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The Truth About The Underdrive Crank Pulley


The underdrive crank pulley dates back many years when hot-rodder's were squeezing every last horsepower out of their rides. The underdrive crank pulley is any form of pulley smaller in diameter than the one originally supplied by the factory. Back in the day cars were carbureted, had mechanical fuel pumps and definitely did not have an electric cooling fan. Installing an underdrive crank pulley in one of these older vehicles probably saved a few horsepower by slowing down the accessories like the water pump, air conditioning compressor, power steering and alternator, which was more than adequate to begin with. Older vehicles sometimes came with an alternator that only produced 30amps! Slowing one of those units down with an underdrive crank pulley really didn't matter as the vehicle contained few electronics that would notice the drop in alternator output.

Today's vehicles are packed with essential equipment that can draw large amounts of power.

  • Electric Fans - 50-60amps initial start up, 20-40amps continuous load
  • Electric Fuel Pumps
  • Electronic Fuel Injection to include ECM's, ECU's, Fuel Injectors
  • Audio/Video/Stereo/Navigation Systems
  • HID Head Lights
  • Factory Installed or Aftermarket Power Inverters
  • Heaters, Air Conditioning, Blowers, Rear Units
  • Rear Window Defrosters - some can suck ~25amps continuously!
  • Power Inverters, Multiple Batteries

Most OEM crank pulleys are 6" in diameter while most OEM alternator pulleys are 2". This gives us a 3:1 ratio between the crank speed and the alternator speed. At an 800rpm engine idle speed the alternator is actually spinning to the tune of 2400rpm. It's very important to maintain the 3:1 ratio or else the alternator can't do it's job, especially a high amp unit that needs every last rpm to operate effectively. 

We've searched the web and have seen wild claims of 10%-15% power gains, better fuel economy and some underdrive pulleys will even make your hair grow back (ok, just kidding about that last part). What is even more amazing than these claims are the prices of one of these pulleys. There are plenty of people eager to spend up to $400 on these devices that only hurt your vehicles performance.

How can an underdrive pulley actually hurt horsepower?

It's pretty simple. For today's vehicles to run at optimum performance a lot of things need to happen, correctly. There are two very important parts of the rpm range when talking about alternator output. Idle and WOT (wide open throttle). If you didn't notice there is no longer a carburetor or distributor on almost any vehicle even 10 years old. What controls your vehicles fuel delivery and timing then? It's called an ECM (or ECU for the import guys!) and it's voltage dependant. You see the cars today are so smart that if your electrical system isn't healthy the computer will simply retard ignition timing or alter fuel delivery at WOT to compensate for low voltage, saving a costly engine disaster. The computer knows that if your vehicles voltage is lower than expected the fuel pump will be delivering less fuel, the injectors will open and close slower and some adjustments will be made to guard against engine damage. Install an underdrive pulley and you may end up underpowered.

If you couldn't tell by now, we don't like underdrive crank pulleys. Since we are in the business of increasing alternator output these have been a thorn in our side for quite sometime. Any time a customer has reported low alternator output on one of our units it was because of an underdrive crank pulley, insufficient battery cable size or cheap battery. More articles on the other two culprits coming soon.

We can't compete with the hundreds of thousands of dollars put into marketing products that you just don't need for your car, but we hope you've at least heard the other side of the story.