Compression check... the ongoing saga
 

It was a brisk 40 degrees F today, so I decided to try out my new compression guage. Here are the results:

#1-136
#2-141
#3-135
#4-139
#5-139
#6-131

While they don't seem that far apart (from each other), they are a hell of a long way from the 170-180 that was reported from my PPI 3 years ago. I've put 13k miles on it since then... none on the track.
Was I duped on the PPI, is my compression guage just a pile o' junk. If those are the accurate #'s should I be worried (even more than I already am).

Thoughts, theories??

PS. Car is an '87 911

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Dan Tolley
1987 911 Coupe
http://www.cheaterswayside.com/911/gallery.asp?sort=0&userid=294

Did you do the check with the gas pedal all the way down? That made a big difference on my car, but maybe it only matters on cars with carbs.
Brad

Your readings are not so bad and far apart, yet lower than the PPI. I would borrow another gauge to compare. On the other hand, the gauge used during the PPI could have read 'higher' for *some* reason, and now you have a truly accurate reading. I would not worry at this point, besides, how does it run?

Did you do the test with the engine warm? If not, that can explain the low readings. On a positive note, they are all less than a 10% variance which is good.

Kurt V
72 911E

a hot engine, and starter speed make a lot of difference, too.

The fact that there are no major disparities between the figures means you're OK. Also, the numbers you show are the numbers I see in the Bentley manual.

Maybe the higher figures are due to carbon buildup? Always wondered about that...

Jw

So the tests should be done on warm engines or cold? I always assumed cold?

Shawn

Ahhh, I'm a fool. I did the check on a cold motor, and didn't touch the gas pedal. I guess all is not lost. It only took about 1 hour. Maybe I'll give it another try tomorrow.

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Dan Tolley
1987 911 Coupe
http://www.cheaterswayside.com/911/gallery.asp?sort=0&userid=294

Dan,

Engine warmed up to normal operating temperature, all of the spark plugs removed, throttle wide open, and fully charged battery!

I think you will get better results, but the consistency of your cold results look good, though not spectacularly high!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

Yeah, consistency is what counts here. Although I'm sure I have less then 1% of the experience someone like John Walker has, I can say I have never seen a "tired" motor with compression results that are consistent between cylinders. A tired motor will have one or more cylinders that give quite different results from the others.

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'83 SC

I suspected the starter speed had some effect on results, although I don't know why. Thanks to John W and everyone on this post as I was wondering the same thing.

Bob Sauerteig
'87 Coupe

Battery/starter has to be strong enough to crank each cylinder(test) quickly and did you crank until guage stopped rising? Other guys are right about warm engine and all plugs removed. My SC with 110k miles has 170+lbs. in all 6. Also,(very important), remove fuel pump relay while testing or you will flood the heck out of it.

[This message has been edited by speeder (edited 10-18-2001).]

From what I've read, you can remove the DME/Fuel pump relay, or just the fuel pump fuse. Is this correct?
I pulled the fuel pump fuse because it was easiest. I noticed no flooding...
I also hooked up the battery charger while doing the test. Just to keep everything charged up.

When I get some time I'll re-do the compression test the CORRECT way, and post the new (hopefully) results.

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Dan Tolley
1987 911 Coupe
http://www.cheaterswayside.com/911/gallery.asp?sort=0&userid=294

[This message has been edited by JDaniel (edited 10-19-2001).]