FAQ on Underdrive Pulleys: Everything You Need to Know

Underdrive pulleys are one of the most effective bolt-on power upgrades available for any naturally aspirated engine — and March Performance has been engineering them for over 25 years. Here are the most common questions we get about our performance ratio pulley systems.

What Is an Underdrive Pulley?

An underdrive pulley is a crank pulley with a larger diameter than stock, which reduces the speed at which the accessories — alternator, power steering pump, water pump — are driven relative to engine RPM. By slowing these accessories down, you reduce the parasitic load on the engine, freeing up horsepower that was previously spent spinning accessories faster than necessary.

How Much Power Can I Expect to Gain?

On a typical naturally aspirated V8, a quality underdrive pulley system can recover between 8 and 15 horsepower at the rear wheels. March Performance's performance ratio pulley kits have been tested on a wide range of applications — some seeing as much as 15hp on a dyno. The exact number depends on your engine, your existing accessory loads, and the ratio change of the specific kit.

Will Underdrive Pulleys Cause Problems with My Accessories?

For street-driven vehicles, underdrive pulleys work well in the vast majority of applications. A few things to consider:

• Alternator output at idle — if your build has high electrical demands (multiple electric fans, big stereo, EFI system), an underdrive pulley will reduce alternator output at idle RPM. This is rarely a problem at cruise or highway speeds, but worth thinking about if your car idles for long periods
• Power steering assist — slightly reduced PS pump speed is imperceptible in normal driving on most applications
• Cooling at low speed — March offers both underdrive and high-flow (overdrive) water pump pulleys. If you're running a street rod that spends time in traffic, the high-flow option keeps cooling efficiency up

Are March Performance Underdrive Pulleys Safe for Daily Drivers?

Yes — thousands of street-driven vehicles run March Performance underdrive pulleys daily without issue. The key is selecting the right ratio for your specific build and electrical load. If you have questions about your application, call our tech team at 1.888.729.9070 before ordering.

What's the Difference Between Underdrive and Standard Ratio?

March Performance offers both standard ratio and performance ratio (underdrive) pulley kits. Standard ratio kits maintain factory accessory speeds while still giving you the show-quality look and CNC precision of a March system. Performance ratio kits reduce accessory speed for power gains. Many builders choose standard ratio for show cars or low-mileage builds where maximum power recovery isn't the priority.

Do I Need a Complete Kit or Just a Crank Pulley?

For maximum benefit and proper belt alignment, a complete system including crank, water pump, and alternator pulleys is recommended. Mixing a March underdrive crank pulley with stock accessory pulleys can create belt alignment and tension issues. Browse complete SBC pulley and bracket kits, BBC pulley kits, and Ford pulley kits at marchperformance.com.