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Posted: Tue Nov 03, 2009 3:08 pm
by cj91legss
How does an underdrive pulley make more horsepower?
I didn't know that...
Posted: Tue Nov 03, 2009 3:21 pm
by jefferson
It underdrives the ps, alt, and ac. The underdrive pulley is smaller which is in effect gearing those things down and freeing up the horsepower that they used to suck up to propel the car down the road.
Posted: Tue Nov 03, 2009 3:27 pm
by cj91legss
I see, i also heard that U/D is for our cars tho... don't recall the reason why
Posted: Tue Nov 03, 2009 3:30 pm
by kimokalihi
It just makes it easier to turn accessories like the alternator and A/C but it also runs them at a lower RPM then they would normally run at.
I would stay away from them. If they made that much more power without taking a hit on your accessory functions the manufcaturer's would certainly put a smaller pulley on at the factory. It would be cheaper to make smaller pulleys to so there's a reason they're not doing it. That reason being that the accessories weren't designed to function at those RPMs with the same demand.
Underdrive pullies are mostly for spinning accessories such as a/c, alternator, and power steering slower so they rob less power from the crank. The problem with this is, at idle, the alternator can spin too slow, causing voltage to drop, lights dim, etc etc. You shouldnt need to spin them slower because the alternator has a voltage regulator on it so it only robs as much power as it needs. The a/c has a clutch on it that disengages when not in use. The p/s prolly does rob power all the time, but spinnign the pump slower will make it harder to turn. The pulley itself is lighter than stock, but usually lacks harmonic balancers which hodna builds into the pulley which can be potentially detrimentl to your crankshaft.
Posted: Tue Nov 03, 2009 6:43 pm
by Legacy777
I've run all three pullies.....stock, LW UD, and LW.
For all intents, the UD pullies are kind of a pain with the accessories. The difference in mass between the UD LW & stock sized LW pullies is negligble. The gearing effect really isn't that much.
If you're going to go with a LW pulley stay with the stock size pulley. On higher HP engines, I'd recommend staying with the stock pulley.
Posted: Tue Nov 03, 2009 8:53 pm
by jefferson
I have had no issues at all with the underdrive pulley other than the ac doesn't work as well in stop and go traffic. No charging issues and I have a lightweight pc680 battery. I don't have any of the stereo stuff so prevalent today however, so that helps. If you have a high drain stereo then the underdrive is not for you. If not then go for it.
Jeff
Posted: Tue Nov 03, 2009 9:12 pm
by Mattheww044
hmm, I will definately look into that. I plan on doing LW flywheel, but not til my clutch goes out and needs a WRX one,so I'd like a LW or underdrive pulley until then (hopefully a few years), since you obviously don't want a LW Flywheel and LW pulley
Posted: Tue Nov 03, 2009 9:37 pm
by jefferson
I am planning on taking some weight out of the stock flywheel when I pull the tranny to rebuild it, which I really need to do soon. I think the light underdrive pulley and the lightened stock flywheel would be a good combo and cheaper than buying a light flywheel.
Jeff
Posted: Sun Nov 08, 2009 9:15 pm
by NuclearBacon
Got mine, been running for a year now? no problems. I use perrin