in HS, and for a short time later in the Navy, and weigh in at about 220 lbs. now, so those kind of pre-requisites would be useful..) I wouldn't take what you get doing it by hand as a good conclusive test, but done with the same vigor on each cylinder, you can at least get an idea of what's going on in there (consistent readings would be a good sign).
Also, given that you'll be doing it cold, I'd put a few CCs of oil in through the spark plug holes to at least lube the cyl walls/rings. It would be nice if you could have the rest of the engine lubed, but since the oil pump's turned by the crankshaft, that's not so easy. (With my Jeep engine, I went in through the distributor hole with a screwdriver mounted to a 1/2" drill and turned the pump to lube the engine that way.) Yeah, now that I think of it, you'd be better-off mounting the starter and using that if you can.

Of course you could do a compression leak-down test. That doesn't require turning the engine. (I think I saw comp leakdown testers selling pretty cheap at Harbor Freight recently.)
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

CrashTest · Posted: Thu Sep 11, 2008 10:28 am Post subject:

Thanks...

let ya know,

sam
_________________
'80 931 primer, brown interior, five lug In pieces now and parting out.

Smoothie · Posted: Fri Sep 12, 2008 11:40 am Post subject:

Here's the Harbor Freight $30 leakdown tester -
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=94190
-Obviously requires an air compressor too.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

My experience is that you can't get conclusive results on a cold engine that hasn't run in a while. The rings simply will not seal correctly. So the actual numbers you get won't really tell you anything conclusive regarding the actual compression. IMO, the best you can hope for is to compare readings from cylinder to cylinder and verify that they are all relatively consistent. If you find that one cylinder is much lower than the rest, that would be cause for concern. If all cylinders are within say 10% of each other, that's cause for optimism. But the raw compression readings are probably useless.
_________________
erstwhile owner of just about every 924 variant ever made

CrashTest · Posted: Sat Sep 13, 2008 1:10 pm Post subject:

This all makes good sense to me. Ultimately, I am just afraid to make any real decisions. My mind swings from a wanting a complete rebuild to the bare minimum to achieve driving the car again.

I wind up looking at the stupid engine on the stand churning over the details for hours.

I was the last person to drive the car about two years ago. It ran well and was reliable for several years before that...

Some one told me once that if you don't know what to do, don't do anything... That's my position for now. Gonna stare @ it some more I guess.

thanks a bunch
_________________
'80 931 primer, brown interior, five lug In pieces now and parting out.

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

If you have the engine out of the car already, why not just do a baseline rebuild? You don't have to go crazy with modifications and stuff, and it would seem like such a waste to have the engine out and not refresh the lower end. You should also consider having the turbo rebuilt while the motor is out. Majestic can do a stock rebuild for $335 (unless it needs major repairs). A bottom end rebuild on your own won't cost you too much more than that in parts. Freshen up the head (inspect the guides, replace the valve stem seals, gaskets, & cam oiler elbow), and put her back together.
_________________
erstwhile owner of just about every 924 variant ever made