Turbocharging

How Turbocharging Works Why it Sometimes Doesn't, and What it Can do for Your Motorcycle

John Nutting

Turbocharging

In the world of go-faster motorcycles, the importance of the bolt-on goodie has reached almost mythological proportions. Use our product, add this component say the advertisements, and your motorcycle will be transformed from the miserable slug that emerges from the factory to the thrilling tool of excitement that a motorcycle is meant to be.

But while some of the products may impart a small degree of improvement just by the simple expedient of bolting them on. modest changes rarely offer more than modest results. Major modifications require that you either spend a lot of time adjusting the rest of the motorcycle to suit, or make a number of compromises in your goal of having the hottest bike on the block.

Except one. The turbocharger. Bolt a turbocharger to your superbike and you can expect the top speed to be increased by 20 mph or more and the standing quarter mile ET to be cut by at least a second. The maximum power output of the engine has been boosted by at least a third. In other words, a monstrous increase. And without altering the engine one bit.

The advantages of such a straightforward and simple modification are obvious.

But if turbochargers can offer enormous power increases for the hot rodding crowd they can also offer advantages in economy of running. A small engine with a turbocharger can develop as much power as a bigger and heavier engine. And because the car or motorcycle as a whole can then be lighter, overall performance, which includes acceleration and fuel consumption, can be improved.

Sounds like a sales pitch, eh? The claims above are valid.

But there is another side to the turbocharger.

Turbos began as work horses for aircraft and industrial engines, because turbos work best under conditions of steady throttle and heavy loads. They don’t respond quickly to changes in throttle opening. Most important for our purposes, the turbo units now on sale are in conflict with the inherent advantages the turbocharging principle offers for motorcycles.

You can trace this in the four-wheel field. Automotive turbos first appeared on exotic racing cars, multi-million dollar jobs. Development of winning engines took years, and the results were very different from simply bolting bits onto production engines.

At present, many car manufacturers offer turbocharging as an option.

We can thank the Environmental Protection Agency for that. Cars used to be like motorcycles are now, with high performance coming from large and efficient powerplants.

But the car rules changed with emissions and mileage rules. The good thing about a turbo is that when it’s working, you can get big-engine power, while when it’s not working, you have low emissions and good mpg.

Thing here is, properly installing a turbocharger on a street-going engine isn’t as simple as just bolting the blower into place. The motor must be strong and it should have lower compression ratio and valve and ignition timing tailored to the turbo.

For the final hurdle, to work really well the turbo itself should have been designed for the engine displacement it will be working with.

All of which goes to explain why turbocharged motorcycles aren’t offered by the motorcycle factories. Superbikes obviously have no power shortage to compensate for; when the factory wants more power, they increase the displacement.

But in 1980. things are beginning to change. All the ’80 models we’ve tried to date, the Honda CB750E excepted, have been slower than the ’79s. There are rumbles of limiting displacement, miles per tank have become more important and the monster motors weigh so much and are so complicated that there's a quiet groundswell of riders who’d like lighter, more efficient motorcycles that still offer high performance.

No surprise that various R and D departments are working on turbos for production. Maybe in a few years . . .

In the meantime, bikers can only use the turbocharger kits that are offered as aftermarket bolt-on additions to their machines.

Unfortunately, there are many limitations on the type of machines that turbochargers are adaptable to. And the reasons are not immediately apparent.

A turbocharger increases the power output of a four-stroke engine in a similar manner as a supercharger: Instead of the engine sucking in the fuel/air mixture with a vacuum created by the descending piston, the mixture is pumped in under pressure.

Normally, because of friction losses caused by the obstruction created by the bends in the inlet tract, the air filter, the valve guide and the valve head, a road bike will draw in perhaps only 80 percent of the engine’s displacement during each cycle.

On highly tuned engines this efficiency, called the volumetric efficiency, might reach 100 percent by harnessing the natural frequencies of the gas columns so that the gas inertia during each cycle aides filling of the cylinder. This is how a tuned engine develops more power; by compressing a higher amount of charge into the cylinder before each power stroke.

By pumping the mixture into the cylinder however, even greater volumetric efficiencies can be achieved, often up to 140 percent. In other words, there is 40 percent more combustible mixture in the cylinder than its swept volume. So the engine acts as if it’s bigger than it actually is. Hence more power.

