It is certain that your budget dramatically exceeds the one-Kevin's budget who had to ask for (and still does) loaner engines in order to examine them. An oem recently sent him a core motor of recent vintage to work with. He does not have $10,000 to run out and buy them. Someday maybe.

He knows just how you feel.

 

What is failure mode in these cases? Low rpm idle, high rpm track use or ...? Will they fail suddenly or has some partly failed cases been found? Do 944 S2 also fail or is it just S model? Both use same valve.

 

I thought the plastic guides wore out first, then once it's metal on metal the chain either snaps (like mine did) or it rips the mount of and destroys everything.

In my 944S the teeth on the cams were damaged and by some blind luck didn't bend any valves or wreck the head.

That sill leaves the question why the guiders are guaranteed to wear out on the 944S/S2 while the 928's don't suffer the same issue.

I was pulling away from a stop sign in traffic. Luckily coasted into a parking lot.

This was shortly after Zimbrick Porsche told me there are no serviceable items in the tensioner to be replaced.

 

The mechanism is mounted on the head. I presume it is receiving the same aerated oil in a 3 bar circuit. A tensioner is designed to have an ideal incompressible fluid in it, not a compressible fluid with air bubbles in it. This exacerbates any problem with an aerated valve train.

The 928 did not come stock with the 3 bar pressure limiter. The racing suggestions by Herr Kristen were presumably for a 928 that had less than or equal aeration to a stock 944.

Coming off idle after a hard run would be the most dangerous time. Deep aeration of oil takes minutes to dissipate, not seconds sitting at a stop sign. Maybe you had just blown someone's doors off.

 

Pretty sure they do, or I have replaced a lot of wear strips for nothing.
I did a set this week, fairly low miles, worn enough that the rollers were contacting the guide.
Frankly they scare me, once they are old, brittle, brown and worn, they need to be replaced.

 

Sorry, I meant to say it doesn't seam to be as often or as frequent with the 928.

 

I've never seen a 928 engine "******" the tensioner off of the head. I've never seen a 928 engine that has worn through the plastic pads of the tensioner. I've never seen even a chain failure.

I've seen lots of "early" 32 valve tensioners that have the "hydraulic" piston completely non fuctional because the top "hat" moved out of place (design flaw in the first 1000 engines)...and they've been like that for a bunch of miles. Why don't those engines "******" the tensioners off?

Sorry, not going to restrict the oil flow, anytime soon.

 

Back in 1998 the Porsche dealer said they've never heard of them failing in the 944S/S2. Told me it was a waste of money to do the job on my 944S.

My local guy, Jim Page @ Page Auto has seen a few 944's lose a chain. I don't recall any of those damaging the heads, mostly bent valves. Mine was the first with camshaft damage that he's seen.

All of them (including mine) what was left of the top pad was rock hard and brittle. Lower pad wasn't much better.

How about a theory Porsche used all the leftover defective units in the 944's?

 

you might remember the holbert chain and sprocket system being a little substandard It shed 7 full teeth on the sprockets, and when i pulled it all apart, the one side, had a tensioner with the piston all jamed up. the crimp fit of the piston was ajar and it was all jammed up. basicallyy, no tension movement. but, it didnt explain the cam gear failure, as the other side was fine and it ws missing almost just as many teeth. I still have those cams. wonder if they are worth fixing the sprockets for. there was a 944 or 968 problem that was fixed by a sprocket shop up in canada that said they could fix the problem. would cost $1000 so, that really wasnt an option vs used 85 cams to be modified by elgin.

my chains were loose, and probalby were the cause, or maybe the cams just were brittle. anway, got new 968 racing chains and they have been fine for the last 7 years of racing! the tensioners looked ok. plastic wasnt grooved too much, so I left them alone.
MK

 

Mark, I was looking through the pics I have of the Holbert engine being disassembled and did not see those. Could you repost them or direct me to the thread where they are located? Thanks.

Aside: Hope this is helpful to people. When the oil in the 928 or 944 circuit has free air bubbles in it, it is subject to stratification by density when it undergoes acceleration. When this mixture goes around turns or bends in the galleries the denser fraction is pushed to the outside and the lighter density fraction, i.e. free air bubbles, is pushed to the inside or shorter radius.

The oil entering the main passageway has just undergone turns and so the air/oil mixture is already segregated. Very shortly thereafter this segregated mixture reaches the side passage leading to the 2/6 or 2 rod bearing circuit. In making this turn the heavier density fraction, oil, will create a boundary layer and will channel the already segregated air bubbles to the shorter inside radius. In this way, the number 2/6 or 2 rod bearing receives a higher relative/local percentage of entrained air than the other passages or in the system as a whole. This is fatal for reasons which are well explained in SAE papers. After passing by the 2/6 or 2 side passage to subsequent passages two advantages appear: 1) the average mix has a lower entrained air percentage by virtue of a disproportionate amount of air being already directed to the 2/6 or 2 passage; 2) the segregated air and oil has a chance to redistribute more randomly -- this makes subsequent corner flow radial accelerations less damaging.

 

here are a few. these the ones? picture of missing teeth, and then the last pictures is the new cams with new racing chains.

 

What variant is DSCN1788 ? There appear to be (at least) two different style tensioner mechanisms.

 

the pictures show both sides of the engine with the stock 87 prototype engine. dont know what variant the tensioners are, but they came on the holbert heads.

the last picture is without the tensioner installed and its the driver side with the new racing chain installed.

 

ARe you just lucky or do the metal pieces just fall down into area and just sit there?

You should just name your racing endeavors by your guardian angels name and be done with it....


andy

 

Your teeth definitely had a fair amount of wear on them. However it almost looks most like impact damage.
What could of caused it, who knows. Possibly excessive chain flutter but who really knows.