There are four ways in which you can increase engine power:
- Increase the engine displacement by boring the motor out or stroking the crankshaft.
- Increase the engine speed.
- Improve the Volumetric Efficiency of the engine.
- Increase the air density.
Essentially, all these engine tuning methods seek to improve the quantity and efficiency of the air flow into and out the engine.
Engine capacity or displacement is measured by the formula (π/4 × bore2) × stroke × cylinders. The bore is the diameter of the cylinder; thus (π/4 × bore2) gives us the area of the cylinder. The stroke is the distance the piston travels from TDC to BDC and gives us the length of the cylinder. Multiplying these two measurements gives us the volume of one cylinder. Multiplying the volume of each cylinder by the number of cylinders that engine has will give us the total displacement of the engine. Thus, by increasing the area, length, or number of cylinders, we can increase the displacement of the engine.
Unfortunately we can't increase the number of cylinders so we're left with the area and the length. We can increase the cylinder area by boring the motor. This is the easiest way of increasing displacement, but is restricted by the thickness of the cylinder walls, and the space between the cylinders. We can also increaser the length by stroking the crankshaft. This is more complicated as it requires the offset machining of the big-end journals on the crankshaft and possibly on the conrods. If the big-end journals of the conrod cannot be ground, you must either find slightly longer conrods that will fit, or pistons with a shorter compression height, i.e., the distance between the center of the gudgeon pin and the piston top. Stroking is restricted by the clearance between the rotational diameter of the crankshaft and the engine block.
Increasing engine speed does not increase the power per cycle, but increases the rate at which power in produced as the number of cycles per time frame increase. In other words, power is being produced more often as the Otto cycle is being completed much quicker. Increasing engine speed above the red line of the stock engine generally requires a complete engine rebuild with forged pistons, stainless steel conrods, stainless steel crankshaft, and a more robust valve train.
The Volumetric Efficiency (VE) of an engine is the amount of air/fuel mixture that is ingested by the engine during the intake stroke, relative to the engine’s displacement. There are a number of factors that prevent a stock engine from achieving a 100% VE. Chief among these are restrictions in the airflow path of the intake and exhaust, valve overlap effects, and reversion.
Restrictions in the airflow on the intake side include the air filter, the throttle body, the plenum and runners, and the intake port. These restrictions can be overcome to some degree by fitting a high-flow air filter, and improving air flow through porting and gas flowing, especially on the cylinder head.
Restrictions on the exhaust system include the exhaust header, the catalyst converter, and the mufflers. Unfortunately, anti-emission legislation requires that the catalyst converter be retained on street legal cars but restrictions in other areas of the exhaust system can be overcome by fitting a free flow exhaust header and free flow exhaust mufflers.
Fitting a free flow exhaust system will also reduce reversion, which is the flow of exhaust gasses back into the combustion chamber. Reversion causes contamination of the air/fuel mixture and takes up space that the air/fuel mixture should fill, thus reducing volumetric efficiency. Too much back pressure in the exhaust system will cause reversion. As Bre suggests in our exhaust guide, fitting a free flow exhaust header that is slightly larger than the exhaust port on the cylinder head reduce reversion but an anti-reversion (AR) header that is specially designed to inhibit reversion would be even better.
Denser air produces more power because it has more air molecules per volume. There are two ways in which air density can be increased – by lowering the air temperature, or increasing air pressure. Unfortunately, we can't really lower the air temperature but we can increase air pressure. The easiest way to increase air pressure would be to drive at lower altitude but this isn't really practical. The other way is to use forced induction. The three forms of forced induction are:
Bre doesn't believe NOS is forced induction, but that's his problem. Forced induction is the easiest ways of improving engine power and if done correctly, you can easily increase power by up to 50%! Now that's major engine power! Hhowever, there is another simple way of improving car performance but not engine performance – weight reduction.