#TechTip: Back Pressure Vs Low End Torque

When discussing exhaust, you’ve undoubtedly heard the mention of back pressure and its effects on low end torque. The common statement is that you need back pressure for low end torque. So, is this true?

Where this idea comes from

An exhaust system’s purpose is to evacuate gasses from the cylinder, and logic would say that a bigger pipe would evacuate more of those gasses. This “bigger is better” idea doesn’t always ring true, however, and there can actually be power loss at lower rpms when the exhaust diameter is increased. This is due to a decrease in exhaust velocity which leads to the presence of turbulence. This turbulence can hurt performance, and that leads people to believe that less back pressure causes a loss in torque.

To better understand this, you should understand what these terms mean:

Back pressure is basically the pressure that opposes the desired exhaust flow.

Exhaust velocity is how fast the exhaust is moving through the pipe. (To an extent, a smaller diameter pipe will increase velocity, and a larger diameter pipe will decrease velocity)

Turbulence is the inefficient tumbling of exhaust flow cause by a lack of velocity

Basically, the optimum exhaust system balances between having low back pressure and high velocity while avoiding turbulence. It’s good to reduce the restriction on flow, but if too much velocity is lost, that’s not good either.

Exhaust scavenging

Another important aspect that ties into good velocity is what’s called exhaust scavenging. Exhaust pulse scavenging works on the concept of the idea that exhaust comes in “pulses” which exit each of the cylinders on the exhaust stroke. A 4 cylinder engine will have 4 pulses per 2 revolutions, while a 6 cylinder will have 6, and so on. Behind these “pulses” exists a small vacuum which helps to pull the next pulse along. The higher the speed of these pulses, the more effective the effect of the scavenging effect is, and the better the exhaust gasses will be pulled through the exhaust system.

So, is back pressure necessary for low end torque?

In short, no. Velocity and scavenging are much more vital to making good power than the amount of back pressure in an exhaust system for a 4 stroke engine.