Power Adders

Horsepower. Speed. Increase the former and you get more of the latter. Acceleration and speed are a performance junkie’s fix and a necessity for winning at the race track. Minor power output improvements can be gained with carburetor jetting and ignition advance curve changes on older cars, or with a cold air intake and performance programmer on newer vehicles, but getting sizeable increases used to mean spending a lot of time and money getting inside the engine with boring, stroking, bigger cams and heads, etc. Today, massive power increases are much easier to get with power adders.

Increasing displacement, high-flow carburetors and intake manifolds, ported cylinder heads, and high-lift, long duration camshafts are all about getting more air and oxygen into the cylinders so more fuel can be burned. But in addition to the complexity and expense there are other downsides to this approach. Most engines can only be made so much bigger with boring and stroking, a big cam may only work well in a certain rpm range, and regardless of modifications you’re still limited by the amount of air that can be pushed into the cylinders by atmospheric pressure.

Power adders like superchargers and turbochargers bolt on with no need to get deep inside the engine. Both force more air into the cylinders, increasing cylinder pressure above atmospheric and effectively making the engine perform like a much larger powerplant. Forcing more air into the cylinders increases the supply of oxygen so more fuel can be burned, thereby increasing horsepower. Forced induction can also overcome intake system deficiencies that would impede airflow in a normally aspirated engine. “Boost” is the term used for the amount that intake manifold pressure is raised above atmospheric pressure.

There are centrifugal, screw type, and Roots superchargers, but all are driven by the crankshaft via a belt. Air is drawn through the air filter, intake tube and throttle body into the supercharger, and is compressed by an impeller, lobe rotors or twin screws, depending on supercharger type, and forced into the intake manifold. A turbocharger consists of a turbine wheel and a compressor wheel that rotate on a common shaft in an aluminum housing. Exhaust flow spins the turbine blades, which in turn drives the compressor wheel. The compressor section of the turbo is part of the air intake system. Air is drawn through the air filter and intake tube, compressed by the compressor wheel and forced into the intake manifold. Because compression also heats the air, both systems usually include an intercooler.

Since the main goal of increasing the amount of air in the cylinders is amassing more oxygen, an easier way to raise the oxygen level is with a nitrous oxide system. Air contains only 21% oxygen while nitrous oxide contains 36% oxygen. When nitrous oxide is released into the engine, along with additional fuel, the heat of combustion causes the bond to break between the nitrogen and oxygen atoms, increasing the amount of oxygen, which allows the added fuel to burn and create more power. There are wet and dry systems. In a dry system, nitrous oxide alone is injected into the intake air stream before the throttle body. Wet systems can inject nitrous oxide at a single point where it mixes with air and fuel or at each intake runner where both nitrous oxide and fuel are injected just ahead of each cylinder.

Another injection system that can increase horsepower is water/methanol injection. Water/methanol injection reduces air intake temperature and boosts octane, allowing ignition timing and boost pressure to be increased. With this system an atomized mist of the fluid is injected into the intake air stream, where it vaporizes, absorbing heat and cooling the air charge. The cooler, denser charge provides more oxygen so more fuel can be burned, and slows the rate of combustion, preventing detonation and allowing more boost and timing advance to be used for more power. The methanol also cools and controls combustion, and as a high octane fuel itself, it effectively raises the fuel’s overall octane rating. Water/methanol injection is beneficial for forced induction and naturally aspirated engines.