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    In this series, I will not repeat the information that is already available on how superchargers work, but instead focus on their selection, installation, use and tuning on older engines. Here I will make various observations and comments on function, and in some cases how to make improvements. New material will be added regularly, and eventually better organized. Click below to jump directly to an individual topic.

Supercharger Topics

Introduction

Choices

Eaton

Eaton data

GMC

GMC manifold

Function

Drive Methods

Engine Design

Carburetor

Illustrations

Links

Supercharger Drives

    A mechanical connection must be made between the engine and the supercharger’s drive end. To keep efficiency high and parasitic losses low, normally the supercharger is mounted with its shaft axis parallel to the crankshaft, although 90° drives have been used successfully. This must be done precisely or wear, vibration and breakage will result, less than 1° misalignment out of parallel should be the target. Remember that this includes both planes: roll (rotation around the wheelbase axis) and yaw (rotation across the lateral or crankshaft axis). Many superchargers are tolerant of pitch angle (inclination around its own shaft axis), which provides some additional leeway for exact placement &c. Vibration-free “isolastic” mounts are theoretically possible, but the complexity of making a subframe to mount the engine, transmission, supercharger &c. together and then to isolate this from the chassis is more involved than the supercharger installation. I suggest that absolute rigidity is the best practical solution.
    The supercharger will almost always rotate at a higher speed than the engine depending on the supercharger design and its capacity relative to engine displacement, in some cases as much as triple the engine RPM.

Drive source
    The supercharger is almost always driven off the crankshaft nose, either directly by a modified hub, damper or pulley) or indirectly by an accessory.

Jackshaft
    The drive ratio need not be accomplished by a single pair of sprockets or belts. A jackshaft or countershaft can “split” the ratio into two components which, when multiplied (not added) together produce the required drive ratio. Where useful, a combination of overdriven and underdriven sprockets may be used to take advantage of existing inexpensive, or easily available components. For example, a similar drive ratio may be obtained by any of the following choices:

Countershaft ratio choices compared

Split type

Sprockets, ratio 1

Ratio 1

Sprockets, ratio 2

Ratio 2

Final ratio

Overdrive, split evenly

28 + 20

    1:1.4

28 + 20

    1:1.4

    1:1.96

Overdrive, split unevenly 

24 + 20

    1:1.2

30 + 18

    1:1.67

    1:2

Overdrive + underdrive

54 + 18

    1:3

20 + 30

    1:.67

    1:2

    This not only gives greater latitude in sprocket or pulley choices, but allows the driven end of the supercharger to be laterally opposite the engine component driving it. This also reverses the supercharger rotation, which is very helpful in many cases where the original application has the supercharger turning clockwise, such as most Eatons.

    Here’s a jackshaft and its bearings built into the case of a twin rotor supercharger to both reverse the drive, and place the air entry forward. Click for a larger view.
    It allows use of prime number sprockets to maximize chain and sprocket life, and avoids using sprockets too small for best life (smaller than 17) or too big for good chain speed (more than 60).
    Don’t use a jackshaft unless you have to - the downside is minor power loss and more potential breakage.

Drive methods
    The supercharger drive is most frequently roller chain, toothed belt (also known as Gilmer belt, cog belt, synchronous belt, &c.), ribbed (serpentine) belt, V belt, flat belt, gear, or direct. There are advantages and disadvantages as to cost, size, power loss, fabrication complexity, adjustment, and safety to each of these, and not all methods are practical for every chassis, engine, or supercharger.

Chain
    Roller chain has been used for almost 100 years. It has high efficiency, absolute speed accuracy (no slippage) which gives no protection to the supercharger against engine back-fires, reasonable strength (in the proper size), tolerates misalignment fairly well, and needs no pre-load tension. Ratios are easy to change since each sprocket tooth change is roughly 6% or less than the overall ratio. Sprockets and chains

are inexpensive and easy to find. Here’s a a pre-unit Triumph twin with a Shorrock supercharger driven from the clutch drum by chain, click for a larger view.
    However, chain drive needs constant lubrication by oil which generally requires an enclosure for cleanliness, is noisy and heavy. There are limits to the minimum and maximum sprocket size, and both RPM and feet per minute chain speed. In general small chain, such as #35 as used in many cam drives, permits more teeth for a given diameter, but reduces strength, requiring multi-row chain.
    Figure “8” chain weighs less than straight link chain with only a minor strength loss.
    At least one sprocket should be a prime number (17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71), or at least an odd tooth count with no factors shared between the sprocket sizes. Click here for more chain data and comparisons: .

Belt
    Cog belt is used on large Whipple, 14-71 &c. fuel cars, but the pulleys and belts are very expensive except where the size is used commercially as in cam drives. Speed accuracy is absolute except that overload, overspeed or inadequate tension will “skip” the belt and damage or break it. Proper tension is essential. Alignment is more important than with chain drive, and frequently flanged pulleys are used. Commercial supercharger

