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This question was asked in the recent post regarding elimination of the center pulley by installing a shorter accessory drive belt. I decided to blow the dust off my Mark's Standard Handbook today to verify what I thought might be the implications of by-passing the center pulley.
My suspicions were correct, drive belt horsepower is related to circumference of pulley wrapped by a belt. This number is called the "Angle of Contact Conversion Factor" and it decreases from unity at 180 degrees of angle of contact, to 0.7 at 90 degrees of angle of contact.
Assuming the waterpump pulley has 180 degrees of belt contact with center pulley installed/long belt, and only 90 degrees of belt contact when using the short belt, then the amount of horsepower which can be transmitted into the waterpump to push coolant through the engine has been reduced by 30%.
What does this mean to those of us who may perform this change? Maybe nothing. Depending on the amount of safety factor/over design used by Saab's engineers there may be no implication at all. It may however, be possible that the belt would slide over the waterpump pulley causing premature belt wear. This might be more likely to occur during periods of high horsepower output from the engine. Severe belt slippage could diminish the pump's ability to move coolant, thus increasing engine temperature slightly.
Belt inspections at earlier mileage intervals would probably be a good idea for anyone going the short belt route, possible replacement earlier than the recommended 60,000 mile interval too. Since this has been done for a while now on 900 Series engines there may be enough history to know if there is any risk of additonal belt wear.
Other comments please.
posted by 198.208....
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