TURBOS VERSUS SUPERCHARGERS
You can’t fit a quart into a pint pot, or so the old proverb goes. But one of the coolest things in bike tuning revolves around exactly that. Well, kind of…
Both turbocharging (as seen on the GSX-R1000 we caned on p52) and supercharging (as embodied in Kawasaki’s incredible H2) are based on squeezing an extra dose of power and torque out of a certain engine capacity. Riding one of those bikes is like riding with a much bigger engine – the H2 in particular is like a tuned ZZR1400 motor in a litre-bike chassis. But how do their builders make a small motor feel like a big one?
Well, simply put – they use a megapowerful fan that blows more air into the engine than the engine would normally take. Imagine one of the 250cc cylinders which sit inside a GSX-R1000 or a ZX-10R. When it turns over on the starter, the inlet valves open as the piston is moving down, and the descending piston sucks air in through the valves. At normal pressure and temperature, 250cc of air weighs about 0.3g. So we have a third of a gram of air inside our cylinder, as the valves close. Of course, it’s actually a bit less, because we squirted some petrol in as well, but if we stick with 0.3g of air then there’ll be an ideal amount of fuel to burn with that. The Kawasaki is very clever, so it’s worked out the correct amount, and the ECU has squirted that dose of gas in. Next, we compress the whole mix, light it up with a spark, then the resulting inferno heats up the air, and pushes the piston down, turning the crank and moving the bike.
Now, let’s fit our fan to the intake manifold, so that it’s blowing lots of air into the engine, at high pressure. Say it’s able to blow air at a measured 15psi – that’s on top of the normal atmospheric pressure. Now, when our inlet valves open, we’re blowing air in there at 15psi more than we did before. The air pressure was about 15psi (normal atmospheric pressure) last time, so by adding another 15psi, we’ve doubled the pressure. And that means we’ve doubled the amount
of air inside the cylinder: we now have about 0.6 grams of air. Guess how much more fuel we can burn now? Yep – twice as much. That means a bigger burn, more heat, and so more pressure on the piston and a stronger turning force (torque) at the crankshaft. That’s the science behind the ‘charging’ part of the supercharger and turbocharger equation. The other half comes down to how we power our fan.
One goal, two visions
Don’t underestimate the amount of work needed to pump this air – 15psi doesn’t sound like much, less than half the pressure in your tyres. But you need to keep that pressure up, while moving vast amounts of air – all going into the engine at 10,000rpm. Your mum’s big hairdryer – even her fancy Dyson vacuum – couldn’t move a fraction of the air that your engine uses, especially not at 15psi.
The supercharger is arguably the simplest design – it uses the engine itself to turn the fan. A drive from the engine – it can be a belt, chain, gear, whatever – turns the supercharger fan, which blows the high pressure air into the engine.
A turbocharger is a little more complex. It uses a small turbine, which sits in the exhaust, and is spun by the high-speed exhaust gases coming out of the combustion chamber. This turbine wheel is connected by a short shaft to a fan on the other side, which blows air from the intake filter through the inlet valves and into the combustion chamber.
Once we’ve made the fan spin, the rest of the process is largely the same on turbos and superchargers. We’ve got a lot of high-pressure air to stuff into our engine – but one bad thing is heat. Compressing air makes it hot and hot air is bad news for an engine: it makes less power because it’s less dense, and will make the already-baking pistons and valves even hotter.
So, if we can, it makes sense to cool the pressurised air down again. The Big CC GSX-R has a charge cooler under the tank, which flows water through the intake manifold to remove heat, a compact 12V pump moving the water through a small radiator in front of the main engine cooling rad. Kawasaki hasn’t bothered with a charge cooler on the H2, relying instead on its superior electronics and, possibly, richer fuel mixtures to keep things cool at high revs.
Fuelling a blown bike is a big old job. You need an ECU which can measure the boost pressure going into the motor, and adjust the amounts of petrol going into the combustion chambers to suit. Kawasaki’s obviously got a bespoke unit on the H2 but the Big CC GSX-R uses a variety of aftermarket add-on fuel computers, because (obviously) the stock ECU doesn’t have any way to measure the boost off the big turbocharger.
