A cold-air induction system.
An alternative to modifying the stock air intake system that we've discussed thus far, is to either design and install your own cold air intake system, or install an aftermarket system. Your cold air intake system can be either a short ram induction system or a cold-air induction system. Of the two, a cold-air induction system would provide more horsepower, but it has its disadvantages. Let's start with the short ram induction system.
The Short Ram Induction System
A short ram induction system may be good for 4 to 8 horsepower and is relatively simple to design and install. It generally consists of a short piece of metal tubing and a high-flow air filter that is usually a conical filter. The metal tubing should have the smoothest possible bends and would usually be slightly larger than the stock intake system. Don't go too big with the tubing as you don't want to lose air flow velocity. In fact it would be best to determine the correct diameter of the tubing by using a water manometer to measure the air-flow rate as we've described in our section on the air filter and air box.
If your car has a crankcase breather hose that runs from the valve cover to the air intake, you may need to make a fitting for it on your short ram induction system and fit a hose of the appropriate length. In most cases this would be the only modification you'd need to make. The air filter would be relatively close to the engine so you shouldn't require any modifications to the engine or body.
The Cold-Air Induction System
Depending on the design, a cold-air induction system could double the power made by a short ram induction system because it would pick up colder ambient air either from below the car's grille or from the car's front wheel arch, whereas the short ram induction system picks up hotter air in the engine compartment; and engine compartment temperatures could be 30 to 50 degrees higher than the ambient air temperature! And warmer air is less dense than colder air, and hence makes less power.
Obviously the cold air intake pipe must be longer to reach the colder air, and it is going to need more bends. However, if there are too many bends, the horsepower gained by getting colder intake air could be offset by a loss of air-flow in the intake system. So try to keep the cold air intake pipe as straight as possible and as short as possible as the longer the pipe, the more friction the incoming air will experience as it passes through the pipe. Also, the intake pipe must be properly sealed so that the cooler air does not escape into the engine compartment where it is of no use.
Finally, you need to be careful where you place the air filter of your cold-air induction system and the air pick up point. An obvious concern is the possible contaminated by oil and dirt, or the filer being blocked by snow or mud. While these aren't really serious problems, if the air filter is positioned too low, it could suck water into the engine, which would be disastrous. Water is far less compressible that gas, and once it gets into the combustion cylinder it will cause hydraulic-lock as the pistons will encounter a virtually solid mass of water. The result could easily be bent conrods, a snapped crankshaft, and a totaled engine! A popular location for the air pick up for a cold-air induction system is the front wheel arch. This is not a good spot as the front wheel arch is a relatively low pressure area compared to the front of the grille, the front bumper, or the front of the bonnet, all of which would be a better location for the air pick up if you want to make more power. Some tuners prefer to use a bonnet scoop to draw cool air into the filter as this reduces the length of intake system.