87 Compression #'s - Help!

Just had the compression checked on an 87 carrera I am considering. The #'s came back at 160 on all cylinders. I am a bit perplexed because I did'nt think that was possbile. Is'nt there normally variation between cylinders? Also should I be concerned about 160? I think I read somewhere that 130 - 175 was good compression?

TIA - michaelw

Come guys, step up and help me out! I really think this may be the car but the compression questions intrigues me? Could they really be the same?

mw

Yes, they can be the same. 160 is fine.
My SC was serviced by the previous owner at Andial. They do a compression test everytime they do a valve adjust. They know what they are doing and are capable of doing accurate compression tests. It tested throughout the years at 150 on each of the cyls.
I recently had them do a compression test on it, just to see how things are going. Even with 125,000+ miles, it still tests at 150 in each cyl.
Sounds like the car you are looking at has a strong, healthy engine!

[This message has been edited by Jim T (edited 02-09-2001).]

Michael,

I don't know for sure, but those numbers sound like a tired engine to me ... for an engine with 9.5:1 compression ratio. Do you have any service records or history on the car available?

My 33 year-old 340 Mopar engine (build date of 2-20-68) with only a valve grind in 1987 and 10.5:1 compression ratio still has 195-210 psi compression test numbers.

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Warren Hall
1973 911S Targa

I don't want to be misunderstood here. I'm not disagreeing with Warren. I would not find those numbers disappointing. While it is true that 175 would sound better, teh fact that they all measured 160 is impressive to me. And 160 is not a low number, generally speaking.

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'83 SC

Super,

It doesn't matter if you do disagree, with me! If your comression test number are similar with a 9.3:1 compression ratio, then so be it. What I do know is that my 8.5:1 engine had higher numbers when I tested it the last time.

I would be glad to hear some test numbers from Carrera owners with the 9.5:1 engine. I don't know them, because I've never worked on any of the '84-'89 cars.

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Warren Hall
1973 911S Targa

I read the buyers guide that pelicanparts has listed on this site (forgot the author) but he indicated good compression on the carreras should run from 140-175. I have only run across 1 carrera that had a compression over 170. I would be interested what bruce anderson states is adequate (and yes I am going to buy his book).

I am only at the very front of the learning curve on porsche as this would be my first. So all input is appreciated. I too would like to hear of other compression #'s.

TIA - michaelw

Todd at Protomotive states, "3.2's usually have between 125-130psi"

Mike
86 Carrera t

On the prepurchase inspection I had done on an '86 3.2 Carrera the #'s came back 155lbs +/- 2 pounds on all six. I then requested a leakdown and it came back with 0% leakage on all six. I'm thinking 160lbs per is a good result. Your biggest concern in a comp test is consistency, not raw numbers. Just my 2 cents

Good lord.
You're gonna get some variance between gauges (which is why unless it is way out of line, the # isn't as important as consistency between cylinders), but 160 is very nice for a Carrera.
160 on EACH cyl is a tired engine?!?! Please.

My '88 Carrera tested at 165-170-170-170-170-165; and my mechanic told me that was good. But I have also been told that:
a. a single compression test does not mean much; it is the rate of change over time that tells you the health of the engine, and
b. compression tests are not meaningful in the absence of a leakdown test.
the source of this info (a 911 newsgroup) did not elaborate.

I just would not be uncomfortable with 160. 160 is a good test. I also believe that symmetry is another excellent indication. A cylinder that is too low has a ring or valve problem. A leakdown test can determine whether the leak is a ring, an exhaust valve or an intake valve. When engines wear out, the numbers get too low. A car with 120 lb cylinders will run poorly. One with 90 lb cylinders may not run at all.

It is interesting now that I see how sharply this pressure climbs with compression ratio. 195-210 is pretty serious. I drove a 340 Wedge. Snappy car.

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'83 SC

The compression on my 1988 Targa with 100,000 miles is:

1. 155
2. 150
3. 150
4. 145
5. 160
6. 155

Keep in mind:

A. the compression gauge was the $19.95 special from Harbor Freight.

B. I had never done this type of test myself before.

Who knows? It could be totally accurate or the gauge could be off or I could be a dolt. But it seems in line...

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E. William Reinecke
1988 Carrera Targa

As a follow up to the above:

Shouldn't normal compression be the compression ratio * the air pressure? i.e., I am at sea level so the readings should be 9.5*14.7 = 139.65? What accounts for the higher readings? I know that ambient temp etc. could make for some changes but not that much??

Thanks!

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E. William Reinecke
1988 Carrera Targa

Thanks guys, it seems from the thread that maybe the leakdown is as important as the compression test and I am surprised that the porsche specialist did'nt even mention that.It sounds as if 160 maybe a pretty good number. I am going to hit my local barnes and noble and see what compression is on a new carrera. That will give me an idea of the degradation in compression over 14 years. The last time I was in B&N there was a book specifically on the mechanical evolution of the 911 (not bruces book). Hopefully that may have the #'s.

Thanks,
michaelb

Good hunting, but as a researcher I'd say it's surprizing the stuff that's NOT out there like you'd expect.

Very exciting. E. William is right as far as I can see. Maximum compression should be 14.7 * CR. How are they higher? Perhaps even at slower speeds there is the interia effect in which air gets packed into the cylinder a little tighter than atmospheric pressure, just because of the air movement and momentum. But does this effect really pack 17 lbs of air in the cylinder? And if this effect is at work, then the speed of your starter motor will impact your readings.

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'83 SC

Super,

I've never seen any kind of formula used to calculate compression test numbers. The other factor/variable you are forgetting is temperature of the entire engine, including cylinders, pistons, heads, and the entire intake tract! That the test is supposed to be done with the engine warmed up and at normal operating temperature means a five-minute delay in getting all of the plugs out ... gives the air in the intake system a chance to warmup to 200-250 degrees F, and should raise the pressure quite a bit. How else could an 8.5:1 engine get a mean pressure of 172 psi? Or a 10.5:1 engine achieve a 196 psi mean?

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Warren Hall
1973 911S Targa

Just another reference fact:
my 1985 carrera had 39,000 miles on it the last time the compression was tested.

It had 195 195 195 195 195 195

Reputable mechanic, I assume it was accurate, my car runs great so I wasn't to concerned about the reading at the time. Hope that helps. Oh yeah, the car is stock original.

R22tech-

195 is amazing. The car must have been very well maintained. I would'nt think there is any way to increase compression (i.e. engine health) over the life of the car without a rebuild. Internally the engine is wearing out from day one and all you can do is slow the progress, sort of like my personal situation.

Thanks for the input! MW

Another important factor to consider in the interpretation of absolute values of a compression test is the cam timing. Few engines have the intake valve closed at BDC. This explains why a 911T frequently will have a reading as high or higher than an early 911S. The Carrera intake value closes at 50 past BTC according to Bentley. I also agree with JimT that there must be considerable variablity across gauges. My vote is that 160 across all cylinders suggests a solid top end - and I assume it drives great!
Best of luck

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Joe Riley
84 Carrera Targa
69 911S Coupe
http://members.rennlist.com/911andson/