I was speaking with a local PCA member the other day and he told me that he had heard of a Kevlar reinforced timming belt for our cars. Unfortunatly he could not come up with the name of the manufacturer and said he would try to get me the name once he contacted his freind. He did add it was more than 3 times the cost of a conventional belt.

I was curious if anyone had heard of such an animal and from what I understand the Kevlar reinforced belts last much longer than standard belts. What benifits would this offer us? I would think if they lasted more years and miles it would be worth the added expense to do the job once every 8 or 10 years vs every 5.

Any input or info would be appreciated as I am due for a TB replacement soon.

Thanks,

 

The most common failure seems to be teeth stripping off of the belt, not belt breakage. I doubt that better fabric would help that much...

 

The only drawback I can think of is the Kevlar belt would be a lot less stretchable and this could be a problem. The 928 aluminium engine expands quiet a bit when warm and the extra stress on the cam “gears” can be detrimental. This is just a theory, so I could be way off.

 

It's my understanding that the factory belt is not the cause of failures. What IS are worn cam sprockets, overtensioning, worn tensioner parts, bad waterpump bearings (from overtension most likely) and finally the belt teeth usually shear off and cause bad timing and valve crash. None of those things would be helped by a stronger belt construction.
The achilles heel just needs some attention periodically to be happy.

 

Interesting,

I was told by many that the belt should be changed every 5 years regardless of mileage. IIRC the general comment was Ozone attacking the belt and it breaking. I guess I took breaking in the literal sense of the word. If the teeth break off I guess that would be covered under breaking.

From what i had read on a recent thread a periodic inspection might be acceptable instead of a replacement. Although I have just over 5 years and 12k miles on my belt, it may be smart to just change it. The car is on its second belt with the first one replaced at 45k miles.

 

I assume the pulleys also play an important role and as they get more used they can cause issues even if the belt fabric is stronger.

Marton

 

Cobalt
5 years is a safe replacement interval....with inspections and tension checks you could go longer...but why?

 

and YMMV .... Anthony, it does break yes, but if I showed you a belt I took off my S2 which had literally lost half its width through its full length as it spun on a seized water pump, still faithfully turning the cams .....

You'd know that the BELT is the toughest part of the system. The thing to fix is the wear on other components. I think I'll post a common tbelt failure mode thread.

 

Thanks all for putting me straight. This is only my second 928, the last one was over 15 years ago, don't have these issues on 911's. The water pump was replaced less than 5k miles ago and I did an inspection on the belt and teeth and they looked like new. Finding the time to replace it seems to be the toughest thing lately. I am unloading several projects that I planned on completing just because I don't have large chunks of time to commit anymore. Like everyone says it is worth the piece of mind knowing it is new and problem free. My wife will kill me if the car isn't ready for the autocross season which is coming up quickly. I would pay someone to do the work but I know all the Porsche mechanics in the area and I know they are competent 911 guys but hear little about their 928 expertise.

 

Heinrich showed me that belt and it is quite revealing. I'm a 928 newb, so it was very interesting to see how the teeth had worn off the belt, and how the belt in general was slowly failing. Yet the car was still running. Amazing.

Heinrich, you really need to get a pic of that belt and post it. -Ed

 

I would be interested to know if anyone has had a belt break on its own?
Everyone I have known to break was from worn cam gears, water pump freezing etc.
If things are kept adjusted correctly will the belt break?

I believe Mr. Bailey reported having a cam gear that was worn through. Most of the gears I see replaced are form sharp/worn teeth from over tension which in turn will shave the teeth off the belt.

So, has anyone had a belt break or the teeth stripped from healthy, normal wear?

 

generally, its the lack of tension that kills the belt. as the pump wears, that can introduce slack, slight belt streatching, etc. usually, a rev to 6k with a loose belt does the engine in. i think that is the mode of failure. i was lucky and skipped 2 teeth at the race track, fortunately no damage. this was on a belt that was probably still in spec tension, but on the loose side.

the system has a weakness and there are trade offs. i guess a chain driven system would be better or more tension rollers to keep belt contact, even if the belt was loose. ???? who knows.

but, considering how long the racers have been going without failure, is a good indication on how reliable the system really is..... (knock knock)

Mk

 

Mark, I've seen only overtension failures. Water pumps don't care abput undertension but overtension kills them fast.

 

So if I am reading this correctly I should be fine with a new water pump (5k miles old) and a periodic inspection of the gears belt teeth and tension. Since all appears to be in superb condition a TB replacement is a guaranteed piece of mind but possibly an unessecary expenditure of time and $$.

 

Crect