Last month we explained how our pal Frank Wallic rebuilt a McCulloch supercharger for his roadster’s Flathead. We also stated that we were going to divulge the information we gleaned from our sources. Though the ’37 installation manual, parts listings, and brochures don’t account for a lot, they increase the amount of information currently circulating about McCulloch blowers by a log scale.

Before we start, bear in mind that McCulloch designations seem willy-nilly. The models usually designated 37 sometimes have 78 stamped on their tags instead—78 being the model designation for the 85-horse ’37 Fords. The year designations also seem to refer to the design date and not the vehicle model year. For example, we’ve seen models with 37 design hallmarks configured for ’35 and ’36 pump-in-head engines in McCulloch literature. And a brand designation doesn’t make a design model specific. For example, in the ’39 models only the ’39 Mercury unit fits the ’39 Ford DeLuxe. It just requires the 85-horse crank pulley for the smaller displacement.


The port adjacent to the oil-pressure sender port feeds the blower with a constant oil supply. The blowers drain by two ways, depending on the model. The ones that mount to Ford manifolds require an 11/16-inch-diameter hole on the driver side between the two leading bolts that mount the manifold to the block. A hose slips over the McCulloch drain, bends down 90 degrees, and presses into that hole. The blowers for the later models drain through a port in the supplied manifold.

McCulloch supplied 1/4-inch copper lines but oil from an unregulated supply will handily overwhelm the return port, fill up the gear case, and gush from the vent under the rotor. The “special fitting” that McCulloch indicated in the 1937 instructions likely has a restricted orifice. But flow metered by an orifice varies depending on oil viscosity, oil temperature, bearing tolerances, and engine speed. So it’s possible to set a level that’s perfect at idle or high speed but not perfect at idle and high speed.

Apparently McCulloch understood this because in the ’39 parts sheet it lists what it refers to as an oil-relief valve, which consists of a plunger, spring, and nut. The name and those parts’ descriptions indicate that the valve bypassed (or relieved) excess oil to the crankcase. Once set, such a valve would better maintain a specific oil level within the blower regardless of engine speed, viscosity, and so on. Unfortunately, small bypass valves are expensive.

Heated Intake Air

Heating the intake air is a good idea on a road-going engine with a wet intake manifold (one that transfers fuel as well as air). Most of us are aware of one benefit: a warm manifold prevents a carburetor from icing. But warm intake air also keeps fuel suspended as a finely atomized mist that more readily burns. It also improves combustion efficiency by accelerating the combustion rate.

McCulloch endowed all but its earliest superchargers with a means to heat the rotor housing. The rotor housing offered in 1937 looks like a hamburger bun with coolant ports on the side. The passenger port connects to a nipple threaded into a bung just below the water neck in the head. The driver’s port connects to a nipple in the upper radiator hose near the radiator or in the radiator itself. That plumbing gives the coolant a low-resistance path around the thermostats.

McCulloch introduced its own manifold and redesigned the rotor housing in 1938. That design plumbs the hot exhaust gases from the crossover ports to a thermostatic-controlled intermediate housing below the blower outlet. The gases then go through the rotor housing.

In 1939 McCulloch reverted back to coolant to heat the rotor housing. The company produced two versions. A Ford-badged one fits all Fords to 1938, ’39 standards, and ’39 pickups. A Mercury-badged one fits the ’39 Mercury and the ’39 Ford DeLuxe passenger cars.

In the Ford-badged models a stubby radiator hose routes all of the coolant from the driver-side head to a neck on an intermediate housing that fits between the manifold and blower. The coolant ascends into the rotor housing and exits to the radiator by a large port cast in the top of the driver-side rotor housing. The high outlet means the Ford-marked blower won’t fit the ’39 Mercury or Ford DeLuxe. The passenger radiator hose fits as Ford intended.

The Mercury-badged design is a bit more sophisticated. The passenger head’s water outlet feeds one leg of a two-branch iron elbow. That feeds one side of an aluminum intermediate housing between the manifold and rotor housing. The other side of that intermediate housing returns the coolant to the second leg of that two-branch elbow, which connects to the upper radiator hose. To the best of our knowledge the thermostat mounts between the second elbow and the radiator hose.

We say that because the thermostat can’t mount where Ford intended it, on the head’s water outlet. A stem descends down into the passenger head water jacket. This stem feeds coolant to a fitting cast into the passenger-side rotor housing. According to the parts list, the fitting on the driver-side rotor housing feeds the coolant to a fitting threaded into a hole drilled in the driver-side water pump inlet. For what it’s worth, this is a potentially good return path for the earlier models to follow as the pressure is lower there.

Yes, we are aware that tuners go to great lengths to cool the intake charge in other blown applications but that’s generally an advantage at boost pressures greater than 8 psi. You’d be lucky to achieve half of that with a McCulloch blower, so take advantage of the other properties that a heated rotor housing offers. It does improve performance.