#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.
Comments
Sooo simple with 4 strokes not like 2 strokes
True that! the fact that there are valves helps a lot xD… The expansion exhaust on a 2 stroke works by pulling air/fuel mix to the exhaust and back to the cylinder to get some kind of “boost” isn’t it? I have a 2 stroke to repair and I have no experience with it … Every info is nice!
Back pressure doesnt seem to be “so” important, can dent a pipe quite much without loosing much power.
https://www.youtube.com/watch?v=azPKIjxmmdU
I am not a native speaker, so it might only be a misunderstanding. I was told that scavenging was the effect that you can have when inlet and exhaust valves are open at the same time and the incoming fuel air mixture is able to fill not only the cylindrical volume but also the compression volume. So the delivery degree is more than 1,0. This would be the case with turbo cars with a higher boost pressure than exhaust pressure.
So I was wondering if the effect I am describing, has a different name in English because I remember that my German engine professor in a lesson taught in German specifically used the English term scavenging.
Does this apply to diesels as well?
Not really with turbo applications as the the turbocharger supplies the initial resistance. I say that since most diesels come paired with a turbo.
Turbo is all the back pressure I need ;)
Removing the contents of my cat and deleting the rear muffler helped my old Volvo breathe, however it might be just the cat being filled up after 20 years of usage. It has a lot more to deliver when accelerating now.
And THIS, Honda owners, is why your car doesn’t need a 3 inch exhaust.