87 930 compression results...so what does it mean?

[porschett]

In the process of dropping the engine to do the top end and cam swap. First step was to take some numbers. Here's how it faired...


3 - 115 psi 6 - 119 psi
2 - 125 psi 5 - 119 psi
1 - 108 psi 4 - 115 psi


The plugs (NGK forgot the temp code) look really good except:
#2 plug was a little white on the electrode
#4 plug was a tiny bit wet and more carbon around the threads


So what do you think is up with #1 and #2 cylinders? What needs to be done while I have the top end out?

Thanks.
David

[porschett]

Anyone please? Bueller?

[john walker's workshop]

the valve grind should take care of that, unless some rings are broken, or pistons/cylinders scored. pull off the jugs when you have the heads off.

[OSI930]

What kind of compression numbers and % variance are acceptable for a 911 turbo?

[porschett]

I'm not sure what are acceptable numbers. I think what you really want to look at is the variance because different people use different compression test techniques, tools, temperature of engine when test was taken, elevation, etc...

I'm concerned with the variance of #1 and #2 cylinders. I even tested the readings many times and had same results. I think I'll take John Walker's advice and not guess until I take the camshaft housing off.

But before that, I'm also going to do a leak down test when the engine is out to see if I hear anything strange.

[fredmeister]

I don't understand the concern if you are dropping the engine to do a swap on top end anyway. The variance is a little off but not badly. Usually within 10% of each other is acceptable. Are you sure the valves were adjusted properly so that none are too tight?
Was the throttle open fully when testing. Were all the plugs removed to get a quick engine rpm at turnover with the starter motor?These will all effect compression readings.
Sounds like reasonable values to me as well.

[porschett]

Thanks for the assurance Fred. The valves haven't been adjusted yet. I just took unmolested readings. Adjusting the valves will be done with the cam swap.

All the plugs were removed and settings taken at WOT. Cranked it over 12 x. I'll report back once I perform a leak down test.

[beepbeep]

I bet you'll find bunch of carbon deposits on pistons once you take off the heads. Readings look OK.

[Cory M]

As a data point, my 930 was tested with 30k miles:
compression was 125 and leakdown was 2% across all cylinders

[porschett]

Hey Beep,

Why do you think there will be a lot of carbon deposits? Is it because of the low compression #s? Wouldn't carbon buildup increase compression?

TY.
David

[Wayne 962]

Read this:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

[porschett]

OK. Finally got the P&Cs off as part of my rebuild. I found the cause of low compression for #1. The 2nd compression ring is broken. The rest of the rings look in good shape. I'll measure the ring gap later tonight.

Question is when I get the new compression ring, do I need to do anything else such as re-hone to #1? Since I'm only replacing one ring out of three, I'm hopping I can just re-clock the rings and I'm set.

#5 and #6 had a lot of oil on the exhaust valves. Further inspection of the positioning of the oil rings, they were pointed down at 6 and 7 o'clock vs. 12 o'clock and the compression ring gaps were more inline vs. 180 degrees apart. Can the positioning of the ring gap normally rotate with time? Or once it's set, it's likely not to rotate?

Things I'm doing:

- ARP studs
- ARP rod bolts
- 964 cams
- port intake/manifold/plenum to 36mm
- headers
- custom full bay IC
- full head job (valves/springs/seats/regrind)

Thanks,
David