jon928se

What's the betting that Brads 20yr old 29k belt still has the Porsche writing on the outside.

From what I've learnt vicariously, 4.5L 928 engines are definitely non-interference (and are the only 928s to be termed OBs)

4.7 Euro engines - 300hp not interference if you are very lucky and there is no carbon build up and the belt lets go at lo revs, 310 hp (S2) are interference when running but may not be if turned over by hand.

No idea about the 85/86 US engines.

I don't recall ever reading about a belt that failed on it's own on any 928 engine without some external cause like WP siezed or Cam/oil pump/crank pulleys got too worn and tore the belt up or idler bushings wore out causing misalignment and the belt failing when it got to be 1/2 it's normal width.

Jon B.

ALL 32v engines are interference.

DougM

I can tell you from first hand experience that some 4.7 Euro motors are definitely not interference. My timing belt broke when accelerating hard @55mph and I had no damage. On top of that, the previous owner had the heads milled the prior year after a head gasket leak.

Tom's 87 928 S4

My 83s was interference as I had to have a valve job when a bolt fell into the TB.

hacker-pschorr

That could just mean you got lucky, like I did.

My 944S broke a cam chain (same difference to the valves). The S engine is almost identicle to the S4 with higher compression and larger valves.

I did not bend one valve.

Mike Frye

So we have a consensus that the 16v 928 engine may or may not be an interference engine.

ROG100

Mike - Exactly the point I am trying to make - Assume it is interferance and work on it accordingly.

ROG100

Point taken - so that makes my 82 Weissach an "Old Bugger". Complete with S spoilers 8>)

Sorry I will stick with OB for "Old Body" - each to there own.

joejoe

My '86.5 sheared the belt on startup bending 2 valves. Car had just under 60k but belt was 12+ year old and had sat for 4 years or so. Was going to replace motor but my brother told me to do it myself saying "it's already broken, can you really do anymore damage?" Pulled motor and fixed and replaced. Definetly learned a great deal on the way.

dprantl

What I have learned over the many years I have been into 928's:

- *All* stock 32v engines are interference, this is 100% definitive
- 4.5 liter engines are non-interference unless the heads have been shaved too much
- US and '80 - '83 EuroS 4.7 liter engines are probably non-interference, but much closer than 4.5 liter engines
- '84 - '86 EuroS 4.7 liter engines are probably interference engines
- Just because someone broke a timing belt and didn't bend valves doesn't mean their model is non-interference
- Just because you can turn the cams 360 degrees at all crank positions by hand with the engine off doesn't mean the engine is non-interference

Just change the belt and refresh the tensioner bits or switch to PK-tensioner and then you don't have to worry about it.

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

dcrasta

One tooth is a lot on the cam its probably close to a > 5 degree difference, especially if its 1 tooth late.. you would notice.

GlenL

Thanks, Dan. That's a good summary.

When I had head work done for my '80 Euro S the shop said one valve was bent. The car had no rev limiter and some genius probably over-revved the engine and floated a valve. Still, tells me things are close in there.

When the cam stops, for whatever reason, it's not going to hold any valve wide open. This leads to random valve damage with more likelihood for more interference.

Tests with the engine off aren't entirely valid as the lifters aren't pressurized.

For the record:

OB -> "Old Bugger" -> Pre-1980 CIS cars.

dcrasta

Most 4V per cylinder motors are Interference design..

brutus

Unless you did the valve job it is quite possible that you were sold a valve job by a shop which THOUGHT you had bent valves. Very few after pulling the heads would admit that they were mistaken and based their diagnosis on their usual 944 experience. Even if you were shown used bent valves they might not have been from your engine. Real hard to call and say, Guess what we pulled your engine and the heads for no reason. So they rationalize and say, well he was going to need a refresh and new headgaskets soon anyway.
That is part of the problem with is it or is it not? The evidence is often hearsay.

Tom's 87 928 S4

Wife was driving it when it went dead. Towed to shop. Cost me $3200 in 1997. I had them put in earlier cams to give it more horespower while they were at it.