Nothing quite approaches automotive nirvana than the act of bolting a supercharger to a Chevy engine. While some would argue, especially on the Internet, that running a blower is "cheating" and not as pure as making big power from a naturally aspirated mill, they are missing some of the huge benefits of superchargers.
As the name implies, superchargers increase the atmospheric pressure inside the combustion chamber. More pressure equates to more air and thus more power. Sounds simple, but there's more to it.
In a naturally aspirated engine, the intake valve opens and the downward stroke of the piston creates a partial vacuum inside the cylinder. The differential between the cylinder and atmosphere causes air to move into, or "charge," the cylinder. A supercharger increases the pressure inside the intake manifold so that the pressure difference between it and the cylinder is even larger. This causes a larger mass of air (air charge) to enter the cylinder. More air means that more fuel can be burned, and the result is what we all crave-more power!
A highly refined version of the classic Roots-style supercharger is this Magnuson unit. Instead of having nearly straight lobes, the rotors are twisted into a steep helix shape. While still commonly referred to as a Roots-type unit, it's actually more of a hybrid of the Roots and axial designs. Instead of air being drawn in through the top like a typical Roots unit, it's drawn in through the inlet port and pushed into the engine along the rotors' axis. Thanks to a vacuum-referenced bypass valve, the inlet pressure is equalized with the manifold pressure. This pretty much negates any parasitic loss while cruising and makes for a very efficient design. The Magnuson can also be run with or without an intercooler, but it will make more power and run on lower-grade fuel with the intercooler. The newer Magnuson units feature the improved Eaton TVS four-lobe rotor arrangement.
The compression added to an engine by a supercharger is measured in pounds per square inch (psi) and is referred to as "boost". If a supercharger is putting out 10 pounds of boost, then it is pressurizing the engine's intake to 10 psi over atmospheric. Most superchargers found on production cars are in the 4-12-pound range, while all-out race applications can put out 20, 30, or more pounds of luscious boost. The limiting factors are money and the engine's ability to stay in one piece under the strain. On a normal gasoline-fed engine it's common to see a 50 to 70 percent increase in torque from a supercharger putting out around 7 psi. And that's part of the allure of superchargers: big gains from a bolt-on system without the need to ever fill a nitrous bottle.
Not long ago that, the Roots design was considered the least efficient of the three basic supercharger types. Today there have been huge engineering advancements, specifically by the Eaton Corporation. Its new TVS superchargers are very efficient compared to older models. The newest design, introduced for the superchared LS9 in the '09 ZR1 Vette, features a four-lobe-per-roter design compared to the typical Roots arrangement of three lobes. Like other Roots systems, the supercharger is especially good at delivering boost at low engine rpm.
But why add on a supercharger instead of just building a bigger engine? The simple answer is efficiency. Engines are great at turning high-pressure gas into rotational energy, but are somewhat limited by atmospheric pressure, which is constant. One way to make more power using the Earth's atmosphere is to increase the size of the cylinder. If you can cram in more atmosphere, then you can make more power, even with a smaller cylinder. Using a supercharger makes an engine react like it has way more cubic inches, and that's a very good thing indeed.
There are several different types of superchargers on the market today: axial-flow, screw, centrifugal, and Roots. Axial-flow compressors consist of rotating, aerofoil-shaped vanes. Since this type is typically seen inside jet engines, we'll skip it for this discussion, though we've always wanted to build a jet-powered Chevy. Then again, who hasn't?
Inside the LS9's supercharger and the newer Magnuson TVS units is a new four-lobe rotor arrangement that's a big improvement over the previous three-lobe design. Where the twist of the rotors on the older Gen V units was 60 degrees, the new TVS rotors are at an aggressive 160 degrees. Also, the lobes' intermeshing design helps quiet the supercharger, even at high rpm. In fact, the new Gen VI four-lobe blower design provides up to 10 dBa less case-radiated noise than the Gen V blower. The rotors are coated, and when spun up the first time they "self-machine" for extra tight tolerances.
The Roots-type blower has been around a long time, nearly 150 years, and was patented in 1860 by Philander and Francis Roots. The brothers didn't have any Chevys to bolt their invention to, so they used them to ventilate blast furnaces, coal mines, and other places that needed air pumped in. The Roots isn't a compressor, but rather a blower. Air compression doesn't happen inside the supercharger. Instead, it occurs between it and the intake valves. The Roots unit blows more air into the intake than the engine can flow out, so that air compresses.
