5 THINGS YOU NEED TO KNOW…
Lower the compression
If an engine is going to cope with the hike in power from a turbo it needs to be set up so that it won’t melt when the dynamic compression climbs as the boost increases. A standard, first-generation Hayabusa with an 11:1 compression ratio can only take 6-7lb boost to give 260bhp, as it’s then nearer 13.5-14:1 and on the limit for road-going fuel. But if the compression ratio is lowered to 9.5:1 it can run 12lb of boost and deliver 450bhp.
Cooling is key
If you’ve ever pumped a bicycle pump with your thumb over the end you’ll know how hot compressed air can get. That temperature increase limits your power gain, but an externally mounted ‘charging air radiator’ known as an intercooler, married to a smaller oil-cooler style radiator in the airbox can drop those temperatures from 150 degrees to below 40 degrees. These coolers will add up to 15% more power for the same amount of boost, allowing a typical four-cylinder Japanese bike to go to 450bhp on road fuel.
Feel the pressure
Boost pressure is a measurement of the air that cannot get through your engine because it’s backing up as back pressure. If you design a turbo system to flow the air, you are going to get more power for less boost pressure.
The way to get road civility is to have the right-sized turbo which will give you a power curve that climbs and climbs at 45 degrees. If the turbo is too small the exhaust gases get restricted and flow faster so you get higher boost pressure for less bhp, which results in a narrow power band that could peak at 7-8000rpm, ideal if all you want to do is wheelspin and wheelie… A bigger turbo pushes that peak further up the rev range. A Stage 2 turbo kit with boost controllers allow you to press a button or flick a switch to go from, say 6lb to 12lb. Good on an empty motorway…
Choose your weapon
Large-capacity Suzukis have been the turbo lover’s choice for decades, so pistons, conrods, beefy nuts and heavy-duty clutches are widely available, but a set of custom-made pistons for a one-off Honda or Triumph will have a 12-week leadin. A turbo bike also holds its money. Take a £3000 GSX-R and spend £5000 on conventional tuning with gasflowed heads, big pistons, a race pipe and a remap it’ll produce 170-190bhp, but it won’t be worth much more than £4000 when you sell it. But a properly turbo’d Busa that’s worth £2500 standard and has a £5500 turbo fitted will hold its money and could go for £8000 upwards.
Cool the compressed air and you’ll gain a bigger boost
Don’t melt your pistons
Who wouldn’t want 500bhp from their Suzuki GSX-R?