Little Roots Of Power

Regardless of how cool the big blowers are and regardless of how much power can be made with them, we still love the little Roots blowers. Based roughly on the original GMC/Detroit Diesel 6-53 series of blowers, Weiand's Pro Street lineup of Roots blowers are definitely the easy ticket to power and cool looks under--or through--your hood. Back in the mid-'80s, Weiand developed these little huffers to meet a new demand for supercharged power from a small package that could almost be stuffed under any hood. The market at the time was street rods and trucks, both of which had ample room under their bonnets to fit a little blower like this. And their installation was made easy by simply installing the correct intake manifold underneath and a drive pulley on the crank. Also coinciding with the design efforts at the time was a big push for street legality with lots of time spent in the emissions lab and on the dyno, proving that these little buggers could add power without polluting the clouds. A California Air Resources Board (C.A.R.B.) Executive Order (read: "smog exemption"), the Holy Grail of smog-legal performance, was granted to Weiand's PN 6500 Pro Street supercharger. And the rest is history.

COST OF POWER
While not as cheap as a nitrous kit, but certainly less expensive than most centrifugal superchargers, these little Roots blowers quickly caught on as "the" blowers to have if you wanted around 600 hp in your daily driver. And the fact that such power could be had legally, the mini-blower craze reached a fever pitch around the '90s. But these blowers have one distinct disadvantage when compared to centrifugals: they're hard to run EFI on. About 99 percent of the time these little blowers are installed with a carb sitting on top of them, which limits their street-legal application to pre mid-'80s cars and trucks. But there are plenty of them out there, so we thought we'd slap together a reasonable stroker Mouse and see how much power we could push out of one of these little things.

BUILDING THE BEAST
To start, any motor that's bound to wear a blower must be built strong or it won't live long. That means a forged crank and good aftermarket connecting rods are suggested for anything over 500 hp and high-quality forged-aluminum pistons should also be considered. Outside of those exceptions, the sky's the limit for everything else, and these little blowers respond well to just about any bolt-ons and performance parts.

One of the coolest things about running a blower is that you can run a lot more camshaft timing along with it. The blower will tend to tame the cam, making the engine idle better, and with more vacuum, too. For this build we chose a COMP Cams hydraulic roller with Isky's HR Rev kit to help us gain a little more top-end power.

Besides the cam, cylinder-head flow can also be increased when running a blower, and typically the bigger the better in this area. Since the blower is trying to pack as much air as possible into the cylinders, you may as well give it every opportunity to do so with a great set of cylinder heads. Ironically, with a big cam and great cylinder heads, you may actually see less boost register on your boost gauge than you would running a small cam and restrictive heads. But that's OK, because the power will be there in lump sums.

FUEL REQUIREMENTS
Fueling a blown street engine is a bit of a paradox. You don't want to run a big race-engineered electric fuel pump because they're not designed for constant use. But you need the fuel flow requirements that the big pumps offer. Luckily, aftermarket companies have addressed the issue by designing hi-flow street electric pumps that can run forever and supply enough fuel for literally any street engine. Typically, a blown engine will need almost twice the fuel than a normally aspirated engine to keep from running lean.

Since the blower needs air to make boost, you also have to install an unusually large carburetor when running a blower. In the case of the single 4 bbl on this 383, we installed a Holley 850 that was worked over by the guys at the Carb Shop to flow in excess of 900 cfm. The carb was also optimized to work with the blower on the street.

POWERFUL RESULTS
When the crew at Speed-O-Motive was putting this engine together for us, there were bets going around as to how much power it would make and whether or not it would live very long. The magic number everyone agreed it would see would be 600 hp, but a few holdouts--myself included--hoped for more like 650 hp. And we were right. On the best pull of the day we walked away with just over 650 hp and 612 lb-ft on a livable 10-psi boost. But note: All this power was made on Shell Performance 105-octane unleaded fuel, not on pump gas. Lower boost pressures around 5-6 psi could probably live on 91-octane, but anything over 10 psi from these high-winding little Roots blowers, and you'd better run some good juice. We also used Quaker State's Q-Racing synthetic motor oil to make sure the higher oil temps associated with running extended boost would be easily managed. Just for kicks, we also ran the engine on regularly available 91-octane pump gas, limiting top rpm to just 5,000 to keep blower temps down, and still managed 562 hp and 556 lb-ft at 6-psi boost. Not bad for a street-worthy small-block on pump juice.

THE BEST--AND WORST--OF THE BUNCH
While we did make some pulls on 91-octane, they weren't nearly as fun as the big power we made on the good 105-octane Shell unleaded. Here's some figures from the dyno using Weiand's 7-inch lower drive pulley and 3-inch upper blower pulley, giving us a 2.33:1 overdrive ratio (233 percent), which means that at the 6,800-rpm horsepower peak, the little Weiand huffer was spinning 15,844 rpm! One of the best things about this is that the high blower speed makes tons of low-end torque. In fact, when we sped up the blower even faster on a following pull we saw a 3 percent increase in power at 3,000 rpm running Weiand's smallest 2.85-inch blower drive pulley (2.46:1 overdrive). But at the top end of the run, with the 2.85-inch pulley installed, the engine made 12.5-psi boost and over 200 degrees intake manifold temp on its way towards serious detonation. Also, when we first installed Weiand's smallest 2.85-inch pulley, all the blower belt did was slip at higher rpms so we had to swap-more like force-Weiand's shortest drivebelt on and got one good pull out of it before the intake manifold temps skyrocketed and we shut it down for safety.

POWER RUN
2.33:1 overdrive Weiand 177ci Pro Street blower, 105-octane Shell unleaded, 383-cid Chevy tested on DTS dyno at Speed-O-Motive and Vrbancic Brothers Racing.