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The modern multiple rib serpentine belt system is a major improvement over the old multiple V-belt systems typically found in classic cars. A single belt allows for more compact packaging at the front of the engine and in conjunction with an automatic belt tensioner device is virtually maintenance free.
For a typical driver, the only time they might think about their belt is when it needs replacing. But if you’re reading this magazine, let’s face it, you are not typical. You are an enthusiast and as an enthusiast you continually upgrade your vehicle. We can’t help ourselves, it’s in our nature.
It starts with normal bolt-ons and the upgrade path from there is dictated by budget and the depth of your individual addiction to horsepower. And from the robust sales of supercharger kits in the current market, a lot of you have a serious addiction.
We put a lot of thought into our exhaust system, wheels, tires, electronics and more, but we never think about the good old serpentine belt until there’s an issue with it. The most common issue is when you have a built engine and a supercharger with smaller pulleys to make more boost than the original supercharger kit was rated for and you encounter belt slip.
Our HEMIs are limited to six-rib serpentine belts. Belt slip occurs when the resistance of a supercharger pulley exceeds the coefficient of friction of the belt that is turning that pulley. An easy way to increase the coefficient of friction is to go to a wider belt; some Bowtie cars are now coming with ten-rib belt systems from the factory. But that’s not a practical path for late model HEMIs as the required pulleys just aren’t being made by anyone.
Belt breakage is another issue that plagues high output HEMI engines, and this happens not just on supercharged engines, but also on turbocharged engines and naturally aspirated engines with modified heads and aftermarket cams. The culprit in both belt slip and breakage is the factory-style belt tensioner. The tensioner that comes from the factory on HEMI cars and trucks will do its job and last for hundreds of thousands of miles on non-modified daily drivers.
If breaking a lot of belts makes you a belt-breakage expert, then we must be belt-breakage gurus. We’ve broken five in a row on the dyno at times, and five or six over a race weekend wasn’t unusual for us. It has caused us more than headaches; we’ve lost races due to a broken belt, and a shot at a championship. In mid-2016 we knew we had to find a fix, which we did and along the way we also picked up more boost and horsepower.
Have you tried to stretch a serpentine belt by hand? If you have then you know that you can’t because they’re really strong. But your engine has no problem stretching the belt (it has more horsepower and torque than you do, of course). The more you mod your engine, the more power you have, the more the belt can be stretched. Late Model HEMIs have very long belts to start with, add a blower kit and you can be looking at a belt that’s 100 inches long -- that’s over eight feet of belt. On high output engines we’ve seen 5-6% of belt stretch at wide open throttle. That could mean five to six inches of belt stretch for your tensioner to deal with.
The job of the belt tensioner is two-fold, to take up the slack when there’s momentary belt stretch such as at full throttle, and to take up long term slack as the belt stretches and wears over its lifetime. The peak force on the tensioner occurs when the throttle is suddenly snapped open or shut, dramatically increasing or decreasing the torque working on the belt. This same effect can occur during shifts on high output engine vehicles.
A belt tensioner is a simple device; it features a pivot point and an internal spring. When a belt is installed, a tool such as a breaker bar is used to move the tensioner against the spring pressure allowing the belt to be slipped over the pulleys. When the tool is removed the belt is under tension and the tensioner pulley is about in the mid-point of its travel. When the belt is stretched under power, the spring moves the tensioner pulley to take up the slack.