Engine Tuning/
Forced Induction

Introduction

The power that a car produces is determined mostly by how much fuel it can burn in a given amount of time. For that amount of fuel to be burnt, a certain amount of air is required. More air equals more power. The easiest way to get more air in is simply to make the entire engine bigger, but that has some disadvantages:

The entire machine becomes heavier, which hurts handling, performance, and fuel economy
The engine becomes bigger, which can cause problems in fitting it in a small-medium sized car
Bigger engines usually consume more fuel than smaller engines

So how do you get more air in an engine which is still the same size? You compress it, either by a turbocharger, which is driven by exhaust gases, or a supercharger (or compressor) which is driven by the crankshaft. Generally a turbo or supercharger creates .7 to 1 bar (10 to 14 psi) of boost, which means you get 70 to 100% more air in and an almost equal amount of extra power.

A supercharger and a turbocharger

Picking the right size

A big turbo might seem like a good idea because it is capable of supplying a lot of air to the engine. But a bigger turbo needs more exhaust gases to spin up to speed which can cause too much "turbo lag" on smaller engines. turbo lag is caused by the fact that it takes the turbo about .5 to 1.5 seconds to provide full boost because the exhaust gases need to spin the turbo up first before pressure is created. A supercharger will provide boost immediatly because it is driven by the camshaft. Because it is, it doesn't improve fuel economy that much, driving it takes about 10 HP of power which is taken from the engine itself rather than from it's waste (the exhaust gases).

Boost control

Turbo boost needs to be controlled for several reasons:

Too much boost can damage engine parts
Too much boost can cause the fuel mixture to become too lean which increases engine temperature (melting pistons are possible)
For the reason above, the ECU would normally prevent this from happening by going into safe mode, which decreases power (imagine a to pspeed of about 80km/h)

There are two ways of doing this: 1. with a mechanically controlled wastegate (controlled directly by the pressure on the intake manifold), which reroutes exhaust gases around the turbo turbine so that boost no longer increases. 2. with the ecu, this also drives a wastegate but in this case an electronic valve controls the opening and closing of the wastegate.
The first method has a clear disadvantage: becuase of how the wastegate works, exhaust gases start "creeping" through the wastegate from a boost level as low as 0.3 bar. This causes the turbo to spin up slower, hurting throttle response and acceleration, and decreased engine efficiency (and therefore higher fuel consumption). This problem can easily be solved by installing a manual boost controller (MBC). A manual boost controller prevents intake pressure from reaching and actuating the wastegate until a set level of pressure is reached. Usually an MBC will also allow you to increase maximum boost.
On an ECU controlled turbo the same can be done by remapping the ECU.

A manual boost controller

Intercooler

If air is compressed, it's temperature increases.We know that warm air carries less oxygen than cold air (which is why a 1 Bar turbo doesn't create 100% extra power) so we want to cool the air down before it goes into our engine. This is done by using an intercooler. Cars that come with a turbo will usually have one unless the turbo is very small and boost pressure is really very low. Air that comes from the turbo is routed through the intercooler which in turn is cooled down by the wind as you drive, much like your radiator which cools your coolant. Factory intercoolers are normally not very big or effective, quite a big gain in power can be made by mounting a bigger and more free flowing intercooler. Performance intercoolers are usally mounted at the front of the car to get the biggest amount of cold air going through it.

A Front mounted intercooler

Blow off valves

On petrol engines, closing the throttle valve (by lifting your foot off the throttle) will cause a high pressure surge in the intake channel. This is because the turbo has a certain amount of mass and can't just stop spinning suddenly. This causes two problems: Increased wear and stress on the turbo and other intake parts, and the fact that the turbo will stop spinning as back pressure suddenly increases. A blow-off valve, or dumpvalve, allows the compressed air to be released into the atmosphere so that the turbo will continue to spin. This decreases turbo lag after, for example, shifting gears, or just lifting your foot off for a second while cornering. A blow off valve can be adjusted so that it doesn't dump air while the engine is running at maximum boost but only 0.1 or 0.2 bar higher. There are also valves that bypass the dumped air back into the intake (to the part ahead of the turbo).