Head porting makes the intake and exhaust surfaces smoother (and larger) so that less turbulence is created and allows the air/fuel mixture to move faster. A port job would typically be accompanied by a larger cam (responsible for opening and closing the intake and exhaust valves). Obviously a larger cam will allow the valves to open slightly longer-permitting more intake and exhaust to flow.
If you want to make big numbers on the dyno you'll need big displacement, as Graves suggested. All modern four-stroke motorcycle engines are oversquare-the bore (cylinder diameter) is larger than the stroke (distance the piston travels in the cylinder) is in length. If you're after more low-down torque you would increase the stroke for more leverage on the crank, while making the bore bigger and keeping the stock stroke will offer more overall horsepower with a possible loss of bottom-end torque. Obviously the two combined will make even more impressive power, but reliability comes into play because the cylinder head loses strength and the ability to effectively dissipate heat as these modifications are made.
One thing to consider is that the wilder the build (larger pistons, cams, etc.), the more expensive and less reliable the engine becomes.
Power delivery is smooth and linear throughout the rev range. Gains can range from 3 (mild tune) to 30 (big-bore kit) horsepower.
Best Application: Racing and competitive applications.
Average expense: $500-$5000.
Exhaust gases power a turbine that sucks in fresh air and forces it back to the intake system-simple, yet highly effective for big horsepower.
Turbo
Lately we've seen loads of fat-tire customs sporting a turbo system, but generally this is more of a bragging device than an applied application. Occasionally the bikes are so overweight that they require a turbo simply to help them lumber down the road, however.
For the most part, turbochargers are saved for the dragstrip racer or top-speed shootout competitor because of the violent top-end power and massive power gains; roadracers would typically struggle with the abrupt variations in power delivery.
At its most basic level, a turbocharger simply uses exhaust gas to compress outside air that's then channeled into a highly pressurized airbox and forced into the intake system. Obviously, more fuel must be paired to the increased amount of compressed oxygen.
A serious airbox is necessary to withstand the pressurized air being pumped into it from the turbo. Billet aluminum does the trick quite nicely.
Power delivery is violent and typically kicks in toward the upper rev range as the turbo spools up. Gains of 100-400 horsepower (when combined with support mods) can be expected.
Best Application:
Drag racers and competitive applications are ideal, but some street riders run low-horsepower systems for style and performance.
Average expense: $3500-$6000 parts and installation.