Definitive Answer on Interference Engines?
Addict
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From: Sydney AUS
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.
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.
Three Wheelin'
Joined: Jul 2001
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From: Louisville KY
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.
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From: Up Nort
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.
Craic Head
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From: Jersey Shore, USA
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.
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.
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From: Double Oak, TX
Mike - Exactly the point I am trying to make - Assume it is interferance and work on it accordingly.
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
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928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
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Joined: Jan 2003
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From: Double Oak, TX
From what I've learnt vicariously, 4.5L 928 engines are definitely non-interference (and are the only 928s to be termed OBs)
Sorry I will stick with OB for "Old Body" - each to there own.
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Michael S. Palmer
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From: Porterville, Ca.
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.
Race Car
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From: Atlanta, GA
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
- *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
Three Wheelin'
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From: Washington "Dc"
I questioned this for years. Then recently, I did the timing belt, wp, and pksnr... I didn't use a flywheel lock, and I purposely spun the cams and crank separately and slowly. No contact. I've been told.. (or maybe ive seen it here somewhere) that if you surface too much off your heads that you can make these 4.7 engines interference... That I don't know. But I'm 100% confident that my '84 4.7 is NOT interference. Also of note... When I pulled the old belt, it was off one tooth. When I got it reassembled in correct timing alignment, it sure seems to make quite a bit more power. I didn't think one tooth effected the 16v engines that much?
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.
Nordschleife Master
Joined: Jul 2003
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From: Minneapolis
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.
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.
Burning Brakes
Joined: Jan 2007
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That is part of the problem with is it or is it not? The evidence is often hearsay.