So whilst look for info to answer on another forum I came across this link about under drive pulleys, it is short but concise and well worth reading if thy are on your Christmas list >>

 

 

http://www.metroplexalternator.com/underdrive-pulley-kits.html

 

 

Here is a quick copy and post of some info from that link >

 

When you install an underdrive pulley kit your engine is not suddenly producing more horsepower. You are simply using less in one area so that it is available for use elsewhere. The typical claim is that these pulleys slow the belt driven components down anywhere from 20% - 30% and 10 to 15 horsepower can be freed up by installing an underdrive pulley kit as well as improved fuel economy and improved torque......all with no adverse effects. 

 

Common Sense Check: If vehicle manufactures could increase horsepower, increase torque, and increase fuel economy by simply installing a smaller diameter stock crank pulley , which would cost them nothing extra, then these pulleys would be standard equipment on every model of every vehicle made by every car manufacturer on the planet. Just saying.

 

The claims sound great but as we all know nothing comes for free....

and there in lies the rub. While it takes less horsepower to spin a thing slowly than it does to spin that thing fast it is also true that the slowly spinning thing can do less work than it could if it were spinning fast. A water pump moves less water when it spins slower and an alternator will put out less power if it spins slower and an a/c compressor will produce less pressure if it spins slower and a power steering pump will provide less power assist if it spins slower.

Personally, I think the more important thing to consider with the majority of crank pulleys on the market, is that they’re solid one piece items with no harmonic dampener.

 

The purpose of the dampeners is to remove/reduce the harmonic frequencies that pass through the crank shaft during the stages of a 4 stroke combustion engine. By removing that dampener you’re increasing the wear on your bearings due to an increase in harmonic vibrations.

Personally, I think the more important thing to consider with the majority of crank pulleys on the market, is that they’re solid one piece items with no harmonic dampener.

 

The purpose of the dampeners is to remove/reduce the harmonic frequencies that pass through the crank shaft during the stages of a 4 stroke combustion engine. By removing that dampener you’re increasing the wear on your bearings due to an increase in harmonic vibrations.

 

Well put, and technically correct.

 

Jeff