Over-the-counter turbochargers have been available for big Jap four-cylinder bikes for as long as there have been big Jap four-cylinder bikes really, but these bolt-on performance kits are only as good as the amount of work that's needed to be put in to make them work properly and harmoniously with the engine. All four members of Nippon's Big Four Boy's Club, namely Honda, Yamaha, Kawasaki and Suzuki produced production turbocharged motorcycles in a mad fit of Eighties one-upmanship: a corporate war between the giants that spawned a trio of technically over-complicated and bizarre motorcycles ... and the world's first successful turbocharged production bike.

Honda were the first into the fray with a turbo version of their CX500, the bike that launched a million yawns and is undoubtedly the all-time train-spotter of the motorcycling world, and to produce a turbocharged version of this was about as useful as giving a knife and fork to a Rwandan. Yamaha also had a bike that was as bland as a family-sized bucketful of the Colonel's fried chicken lips - the XJ650 - and putting a turbo on that particularly boring motorcycle produced nothing more than a boring turbocharged motorcycle that had trouble keeping up with itself. Suzuki also had a middleweight key to the boring cupboard in the form of the GS650, a motorcycle that had the charisma of a French waiter and a level on the exciting scale that ranged well below Leonard Cohen and only slightly above a dead cat but, in a hasty attempt to stay in the game they turbocharged it nonetheless, and called it the XN85, a name as boring as the bike was ugly.

There are a number of ways in which you can install a turbocharger without necessarily cluttering up the clean lines of your Harley as the Buell at the top of the page demonstrates ... ... once you've tidied up the exhaust's plumbing, or course. And this is before it's hooked up to the intake!

Now the Japanese are well known and respected for producing mind-numbingly fast and reliable motorcycles, so to fit turbochargers to three of possibly the most bland and unsporting motorcycles this side of a Harley-Davidson 45 sidecar outfit towing a trailer can only be explained by the nip designers responsible for all three of these wet farts succumbing to the long term effects of eating too many mercury-contaminated raw fish and dolphin sandwiches.

After the other three had shot their corporate bolts Kawasaki jumped out of their silk pyjamas and gave them a severe kick in the organs génitaux with the ZX750T: the only real sports production turbo offering that had any sort of street cred whatsoever. The turbo Kawasaki was by far and away the best of the bunch: a bike with the aesthetics and menace of a switchblade with a simple, effective and compact turbocharging system that would howl the red projectile along at 14Omph and turn in standing start quarter mile times in the high tens.

So what is a 'turbo', what does it do and how does it do it?

Well, a conventional engine's ability to produce horsepower is dependent upon the rate at which said engine can burn the fuel and air mixture that is fed to it by whatever means, be it injectors or carburettors. A turbocharger is a mechanical device designed to increase the mass of airflow into an engine and therefore increase the engine's capacity to produce power. In other words a turbocharger is a mechanical pump designed to force-feed more air into the engine; more air means more fuel, more power, more heat - and more strain on the mechanical parts of the engine.

Leaving the financial outlay facet of the equation aside for a minute, turbocharging - or to give it the correct and little-used title of turbosupercharging - really is a way of getting something for nothing. The gas coming out of your exhaust system is fast and hot, and following its exit into the atmosphere it serves no useful purpose at all other than upsetting the pencil-dick yoghurt and sandals pseudo-environmentalists ... and waking up your neighbours at three o'clock in the morning. A turbocharger's role is to harness this stream of hitherto wasted energy to produce more horsepower from within the engine.

In effect a turbocharger is a supercharger that is driven off the energy generated by the flow of exhaust gas, and differs from a supercharger which is mechanically driven from the engine by-way of belts, chains or gears, therefore using power from the engine to create more power. They both have their advantages in that a turbocharger is basically a turbine driven by the engine's exhaust gases without sapping any energy at all from the engine itself, while a supercharger uses power to make power, and has the advantage that the boost is constant and corresponds directly with the rise and fall of the engine's revolutions. They both have their disadvantages too: the supercharger's mechanics are complicated and bulky, and they've never been anywhere near as successful on roadgoing motorcycles as the smaller, lighter and simpler turbocharging unit has. The turbo has it failing s too, but we'll come to those in a while.

A typical turbocharger is made up of three basic components: a turbine housing, a bearing housing and a compressor housing. Instead of a conventional exhaust system, the turbocharger unit is connected directly to the engine by one or more exhaust pipes and the hot, fast-flowing gases are directed against the blades of the turbine causing it to spin.

This in turn, rotates the shaft and the compressor impeller, inside the compressor housing, which draws the mixture of fuel and air through the carburettor - or fuel injection venturi - and forces it through the snail shell like housing, where it is compressed and fed to the intake side of the engine under pressure. This scenario is what is known as a 'draw-through' application as opposed to the other common method where a charge of compressed air alone is blown through the carburettor(s) or fuel injection venturi and known, predictably enough, as a 'blow-through' application.

