Roots-Style Blower - Torque Monster

There's nothing pragmatic about a Roots-style blower. It's a big, bulky monster that requires hacking up the hood. It's also a technological simpleton, predating Larry King. Worst of all, it's trounced by turbos and centrifugal superchargers, in terms of efficiency. But its drawbacks notwithstanding, when it comes to visual intimidation, the Roots is king. Sure, that's not the most practical reason to drop one on your motor, but if you want practical, go buy a Honda and move into the slow lane. Aesthetics aside, the Roots' real claim to the throne is its simplicity and ludicrous torque production.

While turbo eggheads are throwing around adiabatic efficiency figures on Internet message boards, Roots-blown motors are belching out torque curves as flat as Kansas. Want proof? How about 741 hp and 621 ft-lb from a pump- gas-burning 400ci small-block Chevy? Furthermore, although its peak torque of 621 ft-lb arrives at 5,300 rpm, the 400 produces 95 percent of that torque at just 3,600 rpm. Built by the School of Automotive Machinists, this 400's broad power delivery equates to an average of 587 hp from 3,600 to 6,500 rpm. Throw it in a jail cell with your average naturally aspirated big-block, and there's no question who'll end up on top. Succinctly put, this is a combo worth emulating, and here's how.

Short-Block
Since forced-induction motors must endure extreme cylinder pressure, the SAM didn't mess around with questionable components when assembling the bottom end. At the core is a rugged GM Bow Tie block, to which four-bolt billet-steel main caps were added. The rotating assembly consists of a forged 4340 Lunati 3.750-inch crank, Childs & Albert steel rods, and custom JE 7.6:1 compression pistons. That compression ratio may seem low, even by blower motor standards, but the SAM Founder Judson Massingill says it was the right call for this application. "With a Roots-style blower, the motor's going to have throttle response up the wazoo anyway, so we kept the compression low to keep things safe," he explains. "Having such immediate boost definitely offsets the lower compression, and it's better than having too much compression, since it allows running more boost safely on pump gas."

Just like a production 400 small-block Chevy, the SAM's motor features a 4.125-inch bore and a 3.750-inch stroke. While many engine builders favor oversquare dimensions, it was purely coincidental in this instance. "With a blower motor, you don't need a lot of displacement, and we felt that 400 ci was a good compromise between longevity and engine size," says Massingill. "With a longer stroke crank, the pistons would be pulled farther down the crankcase than we would have liked." Another deviation is the use of 5.850-inch rods in lieu of the more common 6.000-inch units. The idea was to maximize compression height to make room for thicker ring lands, which improve durability in the event of detonation. The shorter rods also allowed for the lowering of the wrist pins in the pistons, increasing space for the deep 30cc dish necessary to achieve the target compression ratio. One drawback of the 5.850-inch rods is that they required shorter crank counter-weights in order to clear the piston skirts. Consequently, a few extra slugs of heavy metal were needed to balance the rotating assembly.

Camshaft
Generally, blower cams aren't all that different from naturally aspirated cams, except for wider lobe-separation angles and modest increases in exhaust duration. For this blown 400, the SAM opted for a COMP 250/261-at-.050 mechanical-roller cam with .630-inch lift ground on a 110-degree LSA. While the duration and lift specs aren't out of the ordinary, the LSA is rather tight. "Most blower cams have 112 to 114 degrees of LSA, so in comparison, we're running a lot of overlap," says Massingill. "What we learned from building power-boat motors is that increasing overlap allows more unburned fuel to escape through the exhaust port at part-throttle cruising. This helps cool the exhaust valve, which improves reliability on the street. In a naturally aspirated motor, a 110-degree LSA might choke the motor up top, but we can get away with it on a blower application, since we're not turning a lot of rpm."

Cylinder Heads
Blowers and large displacement motors exacerbate the airflow demands placed upon a cylinder head, so choosing the right set was critical on the blown 400. The SAM started with a set of AFR 220cc heads, and opened them up to 230 cc. The port work yielded 335- and 235-cfm of intake and exhaust flow, respectively, at .700-inch lift-more than enough to feed the hungry mill. O-ringing isn't always sensible for a street motor, so the AFR's 3/4-inch-thick deck surfaces go a long way in averting blown head gaskets. Achieving the target compression ratio required laying back the combustion chambers to 84.5 cc, so that the AFR's had plenty of meat to facilitate those dimensions.

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.