Originally Posted by Z

Even some of the engine analyzer programs show valve float occurring in a GT with stock components, and that's with the valve springs being in new condition. Valve springs don't last forever.

It's not too hard to find people here who have replaced their suspension springs because the original ones were worn, weak, soft, or sagging. You'll also see where people have replaced or repaired the foam or springs in their seats for the same reasons. How many times have you heard of someone replacing their valve springs for those reasons, even though the valve springs have led a far harder life? Every time the valve springs go through a cycle they're flexed to pretty much their full range, and it's happening whenever the engine's turning. Up near redline, it's occurring over 50 times per second.

It's probably kind of hard to convince most people that they should spend $1k+ for new springs that will look identical to the ones that they'd then take out and throw away though. Even when they do a valve job, the same old valve springs seem to usually get put right back in, without even checking to see how soft they've become.

Compared to the same valve spring in an S4 or GTS, the one in a GT has to flex further with each cycle, faster at the same engine RPM because it has to go further with the higher lobe lift, go to a higher engine RPM with the higher GT rev limiter, and get flexed like that more often at the same road speed, because of the lower differential gearing. The 32v '85-'86 engines have the same cam lift as the GT, but they don't have as high of a rev limit, have higher differential gearing, most of them have an automatic transmission, and have they smaller, lighter valves. Porsche used stiffer valve springs in the later 944/968 engines, and it's probably pretty safe to say they did it for a reason.

Boost can make the problem worse because it adds additional force to the back of the intake valve, which acts against the pressure of the spring, effectively making it weaker to some degree. If you think about the actions of the valves and piston in a cylinder, and what they're doing as the engine goes through it's cycle, the bigger problem may actually be more likely to occur with the exhaust valves from high exhaust backpressure. That's usually even more the case with a turbo, because of the higher exhaust backpressure in them. Porsche put stiffer valve springs in the 951 too.

Of course even normally aspirated cars can and do have exhaust backpressure. More horsepower made often means more airflow through the engine, and therefore more exhaust to get rid of. A GT makes more power than an S4, but has the same exhaust manifolds, so higher exhaust backpressure at the exhaust valve in a GT would be expected when compared to an S4 making lower power. In his thread, Heinrich mentioned the pitting being on the exhaust cam lobes of his GT engine. Just a coincidence?



The oil used and it's temperature have nothing to do with the type of GT cam pitting that's being discussed. Oil related issues would be worst where friction and load are highest. That would be where the lifter is climbing up the cam lobe, with the spring being compressed, and the valve train components being moved to the valve open position at the fastest rate. The pitting seen is after the peak of the cam lobes though. That's the area of least load and friction, and where you'd expect to see the least wear if there's an oil related problem.

Imaging a car speeding up a hill. The driver is pushed down into the seat as he zips up the hill, the suspension is compressed, and the tiires are pressed hard against the road. The car gets to the crest of the hill, the driver feels weightless, and the car goes airborne. It floats along for a moment, then comes crashing back down against the road, like it does in every episode of the Dukes of Hazard. That's exactly what the lifters are doing as they go over the cam lobes in a GT up near redline. They get slammed down against the cam lobe by the valve spring, when it finally overcomes the inertia of the valve train components from when they got flung open. The material of the cams is at least somewhat brittle, and it's just a matter of time, with enough of that hammering, that the lobe starts to chip, and you have the pitting develop.

They aren't specifically 928 engines, but here are a couple of web pages found with a quick search that are talking about basically the same isssue:
http://www.thumpertalk.com/topic/887...t-watcha-think
http://www.circletrack.com/techartic...t/viewall.html
http://www.lindseyracing.com/LR/Part...VESPRINGS.html

Originally Posted by blown 87

when I worked for Valvoline in the lab, we were told that for every ten degrees past 212 the life of the oil was cut in half.
I start really worrying about the oil at 230, anything over that, it is too hot IMHO.
I do not have a oil temp gauge on my 928, but as soon as the carputer is ready for sale and I get one I will have one.

Originally Posted by jcorenman

Good question, any opinions? These cams are getting retired, but here a question: If you pulled off the cam-covers for paint, and found a few lobes that looked like that, would you (a) put the covers back on real fast and forget the paint, or (b) peel off a few more Porsche-bucks for new cams(and lifters and springs), or (c) shrug, paint the covers, and be happy?

Originally Posted by Bill Ball

Is the nose damage Jim shows "acceptable"? I have NO expertise in this, but I thought pitting on the nose is like potholes - it's a sign of material (metal) fatigue, not friction wear, and it just gets worse. The fragments/flakes are relatively large and don't show up in oil analysis but end up in the filter. Maybe a few get caught by the drain magnet. So, it could be current despite decent lubrication and negative oil analysis.

Originally Posted by GregBBRD

Junk. Suitable for welding only, which is suitable for a "used car lot" only.

That damage is so far down into the surfaces that re-grinding would be futile.

Originally Posted by brutus

Stumbled across this info "Comp Cams Recommends Using High Zinc Content Oils for Break-In
A technical bulletin from Comp Cams and the recommendation from Crower to use high zinc content oils for break-in of flat-tappet cams comes in response to the growing problem of cam failure during initial break-in. Joe Gibbs Driven BR is the only fully formulated oil designed specifically for breaking in flat-tappet cams. The high zinc content formula has been used to break-in every flat-tappet Sprint Cup engine built here at Joe Gibbs Racing for the last 6 years. In fact, the development of BR reduced the number of camshaft failures during break-in at JGR from 1 in 10 to 1 in 40.

Proper preparation of lifters and cam lobes by polishing both the lobe and lifter foot aid the break-in process and futher reduce break-in failures. It is also critically important to ensure proper parallelism of the cam lobe and lifter. For improved break-in success, we recommend that you use our Engine Assembly Grease to coat cam lobes, lifters and pushrod tips during assembly. Then use BR petroleum based break-in oil during break-in. We also recommend that you use a tape of chemical polisher to improve the RA finish of your cams and lifters prior to break-in. Some companies sell pre-polished products, so you don't have to buy the equipment to do it in house. Again, there is no substitute for proper lifter to lobe geometery, so once you put the right pieces in the right places, you now have the right oil and assembly grease to protect you investment in time and hardware."

Speaks to the importance of the zinc , and polishing the parts to a fine finish.

Originally Posted by RKD in OKC

So did my valve spring question stump the experts? Or is it because if they are different part numbers, I would ask what the difference in the rates are?

Originally Posted by EspritS4s

So these cams are scrap and have no core value? What do a new set of GT cams run for these days or is custom the way to go now?