There are two essential differences between superchargers and turbochargers. A supercharger takes a mechanical drive from the engine, say, from the crankshaft by chain or gears.

A turbocharger uses the otherwise wasted energy in the engine’s exhaust gases. Even though a proportion of the hot expanding gases in the engine’s cylinders delivers thrust to the pistons, the exhaust gases are still very hot and have, high velocity. Therefore, they can be made to perform work.

In the case of a turbocharger, the gases are fed into a centrifugal turbine which is directly connected to a similar centrifugal compressor, the whole assembly being very compact. The inlet of the compressor and the exhaust from the turbine are coaxial while the outlet from the compressor (to the inlet manifold) and the turbine inlet run tangentially to the turbine and compressor wheels.

Naturally, because one end of the turbo unit is handling hot exhaust gases (at around 1500°) and the other is compressing ambient air, there is a large temperature differential. The compressor side can easily be made from aluminum castings but the turbine housing is made of heat resistant ductile cast iron while the turbine itself is an intricate lost-wax casting in a complex alloy called Iconel. This contains a large proportion of nickel to provide high strength at elevated temperatures.

To provide a measure of responsiveness in an automotive application the turbine and compressor must have small diameters. Like any moving or rotating object the turbo shaft has inertia, so it takes a finite amount of time to build up speed, as well as lose it. Large diameter compressors would take far too long to respond to the> engine.

The diameter of a turbo for cars is around 3-4 in., enabling it to spin up to 120,000 rpm without ill effect. Nevertheless, attempts are made to design the steel alloy turbine casting with the minimum inertia by scalloping the periphery of the wheel.

Even though the turbo rarely spins to such lofty revs (turbo rpm is more often in the 60,000 to 80.000 range) lubrication is critical. Rolling element bearings cannot be used because of the centrifugal loadings on the rollers or balls.

Therefore a simple floating aluminum bushing is used, pressure fed from a separate source, usually the engine's oil pump, sealed at the turbine w ith a piston-ring seal to resist the heat and at the compressor with an O-ring. Rajay Industries, which makes most of the smaller turbos for car and motorcycle applications, recommends that its turbos be supplied with lubricant at a pressure of at least 15 psi. This can be met quite easily by most engines with plain bearings, but bikes such as the Suzuki and Kawasaki Tours have roller bearing crankshafts and oil pressures as low as two psi. so restrictors to build up the pressure for the turbocharger bearing are necessary.

Characteristically, centrifugal compressors (and likewise the turbines that drive them) have to run with high revs, to develop an adequate pressure difference.

Often this pressure difference, invariably referred to as the pressure ratio and indicated on a gauge as boost in psi above atmospheric can amount to three times atmospheric.

The compressor operates by flinging the air outwards at high velocity into a ring diffuser where the speed of the air is dissipated in the pressure build-up. High velocities are therefore necessary to create the pressure differential and these can only be created by the high speed of the compressor. At low rpm. say below 40.000. there is insufficient velocity to prevent the air leaking back past the vanes.

The turbine also requires a particular range of exhaust flow for it to operate adequately. Fortunately the matching of the turbine to the compressor is fairly straightforward within certain limits because they have to flow a similar amount of gas. After all. they are connected together, albeit on opposite ends of the engine’s airflow system.

But it is also crucial that the turbocharger is matched to the engine’s air flow capabilities. A number of turbocharger manufacturers build a wide range of turbos for a number of applications, which might range from small cars to massive truck diesels, and a turbo intended for a large motor will be completely w rong for one half the size.

Manufacturers offering kits for motorcycles have invariably been limited by the small range of turbos available to them. This is because the turbo manufacturers make their products for the most profitable markets, which are the car. truck, aircraft and boat areas, all of which use very large engines compared to motorcycles.

Rajay Industries in Fong Beach, California. makes the smallest turbos so it is inevitable that these have found their way onto bikes. Nevertheless, because the flow' needs of the smallest turbo compressors are at least 100 cu. ft. per minute to obtain a boost pressure of 5 psi, only the largest engines (above 750cc) respond well to the fitting of a turbo.

Even so, a 750 has to be spinning above 7500 rpm to develop this air flow. It will become obvious that very high revs are needed even for the largest superbikes to work well with the available turbos. Up to now. this hasn't been a great hardship since most of the people using turbochargers wanted them to provide the massive power needed in competition work. But now there are convincing arguments to use turbos to boost the power of smaller engines, below 500cc for road use.

