Tony'z911

Ok, I’m wondering about these two subjects both individually and together. First I’m wondering if a car w/ high track mileage (say 8-10K or more) is likely to need a rebuild much sooner than a car that hasn’t seen the track too many times? Is a car w/ 125K miles, 10K+ of which are track miles any more or less reliable than a car with say 125K, 140K, or even 170K miles that hasn’t seen the track too many times if at all?

Next I was wondering about the Rod Bolts in cars that have seen or will see high track mileage. Specifically regarding the 84-89 Carreras w/ the 9mm Rod Bolts- how prevalent is it that the cars that see a lot of track miles, or that are revved hard during daily driving on the street will fail? What would be the danger if either of these owners has installed a chip that extends the rpm range and allows the car to be driven at even higher rpm’s? Are these engines subject to eminent rod bolt failure and what will most likely happen if/when the rod bolts stretch and/or fail? Would you recommend replacing the rod bolts on a 125K/10K- tracked engine immediately as preventative step to what may happen if the rod bolts stretch and/or fail?
Thanks,
-Tony

Greg Hammond

A car with 125K miles that has been driven hard on the track for the last 10K+ miles may be in need of more maintenance than just rod bolts. In my opinion, you're flirting with an expensive repair bill, and the possibility of the engine being badly worn on the inside and needing rings, rod bearings, main bearings, as well as those pesky rod bolts, head studs, and new gaskets all the way around.

Greg

I normally just lurk on the 911 board, but track miles and wear are universal.

Tony'z911

Greg, thanks for the response. That is what I'm thinking too and I really don't want to be spending $6-8K+ on a rebuild next year! Well, unless the price left me w/ some cash to be prepared for that.

BTW- the 10K+ high reving track miles were put on over the life of the car, not just in the last year or two.

Any more thoughts or opinions?
-T

Bill Gregory

A leak down and compression test during the PPI will point out any developing weaknesses in the engine. While taking an average vehicle on the track may cause excessive wear, our Porsche's are far from average. If the owner has taken good care of the engine, has adequate oil cooling, changes oil religiously, perhaps uses a synthetic, I wouldn't let track time scare me away from a Porsche.

Steve Weiner-Rennsport Systems

I'd sure agree with Bill on this; if a compression and leakdown test shows good health, then you should be OK.

One important addendum though, if the car has an aftermarket chip that has changed the factory rev limiter from 6750 RPM, I would be VERY careful. The rod bolt issues are cumulative, due to the use of stretch-type fasteners on the connecting rods and these do not like being stressed that hard.

When you back out of the throttle from 6000 RPM+, the rod caps and bolts are now loaded in tension, not compression, and those poor 9mm bolts are truly stressed and start to deform. Installing a premium bolt like the superb ARP 2000's which have tensile strength of 220,000 psi, is very good insurance.

We have a whole stack of cases, cranks, cylinders, heads, oil pumps and other sundry items that were totally destroyed when a rod bolt finally broke. This event will drill a nice hole in a $ 10K bill with pay phone money left over.

Ed Bighi

Rod bolts are definetely not something you screw with. They are normally not a problem even if you hit the rev limiter at the track while using a stock chip. It is a cumulative problem like Steve Weiner said. Chipped cars with higher rpm limits can stress things. As far as mileage my sc has 202,000 miles on it having only been touched to repair broken studs. Phosphorous bronze guides were put in at that time. Right now I probably have those kind of track miles, but it runs like a clock. I adjust the valves earlier than usual and change the Mobil 1 religiously. But after that is foot to the floor no matter if it is on the track in Arizona at 105 ambient temperature or going accross the desert at 130+mph only slowing down for cops. These engines in stock form can take more abuse than probably anything else. I find that the only weak points are the stock ATE valve guides and the rod bolts if chipped. Again, I don't know about the chip, but my buddies with 3.2's probably have more track miles than you do with no problems.

Clark Griswald

Actually rod bolts are in allways in tension, never compression, regardless of rpm or on/off throttle.

During the combustion downstroke, the compression upstroke, and the exhaust upstroke, the tension is releived. During the intake downstroke, the rod bolts are in tension.

This is the case at all rpm and throttle loads (provided the engine is running).

Steve Weiner-Rennsport Systems

Well Sir, I would respectfully disagree,..

You might talk to the folks at ARP about this, too.

Clark Griswald

Well, which part do you disagree with?

Bolts, when used as fasteners, are not capable of operating in compression. Bolts used as fasteners are designed to act in tension.

The rod experiences both compression and tension at different points in the rotation of a 4 cycle engine.

When the rod experiences compressive loads, it (largely) relievs the tension on the bolt. But the bolt is always in tension, never compression.

It is true that the greatest tension loads on the Rod bolts will occur on the intake downstroke at high rpm when the throttle is closed. Perhaps that is the point you intended to make.

My point is that the condition of rod bolts being in tension is not unique to that situation, they are always in tension, never in compression. Rod caps similarly are never in compression.

Would ARP disagree with that?