danglerb

Twisting the belt, or defecting it, increases the path length for the belt and compliance has to come from something, either the belt stretches or some other part of the system moves to compensate. The Kempf and the 9201 both increase the path length and measure the force that increase requires to estimate the tension of the normal path length. Pretty obviously when the force required to stretch the belt during the measurement varies from stock, the estimate of the non stretched tension is going to be off.

OTOH as long as the tension isn't so high it damages something, or low enough to skip a tooth, does it really matter much? A little variation in valve timing maybe?

The Fixer

Good point Mike

dprantl

Your 928 is a US 16-valve S. If your timing belt breaks for any reason, it is extremely unlikely that there will be any internal engine damage. You are worrying too much.

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

ROG100

With over 150 Gates 928 RB sold and probably well over 100 in service and not to mention the thousands of RB's used on other cars such as 944/951/968, Subaru WRX etc. etc. I have not heard of one issue at all.
This is not a new belt designed for the 928, Gates just applied the belt technology to the belt for our car.
I have learned to trust the Gates engineers as they know what they are talking about, it is after all their business. It is certainly not in their interest to make false statements.

If you use a PKensioner, belt tension would never be an issue 8>).

The Fixer

Very good then, will trust Gates

ROG100

Hi Matt,
Send your racing belt back and I will give you a full refund.
Roger

ROG100

Wrong thread - sorry

dprantl

Sorry, I missed the fact that you are installing Euro cams. In that case, you are correct that this change alone can make your engine interference.

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

jon928se

This is true in theory. However the torsional stiffness of the belt due to it's construction will be very small in comparison to the apparent torsional stiffness observed when deflecting the tensioned fibres by twisting the belt.

I haven't had my hands on a Racing Belt but I can't imagine that it is significantly torsionally stiffer than a normal belt. If it were it would also be significantly stiffer in bending which would then cause lots of issues where it wraps over the smaller tensioner pulleys.

The Fixer

may order a racing belt also to test the twisting force difference between the two.

FredR

An interesting thread- nothing too contentious. My 2 cents worth is that a stock system installed correctly will work nicely without excessive wear on the cam gears but then I am not the hardest user and for sure not the furthest driven 928 owner.

I am seriously considering the racing belt for my next belt change simply because I like insurance of additional heat resistance insurance and I am thinking about the Porkensioner simply because I have a lot of confidence in the stuff Ken comes up with and my stock kit is now 20 years old.

I rather suspect belts fail because of one of two things - incorrect installation or failure to replace critical kit, however there can and will be exceptions. I have not experienced a belt problem but then I have probably replaced way more than I should-I rather feel in our hot climate here time is as much a factor as use not mention that fact that "less use" can also be a problem if the engine sits idle for extended periods [mine does not].

Thus we need to differentiate between "good" and "better". If I was putting higher lift valves and stiffer springs in then it would be a "no brainer" for me.

The only thing I would not do, is run a belt without the covers no matter how I was using it but I do appreciate the dilemma of those trying to save every ounce.

Regards

Fred

Jascd

I had the gates racing belt installed before christmas and since then have had about 5 TB warnings, some just after start (like today), some when on the gas. When it first happened after about 300km, I took it back to the shop and they checked the tension and said it was fine. It happened again after about another 500Km and I took it back for a further check and it was still fine. Not sure what to do now, I suspect its not electrical as there is no real pattern to it but I don't really know. The workshop has suggested tightening the belt higher than recommended to see if that fixes it but that does not impress me at all so I will probably take it somewhere else. I have now 2,000km on the new belt.

Jason

89S4 auto

MFranke

A failing water pump can cause intermittent TB tension alarms.

Also try reseating the ground plug.

jcorenman

Jason, you will likely get some different opinions here, consider the following:

1) Higher-than-stock tension increases stress on all of the belt-related components, so that clearly is not a good answer.

2) You are having a shop do the work, which changes things compared to many (most?) folks here. A shop will typically warrant their work and will typically want to stick with factory-spec components to reduce their exposure, unless they have a lot of experience with a particular part.

3) The belt system is a "system", with the belt, tensioner, alarm, sprockets and pulleys all acting together. Everything has stretch (even the racing belt), the factory tensioner and alarm were designed to work in the context of the factory belt being part of the system. When you tighten the factory tensioner you are partly pre-stretching the belt, and partly compressing the bimetal spring-washers. A stiffer belt means less total compliance in the system, for the same tension. This is a fundamental change to the system.

The factory alarm is set to around 80% of the static tension (my measurements, consistent with other reports here). With the racing belt there will be less total compliance and greater variation in tension. So triggering the alarm, with the racing belt, with normal variations in dynamic tension would not be unexpected.

I suspect most folks here who are using the racing belt are also using a PKT (Porken tensioner). That changes two things: The PKT is spring-loaded and automatically changes length to maintain a relatively constant tension (with hydraulic dampening against compression), and it eliminates the warning. This is a fundamental change to the system.

It has been suggested above that the PKT is a good match for the stiffer nature of the racing belt. That may be the case, but I don't take it as a given: at some level it is simply two different fundamental changes.

In your situation, shop-maintained car and (I assume) relatively stock engine, I think a strong argument can be made for sticking with stock belt components-- factory belt and factory tensioner.

Cheers, Jim

Strosek Ultra

Is Todd using a valve spring having double the pressure of a stock 928? That is extremely much and will put a lot of stress on the timing belt, premature wear on cam lobes and lifters not mentioning robbing horsepower. The stock spring open pressure is according to my measuring 72 kp or about 160 lbs which theoretically will allow the stock engine to spin up to slightly more than 8000 rpm before valve float calculated with a 20% safety margin. Why do Todd need 320 lbs of spring pressure? Is he using a cam profile having a very high deceleration rate at the nose? Can be explained if he is using a high lift profile having short duration. Usually an open valve spring pressure of about 220-240 lbs is sufficient for most performance camshafts up to 8500 rpm for our engines (lightweight valvetrain assumed).
Åke