Nothing makes a bolder statement than a 6-71 or 8-71 blower bolted to the top of a Chevy mill. Even though they've been around for a long time, these Roots-type superchargers continue to be refined and enhanced. Weiand claims to have improved the boost on its units by 14 percent. Its two-lobe rotor design features forged rotor shafts, and Weiand has completely revised its case design and beefed up the front bearing plate for more strength. These work amazingly well on stroked and smaller-displacement mills.
In a twin-screw-type supercharger, compression happens inside the blower. Air is pulled through a pair of intermeshing screws designed much like worm gears. The intake is positioned on the end of the two screws, which are designed to overlap, but not touch, leaving a small gap or pocket. As the screws turn, the gap gets smaller. This compresses the air as it moves along the screws up to the point where it enters the intake. Due to the lower charge temperatures of screw superchargers, they can easily run without an intercooler in lower boost applications.
A centrifugal-type supercharger operates nearly identically to a turbocharger. The main difference is that expelled exhaust gases drive a turbo via a turbine, while a centrifugal supercharger is driven from the crankshaft by a belt or gear. As with any centrifugal pump, the boost created by the supercharger increases with the square of the rpm. That's why this type of supercharger creates very little boost at low engine speeds. In the past there's also been a problem with this type of supercharger making too much boost in the upper ranges in addition to too little down low. Balancing top end and low-speed boost has been improved over the years through better impeller design and unit sizing. The centrifugal design is considered by many to be the most efficient even if it loses out in its ability to produce boost at any rpm like the Roots or twin-screw superchargers. Given its lack of low-rpm boost, it's a popular bolt-on for cars that have engines big enough to provide good off-the-line acceleration without it. If it's bolted onto a smog-regulated later-model car, it's also pretty easy to remove for annual inspections compared to a turbo-not that we would ever condone such a thing.
Today supercharges are more popular than ever, and there's a huge array of bolt-on kits to choose from. Even with gas prices rising fast, they're the hot ticket since they offer the best of both worlds: more power and improved mileage.
The aftermarket has really responded to the demand for easy-to-install boosted power at an affordable price. Weiand offers its 142 Roots-style superchargers for a variety of small-block engines. Its units feature a CNC-machined two-lobe rotor design that produces 3 to 6 pounds of boost, but can be adjusted up to 12 pounds just by changing the pulley.
Edelbrock's new E-Force supercharger kit is a positive-displacement draw-through unit that it claims is capable of boosting power to over 500 horsepower when bolted to a 350 small-block with 9.5:1 compression (8 psi). The unit is filled with Eaton internals and comes with everything you need to bolt it to your ride fairly easily.
Centrifugal belt-driven units, like this ProCharger D-1SC unit, don't make the low-rpm power of their Roots-type cousins, but they generally deliver a cooler boost charge, which allows more power to be made before an intercooler becomes necessary. Their design allows them to move more air with less of a heating effect on that charge. As engine rpm increases, so does the amount of boost pumped out. This is one big reason why they are so popular when the goal is strong mid and top end power.
When you move beyond the typical bolt-on, low-boost supercharger, other areas of the car need to be addressed, particularly the heads and the exhaust system. High-flowing heads and an exhaust system capable of expelling all that atmosphere you're pumping into the engine will help get the most out of your supercharger.
This GM HT383 crate engine was fitted with a Vortech S-trim centrifugal blow-through supercharger system. The Vortech added 200 hp and 120 lb-ft of torque on the engine dyno. Typically these systems can produce gains from 30 to 75 percent, depending on boost levels and volumetric efficiency. This blow-through design encases the 4150-type carburetor inside an aluminum enclosure. According to Vortech, "this results in a consistent fuel curve for more power than an ordinary 'bonnet.'" The enclosure also features front and rear -8 AN fuel ports as well as multiple locations for pressure and vacuum connections.
Typically, non-intercooled superchargers putting out 8 pounds of boost operate with intake manifold temps in the area of 115 to 200 degrees above the outside air temperature. Add an intercooler and that can drop to as little as 28 degrees above ambient. Cooler air is denser air, and that equates to more power. Boyle's Gas Law states that the compression of air will always create heat, and heat limits both how much boost can be employed, the density of the air charge, and even how much timing you can run. The intercooler also acts as a passive wastegate, flattening the boost curve at higher rpm and allowing more boost to be dialed in at lower rpm.
