In Blower Basics (Part 1), we debunked the myth that a supercharger is an exotic upgrade. We even gave you basic boost and compression ratio guidelines for safely adding a supercharger to your stock or mild engine in Blower Basics (Part 2).
But what if you want to take your engine beyond stock or mild?
If you want to take your boost and/or engine rpm past Weiand’s recommended threshold for stock engines, you’ll need to make some modifications to your induction, ignition, and engine. The extent of these modifications will depend entirely on how the engine is to be used. Let’s first review Weiand’s recommendations for stock engines to see if upgrades will be necessary:
Supercharger Guidelines for Stock Engines
- 7.0:1 to 9.0:1 static compression ratio, with an effective compression ratio below 12.0:1 (review Boost Basics Part 2 to learn more about effective compression ratio): The optimum static compression ratio is 8.0:1.
- 3-5 psi boost level: This range of boost has proven to be the best compromise for power and reliability for engines using cast pistons, cast crank, and small cam, according to Weiand.
- 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.
- 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 you’re going beyond the guidelines above, you’ll to start making some modifications.
Guidelines for Higher-Boost Applications
To run boost levels from 6 to 10 pounds, Weiand recommends the following:
- Forged blower pistons with a static compression ratio of 7.5:1
- Steel crankshaft
- Four-bolt main caps
- Steel harmonic dampener
- Stainless steel valves
- Three angle valve job to promote better cooling
- More aggressive camshaft per manufacturer recommendations
- Roller rockers
- Ported and polished heads–focus on exhaust side porting
- Steel connecting rods with good rod bolts
- Chromoly pushrods
- High-output ignition
- High-flow water pump (cast iron or aluminum available – see our complete catalog for applications)
- Minimum of a 2.5-inch diameter dual exhaust with headers.
For maximum boost and horsepower applications (12 pounds or more), Weiand recommends the following engine specifications:
- High-quality forged or billet double keyed crankshaft
- Four-bolt main caps with quality bolts or studs
- Steel double keyed harmonic balancer or crank hub
- High-quality steel connecting rods (H- or I-beam)
- Forged blower pistons and stainless steel piston rings
- O-ringing the block (mandatory)
- Severe duty stainless steel valves or iconel
- Fully ported and polished heads
- Solid or roller camshaft designed for high boost
- Roller rocker arms
- Chromoly push rods
- High-output ignition management system or magneto
- Blueprinted carburetors or fuel injection
- High octane race fuel (112+ rating)
- Minimum of a 3-incn diameter dual exhaust with free flowing street/race mufflers and large tube headers
- Maximum effective compression ratio on gas not to exceed 24:1
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, it’s a good idea to inspect the entire valvetrain. If the valve springs require replacement, factory heavy-duty springs should be used.
For higher boost applications, consider a three-angle, “street-type” valve grind to promote better cooling. 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 to your cylinder heads, 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.
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.
If you’re installing a supercharger on a fuel-injected application, you may need to upgrade to larger fuel injectors and fuel rails to deliver the added fuel you may need based on BSFC (brake-specific fuel consumption). Contact your supercharger manufacturer to calculate your fuel needs or see our fuel injector post to learn more about BSFC. You should also make sure you have a good-flowing air cleaner and exhaust system to allow your supercharged engine to breath easily.
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 camshaft 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. With the use of an aftermarket camshaft, follow the camshaft manufacturer’s recommendations for valve springs. Blower cams can typically be run “straight up.” Note that a blower has tendency to lessen the rough idle of radical cams.
Blown engines make great power in the low-and mid-rpm range. That means most late model OEM electronic ignition systems have the capability of working well with a supercharger and will be fine if you keep your driving under 5,500 rpm.
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. However, 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.
Set initial ignition 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. If detonation is encountered, a boost/retard system that works with manifold vacuum and pressure is recommended. It is also a good idea to run your spark plugs one or two heat ranges colder than normal with a blower to reduce the chances of detonation.
Keep in mind, these are general suggestions–there are no hard and fast rules. Bottom line is you can benefit from supercharging a stock, mild, or wild engine with the right tuning and modifications.