Compression results on '84 Carerra

[bsaffin]

I had an inspection done recently on an '84 Carerra with ~160K miles. The compression results were:

165, 160, 150, 150, 150, 150

I know in Bruce Anderson's book he states that anything within 10-15 % is acceptable, but that a well-maintained 911 engine should be within 5% or so. The difference between highest and lowest in this case is within 15%, but barely. What should I make of these numbers?

Thanks...Bruce

[Bill Douglas]

I think they are good What you are looking out for is a situation like all are similar but one is low at about 130. This is usually how a worn out engine would appear.

[jazzbass]

Did you get a leakdown test? The wrench who did my PPI swears by them, says their more telling than compression (maybe his compression tester was broken). Leakdown in all cylinders should be < 5%.

How is the car otherwise? Does it have the factory installed dashboard cracks and optional 1st gear syncho delete? If not, you're off to a good start.

[Early_S_Man]

Those sound a bit low for a 9.5:1 engine ...

Was the engine fully warmed up when the compression test was done, and was the throttle held fully wide-open while cranking?

[APKhaos]

Do you know what the oil consumption rate is? Thats a good empirical indicator of condition, if you can get an honest answer.

If you go for this one, try running a couple of cans of Techron or BG40K to clean out some of the accumulated crud. The carbon buildup on lightly used engines can be amazing. Change the oil immediately after the treatment - it will be diluted.

Once you get it clean, give it the revs it needs to keep it that way.

[bsaffin]

jazzbass - No factory installed dashboard cracks (this one's not leather) or 1st gear synchro delete ;-) Actually, the synchros are worn somewhat, but it still slides into 1st while rolling to a stop sign. I figure that's not too bad. Unfortunately I didn't get a leakdown test. The interior is a little worn, with odd kinds of things. Carpet on door panel is falling off; Passenger side seatbelt trim missing; rear seatbelts don't retract well, etc.

Early_S_Man - What should the compression numbers be for this engine? Is there a formula to calculate what they should be based on the compression ratio? And, I understand the need for a fully-warmed engine, but what difference does it make if the throttle is open or not when testing compression?

APKhaos - Oil consumption is stated as virtually non-existent between changes. Owner has pretty good records for regular servicing, and under the engine/transmission is very clean.

Thanks all.

[cowtown]

Originally posted by bsaffin
Early_S_Man - What should the compression numbers be for this engine? Is there a formula to calculate what they should be based on the compression ratio?

bsaffin, I'm not S_Man, but I can give you a data point to compare against. I did my last compresion check on my '88 Targa in Feb' of 2002 with about 155K miles. My results were 168-171 on all cylinders except #2, which read 183. I don't know why #2 is so much higher, but it has been consistently so since I began testing with every valve adjustment 6 years ago when I got the car.

Like another poster said previously, it's not so much absolute numbers you look at, it's the variances. Warm/cold engine, spark plugs in/out, ambient temperature, condition of battery/starter, etc., can all affect the absolute psi readings.

Colin

[RickM]

Stupid question: If the compression in one cylinder is so much higher doesn't it throw the engine out of balance?

[tbitz]

Note 165 is only 10% more than 150 (not 15%), and if you compare each cyl to the 'mean value' (154) then the worst is only 7% more than the mean.