GlenL

The Kempf tool shows less at temp because the belt is warm so the rubber is more flexible. The 9201 is steady across temperature.

mark kibort

they both were pretty calibrated together for both hot and cold. kempf tool was at the very beginning of the window, and reading was 5.0 hot. cold, 5.6 vs top of the window of the kempf tool

but what you say, makes sense, as the stretch is more determined by the core fibers, where softer rubber at temp will have an influence in the twisting forces resistance

mark kibort

I agree, and never challenged this point.

You are getting these revelations, because im testing you and pushing you to think!

Now, show me your actively managed system on a race car over some period of time, and we can talk about reduciing risk of our system now. as it stands today, a well tension 928 belt in the stock system does a pretty good job of keeping tension. doing tesion tests with an old conti vs gates comparison, i saw absolutely no differences and the same amout of tension reading was found at and beyond the spec limits, both with kempf and 9201 tools.

PorKen

Do you think that Audi race cars retrograde to a fixed tensioner?

Before you start, don't give me that tired old line of 'it's not the same on the 928'. It's already been proven to work as well on the 928 (since 2006).

The stock system may appear to keep a somewhat consistent static tension, but you are missing the point. Tension is a symptom. Think past tension to belt control. The belt is being loaded and unloaded all the time, creating more or less loose/unpulled belt on the tensioner side which is not well controlled by the stock system.

If you read that Gates PDF again, you'll see that a tensioner/damper is recommended for high dynamic forces. The perfect thing for a V8 engine which is running very hot at high rpm, either WOT or coasting.

mark kibort

Ken, it all makes sense, but i have a real problem deviating from stuff on the 928. makes me nervoius. but, i do agree and understand the changing forces.
audi designed for it in the engine design, we are retrofitting and there is always a chance that we missed something, but after all the stuff you have posted, im starting to believe. maybe it will take me losing some valves before i convert.
at this point, i might actually do it. seriosly. does make a lot of sense, as long as its ENOUGH tension for the quick revs and high revs. thats the issue. the audi isnt the same belt length or the exact configuration, so i worry.
anyone really racing with the system on a 928? not street use, that doesnt count!

dprantl

Have you ever seen a properly tensioned belt with the stock tensioner run on a cold engine with varying RPM as it is warming up sans belt covers? Try it some time and watch how much flapping the belt does, it is very disconcerting.

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

Tom. M

Allright....time for someone to figure out how to measure the dynamic tension... Just a roller and a strain gauge.. Right?.... MK?.... Porken?

PorKen

Sigh. Tom, Tom, Tom. Tension is a symptom. As long as the belt stays on the gears, it's golden. Thousands of Lizard miles, and all of my thousands of WOT runs without a hitch shows that it does.



What would be interesting, speaking of Colin, or one of you other kids with COP/CNP ignition, would be to measure with a strobe light or sensor how much the cam timing changes (retards) at different, especially high, rpm, because of belt stretch, so that the static cam timing can be adjusted to suit.

dprantl

3 years and 26k miles on my porkensioner so far, and I can assure you that I don't drive it easy.

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

mark kibort

I would just be concerned that the tension was enough giving the teeth shape and configuration compared to what it was designed for and how it differes during quick throttle blips.
if someone did a 30 min , all out sprint race, that would be convincing to me.

Gary Knox

This is a bit off the actual subject of this thread, but I chatted yesterday with someone who has probably done a hundred timing belt/water pump services in the past 10 years. Asked him how many of the cars actually had OIL in the timing belt tensioner when it was removed for service. He indicated it was probably less than 5% that had any significant oil, with maybe 1 or 2 being nearly full. Vast majority were bone dry!

IF that is the case, the vast majority of 928's are certainly not having the belt tensioned the way the factory intended for it to be!!

Gary Knox

PorKen

A fixed tensioner cannot expand to take up extra belt 'created' on the unpulled side when the engine is cold, accelerating or at high rpm, so the unmanaged belt flaps. () When the throttle is released, overall tension drops until the belt returns to it's 'normal' shape, according to the static tension it was set too.


