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

 

In other words, you are compromising the reliability of your car for less than 7 HP. Because you can always turn the AC off, and most newer cars automatically turn off the AC at WOT. An underdriven power steering pump will compromise the safety of the car during a sharp maneuver by unexpecdely reducing assist. And the alternator will only draw as much power as it needs, and it needs all it draws.

 

i heard if you get underdrive pullys, your battery will go dead, your car will overheat, and crap like that... idk though

 

Waste of money for the small gains if any. The MFG claims of HP gains are made on a engine dyno with no drivetrain loss factored in.

 

For all those "gains", you still are not making any more power at the crankshaft.

That's one thing I still like about the old days. They actually BUILD UP engines rather than try to take things away to "free up" power.

I'll stay with EFI, though.