Mark Anderson

BTW we are out of "good" used cam gears. It is rare to take a gear off a used engine and have it good enough to sell.

Hilton

Wonder who's making them now, and whether they're doing it in China?

On aftermarket versions - I do think its noteworthy that Porsche haven't elected to make them out of steel (just watch.. now I've said that, the new Porsche ones will be steel). They changed the oil pump gear a long time ago.

Also - would rotating mass be a contributing factor to the known issue of 16V cam breakages? I know the obvious cause is a backed-out or bent cam bolt - but could mass be a root cause?

I will say that for a component as critical as this, the lowest-cost solution may not be the most desireable. e.g. more machining operations on a steel gear to lighten it to factory weight would, for me, make the additional cost worth it.

Oh well - I've got one new pair of Porsche gears to stick in my spares bin, and another pair for my impending TBWP on my red '87.. so hopefully you guys will sort us out with a technically viable and redoubtable solution.

dprantl

The 968 has a steel cam gear. My wife's former 968 with over 150k miles didn't seem to have any camshaft problems, and yes the gear was much heavier than the 928's. Since the 968 head and camshafts are so similar to a 928's, I find it hard to believe the 928 would not work well with steel cam gears. I would pay more for steel gears knowing I would never have to replace it (well, within reason).

Dan
'91 928GT S/C 475hp/460lb.ft

slate blue

By Hilton

If I was having a look at this and had the money to do it. I would out of interest spin up a cam with and without the cam gear assembly and look at the balance. I saw a study done by an F1 team and they along with other F1 engine makers use a damper on their cams. Just like we do on the cranks, the reason is that the lift cam change by 0.5 mm without it. Or basically the cam assembly is out of control due to vibration.

Nearly all race engines use balanced cams, same reason, the F1 engines cams are balanced also of course but lets suppose there is a balance issue that leads to these cams undoing the bolt? I did speak to a Porsche engineer who worked on the Carrera GT project and he told me straight up consider balanced cams for my project.

Also what might work is, a alloy centre with a steel toothed area like the one Hacker and I put links up to. That way it should last a very long time. I am not saying it will be worlds better though.

Greg

docmirror

I'm pretty sure there is a coating on the original gears. Most likely it is a thin film coating of oxide. Yep, plain ole AlO2 Aluminum Oxide. Once a part like this is milled, it's usually put through a quenching process, and then it'll go through a bake cycle where there is a high concentration of water vapor. The process is called low pressure chemical vapor deposition, or LPCVD. You can get decent hardness with an even coat, and it doesn't cost hardly anything to put on. Aluminum with some tempering goes about 4-6 Mhos, and the AlO2 goes about 8.

We've all seen Al gears where the Oxide has worn off and the gear teeth are quite shiny. They'll go another belt change cycle generally, but after that, they teeth are down too far to trust. I would say that tempered Al gears from billet should go 50-70k miles, or basically two belt changes. This is a ballpark, so don't hold me to it. If we want to coat them, that should take it another 20-40k miles, depending on how good the coating process is. Once it starts to come off, it doesn't take long for all of it to come off.

Steel gears would last >100k miles. The changes to the gear design could result in a lighter gear being made, and it would save some weight over the standard old steel gears, but now you're re-engineering something that is pretty critical. I wouldn't want to do it. Given the diameter of the gears, and the weight of the rest of the engine, the gear is insignificant, even though it has a rotational momentum, it's not that important. I have no idea why Porsche decided to change from steel to Al, except for machining costs.

I'd like to see a run of >500 gears. At 2 gears per car, that's 250 cars which should be done every other belt change. Also, some of the cars out there have never had the gears changed, and they'll be coming up for belt and gears in the coming years.

I think we can do away with the recess on the front of the gear that is used as a quick check of the timing. The back tick is needed. The front side humps for the rotor position isn't needed. I've already discussed some stress relief in places, and some strengthening in spots.

123

In a thread that talks about getting the original gears recoated, there were pictures of melted gears and it was stated that they were some sort of magnesium, or magnesium containing alloy, not the aluminum that's been mentioned in this thread. If the original ones really are magnesium, would new aluminum ones be better just because of being aluminum? Anodizing or some other coating on aluminum might be significantly more durable than whatever the coating is on the original gears.

There are enough other cars out there using timing belts that seeing what's being used and what works in those applications shouldn't be too hard.

ROG100

123,
Correct and correct.
Alloy is a mix of materials and in this case magnesium was used in the original alloy.
Roger

fraggle

I'd be worried about weird harmonics if the gears are a different weight than stock.

Otherwise I want blinged-out ones. Like Red.

blown 87

I want ones that are identical or very near to stock so that i can put on and not worry about them.

GregBBRD

OK to all.

I don't want this to turn into a "Carl" thing, so there is not much else to be said.

You go down your path and I'll go down mine.

We will compare notes, when it is all over.

Just write it down on your calendar....I offered.

Ed Scherer

Are there currently people desperate to replace their gears without being able to get any (until the newly-designed/sourced ones being discussed in this thread become available)? If the re-coating process works, do we need to start offering up some used — but likely re-coatable — ones to satisfy that demand?

I've been holding on to a couple used ones that (I think???) are in good shape, but if anyone needs them... I guess I'd let them go (for reasonable compensation ).

Update: These are now gone... to Jim Morton, who was the first person to PM me after this post (amazingly quickly, I might add. ) Based on the additional PMs I got, there's clearly demand for more used gears in good shape, so if you've got some and are willing to give them up, now's the time to help your fellow 928ers.

SeanR

This thread has gone from entertaining and informative, to almost stupid, then back to productive.

Greg, give us some details on what you are doing, the last e-mail didn't show a wanting of collaboration, more of a slamming of market saturation, on a few different fronts.

On my end, I'm going to use what is most cost effective, that includes: Is the **** going to work and work for a long time. Is it going to cost more than what Porsche offered. Is it going to look like a great product that everyone fawns over, only to find out that there have been multiple failures that no one hears about.

I want options, and cheapest is not always best. These are not Bilstien shocks, of which will get produced even when sold out.

atb

I disagree. Black is always faster. Seriously.
Put on a pair of your red cam cogs, rev the motor, then switch them out for a pair of blacks.
The motor will rev faster with the black ones.
I'll put money on it.

NASCAR has its restrictor plates and increased gurney flap angles (okay, they run rear wings now, work with me). SCCA and POC could have the "red cam gear" restrictions.

Kibort to SCCA official - "I'm filing a grievance, that guy is running a supercharged stroker with nitrous assist."

SCCA official - "Check it out, red cam gears. He's good."

Kibort to self - "But he has more net horsepower under the curve"

SCCA official - "Red cam gears kill the torque, it doesn't matter how much horsepower he makes. He's good.

Okay, don't really know where I'm going with this. I'm glad that we have vendors getting after this. I will be in for a pair of new cogs from whomever brings them to market.

ROG100

Greg,
OK I am happy to stop the process and work with you on the project even though I have put a lot of money and time into the work.
A few questions :-
1) ROM selling price?
2) ROM delivery date to me or the customer base?
Either here on the thread or privatley by PM at your choice.
You already know the answers to my goals.
Best,
Roger

depami

Has anyone checked with Spacely Sprockets or Coswell Coggs?