Underdrive Crank Pulley

[davekp]

Do these really give much of a gain in horsepower? Or are they used for some other reason?

[CraigS]

They certainly gain at least something in HP. But they also can cause overheating and insufficient charging. From comments here and there it seems to me that most don't use them and some, who tried them, ended up removing them.

[Bob Cowan]

I put one on my 427W without any problems. I also have one on my LS6 track car.

They "gain" power by slowing down belt driven accessories; meaning that they decrease parasitic power loss. Most of the power gain is in the upper rpm's. The down side is that they also slow down accessories at idle, which can cause overheating and undercharging at idle. The answer to that is get the rpm's up a smidgen when sitting at a stop light.

In a track car that sees a lot of high rpm use, they also decrease wear and tear on those same components.

[NAZ]

Agree with the assessment by Craig and Bob.

If you want to reduce parasitic drag at high RPM without the negatives at idle outlined above use an electric water pump. One of the biggest horsepower drains at high speed is a conventional engine driven water pump. The engine driven pump is a compromise -- it provides barely enough flow at idle and starts becoming inefficient at higher RPMs. Not a real issue for most street driven cars that seldom run high RPMs but for a car that lives much of its time at higher RPM this horsepower drain is low hanging fruit for a tuner. An electric driven water pump, either a conventional pump driven by an electric motor or a pump designed as electrical only can be driven at a speed that is more energy efficient and provides sufficient flow for cooling at any engine load. The gain in efficiency can net enough HP gain you might see it in a time slip but like most incremental gains, you won't feel it in your butt dyno. It takes quite a lot of HP gain to actually "feel" the difference, more than all those simple add-on devices like cold air intake, under drive pulleys, etc. can provide alone.

Want to really make a difference you need to go big like a cam or nitrous or boost but these are expensive and have their own set of problems. The best power adder for a street car is cubic inches. The only thing that can beat cubic inches is cubic money.