The basic reason for forced induction (turbo or supercharger) is not to increase compression or the maximum pressure in an engine cylinder, it is to increase volumetric efficiency (the efficiency of the engine at drawing in air) although forced induction is the most effective method of upping compression.
Adding a turbo to a non turbo car
When a naturally aspirated engine suck air in, it can only draw so much in before the intake valve closes and seals the cylinder. The average N/A engine will pull in around 60% of its volume, so as a result is only 60% volume efficient. The more highly tuned an engine is the more efficient it will be. The best way to improve this is to force the air/fuel mixture into the cylinders (forced induction) thus filling them more. The average forced inducted engine runs at over 90% volumetric efficiency.
Some examples of the result. a 2.0 N/A motor will use approx 1200-1300cc of its capacity. A turbo'd or supercharged 2.0 will use 1800-1900cc, producing more power as it is able to burn more fuel. The more boost you run, the closer you get to using 100% but for some technical reason I don't understand, apparently that's not possible. But the main benefit from increasing boost is to increase the final compression ratio and getting a better bang from the air/fuel mix in your cylinders.
If you increase the amount of air/fuel in the cylinders, then as a result you will increase the compression, which could be too much. In order to keep the final running compression the same, the initial compression (without boost) has to be dropped to compensate. More air will enable more oxygen to be available for burning and with the addition of additional fuel the engine will release more power. Turbo charging is the best way to increase the efficiency of an engine. When adding a turbo to an engine which was not originally designed for a turbo there are some major complications to take into account.
Detonation or knock - this is where the fuel ignites under pressure before the spark happens. This will cause a piston to move in the opposite direction if it has not reached the top dead center and will have disastrous consequences for the engine. As the intake valve will also generally be open the whole intake manifold, injectors and air filter will effectively carry the exhaust of the combustion and many components of an engine will be ruined.
To avoid this from happening you will need to lower the compression of the engine, and or restrict the turbo to a low boost threshold. (Low compression engines plus a turbo will also avoid some of the turbo lag problems inherent in most turbo applications.) The best turbos to add to a non turbo NASP (Naturally aspirated) engine are small. To lower the compression you can go with a re bore and fit lower compression pistons, you can add a stroker kit to alter the compression ratio or you can get a larger head and thick head gasket thus increasing the cylinder size and reducing the compression ratio. You need to aim for a 7:1 compression ratio if you are adding a turbo, anything above 9:1 you will have problems. In all cases you should use the highest octane fuel that you can find as the higher the octane the more resistant the fuel is to engine knock.
If you can reduce the boost pressure to 5-7psi (as opposed to 25-35psi), and used the higher octane fuels available (e.g. Shell Optimax) you should be able to run a turbo on a standard engine with around the 9:1 compression ratio.
When adding a turbo you should also increase the port size, fit bigger valves and go with a larger exhaust header and system as there will be a much larger volume flowing through the engine. Fitting a boost controller will allow you to experiment on a rolling road while attached to diagnostic equipment to find the optimum boost pressure.
Most kits contain only the necessary parts to physically get the turbo onto the engine such as an exhaust header and the necessary intake plumbing to the air filter.
Fuelling will also need to be uprated, with a higher pressure fuel pump and larger injectors. Turbos are expensive but will add the most power for your money. You should allow about 40 hours for fitting.
The induction kit is around £70-£100 with COLD AIR FEED. Exhausts vary but aim for around £500 Sports cat is worth it if you ever have to replace your cat. (Fit both at the same time though!)
Whenever something needs fixing on your car, replace it with a high perfomance replacement so the upgraded become part of the servicing and maintainance costs - thats what I did with my Rover.
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