FredR

This is where my thinking differs from most- you accept it as normal becuase "that is what happens"- I look at system and think about what I expect to happen to see if it correlates with experience- if it does not then I challenge that way of thinking..
The teeth are designed to transmit the shear load into the sprockets- the crank sprocket pulls and the cam and oil pump sprockets are driven, The part of the belt between the teeth is obviously there to transmit the load- not pass it onto the flat of the sprocket. I have never studied toothed belt design in any detailed sense but if the tooth is well and truly engaged into the sprocket at full load why should there be any high loading on the flat such that it wears prematurely relative to the rest of the sprocket?

This made me wonder if somewhere along the line yet another mistake was made wherein someone designed the sprocket but did not allow for the coating thickness that was going to be superimposed. A wild idea, highly unlikely one would think but then Porsche could not make all their flex plate clamps hold and we know how that ended up.

Adamant1971

That is very likely. There are also manufacturing tolerances to consider and at some point, the cost comes in to play as well.

My rule of thumb with this type of wear is to coat the gears, it's cheap and if the gears are still relatively flat it can work well. I will take a look at the re-coated gears on the 89 that I rebuilt a couple of years ago, I'm curious if there is any wear on them yet.

Petza914

Fred, you can probably explain to me why the rounded tooth belt of the newer cars is a better design than the square toothed belt of the older cars.

When I redid my motor last year, I sourced another set of the older square toothed gears for the oil pump and cams. Taking apart my 4.5L, those steel gears without any coating looked just fine and as I understand it, the steel gears don't need to be coated, which is why I went with those again and a new square toothed belt, but with the addition of a Porken tensioner this time. I made them pretty but kept the belt running surfaces clean.

Seems like dealing with a soft metal gear that has a hard coating on it doesn't have advantages over a harder metal gear that doesn't require a coating. I also don't understand why a belt with a rounded tooth profile would grip the gears better than a square toothed gear with a square belt profile, so looking for some education, even though I won't be changing anything.

FredR

Pete,

I was intrigued by this but never found a complete answer. The later design is what I understand is referred to as the HTD design. I figured they may have migrated to this knowing the 32 valve motors were on the way that would increase the drive load on the valve train. Alloy cam sprockets presumably are used to reduce the rotating mass but whether that was a good idea remains to be seen.

Somewhere in my files I have a Gates document that I downloaded that was all encompassing. I gave up trying to digest it.

Mrmerlin

FWIW to verify the airpathway you can prove it to yourself
with a fresh belt and fresh air cleaner and properly installed air ducts
after about 200 miles remove the aircleaner and tap it out on a piece of white paper you will find lots of pieces of black dust
this is the fresh rubber from the belt as it wears into the belt run

Petza914

OK, the rotational mass thing I get. Why does the curved tooth belt work better for high torque than the square tooth though? Seems like a curved surface would have an easier time to slip out of a groove than a flat and square one, unless the curve actually helps the belt teeth to go into the mating surface on the pulley when there is belt flutter at high rpm or something like that.

skpyle

Pete, somewhere, long ago, and in a far off learning center, I read something to the effect of the round tooth profile is better entering and leaving the teeth on the sprocket than a square tooth profile.

I don’t remember if better meant less harmonics in the belt resulting from teeth entering and leaving the sprocket or less wear on the belt and sprocket teeth.

Geza

My first thought on the profile would be there is greater effective contact area (bearing area) with the rounded tooth to distribute the load from belt tooth to sprocket tooth. More contact area yields lower contact pressure, thus wear. Noise reduction may also be behind the tooth profile change.

Regarding the top of the tooth wear, perhaps this is due to the belt slapping the sprocket at a belt resonance. As the engine passes through it's RPM range, the belt will resonate violently - I've seen a video showing this posted here by Ken Porken, I believe. The slapping belt would increase contact to the tops of the sprocket teeth.

