McCulloch made its own crank and water pump pulleys to drive its superchargers with three belts. In 1939 it offered three crank pulleys: one for short cranks to 1938, a second for ’39-and-later Ford passenger cars and light trucks, and a third for ’39-and-later Ford heavy trucks and Mercurys. Darr’s literature shows two blower ratios: 1:6 for 221ci Ford engines and 1:6.6 for heavy-truck/Mercury 239ci engines.
At least in 1939 McCulloch offered two water-pump pulley types: one bored to fit 5/8-inch-diameter shafts and another bushed to fit 1/2-inch shafts (1937 and 1938). The ’35 and ’36 pulleys are exclusive to those years.
Until 1938 all McCulloch-driven pulleys have three sheaves and accommodate the cooling fan. Those models drive the generator from a separate pulley pinned to the driveshaft where it protrudes from the rear bearing retainer.
In 1939 McCulloch eliminated the separate generator pulley, relocated the generator forward on the driver head, and drove it from the rearmost sheave on a four-sheave driveshaft pulley. It offered two versions of that pulley. One had a cooling-fan mount for all Fords to 1938, ’39 Ford trucks, and ’39 standard Ford cars. The driveshaft pulleys for the ’39 Mercury, ’39 Ford DeLuxe, and all ’40 cars and trucks lacked fan provisions as the crank pulley on these cars drove the fan.
As rare as they are, McCulloch blowers outnumber the pulleys. So many enthusiasts like Wallic improvise with two-sheave crank and water-pump pulleys from ’37-41 heavy trucks.
Multi-belt drives present a unique problem: their lengths must match. That’s nearly impossible to achieve but Wallic discovered the solution: banded V-belts.
Banded belts are flat belts made of multiple conventional V-belts. Simply order a belt with the number of V-belts at the length the application requires and cut the belts apart. Voilà, multiple belts that match exactly!
The superchargers that McCulloch produced until 1937 fit the stock Ford intake manifold. The supercharger case is unique unto itself. It mounts to the carburetor pad and the generator boss.
Some very early product literature of the kits intended for pump-in-head engines don’t acknowledge belt tensioners but photos reveal what looks like a slot in the bracket that mounts to the generator boss on the Ford manifolds. That suggests that the whole blower assembly moved vertically to set belt tension.
That of course raises an issue with the space between the rotor housing and the intake manifold; however, the installation paperwork for the ’37 blower explains the role that fiber shims play to take up the gap between the rotor housing and the intake manifold. We know for sure that the ’37 blower didn’t move to set belt tension. Instead the ’37 models (even the ones made that year for the earlier pump-in-head engines) used a belt tensioner that bolted to the timing cover and pressed against the inside of the belts. So the shims existed on those exclusively to compensate for manifold manufacturing variances.
McCulloch delivered blowers with its own manifolds from 1938 onward. That model features an integrated belt tensioner and employs exhaust from the crossover ports to heat an intermediate manifold between the blower and Ford manifold and the rotor housing. It eliminated the exhaust-heat provision in 1939 when it revised the rotor housing to once again use coolant for heat.
McCulloch deemed the stock single-exhaust system inadequate and recommended giving the left exhaust manifold its own pipe and muffler. In fact the company offered a kit to do just that for $14.95 (a bargain even when adjusted for inflation: $249.70).
Supercharging accelerates combustion rate therefore a supercharged engine doesn’t need/can’t use as much spark advance as a naturally aspirated engine. At full advance the stock ignition sparked each cylinder at about 22 degrees before top dead center. McCulloch recommended limiting total advance to 16 degrees BTDC and setting final timing on a car-by-car basis.
According to Darr’s service-data sheets, McCulloch offered special distributor springs. Presumably these springs were stiffer to cause the brake to engage and retard the timing and stave off detonation at lower manifold pressures. McCulloch deemed the remainder of the ignition system, spark plugs included, adequate for supercharging.
McCulloch referred only to the 48-series Stromberg carburetor in the ’37 installation manual. It recommended 47 jets at sea level and 45s at high altitude for the 48. It made no other distinctions for the remainder of the fuel system.
Wallic says he likes the twin 97s on the Speed Gems’ slingshot adapter for their eye candy but admits they’re tough to package (these blowers perch carburetors pretty high as it is). Truth be told an early McCulloch probably can’t use much more than half of what two carburetors can supply.
Blowers like these have a fixed capacity, in this case enough to support about 120-150 hp regardless of an engine’s size or tune. A larger, single carburetor (think luxury cars from the ’40s) can flow enough to support that output. It may require an adapter but it needn’t be thick.
Intake charges forced past intake valves at greater velocity tend to overwhelm the valve springs. Supercharging also generally lets the engine operate at speeds faster than stock. For that reason McCulloch recommended valvesprings at least 25 percent stiffer than stock.