Although average overall belt tension will be lower than stock with a self adjusting Tensioner/Damper; at high rpms, when accelerating, and when the engine is cold, overall belt tension will be higher than stock, keeping the belt better pressed onto the gear teeth (and making for less variations in cam timing).

The T/D piston extends immediately if there is less pressure on it (more belt), but is slow to retract. When accelerating, or at high rpm, when the belt is stretched to it's maximum working tension between all of the gears, free belt on the T/D side is taken up as it passes over the tensioner pulley. If the throttle is suddenly released, the T/D piston is still extended, so belt tension stays higher until the engine revs at a steady state long enough for the T/D piston to retract. (30 sec hot to >1 min cold. It retracts a small distance momentarily to dampen oscillations.)

Some folks have noted a gear whine when accelerating after installing a PKT. This is the sound of the T/D doing it's job, keeping the belt tight(er than stock)!


I think there have been many 'fun runs' all around the country which would qualify. I have been on one or two redline shifting, WOT/brake/WOT/brake, runs which seemed like races, without the passing.

mark kibort

i race, and I cant even simulare the engine operation. pretty tough to do. so, i understand how it works better now, but still want to see it on a 30 min race with throttle blips to make sure things actually work as they should. there are so many dynamics during a race that i cant even begin to make the list. alll you need to do is get a set up on the race track with someone that knows how to drive and see if it survives. it probably would, but i wouldnt be a ginny pig. what if, just what if, during race conditions you NEED The extra tension?? remember my speed GT race in 'o4, i skipped a few teeth in qualifying only making one decent lap and hobbled home. it was still tentioned properly. so, why did it jump. i think i posted the video too. |BUT, it jumped teeth possibly having more tension than the audi style tensioner. now, the more wrap around position might cure this, but i would like to see it work in a pure racing environment.

PorKen

You're almost there. Still have to get past the 'T' word, though.

Belt tension between the 1-4 and crank gears, set when the engine is cold, really doesn't mean a hill of beans once the engine is running. You don't need extra tension when the engine is running at a steady pace. Once you accelerate, with a PKT, the extra belt will be managed as it is 'created' by the strain of the crank turning the cam gears, whereas, with the stock system, the extra belt will just stack up somewhere between the 1-4 and the crank gears.


You skipped, how many teeth exactly? Both cams, same amount?

At high rpm the timing belt does strange things, mostly because it's weight is much higher at speed. For example, take the center cover off and rev it up over 4000 rpm, and you'll start to see the belt come out of the water pump pulley horizontally, farther and farther. Harmonics at different rpm ranges will do strange things to it too.

If you skipped an even number of cam teeth, then it's likely the belt was flapping at just the right frequency to let loose belt reach the crank gear when you let off the throttle. An odd number would mean it was able to creep over the cam gears, also likely when you decelerated.

jon928se

Having spent a long time pondering the advisability or not (from a pure theoretical engineering point of view) of using a PKT the above from PK has clarified it in my mind to the extent that I would now fit one.

Total tension required in the belt to maintain enough wrap on the pulleys is probably quite low, so long as that low tension can be maintained when the belt gets very stretched between the crank, 5-8cam, 1-4 cam gears which potentially results in more belt than there is distance between the crank pulley, stock tensioner and 1-4 cam gear; hence using the the stock tensioner, tension has to be set quite high so that some tension still exists on the loose side when accelerating or blipping the throttle.

From what PK says the PKT basically is a spring and a "shock absorber" that has lots of bump damping but no rebound damping - so the spring can extend quickly (uninhibited by the shock) but only compresses slowly. Thus at constant RPM it can have less tension on the belt than the stock tensioner would have and when the undriven side would go nearly slack with the stock tensioner it maintains the same lower tension.

Makes lots of sense all of a sudden and would also negate my concerns over using one of the Gates Racing belts.