FredR

Pete,

I simply figured that Gates who presumably came up with the HTD designation did so for a reason-the HTD standing for High Torque Drive as I recall. Not sure what model 928 they first appeared on but it seemed to align with the arrival of the 32 valve models and as per WSM they can be retrofitted on your engine or so I understand. What I can say is that Porsche must have found something about the original stock tensioner system that concerned them becuase they changed it somewhere around 1984 to something that is a bit more sophisticated- both models having US patents taken out on them and presumably for a reason. The change as best I can fathom must have been to improve the damping response of the stock tensioner. The original design damper and drive system do not seem to have any notable track record of failure that I have noted but I would not mind a small bet that these two updates may be interconnected somehow given the timeline. Porsche generally do not change things for the fun of it and most probably do so to eliminate problems that have manifested themselves during operation whatever they may be. Then of course there is the infamous belt tension light and the reasoning behind that. I figured that Porsche realised there might be apossibility of the lock nut on the tension bolt coming loose and the nut backing off so they put the alarm on it to advise the driver to check it out. For years on this list when folks talked aobut this you would think the alarm meant "Tough luck pal you have just bent 32 valves"- in reality if there was something really chronic happening the job lot would be history before you even knew it had happened. There is a reason why the alarm says "timing belt service" just as ther eis a reason for its existence.

Whether or not all these things are interconected I have no real idea but something clearly concerned Porsche to put an alarm on the thing- as I am led to believe that is more or less unique on a car engine.

Bottom line- toothed belt design is quite an engineering science in its own right and there ar eprobably no moer than a good handful of people in the world who truly understand the ins and outs of such. I know there is a very tight specification for the tooth profile of the belt and the clearances needed around the teeth of the sprocket. The Belt manufacturer designs the system in totality and they also specify the tension that has to be applied to the belt. Porsche specify what tool is used to check tnesion and also the number of units that is required. How anyone knows whether that is "optimal" [or not] again I have no clue- it is simply a "given" in the community as it were. I have often wondered how much diference there is between the correct tension and the tension at which the alarm triggers in terms of number of flats on the adjusting bolt- maybe this discourse can trigger a resposne in that regard?

Presumably you are happy with the older tooth design system- for sure I have never heard of issues with it. One of these days I will get round to asking Ken how he determined the correct belt tension for his setup. My assumpiton is that both systems must run at the same tension but the PKT goes about it a different way - you just pull the pin and forget about it.

The other point of engineering interest to me is whether the PKT is easier on belt life and sprockets than the stock system. The stock system seems to be just about all in at 120k miles there or thereabouts [sprockets, 2 belts on and a tensioner refurb due. I see you are a dedicated follower of the PKT. I would think by now there should be sufficient data points to be able to form some opinions on that one but heaven forbid it should degenerate into another PKT pissing contest!

Petza914

Interesting discussion guys, thanks.

Yes, I've only had the Porken tensioner installed for a year, but I'm a big fan of self adjusting tensioners that don't need to be checked and adjusted periodically, where belt tension can be maintained optimally all the time. I didn't have any issues with my steel cam gears either and figured it was just as easy to stay with those than deal with coatings wearing out and changing all the gears and pulleys.

Right or wrong, that's the combo I'm running and it seems to work well.

Adk46

Square teeth will produce a higher stress concentration at the corners of the belt's "nubs" than a more rounded shape. The outer nub-free cross section of the belt is highly reinforced, but the nubs are just rubber, including the base of the nubs where the stress concentration operates. As the load on a belt increases, I can imagine that failure may initiate from cracks at this location, growing all the way across and stripping the nub off. Even if it did not strip off, a cracked feature is more compliant - less stiff - so it would carry less load, transferring it to the neighboring nubs.

The stress concentration would potentially be bad for the coating at the corners, too.

Finally, if something anomolous happened, it is easier to imagine a square nub riding up over a square cog of the gear - i.e., jumping a cog.

These are just my own musings, unencumbered by analysis or data. Within Porsche, however, it would only take a chief engineer to muse similarly. "Why do we like square-cogged belts?" he might ask at a meeting, causing subordinates to squirm in their chairs. I knew someone at GE who liked to ask subordinates to write a memo titled "Why we like X", when X was something not ideal but by default going to persist. It was just a rhetorical suggestion, but very effective.
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I used the term "crack" - for rubber. Rubber is formally a brittle material, lacking the ability to plastically deform. It is an unusually forgiving brittle material due to its low stiffness.