Does Your Mustang’s Exhaust Need Back-Pressure?
Jason Fenske from Engineering Explained puts this issue to rest.
Sound is a critical component of the driving experience, and the proper exhaust tuning can help your car make more power. It makes sense, then, that so many people select aftermarket exhaust systems as the first modification they make to their cars.
But there’s a debate about what that exhaust system should achieve. A restrictive system is no good. Everyone knows that. A system that’s too free-flowing, however, won’t make power, either. In the past, it was assumed that this was because back pressure — pressure from exhaust gasses in the system — was necessary to make horsepower.
That’s not the case, though, and here’s why.
Same Results, Different Reason
The time-honored belief that it takes back-pressure to make torque is not an exact science. It more-or-less holds that there needs to be some load on the engine so that it has a force to counteract with the power stroke, but even that is a lousy attempt at rough science.
Scavenging vs. Back-Pressure
Fenske lays out the basic premise right at the start. He says that the video will explain why exhaust back pressure is bad and will also touch on exhaust velocity and exhaust scavenging, which are both good.
To boil things down, the systems that in the past might have been recognized for having enough back pressure are in fact systems that are conducive to scavenging and, therefore, high exhaust velocity. In Fenske’s words “you want some balance of velocity and minimal back pressure.”
It takes a good exhaust design to accomplish this, though, because the best exhausts make use of scavenging. Scavenging is an effect that occurs when a change in piping diameter sends a pressure wave back into the combustion chamber, helping force out leftover gases.
Timing turns out to be the X-factor in exhaust scavenging. As Fenske explains, the desired effect is for the negative pressure wave to “arrive, help push out those remaining exhaust gasses and help pull in fresh intake air.”
Motoring enthusiasts will explain that an engine is nothing more than an air pump, and this is a great example of how good engineering begets a more efficient air pump.
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