Engine

[Chas2]

What is the story with aftermarket pulleys such as those sold by Unorthodox Racing.  All the literature (on the Internet) seems to claim increases in torque and horsepower, but three people I have spoken to who do mod their cars with regularity say don't bother.

It would make sense to me that there is really no increase in horsepower and torque, but just that the engine is not turning as much mass, so it may respond more quickly.

What is the upside and downside?  Especially to the so called underdriven pulleys?

Thanks in advance.

[Weerwolf]

They are called underdrive pulleys , because they drive the acceseries less than the stock component. Basically , the pulley is lighter and smaller than the normal one. Because its smaller , the accesery is turned slower , and in turn , less power is used to drive the accesery. Also , the pulley is lighter , so that also gives an extra bit of power.

The upside is more power.
The downside depends on whitch pulley it is. Either harder steering , less effective airco , or lights that dim at idle.
Also , the standard belt will probably not work with the underdrive pulley.

Sorry about my bad English , I'm tired and sick today.

[Chris Longhurst]

The other thing to bear in mind is that you can get lightweight cam pulleys which are the same size as the factory items. The difference is the weight. Less weight to turn (the pulleys act like flywheels), means less power wasted doing so. It's fractional, but consider lightweight pulleys on lightweight cams, with lightweight driveshafts and lightweight wheels - well - you get the picture. Every bit of weight you drop from the mechanical driven parts of the engine (all the way to the wheels) will free up more engine response.

[Kris]

The other thing to bear in mind is that you can get lightweight cam pulleys which are the same size as the factory items. The difference is the weight. Less weight to turn (the pulleys act like flywheels), means less power wasted doing so. It's fractional, but consider lightweight pulleys on lightweight cams, with lightweight driveshafts and lightweight wheels - well - you get the picture. Every bit of weight you drop from the mechanical driven parts of the engine (all the way to the wheels) will free up more engine response.

You actually get a two fold weight reduction...you will probably find your wallet significantly lightened as well 

[Chris Longhurst]

True
Clever how that works, isn't it?

[Firefly]

there's also some issue about aftermarket crank pulleys causing resonance vibrations becaused they're not dampened or something. not quite sure whether this is true or not.