BUILDING A BRUTE
We’ve looked at what turbos and superchargers do – but what do you need to do to the rest of your engine? Well, the cheapest and easiest option is just to lower the compression with a thicker head gasket. A couple of hundred quid here to fit one will let you use a low-blow turbocharger or supercharger, to make up to about 250bhp, using super-unleaded petrol.
Want more? A lot more? Then your stock pistons, con rods, clutch, crank, gearbox (the bottom end of the motor, basically) all need to be beefed up to deal with the extra grunt going through each component. Since turbo engines don’t usually add any more revs, the power gains are made by increasing the torque – the turning force at the crankshaft.
If you want your engine to hold together then you need a lot of stronger components. Starting with the pistons, which have a doubly tough job: they have to deal with the hugely increased combustion temperatures and pressures, while transferring the extra torque down to the con rod. Forged aluminium is the usual plan, with firms like Wiseco and JE making hugely strengthened slugs for most of the common turbo bikes. You'll also get a choice of bigger bore pistons for more CC, and often a choice of compression – making the top of the piston lower, or machining pockets in it can drop the ratios to the 8-10:1 we want for forced induction.
Next is the con rods, which are simply much stronger, ideally while not adding too much extra mass. Wiseco, Carrillo, Wossner all make super strength rods, often using a deep ‘I’ or ‘H’ cross-section shape, and very high quality, extrastrong steel alloys. For most blown setups, the stock crankshaft is fine. On a Hayabusa, the OE crank will cope with over 700bhp. Upgrades here are expensive and specialist: we’re talking billet crankshafts with longer strokes from firms like APE, and they’ll cost thousands of pounds.
All that extra pressure means it’s harder to keep the cylinder head and barrels attached to the crankcases, and the crankcases themselves need to be clamped together more tightly. When you’re doubling and tripling the power from an engine, the crankcases literally start to get pushed apart by the force of each ignition stroke. The answer here is extra-strong head and crankcase bolts/studs. In extreme cases, you can fit thicker 10mm bolts in place of stock 8mm, with bigger heads and washers, to spread the load more evenly.
Now, we need to get that extra grunt to the back wheel. First problem is the clutch – stock units will slip under a 50 per cent torque increase, so an upgrade is essential here – lockup units use centrifugal weights to clamp the plates together harder as the revs increase. Stronger springs, extra plates, and a tough billet clutch basket may also be needed.
Luckily, the gearbox on most Japanese bikes is pretty strong, and so long as it’s in good condition, will cope with a lot more than stock power.
Do all this, and you have what’s known as a ‘built’ engine, and it will cost you upwards of £5,000 all in. Hilariously, it won’t make any more power at this point (it will actually be slower due to the lower compression) – but when you bolt on your £7k turbo or supercharger kit, the engine internals will hold together when it starts putting out the 300-500bhp that the blower helps it make…