The amount of boost you can run is directly related to an engine's static compresssion ratio. When the boost is combined with the compression ratio, the result is the effective compression ratio. Typically, a 5- to 8-psi boost range (usually produced with the supplied pulleys in blower kits) will work fine for compression ratios in the 8:1 to mid-9:1 range (operating on 91/92-octane fuel). However, this will ultimately depend on other modifications to the car, manual or automatic transmission, gearing, operating temperature, vehicle load, and altitude. If detonation is encountered it can often be controlled with boost retard devices or by experimenting with different-sized pulleys.
Choosing a carburetor or fuel injectors is a crucial step when building a blower-specific engine, because under boost the engine will often need 40-50 percent more fuel and air. Unlike a normally aspirated engine that may suffer only low power from poor fuel delivery, a supercharged engine without enough fuel under power may run extremely lean and destroy the engine. Running too small a carburetor also means that you can't flow enough air to produce maximum boost.
Because more fuel is required to feed a supercharged engine, the fuel-delivery system must be considerably improved. This means large fuel lines of AN-8 or bigger, properly selected and installed fuel pump(s), an adequately designed tank, full flowing filters, and a correctly wired electrical system to operate the fuel pumps.
On a blow-through supercharger system, the carburetor can either reside in a pressurized box or utilize a special carburetor hat. Under boost the false atmosphere (pressure being blown into the carb) requires revamping many of the original carburetor designs to properly supply fuel. A blow-through carburetor generally features sealed caps on the metering blocks, the main well, and the idle well. These carburetors typically feature only annular boosters so that as the signal gets stronger more fuel flows into the engine. As boost is increased by each psi, fuel pressure must too be increased at the same rate. To do this, a special regulator is referenced to boost pressure and raise or lower the regulated fuel pressure, depending on demand.
Reverse-rotation superchargers are generally used in applications where there are fitment issues or on engines designed to spin opposite of most other engines. Fitment issues arise when the area behind the belt driveline is impacted. Examples are 32-valve cylinder heads and exhaust manifolds. In this case the supercharger is mounted in front of the belt line. Consequently, the supercharger must now be rotated in a reverse motion, in which case manufacturers design the inner components of the blower as a mirror image of a standard unit.
Ignition Systems with a Supercharger
On any blown engine, high-performance ignitions are required primarily to provide adequate spark at higher-than-normal engine pressures and speeds. Additionally, it is often a good idea to run spark plugs that are one to two ranges colder than normal. Rule of thumb: the more boost, the colder the plug required.
One of the most important concerns with any supercharger installation is detonation control. This is because under acceleration, detonation can damage the piston ring lands (or other worse yet, damage rod bearings, destroy pistons, or blow head gaskets). A handy device to counteract most detonation problems is an ignition system with a boost-retard control.
Ignition timing is especially critical with a supercharger to not only keep detonation at bay, but also provide good power. For most applications, the distributor should have a centrifugal advance mechanism set up so that the entire advance is in by 2,500 rpm. Typically, 34 degrees should be a safe level of ignition lead to provide close to optimum performance.
There is a lot of power to be gained with a supercharger. The biggest problem, if you can call it that, is selecting the right unit. Although there's little guesswork, and every manufacturer can direct you to a system that'll incorporate all of your needs. Detailing some of the differences is just the beginning, and if you've never experienced a blown car yourself, it's definitely something to be had. During our research for this story, we had the opportunity to ride in Mike Fossati's '64 Chevelle (equipped with the featured 8-71). The power and acceleration was absolutely unnerving. Nailing the throttle produced a hair-raising jolt, and besides making lots of power the supercharger sure pulled a crowd when we had the hood up.
Depending on size and design, centrifugal blowers are capable of significant power increases, and since they're relatively compact (compared to the massive positive-displacement design) they fit under the hood of just about any vehicle.
The amount of observed boost on a gauge can vary substantially. This photo was taken with the throttle closed. Boost is measured when the throttle is open. If the observed boost comes up on the low end, it may mean the engine breathes very well. Another factor that can contribute to low boost is a restricted air inlet or too small a carburetor.
Since adequate fuel volume and pressure are critical to a supercharged engine, it's a good idea to utilize a fuel pressure gauge.
This Magnuson supercharger fits well under the hood of a '02 Corvette. The inlet is forward-feeding on this fuel-injected unit, so it incorporates a rear-mounted auxiliary drive that turns the supercharger's rotors. Inside this engine is a special Lingenfelter camshaft and Lingenfelter CNC'd ported heads, and underneath the blower is an air-to-water cooler to provide a denser air/fuel mixture. The end result is 508 hp to the rear wheels!
This new Vortech blow-through supercharging system (see page 54 for our test) for carbureted small-block applications is a bolt-on package that increases horsepower up to 75 percent. The kit provides a straightforward integration with factory-serpentine or V-belt accessory-drive configurations and fits nicely under the most stock hoods. Options include the Vortech V-1 T-Trim, V-2 SQ S-Trim, or the super high-output V-7 YSi-Trim compressor. The carburetor enclosure allows the use of Holley, Mighty Demon, or Quick Fuel carburetion. All belts, pulleys, and brackets are included.
As supercharger boost is increased, more fuel is required. This fuel-management unit (FMU) signals boost pressure and, on demand, allows more fuel pressure to pass from the pump in proportion to boost pressure. This one is from Vortech Engineering and is available in 4:1 through 12:1 ratio increases. As an example, a 4:1 FMU increases fuel pressure 4 psi for every pound of boost.
This small bypass valve (installed between the supercharger and the air throttle body) allows this Magnuson blower to operate efficiently, in terms of economy and parasitic power loss. A vacuum-actuator control unit operates the valve. When vacuum is high (idle or cruising) the actuator opens the bypass valve, equalizing the vacuum pressure throughout the system. When boost is needed during acceleration, the vacuum is decreased and the bypass valve instantly closes, allowing pressure to increase in the cylinders.
This controller allows normal timing settings for easy starting and reasonable fuel economy under regular driving situations. As the engine goes into boost, this timing setting may cause detonation. So this boost retard controller allows the driver to dial in ignition retard with a dash-mounted knob. This device (Holley PN 91070) operates on a degrees of retard per pound of boost and is typically adjustable from 1 to 3 degrees of retard per pound of boost.
Maintaining normal cooling temperatures with a supercharger requires improvements to the cooling system. This Chevelle is fitted with an aftermarket aluminum radiator and an electric fan to keep engine temps in check.
This cutaway shot of the Vortech carbureted blow-through system shows how the carburetor rests inside of the enclosure (box). The throttle shaft mounted on the side of the box is sealed to prevent any pressure from leaking out.
On full-race or high-boost superchargers (11-plus psi) a cog belt is used for greater strength. These belts are noisier than serpentine belts and install differently. With a cog belt, the installed tension is significantly less than a serpentine belt drive. Incorrectly overtightening a cog belt can raise the belt temperature, causing the belt to get tighter and potentially break the mounting points.
Selecting and installing the correct carburetor is critical for a supercharged engine, because an engine under boost demands higher amounts of fuel and air. Without a carburetor(s) that will provide adequate fuel and air, the supercharger will not be able to make maximum boost.
Ignition timing in a blower motor typically calls for 16-24 degrees initial advance at idle and total advance of 32-34 degrees, all in by 2,500-3,000 rpm.
This carburetor is the race-only Pro model from The Carb Shop (also available: the street/strip Sportsman). It is custom built to match the engine's performance and dyno-tested. Features include a fully rejetable emulsion circuit, specially sealed throttle shafts, and a needle and seat design allowing fuel to exit below the fuel level in the chamber, which reduces aeration.