Why a cold air intake won't increase your car's horsepower
To create fire there are 3 basic requirements that must be met. You need fuel, air (oxygen to be more specific) and heat (every fuel has a minimum temperature at which it will burn). These 3 components form what is commonly known as the fire triangle, if any one of these components are missing the fire triangle collapses and the fire will be extinguished. (picture courtesy of wikipedia: https://en.wikipedia.org/wiki/Fire_triangle).
The powerhouse of your car also known as the Internal Combustion Engine (ICE) does exactly what its name implies, it creates small little explosions which turns a shaft that is connected to your car’s wheels. There’s a lot more to the system but basically that’s the purpose of the engine, burn fuel to make your car go vroom vroom!
Combustion is simply a chemical reaction between fuel and oxygen which creates energy and exhaust fumes. For a better understanding look at the equation below, known as the stoichiometric equation for the combustion of petrol. Take note that this formula would differ depending on the fuel used.
As you can see in the equation above, increasing the oxygen in the engine means that you can also burn more petrol which obviously increases the power output. This is the logic that most car guys use but something often forgotten is that there are two sides to the equation. Increasing the inputs will in turn increase the outputs produced.
Now you’re probably thinking to yourself, “Just add a big exhaust pipe, dummy” but it’s not that simply. An engine has a cycle of burning fuel and expelling the product gases. The speed at which it can perform this cycle is known as the rpm of the engine. If you’re interested in this cycle and the difference between 2-stroke and 4-stroke engines you can find a nice video about this here. With the cycle, after air and fuel is added and burnt the chamber is now filled with the exhaust gases and since you’ve just added a nice air intake you clearly want your engine to perform at its best so you need to remove all of the exhaust gases so that you can add the maximum amount of air and fuel in every cycle.
When the fuel is being burnt the chamber is closed so that the energy pushes the piston down but once this is completed the exhaust valve opens and the gases flow out of the chamber. What you want is to allow the gases the ability to freely flow out of the engine whilst also constricting the flow enough to cause a vacuum effect which helps suck the fumes out. Generally, mechanics use an old rule of thumb that for every 100 horsepower produced, 1 inch of total exhaust pipe diameter is needed. Obviously, this method is a rough estimation and in reality it can be a very complicated problem to solve. Learning how to accurately estimate the exhaust pipe diameter may take many years to learn and master but here are some key factors that fluid flow engineers take into account:
- The volume of exhaust gases produced, this can be calculated by taking note of the amount of air and fuel being burnt and using the stoichiometric equation shown earlier to calculate the volume of exhaust gases produced.
- The temperature of the exhaust gases. Heat causes things to expand especially gases so this needs to be considered.
- Blockages or constrictions along the pipe. This refers to catalytic converters, mufflers and sharp bends in the exhaust system.
These are just some of the things that should be considered when expertly sizing your exhaust system. Since it can be a daunting task to calculate the exhaust pipe diameter we have attached a link to a website that has a reasonably accurate exhaust pipe calculator, you can find the website by clicking here. Simply input your car’s power output and the number of exhausts you want and it will provide you with the diameter exhaust pipe you should use.
Just a small side note, in South Africa it is illegal to drive a car that does not have a catalytic converter because it plays a huge role in protecting the environment. We do not recommend removing it just to get a small boost in performance. There are in fact GT3 racing cars that keep their catalytic converters and still perform phenomenally on the track.