951 Compression Values

951Boost · 10-20-2005, 08:17 PM

Hi everyone, new to the forum

& I know the info must be here but can't seem to find it. I'm interested in obtaining the factory compression values for a '89 turbo. Did a search in this forum under "compression values" and "compression test" and did not find the info.

Also, is there any special protocol to follow for the test that is different from a N/A car.

Thanks in advance.

Zero10 · 10-21-2005, 09:00 AM

Test is the same as a N/A car.
Pull the plugs, block open the throttle, pull the DME relay, and crank the engine.

Good numbers for a turbo depend on your altitude.

The compression ratio of a 951 is 8.0:1, however there is some heating of the air as it is compressed, and normally you see numbers around 10:1. So, if you're at sea level, the air pressure should be ~14.7psi, so you should see compression numbers of 150psi, at 5000 feet, it's around 12.1psi, so you should see 120psi on your compression test. Keep in mind, these are ideal values, so if you see within 10% of them, you're in great shape.

If you see much less than those values, time for a leak-down test, but you probably know that

951Boost · 04:50 PM

Zero10, thanks for your detailed response. So if I'm understanding, at my elevation of 10,200 feet (for future test, I will currently be testing the car at sea level) it should be 9.7psi X 10:1 = 97psi???

Also, regarding leak-down tests... for how long does one take the measurement in each cylinder? Seems like there should be a factory spec on this as if one measures for a short time= low leakdown %, but if one measures for a longer period of time= higher leak-down %. Is my thinking correct & might you have those specs??

Thanks for the help

emwporsche · 10-21-2005, 08:36 PM

no, 150 compression is good for the turbo cars
you can have less (around 130) as long as their even

ae1969 · 10-21-2005, 09:21 PM

10,200 feet...........

Where are you ?

951Boost · 10-21-2005, 09:35 PM

Leadville, Colorado

aka Cloud City

Is my math right & how about that leakdown question??

Thanks

ae1969 · 10-22-2005, 07:08 AM

Well I am curious if thats how low it would actually register at that altitude. Zero and I are at ~3000 feet..... so I would be really curious to see what kind of readings you get.....

I am usually more concerned with a compression value that is >10% different from the rest....

IF they are all even then at least its natural wear and not some major scoring or burnt valve....

Let us know what you get.....

951Boost · 10-22-2005, 09:35 AM

Yup, I'll keep you all posted. I'll be picking up the car next week and testing at sea level then I'll test again at 10.2k when I get home. It will be interesting to see the difference.

If anybody has info on how long to conduct the leakdown test for (factory spec) please let me know, I would really appreciate it.

Zero10 · 10-24-2005, 08:13 AM

I do a leakdown test for 10 or 15 seconds, long enough for the reading to stabilize.

Wow, 10,200 feet, that's pretty darn high up. Honestly, seeing 100psi on a compression gauge scares me, but for your altitude, it would be about right!

If you do a hot, dry compression test, you should see effecticely 12:1 compression, or around 115psi. Please do let us know what you find, as I only recently adopted this idea of using math to determine expected compression values.

951Boost · 10-24-2005, 09:32 AM

Thanks Zero10, I appreciate your response. I will definitively follow up and post my findings in a couple of weeks.

Porsche-O-Phile · Porsche-O-Phile's Garage

Factory spec. per the manual is 10 bar (145 psi).

951Boost · 10-25-2005, 04:35 PM

Thanks Porsche-O-Phile, I just ordered the Factory manuals today. Looking forward to some good reading

nize · nize's Garage

where did you order them from? you know the factory manuals are available for free online, right ?

951Boost · 03:56 AM

Yeah nize and thanks for that link, that was real nice of you. I actually got them all downloaded last night. Even with H.S. cable it took a while. I think their servers must have been overloading. I got the Turbo supplement from Sunset Porsche (800-346-0182) for $65. They had the 6 vol. set for $175 but were sold out & not getting any more. They were very helpfull and nice I highly recomend doing business with them. I ended getting the
6 vol. set from My Porsche for $223 (831-899-8888). I know that's a lot of money but I do my own wrenching so I know I'll get good use out of them. I guess I could have just printed out the .pdf docs & had them bound but I really like the print quality of the Factory stuff. Besides it's my favorite bed time reading material

PS-Been trying to get one of those little pocket sized "Technical Specification" books for a '89 951. They are out of print, any leads??? Also any ideas about a parts manual?

Thanks (and thanks again for the link to the manuals)

Tapage · Tapage's Garage

dude at 10200 feets .. how you feel the power .. ( with little O )