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.
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.
Very nice, compact yet informative article. There is a calculation using cylinder size, number of cylinders and exhaust length that will get you in the ball park for exhaust bore, can’t remember it now though, it’s in a book some where.
Appreciate it. That would be a great calculation to have handy! Let me know if you find it!
If you find it I’d like to know too :)
if you find it can you make a blogpost about it?
I think hot rod magazine listed it too recently, check their archives because it is a relationship between these two things (pick the bigger one) horsepower and displacement. IIRC, it was pretty linear and anything under 400 horsepower or CI didn’t need more than 2.5”
Problems with CT again. Keeps saying “rate limit exceeded try later” and I can’t upvote posts, nor can I comment. (Serms to work fine on the app, but not on computer)
On-topic: So are you saying that the biggest possible exhaust pipe on a Civic isn’t good? That can’t be true.
Lol anyways this was nice post. I kind of guessed that it has something to do with flow and turbulence that the big pipes hurt low-end torque. But I always thought until now that backpressure on smaller pipes gave the better low-end torque
Could back pressure affect how an engine runs? I changed the exhaust on my dirt bike and it won’t start. I cleaned the carb, air filter did an oil change, and even cleaned out some of the fuel lines. Could the exhaust be keeping it from starting?
Is it a 2 stroke or a 4 stroke?
Maybe the jets? You need to rejet the carb when you change exhaust
Now do one for 2 strokes!
excellent, couldn’t of said it better myself
just call ‘em like I see ‘em
A 4 cylinder will only have 2 pulses per revolution in a four stroke engine.
no! i think a 4 cilinder will have 8 pulses pe 4 revolution in a four stroke engine!
Eh, just get some anti-reversion headers
I love these #TechTip ! You give us the basic idea without getting into much detail, which is inviting to read!
Alright here it is. “Basically, a 400hp engine is quite happy with 2 1/2-inch exhaust. It doesn’t matter if it’s blown or nitrous fed as long as it doesn’t make over 400 hp. If you’re going to be much over this in the way of power (i.e. if your base engine is 350 hp, but the bottle drives it up to 500 hp), you will likely need 3-inch duals. The bottom line on power enhancers is, how much of the time will it be run in this mode? If it’s very limited use, stick with 2 1/2-inch and enjoy the sound.” http://www.hotrod.com/how-to/engine/ccrp-9812-2-quarter-inch-exhaust-3-inch-exhaust/