drives are generally much too wide; about 30 to 50mm is about what is needed. Here’s a Norton Atlas with a cog belt drive from the engine sprocket, click for a larger view.
    Serpentine or ribbed belt, as used on OEM supercharger drives such as Ford, GM, Jaguar, Mercedes, Mini Cooper, &c. as well as the accessory drive on most automobiles works well, but needs more tension than a cog type, and pulleys and belts are expensive. Both 6 rib and 8 rib are good. Ford Lightning truck lower pulley sizes, which are available from Ford dealers, breakers, jumbles, yards, &c. may be useful for Eaton applications. The stock Ford Lightning supercharger (driven) pulley is 2.83” OD, the engine (drive) pulley is about 7.50” OD. The Ford retro-fit engine pulley numbers are given in added boost in psi vs. the original pulley size.
    +2 psi boost lower (drive) pulley = 7.90” OD
    +3 psi boost lower (drive) pulley = 8.17” OD
    +4 psi boost lower (drive) pulley = 8.50” OD
    +5 psi boost lower (drive) pulley = 8.75” OD
    +6 psi boost lower (drive) pulley = 9.00” OD.
    V belt has been used in various widths and both single and multi-row, and has more power transmission capacity per inch of belt width than a cog type, but requires more pre-load tension (but less than serpentine belt). V belt drive offers some advantages:
  »  Easy and economical installation
  »  No lubrication required, runs dry
  »  Clean & low maintenance
  »  Elasticity of belts helps shock load dampening
  »  Quiet, smooth operation
  »  Long life expectancy
  »  Good mechanical efficiency (but lower than chain at 96%)
  »  Narrow and light “package”
     Automotive and commercial pulleys and belts can be used or adapted, but watch the manufacturer’s recommendations as to load, tension, maximum RPM and speed in feet per minute.
    Flat belt is not really practical due to width and tension requirements, but it will operate on a very small pulley ,and at very high belt speed in f/m. Efficiency is very high at 98%. Gear
    This is slightly less efficient than chain and has similar and difficult lubrication requirements. Alignment and pitch spacing is critical, requiring a gear box or similar device to be adapted or constructed rather than free-standing gears. Ratio changes are very difficult.

Direct
    This drives the supercharger off the end of an engine shaft using a coupler, either at engine speed or through a gearbox at a proportion of engine speed. The extreme length makes this impractical, although the efficiency is high. Potvin and Moon made front mount direct drives for the GMC superchargers 50 years ago. Many pre-War British and European factory-supercharged automobiles were driven

directly including Bentley, as shown here, click for a larger view.

Drive ratio
    Once the type and capacity of the supercharger have been selected, the relative speed at which the engine will drive the supercharger must be tested to detect gross mis-matches:
    If the supercharger capacity is too small, this requires excessive supercharger RPM either above the safe limit (which will vary with individual models and types), or not practical for the drive method chosen (sprocket or pulley too large or small). Insufficient capacity also means that the supercharger is a restriction in the intake flow path until critical RPM is reached, where its capacity matches the engine’s demand. Below this point power will be less than in normally aspirated mode, both due to restricted breathing and of course the drive load.
    If the supercharger capacity is too large, this permits internal leakage reducing boost at low RPM, inefficient operation, and high power loss, or not practical for the drive method chosen (sprocket or pulley too large or small).     There is a rough method of approximating how fast the supercharger must turn in relation to engine speed. It does not include such factors as supercharger wear, the engine’s volumetric efficiency, intake restrictions, &c. and therefore is only useful as an estimator. To calculate the drive ratio to produce the boost desired with the supercharger selected, where:
R: drive ratio (supercharger RPM ÷ engine RPM).
C: engine displacement, either in cubic centimeters, liters ÷ 1,000, or cubic inches × 16.387.
B: boost, in pounds per square inch (psi).
A: atmospheric pressure, in pounds per square inch (psi); 14.7 is approximately correct for sea
     level and 70° F.
V: supercharger volume in either in cubic centimeters, liters ÷ 1,000, or cubic inches × 16.387
     per revolution; Roots volume is “displacement”, eccentric vane volume is “swept volume”.
O: Cam overlap; subtract 30 from the overlap in degrees, then multiply by 1% (.01). E.g. for 65°
     overlap, O = 65 - 30 = 35, × 1% = .35.

    Where there is a choice between, e.g., an Eaton M62 driven at 100% of engine speed and an Eaton M90 driven at 68.9% of engine speed (62 ÷ 90 = 68.889) with both delivering the same capacity, the larger supercharger is preferred despite minor loss of boost at lower speeds due to leakage.
    This is because the power required to drive the rotors rises as the cube of the RPM. If the power to drive the M62 is 100 units at 100% of engine speed, the power to drive the M90 will be 45.2% more (90 ÷ 62 = 1.451613) due to larger volume per rotation or 145.2 units, but only 32.7% as much power (68.9³ = .326925) at the 68.9% lower RPM for a total of 47.5 units (145.1613 × 32.6925% = 47.457), about 52% less parasitic loss.
    Power loss comparisons are only even partially effective when made between models which differ only in supercharger capacity - not between manufacturers (Shorrock vs. Judson) design types (Eaton vs. Roots), or versions (Gen. II· vs. Gen. V·), &c. To make a comparative estimate, find the capacity and required drive ratio for each choice as described above. Multiply the supercharger capacity of each choice times the cube of its drive ratio. The ratio of power loss is Choice A ÷ Choice B, &c.

Sample drive ratio calculations:
Example 1, a 750cc engine with 70° of overlap, where 10 psi of boost is required using an Eaton M24 supercharger:
    C = 750cc engine displacement
    B = 10 psi boost required
    V = 393cc supercharger capacity
    A = 14.7 psi
    O = 70° cam overlap - 30 = 40, × 1% = .4

Example 2, a 1,000cc engine with 80° of overlap, where 20 psi of boost is required using a Wade RO20 supercharger:
    C = 1,000cc engine displacement
    B = 20 psi boost required
    V = 2,000cc supercharger capacity
    A = 13 psi (high elevation)
    O = 80° cam overlap - 30 = 50, × 1% = .5