Get Blown
In addition to wicked torque production and stunning good looks, one of the biggest advantages of a Roots-style blower is how easily it adapts to a carbureted motor. The Mooneyham 6-71 kit used in this buildup features the blower assembly and an intake manifold. The blower drive assembly is from BDS, and includes the blower snout, blower pulleys, idler assembly, crank hub, and belt. Installing the blower is as simple as swapping intake manifolds, and the carburetors bolt right up to the carb adapter (also from BDS)- no fiddling around with carb hats required.
Speaking of carburetors, although the 400 makes 741 hp, its appetite for fuel goes well beyond that of a naturally aspirated motor of similar power output. Consequently, it utilizes a pair of Holley 750-cfm carbs to keep up with the blower's voracious demands. "The reason you need two carburetors is pretty simple," explains Massingill. "These blowers move a lot of air, and two small carbs improve fuel distribution significantly over one big center carb. With blower motors, it's safer to be on the big side, because you're going to make a lot of torque no matter what. On the other hand, if you're on the small side, things could get ugly."
For pump gas compatibility, the SAM configured the blower pulleys to produce 9 psi of boost, which they say is the sweet spot for a 6-71. "At 8 to 9 psi, an 8-71 wouldn't do you any good," opines Massingill. "The only time you need to step up to an 8-71 is when you want to run 15 to 20 psi of boost. A 6-71 could be set up to make that much boost, but at that level, an 8-71 would make the same boost while turning less rpm, which results in a cooler air charge."
Drop It In
If you can deal with cutting a hole in your hood, then the potential applications for a Roots-blown motor like this are endless. Plunking a big-torque motor into a heavyweight like a GM A-body or a Chrysler B-body would seem like an appropriate course of action, and so much grunt at such low rpm allows for the luxury of running a taller ring-and-pinion set for a wonderfully streetable package. Bench racers will still balk at excessive heat soak, but not everyone's buying into it. "Maybe marketing has something to do with it, but the truth of the matter is that a blower is only producing heat when the motor's under heavy load," says Massingill. "By the time you finish a pass and cruise back up the return road, heat soak isn't even an issue anymore."
Adiabatic Efficiency
Start talking about blowers at the track or at a shop, and it won't be long before the term "adiabatic efficiency" comes up. So what is it? According to the laws of thermodynamics, compressing air creates heat. That means that it's physically impossible for a turbo or supercharger to compress air without adding heat to the intake charge. Consequently, a blower with 100 percent adiabatic efficiency doesn't add zero heat to the intake charge, but generates the least amount of heat theoretically possible. Therefore, adiabatic efficiency is the measure of heat that is actually added to the intake charge, in comparison to the heat added under ideal circumstances. Since blowers generate their own frictional heat on top of that already produced by compressing air (which gets absorbed by the intake charge), 100 percent adiabatic efficiency is nearly impossible to achieve. Turbos and centrifugal blowers can hit upwards of 75 percent adiabatic efficiency, while Roots-style blowers are in the neighborhood of 50 percent. Whether or not it really matters in a street car is up for debate.
Blower Sizing
Common sense says that a 14-71 blower is bigger than an 8-71, and an 8-71 is bigger than a 6-71, but where does that nomenclature come from? The original Roots-style blower wasn't used in cars at all-instead it was designed by the Roots brothers to help ventilate underground mines during the 1800s. It's basic design was adapted for use in diesel trucks by GMC, and was given the 6-71 designation because it was made for a six-cylinder engine displacing 71 ci per cylinder. Hence, an 8-71 features a larger case than a 6-71, since it was designed to feed two extra cylinders.
Mating the carbs to the top...
Mating the carbs to the top of the blower is a billet carb adapter from BDS, which is compatible with all 4150-series Holley carburetors.
The Mooneyham 6-71 has Teflon-coated...
The Mooneyham 6-71 has Teflon-coated rotor tips for improved sealing. When rebuild time rolls around, it can be quickly disassembled by removing these Allen bolts.
To eliminate potential restrictions,...
To eliminate potential restrictions, the Mooneyham intake manifold was gasket-matched for maximum flow.
V-Groove pulleys are bolted...
V-Groove pulleys are bolted to the crank hub to drive the water pump. With enough determination, accessories like power steering and air conditioning can be driven off of them as well.
The BDS crank hub replaces...
The BDS crank hub replaces the factory harmonic balancer. In a Roots-blown motor, the drivebelt and the rotation of the blower are said to effectively cancel out engine harmonics.
While the SAM typically sets...
While the SAM typically sets the air/fuel ratio at 12.5:1 for naturally aspirated motors, it elected to run a more conservative 11.8:1 mixture with the blower.
On 93 octane pump gas, the...
On 93 octane pump gas, the 400 responded best to 32 degrees of ignition advance. MSD offers a small-cap distributor for extra clearance between the distributor and blower (PN 8570).
The stepped 1 3/4- to 1 7/8-inch...
The stepped 1 3/4- to 1 7/8-inch headers were on the small side, and probably cost a few horsepower; however, the motor's going into a '32 Ford, and bigger headers won't fit.
The 7-quart Moroso oil pan...
The 7-quart Moroso oil pan has a remote pan-mounted dipstick. This eliminates interference issues between the dipstick and aftermarket four-bolt main caps.
The 400's blower was underdriven...
The 400's blower was underdriven at 5 percent to achieve 9 psi of boost. A simple pulley sway can crank blower speed up to a 5-percent overdrive.
To maintain proper belt tension,...
To maintain proper belt tension, the idler should be adjusted until there is 1/4- to 1/2-inch of free play on the long side of the belt at operating temperature.
Unlike centrifugal blowers,...
Unlike centrifugal blowers, a cogged belt setup virtually eliminates the possibility of slippage. The absence of a harmonic balancer looks odd, but people have been running Roots-style blowers like this for decades.