Re: Someone explain the difference between underdrive and lightened pullies... (hmsa)
Lightened pullies are just that--same diameter as the OEM pullies, but typically machined from aluminum in order to reduce rotating mass. The theory is that the engine will require less energy to accelerate the mass of the pullies.
Underdrive pullies go one step further by not only reducing rotating mass, but also changing the diameter to slow down the accesories that are driven by the crank pulley. So the crank pulley diameter gets smaller in O.D. (which also lightens things quite a bit) and the driven pullies (typically alternator and power steering) get larger in O.D. (think about the gearing on your mountain bike--small in the front and large in the back makes for some pretty low tire speeds). So, the underdriving reduces "parasitic" losses from the alternator and power steering, thus taking away less power from the engine. All this combined should allow you to put a little more poop to the front wheels. It's not a ton, but every little bit helps, right?
The cons? With underdriving, you may notice some reduction in voltage/current at idle so the headlights may dim just a bit (at idle only).
I think some people will also say that the crank pulley is critical for damping harmonics in the rotating assembly and they are right, but it is not nearly as critical on a 4 cylinder as it is on a longer crankshaft of a straight six like a BMW or an externally balanced V8. Ideally, you'd have the crank pulley balanced along with the crank/rods/pistons, but no one does that unless they are actually doing a build-up.
I've had a set of underdrive pullies (Unorthodox Racing) on my Corrado VR6 and Saturn SC2 for 3 years without a problem. I machined my own set of underdrive pullies for my Saturn since the aftermarket doens't cater too much to that crowd. I actually went so far as to OVERDRIVE my waterpump pulley to compensate for the underdrive crank pulley. I made a lightweight (stock diameter) idler pulley for the Saturn as well. Woo hoo!
Hope that clears things up a bit!
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