Engine Parasitic Drag

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The '05-'10 Mustang GT is fitted with a two-speed electric cooling fan as standard, thus sparing the engine from the burden of driving a HP-sucking mechanical cooling fan. However the remaining belt-driven accessories (water pump ~7rwhp, alternator ~4.5rwhp, PS pump ~3rwhp, freewheeling AC compressor pulley ~1rwhp, and tensioner/idler pulleys ~0.5rwhp) mounted on the front of the engine still absorb as much as 16rwhp @ 6000rpm.
That 16hp could be recovered by removing the drivebelt altogether and running an electric water pump, but that is only practical in drag racing where the vehicle only has to run in a straight line for a 1/4 mile. For every other form of racing or for street use, the drivebelt needs to remain in place but fortunately there are other ways to recover most of that lost HP.

Steeda Underdrive Pulleys

An underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such as a water pump pulley) that is designed to drive a vehicle's accessories at a slower rate than stock. Underdrive pulleys are usually marketed as a performance enhancing item that increases the torque and horsepower output of an engine by reducing parasitic drag caused by belt-driven accessories. The gain in horsepower increases with engine rpm, thus making underdrive pulleys a very effective upgrade on higher revving engines.
Steeda's SFI approved underdrive crankshaft & water pump pulleys for a 2005-10 Mustang GT slow down accessory speed by 25% which results in a gain of ~9rwhp & ~8rwtq. Since this is accomplished without any adverse effects on the Mustang's charging system and engine cooling at idle speed, it's an appropriate upgrade even on a daily driven vehicle.

Steeda Mustang Underdrive Pulleys (05-10 GT)

Electric Water Pump

The mechanically-driven water pump is the biggest HP thief, robbing the engine of 6.7rwhp @ 6000rpm (>40% of the total HP loss). Underdriving the mechanical water pump by 25% reduces the losses to just 2.8rwhp @ 6000rpm. Those remaining losses can be recovered with the installation of an electric water pump.
The Meziere WP346 electric water pump (constant flow of 55 gallons per hour) has an idler pulley and allows the use of the stock drivebelt. However if a Steeda underdrive crank pulley has been previously installed, a shorter drivebelt (Gates K060990 99.08" belt v 101.52" OEM) is required. The combination of Steeda UDPs and Meziere electric water pump recovers 11.8rwhp @ 6000rpm. That's roughly 75% of the HP you'd gain from running no drivebelt at all but without any of the downsides!

Reduce Parasitic Drag

Electric Power Steering System (EPAS)

The stock hydraulic PS system comsumes a maximum of 2.7rwhp with the stock crank pulley and 2.0rwhp with an underdrive crank pulley. This can be recovered by installing a Ford Performance electric steering rack (part no. M-3200-EPAS) with a Motorcraft wiring harness (part no. WC-96240, CR3Z-14300-EA). The stock hydraulic PS pump, fluid reservoir, and hoses are deleted thus decluttering the front of the engine compartment. A shorter drivebelt (6PK2240) is also required.

EPAS testing on 07GT