what is the reasoning behind higher RPM being safer?

 

The pitting is more of a charateristic of the GT cam profile and stock valve springs.

 

Sorry, I get that those are not your cams now. You could try a super fine polishing with scotch brite or 1200 1600 grit sand paper.

 

Mark, I will definitely plan on running higher RPM's!!
Don't know much about Amzoil?, but is Redline that much better than the Mobil 1 I'm running now?

John, thanks for the confirmation. It does seem that the culprit is the GT cam profile and springs. I will be putting in new valve springs and both 944S2 and 968 springs have been mentioned. Anyone know if they are the same or different?

Daniel, no problem, I should have pictures of my cams by tomorrow. Would polishing the cam flanks cause any problems with the factory hardening or heat treatment? (or any more problems than the pitting that's there now )

Thanks again guys for the feedback!!

Rick Redpath
'90 GT, Guards Red / Black & Grey, 140k

 

I do not think I would want to remove any material from the cams either by polishing or sanding the lobes have been "polishing" the lifters and visa versa every time the engine turns over.

 

It was explained to me that the faster rpms tend to remove pressure off the cam lobes. I know this is no real testimate, but the holbert cams, beat as they have been over 4 years and then lots of low mileage abuse in its life as the rescue vehicle, had absolutely no pitting. sure, i was missing a bunch of cam sprocket teeth, but thats another story. but if you look at the rpms that my engine has had over 4 years, during racing hours, its never below 4500rpm, and average probably in the 5000rpm range. where most engines are between 1000rpm and 3000rpm for the majority of their life, in fact, on the freeway, near 2500rpm for hours at a time (ie 80mph)

so, call my miles on the cam are probably 2X or 3X of a street car. (ie 30k miles are like 60-90k miles on a street car.

the only other cams ive seen were off a stock automatic 928 s4 (88) it was pretty badly pitted. (sure high mileage and low average RPM too!)

im thinking oil quality and rpms are a dominant factor. thats my take on it.

Mk

 

Well after much debate with myself and great advice from the list, I finally decided to get the cams for my GT "refurbished". When my motor was taken apart late last year, the cams were slightly (?) pitted on the trailing edge of the ramp beyond the lobe tip (hope I said that right). Worst pitting was .01 inch (10/1000th). I decided since I was going through the whole motor anyway, I just could'nt put the cams back in the way they were.

After some searching in the archives, I saw a few had used Elgin cams, so I contacted them about mine. Since I have GT cams already, I didn't really want any different "grind" on them and just asked for them to be refurbished and nitrided for strength and longevity. Tom at Elgin was very helpful and said it would take 5-6 weeks since they have to send out for nitriding now (something about the County and ammonia gas...).

I received the cams back a couple of weeks ago (it actually only took them 3 weeks total from day I shipped until I got them back!). They look as good or better than new according to expert Porsche engine builder Dan Warner at Exotic Motorwerks. Cost was $500.00 to refurbish the cams + $316.00 for nitriding + shipping. Not too bad IMO, especially compared to buying a new set of GT cams... if you could find some.

Can't wait 'til the motor is back together... should be soon... pics to follow.

Rick Redpath
90 928GT, Guards Red, Black/Grey, motor going back together

 

Thanks for the update!

By the way, what happened to the engine to warrant a rebuild? How are you going about the rebuild?

 

Ney, I had pitting on my GT cams. Not like the pics, but more like some very small dimples behind the top of the lobes. I've seen the same condition on almost every GT camset I have looked at..perhaps 6..I was told not to worry. I re-installed the cams into my race motor and ran another two seasons without any further degradation. I sold those GT cams to another stroker builder who is running them now. Do not bother trying to change out the cams with the motor still in the car...awful job, that! I'd just keep em in. And, in reply to the off topic about which oil..My GT motor had Mobil one in it when I first got the car. It blew up, and then we noticed the cam lobes were pitted. I re-installed those cams after a bit of messing around with some others, in the mean time switching to Amsoil. The last time I switched out the cams for the Devek ones I now use, the pitting was no worse than when the original motor blew. I personally have had 4 motor failures using mobil one, so I no longer trust that oil...not a scientific sample or any real data..just my own decision..seems kinda like it is too watery for real lubrication..and 4 lost motors(3 were motorcycles, one GT) is a bit much for total coincidence..
Don Hanson

 

Swaybar,
Long story regarding warranting a rebuild, but really got down to motor getting overheated bigtime... twice. Once a couple years ago after I tried to convince the dealer that the coolant leak was from the waterpump (still under warranty), but they said no, it was the radiator (which was seeping slightly at the end cap). Within a week, waterpump finally gave out completely while on the freeway with no place to exit for a few miles... motor got VERY HOT... almost as much as I did at the dealer , then about a year ago, had a small hole in upper radiator hose while traveling and drove too far with the car HOT (it was late, dark and no lights or tools to check immediately).

Anyway, too hot, too long = head gasket in my case. Couldn't do just one head gasket, was replacing oil pan and oil cooler at the same time, so didn't stop until engine was pulled and completely torn down. Working with Dan and Yolanda at Exotic Motorwerks locally here in Phoenix for the rebuild. Will have pretty much new everything that could wear when we're done... including some upgrades, coated intake/valve covers, etc.

Rick Redpath
90 928 GT, Guards Red, Black/Grey

 

Don, Great to hear from you... only wished you would've written a couple months ago... I wouldn't have had my cams cleaned up!!

Actually with everything else I was doing to the motor, it almost seemed a shame to leave the pitting on the cams. Elgin did a real nice job on them, plus now they're nitrided and I think it was a very reasonable price. You are probably right though, Jim Bailey said essentially the same thing as did a few others, the cams probably would have been fine the way they were (pretty much exactly how you described the pitting on yours).

When are you coming back to Phoenix (PIR) to run again? Really enjoyed meeting you and seeing your beautiful car run a couple years ago!

Rick Redpath
90 928 GT, Guards Red, Black/Grey

 

Rick

you're not allowed to put your car togeather until Dan gets my car squared away first sheesh

 

Rick,

The 944S2 and 968 valve spring have the same stock number through World Pac, so they must be close. Mark Anderson sells the S2 spring for about $12 each, much cheaper than the World Pac price at $22 each.

You may need to run the thinner lower valve spring retainers that came on the '85-'86 32v S motors. On the big lift cams 10+ mm, the stock retainer @ 4.5mm is too thick for the stiffer springs. Check with your mechanic.

 

Khaled, ah c'mon, quit whining, at least you can drive yours...

Adam, yeah, interesting about the part nos. for 944S2 and 968 valve springs. Dan at Exotic Motorwerks did some digging and found the same thing too. Looks like they are the same, or at least superceded to the same now. After having my existing valve springs checked, they tested within tolerance for new, so I am running with those for now. Hoping nitriding the cams will help with future wear.

Rick Redpath
90 928 GT, Guards Red, Black/Grey