Underdrive Pulley
Melting Slicks
Joined: Nov 2013
Posts: 2,049
Likes: 316
From: USA
The crank pulley powers the AC compressor, alternator, water pump and PS pump. This underdrives them....spins them slower. This creates less drag or resistance on the engine rotation itself and can "free up" a few HP. This makes the most sense for a race car that sees higher RPM often. Potential drawback is the alternator won't produce enough current to keep the battery charged, etc... but even at 25% underdrive, this does not seem to be a huge concern.
Alternator, water pump, PS pump are on one belt... AC compressor has its own. Some pulleys underdrive all of them, some keep AC unchanged and underdrive the rest, etc-- just depends on brand and model.
For what it's worth, I run the Powerbond 10% underdrive model. Underdrives everything except AC which runs at stock speeds/size.
Hope this helps.
Alternator, water pump, PS pump are on one belt... AC compressor has its own. Some pulleys underdrive all of them, some keep AC unchanged and underdrive the rest, etc-- just depends on brand and model.
For what it's worth, I run the Powerbond 10% underdrive model. Underdrives everything except AC which runs at stock speeds/size.
Hope this helps.
Last edited by aaronc7; Dec 7, 2014 at 12:31 PM.
Thread Starter
Drifting
Joined: May 2007
Posts: 1,409
Likes: 110
From: Chandler AZ
The crank pulley powers the AC compressor, alternator, water pump and PS pump. This underdrives them....spins them slower. This creates less drag or resistance on the engine rotation itself and can "free up" a few HP. This makes the most sense for a race car that sees higher RPM often. Potential drawback is the alternator won't produce enough current to keep the battery charged, etc... but even at 25% underdrive, this does not seem to be a huge concern.
Alternator, water pump, PS pump are on one belt... AC compressor has its own. Some pulleys underdrive all of them, some keep AC unchanged and underdrive the rest, etc-- just depends on brand and model.
For what it's worth, I run the Powerbond 10% underdrive model. Underdrives everything except AC which runs at stock speeds/size.
Hope this helps.
Alternator, water pump, PS pump are on one belt... AC compressor has its own. Some pulleys underdrive all of them, some keep AC unchanged and underdrive the rest, etc-- just depends on brand and model.
For what it's worth, I run the Powerbond 10% underdrive model. Underdrives everything except AC which runs at stock speeds/size.
Hope this helps.
I performed what I think was very good due diligence before I owned this car & do not believe it was abused or driven hard, I just couldn't understand why he put on the under-drive pulley. Might be wrong.
Thanks again............
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
An underdrive pulley simply slows down the RPM of all the accessory drives on your engine making them turn slower and creating less HP loss that stock from the accessories
power steering--alternator--A/C water pump
Pluses ----Typically you can expect 5-10 RWHP across the ENTIRE RPM range
Minuses---- an underdrive pulley will make your power steering pump turn slower--which can create low pressure while turning at low RPM or idle--So the steering effort like in a parking lot may become difficult
Also the alternator turns slower so the battery may not charge at all at ilde-Not a bid deal unless you have an aftermarket stereo that requires more amps
The fix for this is to have a tuner RAISE the idle RPM so the car will again turn easier and charge the battery at idle
As far as the A/C goes some under drive pulleys do not change the A/C belts where other do---If your brand does---same thing happens the A/C pumps will turn slower and you may not get cold air at idle
I still think it is a great Mod---easy 5-10 RWHP across the board I always installed one--But always raised my idle speed to compensate for it---
power steering--alternator--A/C water pump
Pluses ----Typically you can expect 5-10 RWHP across the ENTIRE RPM range
Minuses---- an underdrive pulley will make your power steering pump turn slower--which can create low pressure while turning at low RPM or idle--So the steering effort like in a parking lot may become difficult
Also the alternator turns slower so the battery may not charge at all at ilde-Not a bid deal unless you have an aftermarket stereo that requires more amps
The fix for this is to have a tuner RAISE the idle RPM so the car will again turn easier and charge the battery at idle
As far as the A/C goes some under drive pulleys do not change the A/C belts where other do---If your brand does---same thing happens the A/C pumps will turn slower and you may not get cold air at idle
I still think it is a great Mod---easy 5-10 RWHP across the board I always installed one--But always raised my idle speed to compensate for it---
Thread Starter
Drifting
Joined: May 2007
Posts: 1,409
Likes: 110
From: Chandler AZ
tblu92, thanks......the Z also has a Diablo Predator tune, maybe that explains why the previous owner had both the under-drive pulley and the tuner, as your explanation explains.............
