The Underdrive Pulley Project
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.
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Yes you are correct, the OE setup is friction-based. There is no proper aftermarket crankshafts that I know of, but you can find modified OEM crankshafts with a machined keyway.
For example, Massive Speed Systems sells modified OE crankshafts with a 3mm keyway. I think SP63 resells them, or maybe they make their own. Any industrial machine shop can machine a keyway at TDC in the OE crankshaft, and into the oil pump drive gear, really. This is very common machining.
Callies sells a heavily modified OE crankshaft with a laundry list of machining and surface treatment steps. The most distinctive features are lightening (-4.5kg / -9lbs) and stress-relieving as well as precision re-balancing after their machining.
You can buy it directly from Callies, and they are resold by at least Mountune USA and Panda Motorworks. That is the crankshafts I have in my two engines.
My pulleys are naturally designed for the ISO R773 square 3mm keys that are used by both Massive and Callies, and keyed is the preferred way but they can be used on unkeyed setups provided a Ford 1S7Z-6378-AA crankshaft pulley friction washer is added. The washer is mandatory without a key, as my pulleys don’t have the fine serations found on the OE pulley.
Not all cars have washer-less serated pulleys though: the workshop manual says the friction washer must be replaced “if fitted.”
Below is the serated (and modified/keyed) OE pulled that I’m replacing.
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