Traditional 6-71 Roots-Style Performance
A Roots-style supercharger is a positive-displacement air pump. Its rotors move a fixed volume of air through the case with every revolution, producing immediate response when the throttle opens.
This design supports:
- Strong low-speed torque
- Immediate throttle response
- Broad power delivery
- Adjustable blower speed
- Classic exposed-blower appearance
- Dual-carburetor induction
- Street, strip and show-car applications
Unlike a centrifugal supercharger that generally builds boost with engine speed, the 6-71 Roots design can produce significant airflow throughout the engine’s operating range.
Advertised 45–55% Horsepower Increase
The manufacturer advertises potential power gains of approximately 45–55 percent. This is an estimated range rather than a guaranteed result.
Final output depends heavily on:
- Engine displacement
- Static compression ratio
- Blower pulley ratio
- Boost pressure
- Carburetor sizing
- Fuel delivery
- Ignition timing
- Camshaft selection
- Cylinder-head airflow
- Exhaust restriction
- Engine condition
- Professional tuning
A restrictive intake, small carburetors or incorrect drive ratio can reduce boost. Holley’s supercharger technical guidance also notes that restrictive exhaust systems may create a higher boost reading without producing a corresponding horsepower increase. More pressure is not always more power; sometimes it is simply your engine filing a complaint. Holley
Complete Supercharger Package
The advertised package includes:
- 6-71 Roots-style supercharger
- Three-lobe rotor assembly
- Double-pinned rotors
- Small-Block Chevy intake manifold
- Supercharger snout
- 50-tooth Gilmer pulley
- 55-tooth Gilmer pulley
- Idler pulley and bracket
- Three-inch blower belt
- V-belt crank pulley
- Dual-carburetor adapter
- Installation gaskets
- Rear-mounted pop-off valve
Carburetors, fuel-delivery components, distributor, ignition system, air cleaners and tuning equipment are sold separately.
Adjustable Blower Drive Ratio
The supplied 50-tooth and 55-tooth pulleys allow the installer to configure the blower drive for the intended engine combination. Optional 48-, 60- and 65-tooth pulleys are advertised separately for additional drive-ratio adjustment.
Changing pulley combinations changes blower speed and potentially boost pressure. Before selecting a ratio, calculate the blower-to-crank speed relationship and consider:
- Engine displacement
- Maximum engine rpm
- Compression ratio
- Fuel octane
- Charge temperature
- Desired boost
- Intended street or racing use
Do not choose a pulley solely because it produces the largest number. Detonation is very efficient at converting pistons into smaller pistons.
Engine Preparation Is Essential
A 6-71 is a large supercharger and places substantial load on the crankshaft, rotating assembly, cooling system and fuel system. Holley recommends a purpose-built engine for a blower of this size rather than treating it like a mild accessory for an unknown stock short block. Holley
Recommended supporting components may include:
- Forged pistons
- Appropriate blower compression ratio
- Forged or blower-rated connecting rods
- Strong crankshaft
- Proper bearing clearances
- Boost-compatible piston-ring gaps
- Suitable camshaft
- Upgraded fuel system
- High-capacity cooling system
- Boost-referenced ignition controls
- Wideband oxygen-sensor monitoring
The engine builder should establish safe compression, boost, fuel and timing requirements for the specific combination.
Crankshaft and Balancer Requirements
A large Roots blower places considerable torsional load on the front of the crankshaft. Holley specifically advises using a double-keyed crankshaft and a steel, SFI-rated double-keyed harmonic balancer with a 6-71 or larger supercharger. A stock cast balancer and single crank key are not considered adequate for this load. Holley
This preparation is not merely a racing upgrade. It helps prevent damage to the crankshaft keyway, balancer and front drive system.
Dual-Carburetor Induction
The supplied adapter accepts two square-bore four-barrel carburetors. The listing suggests approximately 600–850 CFM per carburetor as a common range, but correct sizing depends on the engine and blower combination.
Carburetors used above a Roots blower should be prepared for forced-induction service. Fuel pressure, floats, metering, power valves and linkage must be selected and adjusted correctly.
Professional synchronization and tuning are strongly recommended.
Installation Considerations
This is a substantial engine modification, even though it is sold as a bolt-on package. Installation should be handled by an experienced performance shop or engine builder.
Before installation:
- Verify engine health and compression
- Confirm crankshaft and balancer strength
- Measure hood and firewall clearance
- Check distributor compatibility
- Confirm fuel-system capacity
- Inspect the cooling system
- Establish a safe initial pulley ratio
- Set conservative ignition timing
- Confirm belt alignment
- Check rotor and case clearances
- Use a wideband oxygen sensor during tuning
Additional engine-building and forced-induction information is available through My Pro Street Online and the automotive technical resources at Pro Street Online.
Racing and Emissions Notice
This supercharger kit is advertised for racing or competition-oriented use. No California Air Resources Board Executive Order number was supplied.
Do not advertise the kit as emissions legal or street legal unless its exact manufacturer and part number are covered by an applicable exemption. California buyers can check the official CARB aftermarket-parts database.
Local emissions, registration and equipment regulations should be reviewed before installation.
Product Specifications
- Supercharger type: 6-71 Roots style
- Rotor configuration: Three-lobe
- Engine family: Standard-deck Small-Block Chevrolet
- Advertised power gain: Approximately 45–55%
- Included pulleys: 50-tooth and 55-tooth
- Belt: Three-inch Gilmer-style
- Carburetor mounting: Dual four-barrel square-bore
- Carburetors included: No
- Fuel system included: No
- Ignition system included: No
- Gaskets included: Yes
- Warranty: 12 months
- Intended use: Performance and racing applications
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