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Blown SBC in Chevy II.

Chevy 671.gif871.gif


[edit] Overview

Superchargers have been used through the years to increase the performance of the internal combustion engine. They've been used on aircraft, industrial/construction equipment, as well as personal transportation.

[edit] First of all, why a blower? What good is it?

Engines, for all intents and purposes, are air pumps. The more air you can put through them, the more power you can get out of them. Consequently, a blower can and will push more air into a motor than could be ingested by the motor depending on atmospheric pressure alone. By increasing the pressure of the intake charge, you push more air into the cylinder. More air, when mixed with more fuel, makes for a stronger combustion burn. The stronger burn pushes harder on the piston, which pushes harder on the crankshaft, which creates more torque/hp. This article will only address mechanically-driven blowers or superchargers. Turbochargers will be covered later in another article, probably by someone else, because although I know they work very well, I just can't get wrapped around the maps used to size them. Maybe later.....

[edit] Parts of a blower

Blowers are very simple in nature and have very few parts. Centrifugal blowers are made up of an intake (scoop), case (to contain the vanes and hold the drive pulleys), vanes, drive (pulleys), and a exhaust manifold mount (usually, incorporated into the case. Roots-type blowers and screw-type blowers consist of a case, two rotors and a set of gears.

[edit] Types of blowers

There are several types of blowers; Roots, screw and centrifugal to name the most popular today. In the past there have also been sliding vane (Judson) and axial flow (Latham) units, piston type (like the unit on the top of your air compressor) and possibly other types. Let's look at each one and list some sites for finding them.

[edit] Roots type

The Roots type supercharger or Roots blower is a positive displacement pump which operates by pulling air through a pair of meshing lobes not unlike a set of stretched gears. Air is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The supercharger is driven directly from the engine's crankshaft via a belt or by spur gears.

It is named for the brothers Philander and Francis Marion Roots of Connersville, Indiana, who first patented the basic design in 1860 as an air pump for use in blast furnaces and other industrial applications. In 1900, Gottlieb Daimler included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available.

These blowers were never designed to compress air, just to move air from one place to another, sort of like your house fan. You'll find several explanations of why the Roots brothers originally designed this blower, but the one I like the best is that it was designed to blow air down into mine shafts so the miners could breathe. No internal compression takes place in the blower, but the restriction of the closed intake valves in your motor allow the buildup of pressure in the intake manifold and head ports. The centrifugal and screw type blowers make pressure inside the blower.

There are Roots designs which have 2 lobes on each rotor and others with 3 lobes on each rotor. There are also variations where the rotors are "twisted" into a helix along their axis to help prevent the "backwash" of air between the rotors and reduce "buffeting". Here's the aerospace engineering definition of buffeting afforded by "The beating of an aerodynamic structure or surfaces by unsteady flow, gusts, and so forth. The irregular shaking or oscillation of a vehicle component owing to turbulent air or separated flow."

Here's an animation showing a straight-on view of the rotors in a 3-lobe Roots. Air entering is in blue and air exiting is in red:

The Roots types we are most familiar with came as original equipment on GMC two-stroke diesel engines. The number of cylinders and the cubic inches per cylinder is the way the engines were designated, so the blowers just naturally were called by the same designation. For instance, a 4-71 blower would have come off a 4 cylinder inline motor with 71 cubic inches per cylinder for a total displacement of 284 ci . This blower would be a good choice for a motor of around 300 cubic inches and could be over driven (by juggling drive and driven pulleys) to feed a larger motor or under driven to feed a smaller motor. Here are some examples of the GMC motor designations:

  • 2-53
  • 3-53
  • 4-53
  • 6V-53(V-configuration rather than inline)
  • 8V-53
  • 12V-53
  • 2-71
  • 3-71
  • 4-71
  • 4Twin-71
  • 6-716Twin-71
  • 6Quad-71
  • 8V-71
  • 12V-71
  • 12VTwin-71
  • 16V-71,
  • 16VTwin-71
  • 6V-92
  • 8V-92
  • 12V-92
  • 16V-92
  • 6-110
  • 6Twin-110
  • 12V-149
  • 16V-149.

Add to those the turbocharger, intercooled turbo, aftercooled turbo, etc.

The most popular blowers for hot rod use today would be the 4-71, 6-71 and 8V-71 (just called 8-71) units. There is some work to be done on a stock (fresh off the diesel) "Jimmy" blower that you might find at a swap meet or on eBay. The rear case cover bearing bosses need to be strengthened with steel collars and the rotors must be clearanced to the case interior. These and other oiling mods or sealed bearings will be addressed in Pat Ganahl's book. If you're considering a blower, it is well worth the investment to buy and read this book.

By the way, speaking of swap meet or ebay, did you know there is a "small bore" and a "large bore" 6-71? Click HERE for an explanation of the differences.

Of course, most of the blowers bought today are purpose-built aftermarket units that have the proper clearancing and strengthening mods built in. Some manufacturers use teflon strips on the rotors to tighten up the rotor/case clearance and increase the efficiency of the unit.

[edit] Resources

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