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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.

Size mat­ters

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?

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