3/17/2009 - Charts and Guides
Many people have the impression that a supercharger is an exotic performance part found on wild street machines or racecars. There's also the impression a supercharged vehicle is difficult to drive on an everyday basis. Nothing could be further from the truth on both counts.
First, a supercharger is nothing more than a large air pump that can provide greater than atmospheric pressure (boost) to an engine. When was the last time you thought of an air pump as exotic?
Second, when building an engine for supercharging (other than a racing application), it's generally built for low to mid-range torque and power just as a stock engine would be. Because an engine may be left stock when utilizing a supercharger, your vehicle would be no more difficult to operate or maintain. As you can see, a supercharger isn't really exotic. It's really quite practical.
The guidelines below have been established to assist in building a basic street supercharged engine. Engine durability and dependability are two factors given strong consideration in these guidelines as current supercharger kits are developed for everyday use. However, superchargers are quite capable of reliable use in competition.
The extent of the engine preparation will depend entirely on how the engine is to be used. A supercharger can be installed on a stock engine with cast pistons and a cast crank as long as moderate boost (below 8 lbs.) is maintained and any detonation is strictly controlled. Engine speed should also be limited to 5,000 rpm. Detonation on cast pistons can easily break ring lands. Too much boost and/or detonation on a stock or worn engine can cause piston damage or burned valves.
Most late model "smog" engines work well with a supercharger due to their lower compression ratios and smaller cam profiles.
Supercharged Engine Guideline
1) 7.0:1 to 9.0:1 compression ratio: The optimum compression ratio is 8.0:1.
2) 4-7 psi boost level: This range of boost has proven to be the best compromise for power and reliability.
3) Engine rpm: When using stock cast pistons, the engine should be limited to a maximum of 4,500-5,000 rpm. Exceeding this limit may over-stress the cast pistons causing failure. Blueprinting an engine using the proper components will allow higher rpm reliability and will maximize a supercharged engine's potential.
4) Detonation (pinging): Detonation is the single most destructive force in a supercharged engine and steps must be taken to eliminate it. This may include lowering boost level, retarding timing, installing a boost timing master, increasing fuel flow to prevent leanout, and/or using a fuel additive to raise octane level. The cooling system also needs to be in good condition to prevent overheating, which may lead to detonation.
If an engine is to be driven hard or under load, as in towing, a thorough blueprinting should be considered. Forged pistons, with their inherent strength and ability to withstand higher temperatures, are recommended. Follow the piston manufacturer's recommendations for piston-to-cylinder clearances.
A compression ratio exceeding 8.0:1 is not recommended, nor is it necessary for brisk performance from a supercharged engine. If raised to this level, fuel, ignition timing, and total boost become critical factors.
Next consideration would be the piston rings. They should always be the best quality available because the piston rings take as much abuse as any other component in an engine. "Moly" or "Double Moly" piston rings (iron piston rings coated with Molybdenum Disulfide) are an excellent choice for supercharged street engines. They seat quickly and wear well. For hot street or competition, where higher boost will be used, chrome or stainless steel piston rings should be considered.
Consideration should be given to using heavy-duty fasteners especially on the connecting rods and main caps for added durability and strength. If the engine will be run with a high boost level (12 psi or more), high-performance head gaskets with built-in stainless steel O-rings are recommended because they can withstand the higher combustion pressure and temperatures encountered in a supercharged engine.
Cylinder Head and Valvetrain Preparation
Weak valve springs or burned valves can lead to backfires. When an engine has more than 50,000 miles on it, inspect the entire valvetrain. If the valve springs require replacement, factory heavy-duty springs should be used. With the use of an aftermarket camshaft, follow the camshaft manufacturer's recommendations for valve springs.
For proper cooling of the valves, use a three-angle, "street-type" valve grind. With the additional combustion temperatures normally generated in a supercharged engine, the wider valve seats will provide better cooling of the valves, and the three-angle valve grind will provide better sealing of the valves.
When any port work is being done, most of the effort should be directed to the exhaust ports. The supercharger will overcome most minor restrictions on the intake side of the cylinder head.
The use of O-ring head gaskets requires receiver grooves in the heads and block milled by a competent machine shop.
A supercharger can overcome inadequacies in a stock cam up to about 4,500-5,000 rpm. You will typically find that performance with a blower will not be significantly enhanced below these speeds with a cam change. However for optimum performance at higher rpms, a more aggressive cam will provide substantial power increases.
For best performance with a blower you should look for a cam that has higher lift and longer duration on the exhaust side. Street performance with a blown engine is usually best with a cam that is ground with a 112 to 114 degree lobe separation. Blower cams can be typically run "straight up." Note that a blower has tendency to lessen the rough idle of radical cams.
Air Cleaners: Good quality air cleaners should always be used on a street supercharger. Allowing dirt or debris to go through the supercharger may score or gouge the rotors or case.
Exhaust System: The less restriction the better. Use large tube headers with low restriction type mufflers. Low speed torque will not suffer by using larger primary tube headers as is typical on unblown engines.
Carburetion: At full throttle a blown engine can require 50 percent more air than an unblown engine and as a result needs a larger carburetor(s) in order to make maximum power and boost. If your blown engine is primarily driven on the street at moderate engine speeds (under 4,000 rpm) you won't need a larger carburetor(s).
Typically the carburetors(s) will need to be enriched by 5 to 10 percent on the primaries and 10 to 20 percent on the secondaries. The idle mixture screws may need to be enriched by one or two turns. In either case, the carburetors need to be jetted properly to prevent a lean condition. A lean condition can lead to overheating and detonation.
For initial start-up, it's better to have a slightly rich condition to help prevent the engine from overheating. After initial start-up, check the spark plugs for proper reading (color) and adjust the carburetor(s) accordingly. You want to see a medium to dark tan color.
Ignition System: Set the initial timing at 6 to 10 degrees BTDC. The distributor advance curve should be calibrated to give a total advance of 28 to 34 degrees by 2,500 rpm. Most late model OEM electronic ignition systems have the capability of working well with a supercharger. Some distributors with computer controlled advance curve and timing may not be compatible with a supercharger because of the preset timing and sensors they require. Any of the aftermarket high performance standard or electronic distributors should function well when properly calibrated. A quality electronic unit would be the preferred choice for best all around performance and reliability. If detonation is encountered, a boost/retard system that works with manifold vacuum and pressure is recommended.
Supercharger Drive Ratios
The reason we cannot provide an exact boost figure is that camshaft profiles, cylinder head configuration, and carburetor size can all have an effect on the amount of boost that will be shown on the boost gauge. To illustrate, if you have small port heads and a stock cam, at higher engine rpms the blower will be unable to overcome these restrictions and boost will build up in the manifold producing an artificially high boost reading. Conversely, changing the cam and heads will make the boost reading go down but the power will increase at higher engine speeds.
Boost is a direct result of three factors: engine size, blower size, and the speed the blower is driven in relationship to the engine speed. Bigger blowers driven at the same speed as a smaller blower will produce more boost.
Normally decreasing the upper pulley by one tooth or increasing the bottom pulley by one tooth will raise the boost one or two pounds. Conversely increasing the tooth count on the upper pulley or decreasing the teeth on the lower pulley by one tooth will lower the boost by one or two pounds.
If you desire a substantial change of boost you can just interchange the top pulley for the lower pulley and change the blower from underdrive to overdrive. However, swapping pulleys will approximately double the amount of boost you will get. This should only be done for extreme high-performance race engines by those with substantial supercharger experience.
Thanks to Holley Performance Products Inc. for contributing to this guide.