JHowell37

I'm willing to bet your Mercedes with bent valves is probably one of the late 80's-early 90's 560 models that used the 5.6 liter V8. If that's the case, the problem was most likely due to an owner who never addressed the recall on that engine due to an inferior single row timing chain.

JHowell37

To answer the original question here, when these engines were designed, several decades ago, almost everyone used timing belts for their overhead cam engines. That was just how it was done. There are applications where belts are advantageous, but contrary to what some folks believe, there are really no advantages to a belt on the street. Bubba NASCAR can get away with a timing belt because in a competitive environment, every HP counts, the belt gets replaced after only a few hundred miles of service, and finally, the belt is maybe 20 inches long. Most car makers have steered away from timing belts in recent years. They're a maintenance issue most car owners don't want to deal with, and honestly, they shouldn't have to.

But let's be honest. If the 928 had timing chains, do you know how many of them would be driving around with timing chain covers welded to the front of the engine because that would be the only possible way to keep them from leaking?

hacker-pschorr

I totally forgot about my firends C4GS - he started to find metal chunks in his oil one day. Turned out to be his timing gear.
How much was this bet?
Duel chain setup on this car - what actually happened is the tensioner failed causing the cam(s) to jump a few teeth crushing the valve's. The chain is still in one piece.

Evil Patrick

I like gear drives. I don't have one installed, but it's available.

Tom in Austin

I think it's true that belts were the "in" thing when Porsche was developing their water-cooled engines in the 1970s. The fiber-reinforced belt material was a relatively new solution that manufacturers were attracted to. What does Cayenne use today, is it a belt or a chain? That would give an idea of which solution has proven superior (by whatever criteria) since then.

BTW, if a Vette owner questions the use of a belt in a 928, maybe he can explain just exactly what a pushrod is good for ...

heinrich

The timing belt on a 928 engine is the toughest part. It's the water pump and tensioner system that are the weak links. I think we should have ALL our timing gears made of modern plastics and we'll stop having failures. Porken?

N421LV

Thanks for the responses, guys.

heinrich

Btw every single tensioner part on a 928 is a SERVICE PART, in other words it was intended, and it today holds true, that at each service interval, the whole lot should be tossed out as wear items. This way, with COMPETENT technicians, there will be no timing belt failures except the stupid weak water pump ones which are far rarer.

hinchcliffe

sorry to jump OT but did Porken ever finalize his Belt Tensioner?

heinrich

he did but he's testing it now...

Bill51sdr

Totally agree! I have worked on older Lexus LS400 V8's and the timing belt configuration would be very familiar to anyone on this board... with the exception that Toyota used a common tensioner and a wider belt. They have no tensioner problems that I know of. Porken is doing pretty much what Toyota did regarding the tensioner and I applaud him

123quattro

I think the main differences between chains and belts are noise and accuracy. Chains are obviously louder. I think that's the main reason why OEMs stay away from them. Also, a belt gives a little more accurate cam timing. That's not so much an issue with a street engine.

PorKen

I already have enough leaks - I can't imagine having a huge magnesium chain cover!

I would like it if the 928 belt were a bit wider. The belt from the engine from which the PorKensioner is derived is ~4mm wider that the 928.

danglerb

I was arguing with my Chevy mechanic pushrods vs timing chains or belts. One of his points is that pushrods allow a much smaller and lighter design, the LSx motors are about 200 lbs less than a 928 motor, and a fair amount smaller.

Makes me wonder how much of current DOHC design is engineering and how much is marketing?

danglerb

Could the whole front end be redone practically, wider timing belt, single serpentine accessory loop?