Another way to cool the intake charge is through "chemical intercooling," like what's offered by Snow Performance. Its Boost-Cooler water/methanol injection systems help suppress detonation so more power-producing boost and timing can be used. Water, with its high latent heat of vaporization, cools the intake charge and combustion. Methanol cools the charge and combustion, but also acts like an extremely high-octane fuel as well as adding more oxygen to the combustion.
For those who like to kick it old school, Magnuson offers this slick Gen 1 Classic unit. And while the name may say classic, the internals are top-shelf Eaton Gen V components. Unlike Magnuson's other blowers that push air along the axis of the rotors, this unit draws atmosphere down through the carb and into the intake more like a traditional Roots blower.
Even today, the Roots-type blower is still the way to go in the top drag racing classes. When bolted on top of a huge-cubic-inch engine, the sky's the limit in regard to power, with 8,000 horsepower not being uncommon. At that power level it can take around 500 hp to spin the blower when it's making maximum boost.
Blow-Through Vs.Draw-Through Carb
In a draw-through application, typically on a Roots-style blower, the carb is located on top of the blower. As the blower operates, air is pulled, or drawn, through the carburetor and into the engine. The carburetor works about the same as on a naturally aspirated engine with the exception that its power valve has to be referenced to the intake manifold pressure. This makes tuning relatively easy. The main issue with this type of system is packaging in regard to hood clearance.
Centrifugal supercharger systems employ a blow-through system. Here the blower creates pressure before the carburetor and air is forced through the carb and into the engine. While it's a snap to package this type of system under just about any hood, the downside is setting up the carburetor properly. Setting up a blow-through carburetor takes quite a bit of skill. If you go this route, then the best idea is to find a company that has tons of experience dialing in blow-through carbs. (See the story by Dan Ryder and Mike Harrington in the next issue for more on building a blow-through carburetor.)
The Whipple Supercharger
While similar in appearance to a Roots-style supercharger, the Whipple supercharger really belongs in its own category as a screw-type compressor. The housing on a screw-type supercharger will have an opening at the top back (inlet) and lower front (outlet). As the screws turn, air is trapped in the lobes and pushed forward, compressed, and moved into the engine.
A screw compressor is a positive-displacement unit that uses a pair of intermeshing rotors to produce compression. The rotors, comprised of helical lobes, are affixed to a front and rear shaft. One rotor, called the male, will typically have three lobes. The other rotor, the female, has valleys machined into it that match the curvature of the male lobes. Typically, the female rotor will have five valleys. The rotors never touch, but are timed by a pair of gears operating in a lubricated chamber that's separated from the rotor chamber. With the three/five rotor combination, the male rotor turns three times to every one time of the female rotor.
The number of lobes on the male and female rotor will vary from one compressor manufacturer to another. However, the female rotor will always have numerically more valleys than the male rotor has lobes. Because of the number of male lobes, there are three compression cycles per revolution, which means the resulting compressed air has small pulsations compared to a reciprocating compressor. By the time the compressed air leaves the package it is, for all intents and purposes, pulsation-free.
Too Good For Its Own Good
Back in the mid-1980s Norm Drazy worked his butt off to develop a large screw-type supercharger. His invention utilized a four-lobe male rotor and a six-lobe female that turned 30-percent slower. His unit was called the PSI, and Norm was hoping to introduce it to Top Fuel dr ag racing. Like the Roots, the PSI fills from the top and discharges at the bottom; but unlike the Roots, the PSI employs internal compression. This is why the PSI requires less horsepower to turn than a traditional Roots supercharger.
The PSI on a little 385ci engine made 1,260 horsepower on alcohol at 28 pounds of boost. This was the same power as a 14-71 unit made on a 552 cubed engine running 36 psi. Not only that, the outlet temp was a comparatively frosty 85 degrees compared to 150 degrees for the 14-71. It was all sunshine and rainbows until the NHRA banned the PSI in Top Fuel. The PSI, and the new larger Whipple screw compressors, did find a home in Top Alcohol drag racing, where they have become standard fare.