I just started a new project which consists in removing the AC compressor and replacing the heavy stock harmonic balancer / crankshaft pulley with a lighter chromoly pulley that was developed by TTV Racing for a Duratec project with Ford. I trust it’s light but not too light as some alloy pulleys might be.
The pulley has a single 6-groove race and drives the accessory belt approx. 20% slower than stock, so the alternator and water pump will rotate that much slower and induce less drag / less power losses, and less rotational inertia. The crank pulley alone is about 1.4kg (3lbs) lighter than stock.
I have to convert the entire serpentine path to the new 6-groove belt which means new idler pulley, new tensioner pulley, and new alternator pulley. Curiously, the water pump already has a wide pulley but I’m replacing it with a light alloy version.
The AC compressor is a heavy beast and the stock crank pulley is quite heavy too, so together with the condenser and some piping there will be a gain of at least 15kg or so, including approx 2kg of rotating masses between the lighter crank pulley and a new light water pump pulley as well as the removal the AC pulley and its belt. That’s significant as far as inertia, as the crank pulley obviously rotates at engine speed while the water pump and AC pulleys rotate even faster. There is also a general theory about cavitation in water pumps which may or may not be an issue on the EcoBoost, but slowing down the pump gives some headroom for more RPMs.
Together with the reduction of rotating masses, the underdriving also free up some actual ponies. So it’s free horsepower and less engine inertia, to accelerate the car harder, and a bit less “regular” weight too, by removing some of the AC components. The main drawback is less comfort during hot days.
I’ll post the part list once the conversion is sorted out.