The likelihood of this happening without the keen interest of the motorcycle manufacturers is remote. One of the main marketing pitches for offering a turbo kit is that it can transform the performance of the bigger superbikes into something approaching an out-of-control Frankenstein’s monster. To persuade a company like Rajay Industries, which is, with all respect, a small concern within the turbo field, to invest the $250,000 needed in development and tooling for a completely new small turbo for a smaller bike that wouldn’t have such stunning performance is inviting financial disaster.

The use of turbochargers for road motorcycles and cars is still in its infancy. One of the abiding criticisms of a turbocharged engine is call “lag.” Not only does the turbine have inertia and take a finite time to build up speed, it also depends on the engine developing exhaust energy to drive it. This will not occur unless the engine is running at a large throttle opening against a load such as in hard acceleration. And, in the case of motorcycle engines using automotive turbos, only with high revs.

It is therefore inevitable that there will be a short period between dialing on the throttle and obtaining the desired amount of power; a certain volume of gas has to make its way through the engine to the turbine before the turbine speeds up to make boost in the compressor. Then the compressor will push more gas through the engine, and so on.

But an application such as drag racing provides a perfect backdrop for the turbocharger. Engines are working hard and there is no letting off on the throttle (as in road racing when the rider is braking for a corner) during which the speed of the turbo can drop.

Additionally, once the turbo is spinning at its optimum rpm, inertia of the turbine and compressor wheels will tend to keep it that way, even during power shifts. In fact, after a shift to a higher gear, a turbocharged engine will make more power, since a characteristic of a turbo running at constant rpm is that it will make more boost with a lower flow, such as when the engine is turning lower rpm after a gear change.

At normal freeway speeds, a large engine will make no boost at all because the throttle is barely open. Only during acceleration will the engine start to perk up.

Fortunately it is possible to persuade engines to start developing boost at low'er revs by adjustment of the turbine housing shape. If the exhaust gases are compressed into a smaller area on entering the turbine blades the turbine w ill spin faster. This can make the engine more responsive, developing boost below 4000 rpm but at the expense of maximum power. At high revs however the turbine housing will choke the exhaust system and limit flow.

It might have become apparent that with the high pressures in the inlet manifold, how' tortuous the inlet mixture's path becomes is largely immaterial. In fact you could say that once an engine is turbocharged, provided it can stand the revs, it doesn’t really matter if it has double overhead cams, overhead valves, sidevalves or whatever. The mixture will be blown into the cylinder regardless. But it is important to ensure that the exhaust system delivers as much smooth, high pressure, hot exhaust gas as possible to the turbine. Therefore the exhaust system must be as short and with as few restrictions as possible. Neither must it leak, so all the joints should be strong and flex free.

The only limitation on power with a turbocharger is the ability of an engine to resist the massive heat input and the inevitable detonation that can occur when very high pressures and temperatures are developed.

To limit detonation and the creation of too much boost a waste gate is used in the exhaust system. This is a simple manifold pressure-operated valve that releases exhaust gases to the air when a certain preadjusted amount of boost is reached.

At very high boost pressures, the only way to prevent detonation is to either use a special, high octane fuel such as H & H racing gas or use water injection, or both. Injection of water into the intake of the carburetor lowers the temperatures that high pressure ratios create and therefore also lowers the combustion temperatures.

Lowering the compression ratio of the engine, particularly when the normal compression ratio is over 9:1, can help in limiting detonation, but at the expense of low-end and mid-range power. Similarly with ignition retard. Because the combustion pressures peak more quickly with the high charge density in the cylinder, the ignition should be retarded to suit. In fact General Motors uses a detonation sensor that automatically retards the ignition at the point of incipient detonation to avoid engine destruction.

Such sophistication isn’t needed for turbos on motorcycles, at least not yet. Perhaps not ever. Turbocharging does add complexity and cost. It may be that motorcycles never get so crippled by legislation that they need turbos to get back some of the losses. Or turbos may come into common use and we'll all be enthusing over 500cc superbikes that provide lOOOcc performance.

Meanwhile we can sample those offerings being made by today's aftermarket turbocharger companies.

They are not perfect.

They are a quick and effective way of getting big performance.