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Old 11-24-2008, 01:41 PM
KarTek's Avatar
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General technical question on belts...

This is a pretty general question but it relates to performance and supercharger drive belts and pulleys.

So, is there a minimum radius that drive belts are supposed to turn around? I'm talking about regular 6 and 8 rib accessory drive belts now. Most pulleys I see are about 3" or larger with the smallest being the alternator.

I'm just wondering if a 2" pulley is too small of a radius and will stress the belt.

Thanks!
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Old 11-24-2008, 10:52 PM
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That is a tough question. It really depends on load and it is more related to belt contact surface. This dictates more wrap around a small pulley under the same load as a large pulley.
If you are planning on running a 2" supercharger pulley at a high boost level it will slip. I ran about this size on a early version of my car with the slk supercharger. I was only running 6lbs of boost though and the rpms were to high for the supercharger. I currently run a 2.75" pulley and wrap almost as much as possible. I made the mounting capable of adjusting the idler pulley to change the wrap and the belt length.
Back to the rpm. You really have to consider this. With a 6" crank pulley and a 2" drive pulley you are at 3:1. So at 6000 rpm you are driving the 2" pulley at 18000 rpm. Most superchargers do not like this or anything else.
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Old 11-25-2008, 08:15 AM
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My SC pulley is a 2.55" Votech part adapted to an Eaton M90. That gives it a top speed of 13725 at 5000 engine RPM which is just under the 14k limit where it starts to beat too much heat into the air. (Keep in mind it's Diesel too) The way I plan to route the belt will provide over 50% contact with the SC pulley.

However, to make the belt change direction (over 90 deg. turn) and run up to the SC and back down, I will have to use some small, possibly 2" pulleys. My only concern is that belts may have some sort of rating where you're not supposed to bend around less than so small of a radius. I don't want to base the entire design on a weak point where the belt will have frequent failures.

I do know that cam timing belts have small idler pulleys but then again, they travel much slower and the belt is just changing direction a little rather than over 90 deg.
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Old 11-25-2008, 10:22 AM
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Thats a pretty big blower to turn with that pulley size and six ribs. I do not know for sure but I would be willing to bet that that will slip at any significant boost. Whipple was very concerned on 6ribs that mine might slip under 3".
You could call Magnusun, who deals eaton blowers and ask them.
You might get away with it though since you will not be revving very fast.
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Old 11-25-2008, 01:09 PM
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Yeah, I realize it's pretty small. If it's a problem, I'll just go with a larger pulley. I need at a minimum about 8 PSI out of the blower. I'll get it tweaked in after it's installed.

Right now, I'm mainly concerned about turning those relatively sharp corners with the belt.
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