Belt Drive Systems Contain More Technology Than You Think
If there is one thing that is similar on all race engines regardless of brand or size, it’s the endless amount of pulleys and accessories which are often mounted to the front of an engine. Alternators, fuel pumps, oil pumps, and vacuum pumps are just some of the things that have to be driven by the crankshaft. However, it’s not just as simple as sticking a pulley and a belt on the item. Since the dawn of time, belt drive systems have changed dramatically in allowing an item to pump or charge without draining horsepower from the engine. And there are enough options to drive you crazier than you already are.
Currently, there are three different types of drive belts available for use, and each has benefits all their own. The most common is a V-belt, so named for its cross-sectional look. It’s the most commonly used belt and has been around for quite some time.
A tooth belt can have either round or square teeth, and each has its own benefits. Lastly is a serpentine belt, the same type that’s installed on most new cars today.
The first step in choosing a system is to decide what type of drive belt you intend to use. The type of accessory you’re intending to drive plays an important role in just which belt to use.
“You must first consider the drag load,” says Moroso Performance’s Scott Hall. “In most cases, an alternator has very little drag, while a vacuum pump does too with it having just slightly more drag load doe to the friction of the internal blades. An external oil pump, be it either a wet or dry sump pump does have a considerable amount of drag. For that reason, a cog-type belt must be used to prevent slipping. An external mechanical fuel pump falls into that same category even though it may only have a seven or eight pounds of pressure, even though some fuel injection pumps must provide 100psi or so.”
Both a V-belt and a serpentine belt require tension to keep it from slipping. This is accomplished by having one of the accessories adjustable to tighten up the tension. New cars today use something called a tensioner pulley that uses a spring to keep the pulley tight against the belt. This accounts for belt stretch by just holding pressure against it. In a racing application, where high rpm is called for, the tensioner can bounce and cause the belt to slip or fly off and therefore not used.
If you’ve ever viewed your street car’s tensioner while revving the motor up, you’d see just how much it can bounce and flex. “You should never use a tensioner in a racing application,” says Hall. “It’s only used to keep a serpentine belt tight in a 100,000-mile vehicle.”
As we mentioned earlier, the most widely used belt is the V-belt. Unfortunately, it has its limitations, and they start with the very thing that racers use to their advantage: Engine RPM.
Because of its name, the V-belt has a rather large cross-sectional size. As engine rpm increases, the weight of the belt along with centrifugal force can cause it to fly right out of the pulley.
“It’s for that reason that we manufacture and sell a deep groove pulley,” says Hall. “With a deeper groove, the belt sits down inside it further, which means that it must ‘fly’ out further in order to come off the pulley. By using a narrower belt, you’d also accomplish the same thing simply because it will have a smaller cross-section. However, you have to be careful that the belt isn’t bottoming out in the pulley groove. A V-belt is supposed to make contact with the sides of the belt against the inside side sections of the pulley.”
Most V-belt systems are suitable to around 4,000 to 6,000 rpm. After that, the standard over-the-counter belt will stretch and create problems and cause the belt to slip off the pulley. “That’s why it’s imperative to be sure to use a high quality belt,” Hall says.
Now because a V-belt requires tension and can slip with improper tension, it can also create a side load on the accessory being driven. Some units will not tolerate a side load or sustain a potential loosening of the belt condition, such as a dry sump oil pump. Because of this, in addition to the load imposed by the oil pressure, a tooth belt is required to drive the pump. A tooth belt doesn’t require any tension to keep it from slipping. The teeth of the belt interact with teeth on the pulley to drive the accessory. With no tension required, it also can be run a little looser, which allows less drag and can lead to a slight horsepower gain, or at least less of a loss due to drag. This same principle applies to that mechanical fuel pump used in fuel injection, or alcohol-carburetor engines.
Tooth belts are available in two distinct styles, square or round. A traditional square tooth, or gilmer belt, obviously has a square tooth. The gilmer belt was a trade name originally manufactured by the L.H. Gilmer company for a belt used for transferring power between two pulleys.
Strange as it might seem, a round tooth belt has (what else?) a round drive tooth. Both units work equally well; however, a square tooth belt contacts the pulley teeth tightly. Should any debris become lodged between the pulley and the belt, there is no allowance for it, and the belt can be dislodged.
A round tooth belt, while still contacting the pulley tightly, has a very small area where dirt can become lodged, and eventually dislodged. That area, albeit very small, is just large enough to self-clean itself. In addition, some round tooth belts have the bottom of the round section cut off, which increases that area.
Because of all of the above, it would seem like a round tooth belt is the way-to-go for a race engine drive system, but the availability of pulley sizes could be a drawback. A critical step, and an often overlooked one by many, is that each accessory item is more efficient when operated at a certain speed, in addition to having a maximum safe operating speed. Knowing that number can be beneficial in choosing the proper pulley.
As an example, NASCAR engine builders will use a large enough pulley on the power steering pump, so that the only time the driver has power steering is when he is at speed. Coming down the pit lane, or whenever the engine rpm is low, there is no power steering. If they were to install a smaller pulley, chances are that the pump could self-destruct by running at too high an rpm, in addition to causing a horsepower drain.
Tooth belts are also available in varying widths. A typical belt-driven fuel pump that puts out roughly 7-10 pounds of fuel pressure, can get by running a 1/2” wide belt. While a dry sump oil pump pumping out 50-90 pounds of pressure, and trying to force cold and thick oil through your engine, should use at least a 3/4” wide belt, and most racers are using a one-inch wide one at that.
“If your fuel pump belt breaks,” adds Hall, “the worst case scenario is that the engine will shut off. However, should the oil pump belt break, you’ll end up losing the engine because of lack of oil pressure.”
It really all boils down to tooth engagement. A round tooth belt has more engagement, and therefore an application like that could get by with a narrower belt that a square tooth one. Moroso and several other companies offer belts and pulleys to fit most applications.
One other thing Hall pointed out is just what is happening to that belt when on a two-step at the starting line of even a three-step used during a burnout. “Under acceleration, the belt is basically in constant pressure,” he says. “However, when the engine is in two-step mode, the engine is basically starting and stopping which causes the belt to do some pretty wild things. Here is where the use of the proper belt makes a difference.
So are tooth belts worth installing?
It’s really a matter of personal preference. However, tooth belt technology is filtering down from the professional classes, and it’s worth a look at them.
Choosing the correct belt system to use isn’t as critical as which type of bearings or piston rings to use. But it all becomes part of a combination that can either make or break horsepower. It shouldn’t be something that keeps you up nights, and with the technology and products available today, it’s an easier decision than you might think.