Motorcycle Superchargers

 
 

Please note, you will need the Flash 5 player to view the supercharger animation's. Get it here if you can't see them.
 

Generally speaking dragbikes use one of two types of supercharger;
1) Vane type, typically the Shorrocks (top right). In these units rotating vanes draw in the charge, and because the vane shaft is eccentrically mounted, they actually compress the charge.
2) Roots type (bottom right), typically the Marshall cabin blower (so called because they were used on aircraft to pressurise the cabin). In this type contra-rotating lobed rotors transfer the charge through the supercharger casing, and technically speaking, displace rather than compress the charge.
Related to the Roots type, are screw type superchargers such as the Sprintex, Whipple and Eaton which do compress the charge.

Vane types are more bulky and will not stand being over-revved for long without disastrous results. They do however tend to heat the charge less and provide a smooth flow, an essential feature for a bike engine which may only have one cylinder.

Roots types impart extra heat to the charge, principally due to the churning nature of their operation. They also have a pulsing delivery, particularly the two lobed type, which can cause problems. Frictional losses are lower than the vane type, one must remember that power is consumed driving the blower. A figure of 250bhp is often quoted for top fuel cars.

  

 

Cross section of the Shorrocks vane type.

Cross section of the Roots lobe type.
 


Both the Marshall and Shorrocks have been out of production for many years, and are not the easiest things to find. However for smaller engined bikes they are ideal.
I'll deal with the Shorrocks in detail in its own section, so back to the Marshall. This has a swept volume of about 600cc, and is more suited to engines under this capacity for drag racing purposes. In fact Mick Hand's amazing World record holding 250 Honda uses one of these units.

  


At the other end of the spectrum George Laycock's 850 Norton successfully uses one for a more modest power boost. The pulsing nature of the flow can be clearly heard on the Norton as it 'hunts' whilst ticking over.

The other common type used is the Wade RO20, which has 3 lobed rotors for smoother flow and is suitable for engines up to around 1200cc.
 
 

All Roots type blower rotors are linked by gears in the end casing, usually operating in an oil bath. The advantage of this is that oil is not lost into the inlet manifold as in vane type blowers.

Efficiency of the Roots type tends to be lower though there are ways in which this can be improved. Teflon, PTFE or similar sealing strips can be inserted into the rotor tips to improve sealing. If the rotors are fitted to steel shafts, the unit can be run as high as 11000rpm. Charge heating will be much greater, but if the engine is run on methanol or methanol/nitro mix, the high latent heat of evaporation of these fuels will have a beneficial cooling effect.

Because the efficiency of this type of blower falls off at higher boosts, various methods have been used to achieve the dramatic boost pressures required for maximum power.
In the late 1930's Mercedes Benz used compound supercharging on there Grand prix cars. In this system one unit pumps into a second smaller unit increasing boost pressures to as high as 38 psi. Methanol was the fuel used, as otherwise the charge heating would have reduced efficiency dramatically.

  

How Mercedes Benz did it.

You may not want to contemplate this sort of thing on your dragbike! The 2 compressors are in the top half of this 1939 1500cc Mercedes Benz Grand Prix car power plant. Rotors in both units are 105mm diameter, though the rotors in the primary are 165mm long and in the secondary 95mm. Compression ratio of the V-8 was 6.99:1 and the blowers ran at 1.25 engine speed.
  New Roots type superchargers are available from former Top Fuel Bike racer Derek Chinn. These are available with 2,3 or 4 lobe rotors, Teflon rotor seals and in a number of swept volumes to suit any bike.
 
  Find out how the Roots blower was accidentally derived from a water wheel in the USA here.
 
 

Moving on from the standard issue Roots, we come to a later development of this type which use helical rotors. This allows for compression of the charge thus improving the efficiency of the blower.

Fleming Thermodynamics of Glasgow produced the Sprintex screw type some time ago. They went out of business and production was taken over by an Australian company. There are two sizes available and the smallest (S82) is most suited to bikes of around 1000cc+. The manufacturers claim a 50% improvement in efficiency compared with 'normal' Roots types.

Manufacturers of the Opcon Autorotor claim very high efficiency for their blower, even at boosts as high as 45psi. The smallest unit is very compact and suited to smaller bike engines, maximum blower speed is 18000rpm!

The other major manufacturer I'm aware of is Eaton who produce some very nice units. Magnuson in the USA are a major supplier of Eaton superchargers and also supply reconditioned models.

  

Many of the latest screw type blowers are sold for road cars, in direct competition with turbochargers. Because of this many are now equipped with integral relief valves to allow the engine to, in effect, run naturally aspirated when under light load.

Rotors of a screw type supercharger

This what the rotors in a typical screw type blower look like. Well its as close as I can be right now.
They are contra-rotating as in the Roots type, though the charge is usually drawn in at the end of the casing. Discharge is at the other end but, usually out of the side of the casing.
 

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