Mileage on re-ringed Alusil's
 

I'm rebuilding my 79SC and considering re-ringing my Alusil cylinders:

How many miles do you guys have on your re-ringed Alusil's?

I've read a lot about re-ringing them but not too much on their longevity.

Thanks

Stu

when you reuse old pistons and cylinders and re-ring, you basically are taking the cheap way out. it's your call if you want to go that way on a "rebuild", and you can't expect to get the same mileage out of them, as opposed to going new. it's anybody's guess how long they will last. they probably already have 6 digit mileage on them. many have done it with good results though. some smoke a bit more than you would like and some don't.

John's got it right - it's a gamble. Sometimes it works fine and other times it doesn't. I've done it before with good results. I know of people who have done it with less than good results. You typically hear about the success stories much more than you hear about the failures (people are less eager to tell about the failures)...

-Wayne

I had my engine re-ringed (along with a lot of other rebuild work) and with approximately 1000 miles on the odo (including 1 track day), no smoke and runs great. (I've not done a leak down however).

TonyG

John, Wayne (hmmm...): Could you say that an engine with low miles and broken/damaged rings would be a better candidate for this gamble than a higher milage?
It could be that those who have done a successfull re-ringing had low milage cars.

low miles would be better. broken rings can do damage to cylinder walls and ring lands, but sometimes you get lucky.

My 79 SC was reringed for the PO by a Porsche authority in Gaithersburg, MD with the result that after only 9,000 miles it smokes like a Indian pow-wow making peace. Leak down was 98% for four and 96% for two. Burns about a quart in 500-600 miles.

How many miles before the rering?

Although I reringed my alusils with success, they only had 115k on them and the cylinder walls were in great condition. The cylinder that was sent me for testing had 220k and there was significant pitting around the combustion end. Clearly that cylinder would have been inappropriate for reuse.

Ahh ha! Now, that's the type of stories that I've heard are common with re-ringing the Alusil cylinders. The engine works fine for a few thousand miles, and then goes downhill quickly after that.

-Wayne

John, You probably wouldn't know if anyone measured the pistons and cylinders to se if they were in spec. before they chose to re-ring. That would be criticle knowledge in the decision. Maybe they never had a chance?
Are you disatisfied with 98/96?
Again, you pay for what you get. I would think Roger would have checked things out before he did thia with a customers car. Maybe his customer, like many, just told him to do it the cheapest way possible?
Anyway, just some thoughts.

Re-ringed my Alusils about 50k miles ago. Compression 120psi roughly all cylinders, taken ast week. No oil smoke even little on initial cold start. I think the reason for the lower compression numbers was the fact that they measured compresion ratios were in the average 7.9 or 8.0 to 1. Not the 8.5 to 1 specified by Mahle or Kolbenscmidt!
Anyway, I was working at Mahle at the time and had the test lab measure all pistons and cylinders to factory specs using the proper equipment. Al within tolerance range allowable. Some surface scratches on the piston skirts but no scuffing damage.
All I did was use steel wool and wet and dry sandpaper to the cylinder walls to break the glaze/clean up the surface then re-ringed with Goetze rings.
Seems great to date 5 years and 55000 miles later. Only disappointment is the lower than thought compression ratio.
Anyone thinnk 120psi compression is in the ballpark or is that a little low?

A lot of things can effect a compression test. You can go 1 step further and do a leakdown which might also quantify a leak and indicate where from. Could be a lot of things other than the rings!

Answer to question one: 130,000 miles.

Answer to question(s) two: I have no idea. The facts are the facts.

Since the engine gets 500-600 miles per qt and the compresson is up, and yes, 98-96% is good. Valve guides maybe? There seems to be some seepage around the head gaskets. Whatever I paid for it, I got it and now must live with it since I didn't make any of the decisions. Of course, maybe its not broken in yet. Only 9000 miles with a HPDC and two day DE on it!!!:confused: :p

Yes, read about compression tests here:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

They are basically only accurate to tell if one cylinder is under performing the rest...

-Wayne

True, true. How some ever.... If special care is taken when doing a leak down test (differencial compression test) in addition to which cylinder may be suspect, air leaking into the crankcase denotes rings, into the intake suggests a bad intake valve, and so it goes for exhaust. After you do a number of these your ears get calibrated. Take your time. Experience pays!;)

Ran a leakdown test and no problems. Just the low compression pressure bothers me when compared to what I have read most people get with 911SC engines. Usually the range is 135-140psi, from what I have seen. My gut tells me that my engine is on the low side, but you need to be more concerned with the spread of the values between cylinders more than the actual overall psi. As long as they are within 10% of each other, you should be ok. Engineering calculation tells me that at 8 to 1 compression, you should expect to see 112psi per cylinder......mine read 120psi on average so I guess I am ok.
If people are seeing 140psi that would equate to 9.5 to 1 or so. Kind of makes sense, since the Euro SC were supposed to be at 9.8 to 1, but tell me if I am wrong.
I have also heard that Mahle advertises the compression ratio higher than reality. My engine actually measured at barely 8 to 1 when I rebuilt it, which is 0.5 off the rated 8.5 to 1 for US 1979 models.
Anyone see the same thing from their experience?

Are you at a higher elevation? Maybe the less dense air is equating to less actual compression? I know here in LC, our atmospheric pressure is 12.7 psi as opposed to 14.7 at sea level.

A 944 turbo with factory 8.5:1 compression with a stock cam willl read about 145psi-150psi at sea level.

My previous 944 turbo with stock pistons and stock combustion chambers, but with a big cam (for a turbo cam) measured about 125psi-130psi at sea level.

I would think that at 8.5:1 on a 911 with stock cams should read pretty close to at least 140+psi at sea level (but I could be wrong)

TonyG

There are so many things affecting compression tests, that you cannot compare them. For one example, an engine running super-hot cams will show really low compression because there's a lot of lobe overlap.

Stop trying to figure out if low compression is a problem - it's really only a useful number in relation to the other cylinders...

-Wayne