Tech Tips

Supercharging your Motorcycle

Written by  October 31, 2004

I have debated on getting into this subject so close to winter, but it is an area that must be translated into a language that everyone can understand.

My first exposure to Supercharging has always been around the Top fuel dragster arena, until I was at F.O.G. Cycles in Kansas City, Missouri, during one of their renowned street party/bike shows. This is where I met Thomas Eddy of Kansas City North. He had the most interesting Harley at the bike show. His custom Harley was equipped with a Magna Charger supercharger. I spent some time discussing what this performance upgrade has done for him and all of the high performance parts he has added to get the unique power that his bike is capable of. Not only was his bike gorgeous but, I would have loved it if he would have to let me take it out for a spin… (Hint, Hint).

So I decided to go forward on this subject and consulted with Larry James of James Racing. We decided to give our readers the basic types of supercharging and components and then let you know what this upgrade will do for your motorcycle.

There are three basic types of 'superchargers,' the most popular being the exhaust-gas driven turbocharger, then the mechanically driven supercharger and the pressure-wave supercharger. The mechanically driven supercharger is broken up into two groups, the mechanically driven centrifugal supercharger and the mechanically-driven positive displacement supercharger such as the screw-type and roots-type.

1-Roots Type – The roots type supercharger is the oldest type of supercharger and is known for its ability to produce large amounts of boost while spinning at very low speeds. On a motorcycle application, a roots type supercharger can often make it's full (peak) boost by 2000 engine rpm. This characteristic has contributed to its success and popularity on the top fuel racing circuit and has made it ideal for use on smaller 2, 4 and 6 cylinder engines that traditionally struggle in the lower half of the rpm. Another characteristic of the roots type supercharger is its simplicity of design. The roots type supercharger has very few moving parts and spins at low rpms, making it one of the more reliable and durable supercharger designs.

2-Screw Type - The twin-screw type supercharger is a positive displacement type device that operates by pulling air through a pair of meshing lobes not dissimilar to a set of stretched gears. Air is trapped in pockets between the lobes and carried between the intake sides to the exhaust. The supercharger is typically driven directly from the engine's crankshaft via a belt.

3-Centrifugal Type - The centrifugal type supercharger is practically identical in operation to a with the exception that instead of exhaust gasses driving an impeller, there is only a compressor housing, and that is driven from the crankshaft Via drive belt.

Out of the three basic supercharger types the Roots is considered the least efficient. However, it is simple and widely used and thus is invariably the most cost efficient. It is also more effective than alternative superchargers at developing compression at low engine rpms.

All three of these systems create 50 to 100 percent boost pressure. So you can see that the difference between supercharging so far is, (lets say you have a Harley Davidson that has 70 horsepower, it is not unlikely to see an increase in power up to 140 horsepower). Common Street Blowers run normal boost pressure usually around 8-12 psi. Full race blowers can run 30-35 boost psi with Nitro Methane fuel. So you can see that Superchargers do what they claim and considerably alter horsepower by almost double. You have to consider that supercharging is not for the novice rider, but according to Tom Eddy, one great factor in his system is how quiet and cool the Supercharger runs. Plus it is a blast to ride. We will have to get that bike on a dyno yet!

When you consider the differences, you gain extra horsepower but you will also loose functionality (what do I mean?), if you currently own a Nitrous Oxide system, you will be tasked to refill the bottle after several applications, which can be costly and not always predictable. When you deal with Turbo charging there is a phrase called, Turbo Lag . This is the amount of time it takes for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly driven compressor in a supercharger does not suffer this compressor using waste energy from the exhaust gases. Compressor and turbine have the same shaft; turbochargers are generally only about 80 percent efficient. It still takes some work for the engine to push those gases through the turbocharger turbine (which is acting as a restriction in the exhaust) As long as the oil supply is clean and the exhaust gas does not get too hot, a turbocharger can be very reliable, but careful maintenance is important. Replacing a turbo that 'lets go' and sheds its blades may be expensive. Regular cleaning of both the turbine and the compressor sides of the turbo is recommended to remove any build-up of soot and dust, to which a turbo may be sensitive because of its high rotational speed.

When you factor all of these items into text you will see that Supercharging by far seems to be the most reliable and direct source of boost that can be achieved. And it looks really cool as well! I have not been lucky enough at this point to ride a bike with a Supercharger, but after discussing options with my favorite riding buddy, we have decided to make it a winter project that will not only be a fast Heritage Softail when completed, but a really great tire smoker when done. We will keep you posted!

When you consider upgrading to a system like this, I recommend taking advice from racing experts, such as Larry James of James Racing. You must build the engine that you super charge to be able to handle the boost pressure that will be obtained in the combustion chamber. You need to consider more than just bolting on this system and off you go. You will have to change compression ratio to a lower number due to detonation, (NOT pre-ignition). We will cover that at a later time. The ignition system and fuel systems must be replaced with a high spark high flowing system. Just as Nitrous applications you want to stay on the rich side, not the lean side. If I can suggest, when you spend $4,800 to $5,200.00 on the actual supercharger and the cost of an expert technician to install and tune, it sure would be a sad day if the valves and pistons could not handle the new power and blow up. So Larry and I agree that when installing an application such as this, you must start with higher performance valves, pistons and rings. You will be glad you did and farther ahead down the street or track.

Please e-mail me with questions on this subject, I am here to help!

B-safe out there.

Dave Miller

I also want to thank Mr. Larry James (Tech Advisor) for helping me bring this to light.