Anthony, just did a search on this forum and had 200+ hits for "valve guide wear" so there is a bunch of reading available, not necessarily all relevent.

 

My 79 SC with max moritz 3.2 P/C's and Carrera crank used about 1 quart every 800 to 1000 miles. Oil consumption has always been an accepted part of owning an air cooled Porsche. Even the 928 and Cayenne consume oil at higher intervals than most cars. I do recall the 3.2 liter engines having valve guide issues and IIRC Porsche made changes to adjust for this. IIRC it had something to do with the seals which were updated. Although we are talking a long time ago and I don't recall the facts without doing research so I have no answers now. I looked up my notes last night on my C2 as I document every quart I add and in fact I did add a quart after 2000 miles of track driving. My turbo gets it's oil changed once a year or every 1800-2000 miles with no toping off.

I spoke to my mechanic last night who does a lot of performance engine builds and he said he has seen some engines come in with worn valves but these engines are usually beaten hard. 100% track miles or poorly maintained. His comment was that it all depends on how the car was treated and it is something that afflicts all air cooled engines. Treated poorly, valve guide wear; treated properly and he says they last. Also if temperatures are allowed to soar on these engines it will create premature wear.

I did a search for the same thing on the 964 and 964 turbo forum. Came up with 61 hits on the 964 forum and 5 on the turbo. The general concern that I get from doing a little reading is excessively high running temps results in valve guide wear. One of the contributing factors in the 964 appears to be the poor quality of corrosion prevention of the heat baffles. They have a tendency to deteriorate and do not flow the air properly over the cylinder heads causing premature VGW. For the most part a well maintained engine seems to have a life of around 100k miles on average without issue. I see nothing of any great circumstance that would make me think the 964 n/a or turbo have issues beyond those of all air cooled engines.

Also to add that the turbos run hotter and therefore rebuilds may be needed early because of the extra heat but this goes for the 3.3 as well as the 3.6 l single and TT motors. Although there are turbos out there with well over 100k miles and still running.

Edit: Interestingly as I read some of the comments on the 993 forum it would appear the 993 3.6 l seams to have a higher rate of VGW than the 964. the 993 uses a different valve guide than the 964.

 

There are different schools of thought on this; some people believe oil consumption is related to the initial break-in of the engine, according to how well the rings seated (or didn't). Some of the early 993 engines had problems in that regard where the rings never seated. Others feel short trips on a cold engine causes the guides to wear. Some debate the role of the undertray. Others believe if you rev the heck out of it all the time it will wear less than if shift like your grandmother would. Who knows the truth?

I think the bottom line is how well your particular car runs and whether or not it's really drinking oil. My 97 runs much tighter and "better" now that I have had the top end done, as well as using no oil at all anymore, but it was really running quite well before, and I wouldn't have done it if not for the SAI system issues. But all in all, it was $3500 well spent.

My favorite quote from my favorite mechanic -- years back when I had a 911SC with a broken head stud that ran just fine irregardless -- is "Just drive it and quit worrying about it."

 

IMO initially break in of the engine is critical. I will say that with the LWF my C2 runs far better than many 964 and 993's I have driven. No 2 pre 98 911's I have driven are alike, unlike the newer cars which seem to be a clone of each other. I also don't believe in the under tray as we can't hit sustained high speeds in this country and it would really only impact top end. I remove mine and prefer to let the engine breath.

I am not worried by any means. I know these cars well after nearly 30 years and fully understand the pros and cons. I just like to educate myself about them and even after 4.5 years on this forum I keep learning new things all the time.

 

I agree, break-in is especially important. That topic itself is a whole can of worms. My first new motorcycle back in '78 I broke in gently and "by the book", it always used oil. Years later I learned you need to be mildly aggressive without abusing the motor, to get the rings seated. However, I thought Porsche broke in the motors before installation or are they just give a short bench test?

Anyway, from the 3.2 and on, each increase in HP of the air-cooled motors has brought higher heat output and increased valve-guide issues. The best solution for the 993s looks to be good aftermarket guides. With 93k currently on my car I'm sure I've got some wear but consumption is less than a quart in 2k so unless I develop an SAI problem I don't expect to have to touch the top-end for quite a while.