Why did Porsche opt for a timing belt vs. chain?
#16
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Originally Posted by Hacker-Pschorr
Let's do a 180 - why use chains? I have a Mercedes Engine with bent valves wth a cam chain.
#17
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Originally Posted by N421LV
I'm sure this has been discussed before, but I often get asked this question by non-Porsche types (vette owners, in particular
) when they learn about the admittedly short replacement interval of our timing belts. So...all you engine design experts out there....any ideas as to why Porsche went with a timing belt instead of a timing chain on the 928 & 944? What are the tradeoffs?
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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?
#18
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Originally Posted by Bill Swift
Corvettes break timing chains/gears too. A guy in my SCCA region had a C5 Z06 that did just that. The engine was toast.
Originally Posted by 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.
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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.
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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 ...
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 ...
#21
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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?
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#23
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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.
#26
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Originally Posted by Wade T
Belts are superior. But after working on Toyotas many years for I can honestly the 928 TB design is far more complex than need be. ![Stick Out Tongue](https://rennlist.com/forums/images/smilies/tongue.gif)
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Last edited by Bill51sdr; 12-29-2006 at 01:35 PM.
#27
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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.
#28
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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.
Originally Posted by JHowell37
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?
![](https://members.rennlist.com/porken/PorKensioner86Running2.jpg)
#29
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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?
Makes me wonder how much of current DOHC design is engineering and how much is marketing?