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Is This True about A Compression/Leak Down Test?

I'm looking at a 91 964 Turbo and called a well known shop with 40 yrs experience to get a PPI done. When I asked about the compression/ leak down I was told on these 964/Turbo cars it wont be accurate, the reading will be all over the place. He also told me Bruce Anderson had a write up on this in Excellent magazine.

So where does this leave me? How do I know the engine is in good shape?
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Old 08-09-2010, 02:59 PM
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that's ridiculous. In three words, find another shop.
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Old 08-09-2010, 06:47 PM
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Isn't the 965 motor pretty much the same 3.3 liter as in the earlier cars? I'd like to hear the wrench's explanation why compression and leakdown in a 965 motor would be any less accurate than in any other Porsche motor. (Maybe it'll teach me something.)

I think the bigger question is what numbers are we supposed to be getting for compression? I've never seen anyone that from any official source.
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Old 08-09-2010, 06:57 PM
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Interesting, I would like to know why or what his reason for the statement was.
Just curiously. Now, I am not accusing the man of anything but I will say that some of the most unpleasent job's to be carried out via such testing are on Rear or Mid engined cars.
But I can't imagine someone stating such facts.

1. Do you have a history with this shop, I mean are you a long time client.
2. If so and you are a long term client then you should trust the shops word so long
as they have not wronged you in the past.
3. And not to be a jerk, but make sure you did not miss qoute or that you did not have a miss understanding or take a statement out of context. Sometimes customers asking something and they hear a portion of a statement and run with it as soul fact and cause themself a panic attack. ^_^
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Old 08-09-2010, 06:58 PM
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Hello there, As a shop owner , I have performed many PPI's , And 90 percent of the time a leakdown test is requested, It can be used to verify the amount of air lost (expressed as apercentage) and of course it can tell us which components are suspect, (rings, valves etc.) ...On a healthy engine the numbers will be close to each other, That being said , in evaluating a porsche for my own purchase I can remember only doing a leak-down once , after driving that particular car it was ovious that there was a problem and this test was used to verify my suspicion. In six other ocasions I purchased without doing a leakdown test and had no regrets.

Of course I will provide the customer with all the relevant info and help them select the procedures to be used,

Two things I always insist in verifying: 1- pull rocker covers ,inspect studs
2-Drain oil, Insp. magnets, cut oil filter
inspect element
Going slightly off topic, Turn the ignition switch on and verify that all the warning lights are in working order, (specifically : ABS, AIR BAG, CH ENGINE)

Over the years we have found cars with missing bulbs,
Also if the car still has the dual mass flywheel check the clutch operation,

Hope everything goes well, 965's are a great car!

Lou / AERO DYNAMICS
Old 08-09-2010, 08:23 PM
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I can offer up a few opinions about leakdown/compression tests and well known/respected p-car shops with years of experience based on my personal experiences with both of them.

I think the shop's statement that the "numbers will be all over the place" is BS. This shop simply doesn't want to do the job. This shop sounds a bit lazy to me, maybe they don't need the billable labor hours. These tests are not a cakewalk on a turbo porsche, but they aren't very difficult either. I've performed both compression and leakdown tests myself on my 930. The leakdown probably took me 2-3 hours in total, and that was the first time I've ever done a leakdown test on any vehicle. A shop with 40 years experience should be able to do it in half the time.

These tests are certainly one of the best ways to assess the health of the engine, but I think they will only tell you so much. Good numbers are certainly no guarantee that the engine does not have some real issues. Perhaps the shop's BS answer is really a way to dodge this issue: if they give you a test with "decent" leakdown numbers and you buy the car but later find a problem, you may come after them.

Bad numbers, or more importantly significant variances between cylinders are probably a sign of something very serious and obvious. You should have them done to rule out the obvious, but realize that they are not a guarantee. I think that many people have elevated "the car passes a PPI with leakdown with flying colors" to be the equivalent of a certified pre-owned vehicle.

My personal experience with leakdown/compression testing on a 3.3 liter turbo:

44% leakdown on #6. That's a number that would have most people run away screaming Look closely and you can see how large the top ring land is...this piston got very hot and melted a bit, deforming the crown and enlarging the top ring land. The 2nd ring land was compressed and the ring would not rotate properly.



10% leakdown on #5. This was the 2nd worst leakdown number of the bunch, but its perhaps not a number that everyone would say "run away"


%6 leakdown on #2...3rd worst of the bunch but the most obviously damaged ring. %6 is not a such bad number and was actually very close to the "good cylinders"


All other cylinders had intact rings and had 2%-4% leakdown numbers which I believe would be "flying colors" territory.

Incidentally, a compression test I performed about 1 year prior to the leakdown showed 135-140 psi on cylinders 1-5 while 6 showed 120psi. So I knew something was up with #6, but the problems with #2 and #5 were not obvious to me with either of these tests.

I should also mention that this car had no visible smoking and ran very strong prior to the teardown. I can't imagine that anyone who test drove it would conclude there was anything wrong. Oil consumption was maybe the only telltale, but wasn't too bad for long trips on the highway (when mostly off boost) and well within "normal" limits.

