Underdrive Pulleys

[LilYellowZQ8]

Written by Jedi Master.

Underdrive Pulley(s) :
An underdrive pulley(s) reduces the amount of energy lost to parasitic drag from the engine accessories (air condition, altenator, power steering pump, etc) as well reduces the physical weight that the crankshaft must rotate. What this means in reality is that there is less horsepower lost in doing work other than driving the wheels, which inversely means, there is more horsepower at the rear wheels to use.

The crank pulley itself replaces the factory crank pulley, and is otherwise much like a stock part. Or there are sets of 3 styles, one uses just a crank pulley in a polished or black powder coated along with and alt. pulley that may or may not be and an overdrive pulley. Alt pullies in order for them to replicate voltage at idle like stock, must be of and overdrive nature, meaning they are 25% smaller in diameter than the stock one. Then also are the 3 piece using and crank, alt and water pump pulley, these are mostly polished show pieces, but wil come sometimes with a underdrive crank pulley.

Alternators:
How much horsepower does an alternator pull ? You can mathematically determine the horsepower cost at any given load. (Remember, alternators respond to load. If there is no load present the alternator is basically freewheeling.)
Amps x Volts = Watts
Watts / 745.7 (one HP) = Electrical HP Produced by the Alternator
HP x 15% Efficiency Loss = HP Loss
HP + HP Loss = Total HP Used
Example:
57A x 14.9V = 849.3 Watts
849.3 Watts / 745.7 = 1.14 HP
1.14 HP x 15% = 0.17 HP
1.14 + 0.17 = 1.31 HP Total

Water pump:
Most water pumps when they’ve reached operating pressure will use a maximum amount of 6 h.p., depending on size of the system.

Power Steering:
Power steering pumps when its in a no-load situation, not requiring assist, straight line driving will be 1.5 horsepower or less. Under load such as turning, the amount of horsepower required will be 3.8 + or - horsepower max at rpm’s exceeding 5000 rpm’s.

Air conditioning:
Air conditioning compressors when under a no-load, not on, use very little in terms of horsepower, usually under 1 horsepower. Now in and operational load the compressor depending on setting will use up to 25 horsepower and sometimes more depending on the unit.

So using these numbers you can safely calculate the amount of horsepower that can be returned to useage. This can be anywhere from just under 7 horsepower to over 20 running A/C and high powered stereo systems.

[LEADFOOT]

not trying to cut the jedi masters throat,just wanting to correct what he wrote. he wrote; "Alt pullies in order for them to replicate voltage at idle like stock, must be of and overdrive nature, meaning they are 25% bigger than the stock one." the alt. pulley that comes with the set is a little bigger that the stock pulley but it doesnt bring the alt. rpm's back up to speed to compensate for the loss in rpm's due the smaller crank pulley. it slows down the alt. even slower than the change in crank pulley did. in order to bring the rpm's back up you will need to go with a smaller alt. pulley. i bought a set of underdrive pullies from Jet, after i put on my elect.fans. Big mistake. i called Jet and asked them why the alt. pulley was bigger and they couldnt give a straight answer, other than you cant put a smaller pulley on the alt. then i called March who told me that Jet buys there pulleys from March, and he said i can put on what ever pulley i wanted to as long as i dont over drive the alt.( go over the max rated alt. rpm's). so i bought a 1 7/8 inch alt pulley from march and then of course had to buy another belt. then i called Jet and told them that were a bunch of dumb a**es. the new pulley brought up the alt. rpm's almost to stock, but it is the smallest alt pulley you can get and really close to the max alt rpm. needless to say i took the pulleys off about 2 weeks after all that because at the time i drove it for work about 50 to 100 miles a day and in the middle of the summer in FL the a/c is a much needed commodity. the a/c just wasnt as cold unless i was trucking down the highway. 1 last thing if you have elect. fans do not get underdrive pulleys. i even bought a 150 amp alt. from power master and that didnt do anything at idle because if the alt rpm's arent where they need to be to begin charging it doesnt matter how big the alt is. i only shaved 1.5 tenths/sec. off my 1/8 mile time buy putting the u.d. pulleys on and thats with elect. fans. the nitrous i want to install will eat up that time and then some.