Rennlister's

I have an 84 US 928S that has an aged timing belt. Only 12k miles on it, but the belt is 13 years old, so I do need to have it replaced.

Here's my question - I understand the US '84 engine is a non-interference engine. However, the PO had the heads shaved pretty close to the wear limit and installed high performance cams. I'm not sure whose cams were installed, so I don't know the profile. I just know the work was done in 1985 or so and that the car sounds like a boat engine - low rumble!

Anyway, how worried should I be if the belt breaks? Do you think there would be enough clearance between the valves and piston? If the cams were "high lift" would that put them in touch with the piston?

Matt

 

Real worried.

Take a weekend and change the belt. $50.

 

If you change the belt, remember to watch the tension very closely for the first few hundred miles. The current crop of new belts stretch at first and the factory tensioner won't take it up. I am just finishing up rebuilding my motor that bent a valve after I changed the belt 'just because' and did not watch closely enough for stretching. Of course your best bet is to upgrade to a porkensioner when you do the belt, but now you are talking something like $400 (as I recall) rather than $40. Also I think there may be two brands of belts and only one stretches, but I don't know the detail.

 

Sounds like interference to me.

 

They say you should change the belt every 5 years, or every 40000 miles, which ever comes first.
your belt is 13 years old, you need to change it.

 

All of them will stretch, some more than others, what is a extra 50 bucks or so for a Porsche belt?

 

I think the 84s are interference. remember, we did a test and there was contact at .3" with the piston, so at near .4" lift, there would be some contact if the belt broke.

 

In theory the 84 US is NOT interference......BUT with the mods you speak of in this 928 I would say it is & I also think you should change the belt and waterpump as a precaution....since $300 in parts in far cheaper than a rebuild

 

Roll of the dice since you don't know any details on valve to piston clearance.

 

I am pretty sure Porsche does not make belts. I think the vendors are conti and gates or something like that. If you buy from the dealer you are getting the same thing and probably paying quite a bit more.

 

I believe Gates is the manufacturer of the OE Porsche belts.

 

I would suspect your engine is now of the interference variety. My US 84 with heads/cams is interference.

 

I believe Roger @ 928'sRus has a new non stretch belt he developed in conjunction with the manufacturer.

 

I've never seen significant stretch. Skipped teeth, not properly indexed cam timing gears, yes ... but stretch ... not. I've used all the belts available (not yet tried Roger's new one)... I've owned MANY broken belt 928's ....

 

I have seen it 1x and perhaps 2x. On my cab, the belt certainly stretched. It was a new belt with less than 500 miles. It was properly tensioned at install, and all of the rollers and pullies and plastic bushings were good. It loosened up enough to skip a bunch of teeth. On my '82 I once had a similar issue, but one of the plastic bushing on the tensioner arm pivot was suspect, so it might not have been the belt.