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'86 930 PPI Tomorrow, What should good compression and leak down numbers look like?

Hey gang,
Been lurking here in the shadows for a while prior to registering, sort of like a long time listener, first time caller so to speak...

So after owning a 911 years back and always dreaming of owning a 930 one day, I bit on what looks to be a very sweet all original '86 911 Turbo with 47K miles and all the history one could ever ask for. The deal certainly seems fair if all checks out, but certainly there are both more and less expensive cars available in this market. While on the surface every thing looks litterally perfect, if there are any skeletons found to be hiding in closets causing unexpected SURPRISES, there would certainly be far better deals to be had.

The full PPI with road test, compression test and leak down is set for tomorrow morning. While I have done a few primitive compression tests and even primitive leak downs on some of my own projects over the years just enough to be familar with the process(es), I was hoping some might chime in with what I should be looking for in terms of good results that should expect to see on a car of this vintage with 47K that seems to have been extremely well cared for. While I am totally pschyed about owning this sweet car and now even my wife is starting to share my enthusiasm as well (I never thought that would happen), if it looks at all like the engine is suspect as apparently some are at around the 50K mile mark, I might very well be inclined to cut my losses and just try to find another 930 with a solid power plant.

Is it just me or is the #6 often times the most usual suspect?

Thanks so much for any feedback!

Cheers, Dave, AKA RideSober

Old 11-15-2010, 04:20 PM
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Leak down

Dont worry about compression as much since you are doing a leakdown. If your engine is below 10% then I would say you are good to go. If it is below 5% then your engine is almost brand new.

The best time to do a leakdown is when the engine is cold. Start the car and let it run for 30 seconds. Shut it down and then go to town. This will be the most accurate numbers... from what I have heard.

With that low of miles if it has been well cared for you should be around 5-7% or less.

Best of luck with the PPI and the new car.

Bryan
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Old 11-15-2010, 04:37 PM
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I think you should accept no more than 3-4% leakdown personally. My 930 was less than that with 40K on the clock.

I cannot imagine why anybody would settle for 10%.
Old 11-15-2010, 04:54 PM
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Thumbs up

Thanks guys, very helpful. I'll post some feedback after I get the results.

I did read Wayne's article here about compression and leak down testing, and in the portion about leak down testing it was stated:

"A newly rebuilt engine should have leak down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine but are still adequate for good engine operation. Leakage numbers around 30% indicate that there are problems brewing, and that a rebuild may be necessary."

I guess if the car is already yours, and you are testing it to see if you need to do a rebuild, then OK, I guess those maybe some marginal but acceptable tolerances. But in my case I think I had already pretty much resolved in my mind I would not accept much more than about 3-4%.

Thanks DDDD for informing me that is what you saw at 40K, and that is what I would have expected. I certainly do hope they don't find any more than that because I really do love this car.
Old 11-15-2010, 08:45 PM
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Has anyone ever seen published numbers on what compression values should be? I haven't.
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Old 11-15-2010, 09:25 PM
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Dave I bought my 87 930 4 years ago in the low 40ís on mileage. Leak down was 3% across all 6.

I have 46k on the clock now and the dam thing pulls like an OX! No smoke on startup ice cold.
Like Bryan said anything south of 10% is just fine IF the car appears loved.

Funny you mention #6, I owned a new Boxster that bleed like a biotch through that cylinder, but only sometimes.
Mech told me that it depended on where the piston/valves were positioned when you shut the motor down :/

Good luck!
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Old 11-15-2010, 09:41 PM
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Well the results are in, and they are not good...

I am really disapointed, but I guess it is far better to learn the bad news now and avoid an expensive mistake. Two of the cylinders had extensive leakage, the number 2 was 18%, and the number 6 was 32%.

Even the Porsche service manager said you would have never guessed by the way the car ran...

Gosh, what a beautiful car.

Last edited by RideSober; 11-16-2010 at 02:46 PM..
Old 11-16-2010, 02:44 PM
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Well, if just the head studs broken or leaking valves or gaskets on two cylinders that is not to expensive to repair. Now the owner knows the motor needs repair if he /she will just think about what options are available. Possibly the seller might make you a price reduction to cover most or all of the repair to make it right. After all it is a great looking car.
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Last edited by voitureltd; 11-16-2010 at 02:58 PM..
Old 11-16-2010, 02:56 PM
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Actually I had a long talk with the the Porsche service manager who did the PPI. He had done some previous work on the car, and he was pretty certain the car had two burned valves.
Old 11-16-2010, 05:18 PM
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With a properly performed leak down test, they should be able to tell you where the leakage is from. No matter, those numbers are not good and the cost to repair is not cheap. IF all you have to do is some head work, studs, that would be the absolute best case scenario BUT...

...you're going to want to measure the cylinders, and given the issues already found by the leakdown my money is on they will be out of spec. New pistons and cylinders are 4k just for parts.

Take 15k off the price and then maybe you can still consider this car, otherwise start shopping for a car with a freshly rebuilt engine. That's what I'd do in your shoes, and they are around.

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Old 11-16-2010, 05:41 PM
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