Tech Tips

Turbocharging Your Motorcycle

Written by  November 30, 2004

After my recent article and defining the available superchargers that are on the market today, I was asked to follow up with an article on turbochargers. I have loved these systems since I saw them on the racetrack, and decided to sneak this in before winter is in its full fury.

If you have decided to increase the horsepower of your motorcycle, have you considered a turbo? Using a turbocharger can increase the output of an engine by 30 percent or more, without a significant increase in the weight of the vehicle. And rest assured that your face will never be the same when your bike hits warp speed and the turbo is at full boost. At least that’s what I have been told by my favorite drag racing buddies.

To find out more about how a turbocharger works and to understand how each part functions, I paid a visit to my official tech consultant, Larry James, of James Racing.

I want to make this as simple as humanly possible, so let’s start out with some frequently asked questions about this system and what it can do when installed properly.

How does a turbo system work?
An engine that is equipped with this system produces exhaust gases that exit through the exhaust ports of the cylinder head. These exhaust gases flow through a turbo manifold just like it does through a normal exhaust head pipe, then flows into a turbocharger unit.

As the exhaust gases enters the turbine housing the velocity of the exhaust gas 'spools' (spins) a propeller bladed-type wheel called the turbine wheel. As this turbine wheel begins to spin, it turns and drives a common shaft. This shaft has another propeller-type wheel on the other end, called the compressor wheel.

The compressor stage of the turbo begins to suck air in as the compressor wheel begins to spool. The compressor wheel spools faster as RPMs (Revolutions per Minute) increase and the air becomes compressed. The faster the turbine wheels spin and the faster the shaft speed turns, the greater the pressure that is obtained from the compressor stage of the turbine.

The air then travels to the throttle body from the carb and back into the motor. As more air is force-fed into the motor, an additional amount of fuel must also be added. The amount of fuel must be proportionate with the amount of air that is supplied to the motor. Bottom line - more air and more fuel equals more power; the same as supercharging!

What are the main components of a turbo system?
The turbocharger is an exhaust driven compressor with three main components.

1. The turbine stage - drives the compressor stage, and is the side connected to the exhaust manifold.

2. A center section - houses the common shaft, bearing, and seals. It is also were clean oil flows to keep down the temperature of the turbocharger. This is a very important section because dirty oil will cause the unit to wear prematurely.

3. The compressor stage - is where the positive charged air is created and discharged. The turbo has two propeller-type wheels connected by a common shaft. The turbine wheel and the compressor wheel both spool up to create positive boost pressure.

What other components make up a turbo system?
The following components are also included in a turbo system:

High volume fuel pump – Normally, the stock fuel pump cannot supply the fuel a turbocharged engine needs. A high flow fuel pump, such as a Holly, must be added. This will give a greater fuel flow and rule out a lean condition.

Fuel regulator - As stated before, when more air goes into the motor, more fuel is needed. A boost dependent fuel regulator regulates the amount of fuel that goes to the injectors. As the turbo begins to spool, boost pressure builds up and as the boost pressure increases, the boost dependent fuel regulator pushes more fuel to the injector, which is sprayed into the motor.

Waste gate - The waste gate controls boost pressure. It is an exhaust bypass valve that opens and closes to let out or retain exhaust gases. It maintains the turbocharger’s shaft speed through this open and close action. When the waste gate opens, exhaust gases leave through the down pipe and then through the exhaust system, which slows down the shaft speed. When the waste gate stays shut, the exhaust gases increase the shaft speed by spooling the turbine wheel, which creates maximum boost pressure. Without this component, the unit will over boost and detonate the engine (Owee!).

Exhaust manifold - The exhaust manifold holds the turbocharger and mounts it to the motor. The exhaust manifold directs the exhaust gases from the exhaust ports to the turbocharger inlet. It is typically made of cast iron, mild or stainless steel.

Down pipe - The down pipe is connected to the side of the turbine stage of the turbocharger and directs all exhaust gases from the housing into the exhaust system.

Blow-off valve - A blow-off valve is a spring loaded valve, which is normally placed on the pipe between the intercooler and the throttle body to prevent compressor surge. The blow-off valve helps increase the life of your turbocharger unit and increases responsiveness. Blow-off valves give off a distinct yet impressive whining sound.

Intercooler - The intercooler is a giant heat exchanger that cools down the temperature of the charged air. Normally, when charged air is created, it is very hot, so it must be cooled down. The colder the air is when it enters the motor (making the air denser), the easier it will combust, and the more power it will create (most of the time). Not all turbo systems use intercoolers, but it is always better to have one.

What is turbo lag?
Although a turbo system is very reliable, there is a factor called turbo lag you need to be aware of. A lag is sometimes felt by the rider of a turbocharged motorcycle as a delay between when they twist the throttle and when they feel the turbo 'kick-in.' The symptom is the 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. On light loads, or at low RPM, a turbocharger supplies less boost, and the engine is more efficient.

Lag can be reduced by reducing the rotational inertia of the turbine, for example, by using lighter parts to allow the spin-up to happen more quickly. Another way to reduce lag is to change the aspect ratio of the turbine to reduce the diameter and increase the width. Lag is also reduced by using a precision bearing rather than a fluid bearing, but this reduces friction rather than rotational inertia.

Is turbocharging right for me?
In theory, turbocharging is a great way to increase horsepower. The unique thing about turbo systems is that it comes in two flavors; carbureted and fuel injection. Drag racing enthusiasts prefer the carbureted-type induction. I have heard great things about Suzuki’s flagship “Hayabusa,” because it comes stock with digital fuel injection. Turbo systems work excellent with that model, accordingly to Larry James.

When you factor in the horsepower gains, turbocharging is a safe and excellent choice for performance upgrades. So, if you decide to take your motorcycle to that next level, I want to make it very clear that you must lower your compression ratio and install heavy duty valve springs, connecting rods and a higher volume fuel system. You can run stock pistons, but be sure to check the compression ratio of the model you currently own by referencing a good service manual. If you follow these tips, everything should work well for you.

A special thanks to Larry James for his technical assistance in helping write this article.

Next month; “Winterization Made Easy!”

B-safe out there!

By Dave Miller