incase anyone has morbid curiosity here is a pic of a blemish on one of the exhaust cam lobes from my GTS engine which is being rebuilt. the pitting is quite obvious in this pic and this is the worst example.

i know this has been a problem with GT cams but i thought the GTS had a milder grind and therefore were immune. i am sending the cams out to be reground with a smaller base circle and hopefully this will push the engine output to aprox 400 bhp with a 2.5" exhaust.

 

Curious - was there any visible or measurable errosion on the corresponding lifter surfaces? If the lifter was untouched, what would the prognosis be for leaving such a cam in service? ( hardly anyone routinely lifts a cam cover to take a look).

 

Had the same issue w/ my S4 cams. I was told by a very reputable source that it was not that uncommon and not to worry about it.

 

On mine there is smaller (largest is 1/5 or smaller compared to Nick's picture) pieces missing on maybe three lobes. Many of the lifters had color distortion but surface was still smooth. Nick, keep forum informed how regrind works. Maybe there's hope on rescueing these as well although plan is to use GT cams.

 

Yep.....from what I understand also the pitting was common for all 32V motors.....mainly S4's and above though.

 

What caused this? It looks like an internal flaw?

 

The cam "ramps" are not idealy suited for the spring rates, and the problem increases as the springs wear! CHECK your springs before you rebuild your engine!

With cams, heads and exhaust, you will easily make 350-360 rwhp or 410-420 BHP.

Good luck with the project.

Marc
DEVEK

 

marc, thanks for the input - i will have the springs checked. from your comment it would seem the problem is remedied through stiffer springs or a more moderate camshaft. unfortunately my budget wont stretch to major head work, not with 32 valves needing consideration, unless there are some simple fixes.

 

Isn't there the possibility to have the cam lobes resurfaced? My understanding was that you can have material added to the lobes and then they are reground. Shouldn't the added material surface be harder?...as with welds?

Mark, weren't the GTS cams toned down due to the GTS being in decllne? I had heard that the GTS heads didn't need any work to accept larger cams. Just drop 'em in and some nice hp gains are achieved.

 

Here is a close up of my former GTS cams. The picture is indicative all of the lobes. The car had approximately 60-65K miles when they were taken out.

GTS Cams

 

keith, louis otts website has a dyno for a GTS running GT cams so at least that much is possible without other modification. from the comments ive read here welded cams prove unreliable in 928 engines. 928 developments list some kind of 'plasma coating' process as an alternative.

garth, its hard to tell but is there any pitting there? with mine the pitting was only present on the exhaust cams, this engine has 36000 km.

what is the bare minimum that can be done to the heads to increase flow, a good valve seat cut?

 

Thanks Nick...can you give me the website address for Louie Ott?

 

louis web site is: performance928.com

ive spoken to a couple of machine shops concerning valve/head work and there are two potential options i may explore

option 1) 5 angle valve seat cut with valves faced to match - pricey

option 2) old fashioned cutting of the valve seat to reduce the shoulder width

any opinions?

 

The Dyno on Louie's web site is my car.

There was no pitting at all on the cams. There was some discoloration, but that appeared normal. If you follow the "GTS Link" it will take you to about 16 other pictures of the cams that can be enlarged to show detail. Tim Dey did the GT cams into his GTS as well. We both experiences some idle issues. He put a GT brain in his GTS and it runs well. I haven't done that. The car will die when decelerating if the clutch is depressed early. I let the car slow down, in gear, to idle speed and have no problems. I've gotten so use to driving it this way that it isn't a problem to me.

 

from what I've heard, the more angles and closer you get to a continous seat profile the better the flow at low lift, but it's more prone to carbon keeping the valve open also... my gt cams looked pretty much like your gts stuff, but strangely my 85 cams were pristine... elgin did the grinding on my gt stuff and they looked pretty nice afterwards, reduced the base about .020" which is apparently pretty much ok w/the hydraulic lifters and stock springs, street stuff... for welding, you might talk to them as there are some tricks to insuring the welding is done right(materials,etc)

w/the change in profile, they claim power will be in the 3k-5800rpm range and it's good for about 30hp on a stroker, so it's pretty decent for street stuff... think mark A is running nearly 2x that, but he's got a real track monster too...

http://www.myinfoware.com/928/85shark has some pics of the heads w/968 valves...not much room for much else, but if you're doing a complete valve job it's not a lot more to replace the intake valves and do new seats... apparently, can still use stock hardware for street stuff too.... kinda cool stuff... have fun!