The blow-through turbo is fast becoming the turbo unit, with the added advantage of intercooling - something you only previously saw on the grill badge of Volvo truck. The advantage of blowing the air through the carburettor or fuel injector venturi is that this method gives far superior throttle response and therefore a great deal less 'turbo-lag' than the draw-through turbo. Turbo-lag is the big problem with turbos, and is the amount of time from when you open the throttle to when the turbo kicks in, and with a blow-through turbo there is only compressed air moving through the carburettor or EFI venturi where it picks up the fuel. There is a lot less preheating of the fuel in the turbocharger itself and, because the carburettor or injectors are usually in their conventional position within close proximity of the cylinder head, the throttle response is faster and turbo lag is greatly reduced.

Pressurised air is hot and the purpose of an intercooler is to cool the incoming pressurised air, thus lowering the temperature of the charge prior to its passage through the carburettor or injector venturi, in much the same way that a radiator cools the liquid in an engine by passing it over a greater surface area allowing it to cool down in the airstream. The main advantages of an intercooler are a reduction in combustion chamber pressure and lower running temperature for the engine as a whole: both factors reducing the strain on the working parts of the engine and adding to its longevity and, although bulky and difficult to conceal on a motorcycle, intercoolers will give a definite increase in brake horsepower when fitted in conjunction with a blow-through turbo. It's an easy estimation that 15% of the engine's available power - 1% per 10 degrees F - will be lost through heat on a blow-through turbo application without the benefit of an intercooler. There are various methods of cooling intercoolers: an air-cooled engine merely passes the air through one or more 'radiators', whereas a liquid-cooled engine has the advantage of using its coolant as a jacket for the intercooler 'radiators'. There are other options such as freon gas or dry ice, but on a road-going motorcycle these methods would be too temporary and / or too complicated. There is however another vital component within the turbocharged package and that small, but perfectly formed item is the wastegate. As its name suggests the wastegate is a preset pressure release valve that expels surplus exhaust gases, and on many modern turbo units wastegates are an integral part of the turbocharger assembly.

There are many different turbocharging units on the market that are either produced specifically for motorcycle use and are available as bolt-on kits - as Steve Kelly has described elsewhere - or you could have a go at adapting one of the small turbo units that are fitted to many imported cars.

Many people with varying degrees of success have tried the latter suggestion, but unless you really know what you're doing a bolt-on kit is the obvious answer. Even then, turbocharged motorcycles do not suffer fools gladly and the slightest leak on the intake will melt a piston like a Mars Bar under a sun lamp. Bolt-on turbo kits have been around to fit Harleys for years, but whereas a turbo on a big Jap four if anything enhances the look of the bike, all of the street bike Harley applications that I've seen look like something from Willi Wonka and the Chocolate Factory. The only turbocharger unit that 1 have previously seen on a Harley-Davidson that both worked well and looked good was the Australian-built and developed Natli Turbo: but I've got to admit that 1 am impressed with what I've seen of the Aerocharger set-up that Steve Kelly tested on a Harley in Daytona. With a response time of 0.7 of a second, the near-absence of any perceptible turbo-lag at all would suggest that Aerocharger have conquered the previous problems with delayed throttle response, that has long been a fault with many of the turbos on street bikes. Although I haven't tried the Aerocharger unit myself 1 have ridden Luftmeister's turbocharged, fuel injected and intercooled 166bhp BMW R1100S, and that too was without any discernible turbo-lag.

But the big question is why would you want to turbocharge a Harley-Davidson in the first place? Stock Harleys are slugs. We all know that, and if you wanted a really fast bike you wouldn't have bought a Harley now would you: you'd have bought a Suzuki GSX- R1100 for less than half the price of a Softail and been able to piss on any street Harley that's ever been build and if you doubt that you're not living in the real world. It's not a difficult concept to understand - a well breathed-on FXR will produce, say 100bhp and weighs around 600 pounds - a late model GSX-R1100 produces 150bhp and weighs around 500 pounds. Get a Harley up to 150bhp and down to 500 pounds and you might be in with a chance, but there'll be precious little left that actually is 'Harley-Davidson'. The usual route to a little more power that can make a Harley fun to ride is the well-known carb-exhaust-ignition scenario that we as owners - and the aftermarket suppliers - know only too well and I would have serious misgivings about sticking a turbocharger of any description on a stock Harley-Davidson engine without first making a just few modifications to help it handle over twice its original power output. For a virtual one hundred percent increase in brake horsepower that an Aerocharger turbo set-up would give, it does seem to be reasonable value for money at just under three thousand Yankee dollars (approx 2K sterling) ... but then you are going to have to think about beefing up the engine because I don't care what anybody says, doubling the bhp is going to put a strain on those poor old standard Milwaukee engine internals, not to mention what it'll do to the transmission and clutch ... and then what about the suspension and the brakes? Making Harley-Davidsons go fast is good fun, but it's addictive and expensive: I know, I've done it and these new generation turbocharger set-ups could be fun ... and addictive ... and expensive, but then what else are you going to spend your money on? Pot pigs and toilet seat covers?