Is All Exhaust Backpressure Bad

The most basic understanding of an engine centers around how air flows through it. Air gets drawn in, compressed with fuel, then pushed out. The idea is simple enough, but the physics behind it are incredibly complex. Decades of debates have turned half-truths into facts, and Banks Power has swooped in to correct the record and clarify a few of the myths, specifically about exhaust, that have been parroted for a long time.

I know these myths exist and can catch anyone, because I have been guilty of saying things like "an engine needs some exhaust backpressure" with a straight face myself. I've since learned better but the books that taught me aren't as illustrative and clear as this Banks video breaking down the ins-and-out of exhaust backpressure.

Backpressure is created by any restrictions or impediments to the flow of exhaust gases on their journey from the combustion chamber to the tip of the tailpipe. Some are necessary, like bends in the exhaust to route around suspension or running gear, while others are not required but sure are nice, like mufflers or turbochargers. Backpressure is nearly unavoidable but can also be leveraged for our benefit. The force to push exhaust gas out from the combustion chamber comes from the power stored in the crankshaft, and short of somehow creating a vacuum that pulls the exhaust out (we'll get to scavenging in a minute), there will always be a little power loss from expelling exhaust gases.

Which means the attempt might be to eliminate backpressure but the reality is trying to minimize it. Maybe that is where the myth of needing a little resistance comes from. Banks theorizes that the myth started with exhaust salesmen in the 1950s who were tasked with selling mufflers that were more restrictive than stock and said anything to make the sale. Could this have worked and worked well enough that people are still repeating it 70 years later? Maybe. We may never know.

What we do know for certain is that backpressure is not needed and cleaning up the path of exhaust flow is a relatively easy way to free up some horsepower. A well designed exhaust will actually use the pulses of hot gases to help evacuate other cylinders which allows even more efficient running. This is called scavenging and is actually negative backpressure. It can be great, but takes a lot of engineering to achieve. Log style manifolds will never have this, and most affordable header designs don't do it either. Banks has a great bench-top demonstration of how a header can scavenge.

So, is backpressure needed? Short answer, no. It's not always the lowest hanging fruit to freeing up power, but if you get into tuning your engine on a long enough timeline you will probably end up thinking long and hard about your exhaust. I know I did and am currently enjoying an increase in performance along with the great sound.

***

Check out the Hagerty Media homepage so you don't miss a single story, or better yet, bookmark it. To get our best stories delivered right to your inbox, subscribe to our newsletters.