mch-22071_wIf you’ve got a stock vehicle, chances are your pulleys and pulley system are fine. Still, there are a variety of reasons to purchase new pulleys for your engine and accessories: appearance, reliability, and overall vehicle performance. And if you’re adding power steering or air conditioning to a vintage vehicle? You definitely need to make sure you’ve got the right pulleys powering your accessories.

We talked to the experts at March Performance to get the low-down on the things you need to know and understand before choosing your pulleys. Of course, the guys at March, Zoops, and other pulley manufacturers can assist you with your selection, but here are a few things you should know:


Underdriving, Overdriving–and What it Means

To understand underdriven or overdriven pulleys, you first need to know about pulley ratios.

Pulley ratio essentially compares the size of a driven pulley (water pump, power steering, or other accessories) to the size of the driver pulley (crankshaft pulley). This ratio can be found by dividing the diameter of the smaller pulley into the larger one. For example, a 6-inch water pump being driven by a 7-inch crank pulley would yield a 1.16:1 pulley ratio. In this scenario, the water pump pulley spins 1,160 times for every 1,000 turns of the crankshaft pulley. Because the accessory spins faster than the driver pulley, it is referred to as overdriven. Conversely, an accessory that spins slower than the driver pulley is said to be underdriven.

In a belt-driven pulley system, it takes horsepower to turn the pulley. To gain back some of the power used to turn the pulleys–often referred to as parasitic power loss–many people opt to use an underdrive pulley system. This is typically achieved by swapping in a smaller crankshaft pulley to use with slightly larger accessory pulleys.

“When you choose to go with a underdrive pulley set you are basically slowing your accessory speed down,” said Casey Burnett of March Performance. “Reasons for doing this would be to gain horsepower or to help aid in belt throwing. This could also help increase a vehicles fuel mileage.”

Burnett says underdrive pulley sets are excellent for applications such as race cars or towing vehicles that function consistently in the mid- to higher-rpm range. However, much depends on the amount of accessories being used.

“In a vehicle that is only running a water pump and alternator and you have electric fans, the horsepower gain is minimal for underdriving–maybe 1 to 3 horsepower,” But in a vehicle that is driving an alternator, A/C compressor, power steering, and has a mechanical cooling fan–you could see 15 to 20 horsepower gains.”

Burnett is also quick to point out that underdriving your pulleys too much can have negative effects.

“If the accessory is slowed too much, it may not function properly at lower rpm,” he said. “You could experience lack of charging, increased engine temperature, lack of power steering assist, or insufficient air conditioning output, depending on the accessories.”

While underdriven pulleys provide power advantages, overdriven pulleys also have their place in certain vehicles. For example, a vehicle with high-power stereo equipment or lots of electronics would benefit from an overdriven alternator pulley, which spins the alternator more quickly. A car that spends a lot of time at low-rpm or cruising speeds might benefit from an overdriven water pump to deliver increased cooling. As with everything, it’s all a matter of striking the right balance.

“Just like underdrive pulleys, overdriving components too much could also cause issues like increased engine temperature at high rpm or premature component failure,” Burnett said.

To strike a balance, March Performance offers pre-packaged pulley sets to deliver the desired performance effect you’re looking for with the unintended side effects we mentioned above. If you’re piecing together pulleys on your own, you’ll want to proceed with caution. For example, you wouldn’t likely want to underdrive your alternator by any more than 10 percent; other accessories shouldn’t be underdriven beyond 25 percent. It’s best to consult the experts to make sure you get the right balance.

“The biggest advice I can offer is not to guess on the parts needed,” Burnett said. “Tech lines are typically free and I suggest strongly in using them to guarantee you have covered all your options and order the correct parts the first time.”

Image courtesy of

Image courtesy of

V-Belt or Serpentine

Another thing to consider when choosing a pulley system is V-belt versus serpentine designs.

A V-belt pulley relies on a single groove which mates to a V-shaped belt surface (see image at right). A serpentine-style pulley works with a drive belt that has several smaller ribs (typically a minimum of six ribs), giving the pulley contact with a greater surface area on the belt. The result of this design gives the serpentine pulley system greater traction with less slippage than the older V-belt style.

“The reason all cars manufactured in the 90’s and up (have serpentine systems) is a reliability factor,” Burnett said. “Serpentine belts have more belt-to-pulley contact than the old v-belt style and also can be used to create more wrap on the components. Its higher tension capabilities also make serpentine belts more efficient and reliable, and good quality serpentine belts will have little stretch compared to V-belts.”


Serpentine pulley.

Because of its resistance to stretching and slippage, serpentine systems are better able to handle today’s higher-revving or high-horsepower engines. The reduced slippage means greater engine efficiency (both power and fuel efficiency) with less load on the engine. In addition, a serpentine system, with its single-belt routing, offers a cleaner, more modern look than multiple V-belt setup. And because a serpentine belt is wider than a smaller V-belt, serpentine pulley systems are stronger and require less belt changes.


V-belt pulley.

Still, Burnett says V-belt pulley systems have their place.

“I could justify staying with V-belts if the vehicle application is a numbers-matching, complete original and any updates could affect the value negatively,” he said. “Or if the application is used minimally and functions fine for its intended use.”


Your Ultimate Choice

Because of the factors we discussed above, your best choice for pulleys is often based on these things (among others):

  • Your engine
  • Your vehicle make and model
  • Future plans for vehicle–towing, show car, racing, etc.
  • Which accessories will be used–power steering, air conditioning, etc.

You’ll also need to factor in the accessories you already have. For example, is your water pump a long or short style? What kind of A/C compressor do you have? Companies like March Performance make pulleys that accommodate a variety of accessories. You can also find complete drive systems that include accessories you might need, including A/C compressor, alternator, and more.


A serpentine system uses a series a pulleys powered by one drive belt.

Finally, if you decide to convert from a V-belt to a serpentine system, you’ll also find a variety of serpentine conversion kits on the market. For custom applications, such as street rods with limited space under the hood, you’ll need to be aware of potential clearance issues.

“The aftermarket parts industry is always changing, and there are always new parts to take into consideration for fitment,” Burnett said.

Hopefully, this guide gets you going in the direction.




Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.