Once again, I received new information from a supposedly informed source about just which 928 engines are "interference engines" and which are not. Thus far, I have been told, definitively, by people who should know, that:

1. All 928 engines are interference engines,
2. Only 32 valve engines are interference engines,
3. Only the 4.5L engine is NOT an interference engine,

and the latest, ABOLUTELY CORRECT INFORMATION, according to this source, is that:

4. Only the 1983 US engine is NON-interference.

Does any else on the forum wish to contribute their ABSOLUTELY CORRECT INFORMATION, along with supportive documentation, to this list? It would be nice to know, accurately, which engine is going to self-destruct when the timing belt goes, and which will simply need to have the belt replaceds rather than several pistons and valves.

I am absolutly not an automotive engineer, but logic would seem to dictate that the lower compression engines, i.e., the 4.5L at 8.5 to 1 would be the most likely not to be an interference engine.

 

AFIK all 32V are interference and (notwithstanding the carbon buildup debate) all pre-85 US-spec engines are non-interference. Euro engines are probably interference. Not so sure on ROW engines (i.e Japan/Canada spec).

 

so far as I know all the 32v engines are interference.

The 4.5L 16v is not interference and the early lower compression 4.7Ls are also non; the later 4.7L with higer compression (e.g. S2) are....

Marton

 

Anyone know if a US 4.5L w/ Euro cams is a valve bender or not?

 

Doesn't make sense as '84 MY US use exact same engine.

Truth is probably that all engines which have 45 degrees market into damber are and all those which do not have aren't. This when there is absolutely no buildup on piston tops.

Depending on buildups height any engine can be. But only '80-83 ROW S are really close. All other early engines will escape damage with light to moderate buildup.

 

Wasn't there some speculation that the ROW low compression green 32V might be non?

 

What is officially non-interference could become an interference engine with enough carbon build up.


Edit - Sorry Erkka, you already said that!

 

84 S2s do not have 45* mark and I don't think 85-86 S2 have the 45* mark.

I know of one 80-83 Euro S that had multiple belt failures with no damage.

 

AFAIK, all 32V are interferance,
all 16V are not,
I say all 16V as I have a 1985 euro S2 and when I did the timing belt service noone could tell me if it was interference or not, so with the belt off and the cams at TDC I manually very slowely turned the crank over, there was NO interferance, so I moved the cams 180 deg tried again no issues, cams back to TDC and still no issues. Once I completed the timing belt service which included all seals, as well as the dual distributor belt the engine fired up on first revolution and ran flawlessly.

and with the comment on the engines which have the 45 dg mark, I think that says only 32V engines again.

 

My US 81 is not - I know this to be 100% fact. When installing the timing belt for the frist time I rotated each cam multiple times triyng to get them lined up - the valve's never touched the pistons.

Most 944 owners think the 951 is an interference engine - this is also incorrect (unless there is enough carbon buildup to break a valve).

To throw a monkey wrench into this, soon there will be a few 5.5 liter 32V 928's on the market that will be non-interference engines.

 

I think you are the first S2 I've heard of that hasn't crunched the valves, although you did it not at high speed. It would be odd if Porsche could get an older higher compression engine to be non-interference, but not all the lower compression engines after it.

 

I thought '84-86 310 hp S was known valve killer including 45 degrees mark and but quess not. Since it doesn't have 45 it should be safe to rotate cams and crank individually. It might still destroy valves when belt goes at 6k rpm.

Compression ratio is only one part of equation. Shape of piston, head and valve angle are also important. 32V have totally different design than 16V.

 

True, but the way Porsche increased CR on the S2 pistons was to shrink the valve cuts to almost nothing. 32V engines have larger cuts outs for smaller valves.

 

euro 83s are deep cut pistons, i mean REALLY deep cut. while the 84 euro is very shallow. 8cc vs 2cc.
all the US 2 valve stuff is non interference.
mk

 

Tell ya'll what - I'll not change the timing belt on my '83 euro until it snaps. In the mean time, I'll make sure to give it plenty of exercise to keep everything nice and clean of carbon deposits. Sound like a plan?