Roots Blower Sizes

Different blower case lengths, often bigger than standard sizes, are available through several of the manufacturers. This can lead to misleading information about how big a blower is with a larger case length. In addition, two of the blower manufacturers provide CFM claims that translate to blowers that are a lot bigger than the standard geometric displacements. CFM values are temperature dependent. As air is compressed in the blower, it heats up. Output CFM values can be greater in volume due to that temperature increase, but the weight of air remains the same. Air weight is dependent on the geometric displacement of the blower. For fuel system tuning, the weight of air from the blower is key, not volume.

Standard blower designations and sizes common in motorsports for both standard & high helix rotors

blower designation

rotor length

blower displacement with rotor seals


cubic inches






















Misunderstanding from Longer Blower Case Lengths

Case lengths may be longer than these rotor lengths.  Some are over 22 inches.  The difference is made up with an internal plug filling the case volume, usually on one end of the case, shortening the case length.  That plug may have additional ports.  Added ports may be feeding from the top or discharging out the bottom.  One or more plugs may be used to make up the difference between the rotor length and the case length.  A plug or plugs may be located at the drive end or rear of the blower.

Geometric Blower Displacement from Rotor Length is Key

The presence of plugs with ports does not increase the geometric displacement of the blower.  The geometric displacement is the same for standard or high helix blowers.  That is defined by the geometric limits of the interlocking rotors.  Some blowers are equipped with front and rear gear drives that may also appear to extend the case length.

6-71 Example

The 6-71 blower with a 15-inch rotor length is specified in many motorsports’ classes.  Many of these are equipped with longer cases with internal plugs and/or rear gear drives.  They may appear illegal; however, the 15-inch rotor resides in a legal blower regardless of the external appearance.

14-71 Example

Likewise, the 14-71 blower with a 19-inch rotor length is specified in many other motorsports classed.  Again, longer cases may be used with various internal features hugging around legal 19 inch rotors.

Fuel System Tuning

Fuel delivery need is based on several features: blower displacement, blower efficiency features, and ram air (if so equipped with a forward mounted air inlet).

Blower features such as high helix or internal ports affect blower efficiency or pumping horsepower.  The gain in power, reported for these, is from a reduction in drive horsepower from the crankshaft, not from an increase in displacement.  Some inlet features are added to increase air flow from interrupts that would otherwise reduce the flow from the geometric air flow.  But air flow from the geometric volume is not greater.

For ram air from a forward mounted air intake, air flow increases with greater racer speeds.  This can be considerable.  At high speed, the pressure going into the air scoop or injector hat can be significantly above atmospheric pressure.  That is multiplied by the boost ratio of the blower to the engine.  For example, assume a blown alcohol engine with 2 atmospheres of boost.  If an increase of 2 psi pressure occurs in the inlet hat from high racer speed, that can increase boost by 4 psi.  Fuel should be increased in proportion.

Drag Racing ProMods & Ram Air Example

Drag racing ProMods with Roots blowers and large inlet blower hats are popular.  Dyno horsepower is typically around 3,000.  Yet closer to 4,000 horsepower is needed to get the speeds and ETs generated by these racers.  The extra horsepower is generated by ram air as the speed increases with appropriate fuel system jetting.

Summary of Fuel System Delivery

Fuel system delivery for different blowers:

  • Fuel system delivery needed for the blower displacement would be the same for standard or high helix rotors.
  • Fuel system delivery needed for blowers with improved inlet, end port, and outlet features may be greater where these features increase air flow that overcome losses that would otherwise reduce the geometric air flow.
  • Fuel system delivery needed for ram air increases with speed; the increase in the weight of fuel for ram air is in proportion to the increase in the weight of air from ram air.

More information is in our books: Fuel Injection Racing Secrets, 5000 Horsepower on Methanol, and Blown Nitro Racing on a Budget.