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Crank Pulley Failures on SCed Cars?

 
Old 07-13-2004, 05:32 AM
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GoRideSno
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Default Crank Pulley Failures on SCed Cars?

Has anyone heard of such?

Quick Carl's post got me curious about this.

I have only heard of something related and it was a crank bolt being snapped. IIRC it was a Koening SCed car and I believe it was probably because the SC belt was far forward on the pulley and the large diameter of the pulley required for the centrifugal.

I'll share my thoughts on pulley design.

I specifically designed my crank pulley so that the SC belt would exert the least amount of force possible on the pulley as well as have a high strength to mass/apparent-mass ratio. This is achieved by having the SC belt ride as close to the engine/fastening point as possible, having a diameter that is as small as possible and having a continuous diameter the length of the pulley. Of notable mention the crank pulley is bolted to the stock steel AC pulley for even more strength and the entire assembly is only 1 oz heavier than stock. I am looking at moving to not using the AC pulley bolting method by having a thicker wall on the crank pulley and using a longer bolt for ease of manufacturing though.


Andy K

Last edited by GoRideSno; 07-13-2004 at 06:35 AM.
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Old 07-13-2004, 10:10 PM
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Hmmm, guess there haven't been any.
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Old 07-13-2004, 11:02 PM
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I have seen the Koenig car and the resulting damage. (car is fine now BTW)
Looking at it all, Bill and myself pretty much came to the conclusion that the real cause was the use of the original stock crankshaft BOLT.
Yes, it will help if the rotating force of the SC belt is closer to the block, but i think the biggest hidden issue is the age of the crankshaft bolt itself and the fatigue already placed on it by numerous "torque downs" .Im putting a new one on mine regardless.

IMHO, your asking for trouble on any SC set up if you are running the original, stock, 17+ yr old bolt. Unless you have records, even then it may not be a true reflection, you don't really have a good idea how many times that bolt has been taken on and off. Remember, this bolt is torqued to over 270ftlbs if i recall.

I saw Carls crank pulley at the OC-IC, it is one beefy pulley and a nice looking one as well. From what i gathered from talking to Carl and hearing of his research and connections in the industry, IMHO, it (the pulley) aint gonna fail. I made a suggestion to him to throw in a new crankshaft bolt for good measure with the kits or just perhaps recommend a new one during the install.
Pretty cool watching a 78 16v toss down more power than your average S4.
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Old 07-14-2004, 01:15 AM
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I can vouch that Tim Murphy will send a new correct length crank bolt with his kit for the 89 S4... The new bolt is a higher tensile strength than the original.
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Old 07-14-2004, 03:08 AM
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Tony,
I'm not worried about Carl's pulley. The way he had worded is his post about Wichita it sounded like someone besides him had had a pulley failure. It got me wondering if there had been one and what the circumstances were.


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Old 07-14-2004, 10:53 AM
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Originally posted by Old & New
I can vouch that Tim Murphy will send a new correct length crank bolt with his kit for the 89 S4... The new bolt is a higher tensile strength than the original.
Yup, the Murph kit comes with a new "Jaysus bolt". You just need a torque wrench with a 7 foot handle.........
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Old 07-14-2004, 01:57 PM
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What about the cyclical bending loads on the crankshaft? Could this induce fatigue failure of the crankshaft itself? I've heard of this on other cars, but never on a 928.
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Old 07-14-2004, 03:21 PM
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John...
On a 928 the crank bolt would snap long before there was any such thing as crank fatigue.
The end of the crank actually snapped in the testing of the Ford GT. The factors that caused it were a chamfer on the end of the crank that was not needed (left over from it's original use in ford trucks) and supercharger and all accesories including dry sump being driven by the same belt.

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Old 07-15-2004, 12:57 AM
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I did not mean to raise worries - I was trying to dispel any.

The kits from Albrecht in Austria use a cogged crankshaft pulley and belt (Where Tim Murphy and I use serpentine mutliple-ribbed belts). The Albrecht supercharger kit has had a pulley failure or two reported in Great Britain and the Nederlands. Not a lot - but still...

The FAST kits from Texas in the US have had some flexing issues, I'm told.
I've seen pictures of a mounting bracket failure of their main bracket, but have not ever seen one personally. When you flex a aluminum bracket, it "work hardens" until it cracks - that's what was happening there.

These are the only failures I am aware of - but to aleviate any worries, we engineered our pulleys and brackets differantly, and triangulated our mounts to eliminate flexing. Materials are different too - both Tim and I are using steel brackets as the primary mounting bracket - not aluminum like FAST. It's just NOT gonna break.

We also slapped a two-year warranty on our pulleys and brackets for you guys to give you that sleep-good-at-night factor that you want.
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Old 07-16-2004, 01:21 AM
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Originally posted by John..
What about the cyclical bending loads on the crankshaft? Could this induce fatigue failure of the crankshaft itself? I've heard of this on other cars, but never on a 928.
And you probably never will.

I've heard turbos are bad for exhaust valves too.
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Old 07-16-2004, 01:30 AM
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Thanks Carl.

Makes sense that it was the cogged type pullies that failed. There is much more stress on those. IIRC one Mfgr of SCs mentioned that cogged belts would cause more wear on the SC even.




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