To save costs, your typical stock exhaust uses small diameter, crush bent pipe. Crush bends are easy to make in mass production. However, crush bends can reduce the flow of a pipe by up to 50%. Your typical exhaust system made by the local neighborhood muffler shop is also crush bent. The best exhaust systems, like most Japanese pre-made exhaust systems, come with mandrel bends. Mandrel bending is done by a special machine that uses a non-crushable insert or mandrel that goes into the pipe while bending to prevent it from becoming crushed.

Some self-proclaimed motor gurus state that you should not run too large of an exhaust tube on your car because engines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. Remember this, it is one of the most common performance misconceptions out there. Read on and you will be able to argue with and break off any self-proclaimed expert on the subject!

You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts.

If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation.

The stock exhaust system on most modern rice rockets is often so well designed nowadays that just switching to a high performance cat back exhaust often does not allow for huge gains of power. With the exception of turbo cars, you can usually expect only about a 2-10 hp at the wheels difference on a stock car. The power gain will usually be the greatest from just below the torque peak to the tach�s redline. On cars that are modified with headers, intakes, cams, headwork, etc., you can expect bigger gains with the addition of a cat back exhaust due to the higher flow volume that these mods produce.

As turbo cars are very sensitive to back pressure, you can expect much larger gains with them, especially if the boost is turned up. A free flowing exhaust usually allows the turbo to spool faster also. A turbo car usually gains from 8-30 hp at the wheels depending on how bad the factory exhaust was and how high the boost is turned up over stock.