Good luck with the search
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Old 08-09-2010, 08:35 PM
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Great post, Jacob. Thanks for scaring all of us even more.
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Old 08-09-2010, 09:12 PM
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What Lou said....... He is right.

Cheers
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Old 08-10-2010, 12:14 AM
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Arrow Probably the case...

Quote:
Originally Posted by jwasbury View Post
... I think the shop's statement that the "numbers will be all over the place" is BS. This shop simply doesn't want to do the job.
Prior to buying Das Babe, one of the top instructor, racer and machine shop owner (guess who is paying his racing addiction ) at Mont-Tremblant did mentioned the same thing... A lot of shop don't want to do them or down know how to do them properly.

I witness the compression test (161-169 Hi/Low; 5% Variation) and had the plugs replaced (my first while in there and the car was not even mine yet)
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Old 08-10-2010, 02:19 AM
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So it is true a leakdown/compression test is not the final word on an engines health. In fact it could give false security to the potential owner.

I believe the shop that made that statement was conveying exactly that.

for the record i'm not saying don't have it done, just don't take the results as gospel
Old 08-10-2010, 03:17 PM
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Thanks for all the info guys. I've never used the shop, I called them because the owner would be in that area for business and he offered to take it to them. He too has never used them, the shop was recommend by someone he knew.

I really like the car, overall its very nice, only thing I noticed was some oil under the engine and it had a small pull to the right.

It has 73K miles on it but looking at it you would never know.
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Old 08-10-2010, 04:30 PM
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Quote:
Originally Posted by BMAN View Post
only thing I noticed was some oil under the engine and it had a small pull to the right.
hahaha, you just descibed every 930 on the planet
Old 08-11-2010, 07:57 AM
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Quote:
Originally Posted by drmatera View Post
hahaha, you just descibed every 930 on the planet
If the car pulls to the right be careful.. most do not pull to either side.
Good chance it's not uneven tire wear or internal belt seperation but may have been in an accident and the chassis or suspension is tweaked or bent, even if you see no cosmetic signs of it.
If there is uneven tire tread wear ask why.

Most of them leak or sweat oil in more than one place after a bunch of years and miles but not all of them.
Old 08-11-2010, 12:29 PM
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This will not be my 1st Turbo Porsche. My 930 had a small oil leak and it never bothered me. This one runs aftermarket wheels & tires and I really didn't drive it long enough but I think it was the crown in the road. All the tires looked great, even tread all around.

The car gets dropped off Monday night for the PPI on Tuesday and I've told them to do the compression and leak down, look into the oil leak and alignment.

Here's my old 930

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Old 08-11-2010, 02:43 PM
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BMAN, good luck with the PPI. I remember when you were selling your old 930 above... loved the look of it.
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Old 08-11-2010, 02:50 PM
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Quote:
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BMAN, good luck with the PPI. I remember when you were selling your old 930 above... loved the look of it.
Thanks Garrison... I sure hope it goes well, I really want this car
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Old 08-11-2010, 02:52 PM
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Quote:
Originally Posted by BMAN View Post
Here's my old 930
It seems that so many people sell these cars only to turn right around and buy another one. What is the story this time?
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Old 08-11-2010, 03:09 PM
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No story just stupidity for selling it. But its ok, if I get this one all is good
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Old 08-11-2010, 03:14 PM
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Since this thread will be here a while and others will be buying these cars...

Just a comment. No one addressed BA's comments about leak down tests. He wrote about this more than once. Mainly, it is easy to have something very small screw up a leak down test and make something think an engine has big problems. He experienced more than one where an engine ended up being pulled and taken apart... for no good reason. Small example, you remove the plugs and in the process of turning over a very small bit of carbon came off the plug. It gets somewhere....and.... Do you see where this is going?

You will also notice that the engine shown above showed a problem with a simple and much less expensive compression test.. a year prior to all this.

I will suggest the following to everyone. A lot of people are going overboard. Do you really want to pay the addition shop fees for a leak down? Frankly, I have seen a great compression and leakdown on a 930 that had several broken rings! In fact, a great many of these good running, non oil comsuming, non smoking.. .fine running 930s have broken rings! Why? People like ... boost. These engines take the beatings enthusiasts give them and keep on going. They only go bad when they are really hammered or the owner is absent-minded while on boost. I have seen a bunch of these engines ruined when the owners ignored the overboost had rusted shut! They sure enjoyed the rocket ride while it lasted... but those were awfully expensive rides.

I suggest common sense. A good inspection with a compression test and throughly inspecting for signs of neglect and abuse. If it passes these with flying colors, you are wasting money including leak down. You want leak down? Fine. It's your money.

911s since 1968.
930s since 1985.
JR
Old 08-11-2010, 03:55 PM
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Quote:
When I asked about the compression/ leak down I was told on these 964/Turbo cars it wont be accurate,
I took the OP to mean that the shop was not interested in doing either test--not just the leakdown.
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Old 08-11-2010, 04:03 PM
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