Finally, the other mod needed on a blown engine is to reduce the compression ratio of the engine. Normal bike engines can go up to about 13-14:1, before the charge starts to pre-ignite, knocking, losing power and damaging pistons. If you pressurize the air beforehand, you’re increasing the effective compression ratio, even if the geometry of the cylinders and combustion chambers doesn’t change. If you don’t cut the static compression, you’ll not be able to add much more intake pressure before pre-ignition and knocking wrecks your engine.
At its simplest, you can just fit a thicker head and/or base gasket, or a spacer plate, which moves the piston down the cylinder a couple of mm at Top Dead Centre (TDC), lowering the compression. The best solution though is special low-compression pistons. Using these, plus super unleaded fuel on the road can let you run up to about 16psi boost in a turbo motor. That gives the 300bhp we had while riding the K5 GSX-R. Fill the tank with high-octane race fuel, and you can bump the turbo boost over 30psi, which lets the Big CC bike make a stunning 492bhp – enough for a certified 240mph at RAF Elvington. Not bad going, eh?
Turbo or supercharger?
So which method of ramping up your bike’s output is best? As with anything, it all comes down to the equipment you use, who’s fitted it and what it is you’re trying to achieve. Both systems have their pros and cons.
Superchargers are easier to package into a powertrain, especially for an OE manufacturer. Look at the H2 – it’s just a small hump above the clutch, which houses the gear drive to the centrifugal blower unit. Senior Kawasaki tech folks have told us they looked at turbos, but that installing one in the best spot (close to the exhaust ports) interfered with the front wheel in a sports bike, and that the oil supply, coolant feed, support brackets and pipework was too much work for production. The Big CC bike works well, but has a lot of special handmade parts. Those would be hard to produce economically to the standard needed for a warrantied road bike.
On the other hand, a supercharger is a literal ‘drag’ on the engine. It takes power to turn the charger, several tens of bhp depending on the install, and that is taken off the engine’s final output. A turbo, by contrast, uses ‘free’ energy in the exhaust gas flow to drive its blower. That energy is otherwise wasted – indeed, a good turbo actually helps silence an engine, by absorbing much of the energy that makes the exhaust noise. Superchargers do nothing to cut the exhaust noise – and the stronger combustion makes it much louder. So supercharged bikes need a lot more silencing to make them road legal.
The other big difference is down to the point that the blowers start to work. A supercharger is spinning all the time (unless you have some complicated drive system that can be switched on and off), so it’s constantly pressurising the intake air. That’s good for low-down response, but it’s inefficient – even when you’re trickling along in traffic at 40mph, the charger is working away, sapping power and making more heat. And at low-middle revs, the intake plumbing has to get rid of the excess pressure – on the H2 around 6,000rpm, you can hear the dump valve constantly chirruping away as it bleeds off the extra, unusable pressure.
Turbos, on the other hand, depend on engine load – throttle position and revs, to make their compressors spin. The turbine wheel is spun by fast-flowing exhaust gas, so at small throttle openings and low revs, there’s not enough gas flow to spin the wheel very fast. On the Big CC GSX-R1000, there’s not much boost below about 7,000rpm. That makes the turbo bike feel a bit flat, but it also means that the bike is well-mannered, even economical at low revs. If you wanted to commute medium distances on the turbo Gixer, you could easily do so – in top gear, it will do almost 90mph without going into boost. Riding to Avebury to meet Al on the Gixer, I sat on the M4 at average speed and got more than 35mpg when I filled up. The other side of the coin is that the Gixer is incredible when it comes on boost: even in road tune spec, it has an incredible rush at 7k, like nothing else I’ve ridden. And for that reason I’m more inclined towards a turbo than supercharged bike. But that’s just me. I don’t think you’d feel disappointed by either option, they’re both insanely awesome.