Does Your Dodge Need Exhaust Back-Pressure? (Video)
Turns out there’s more to getting exhaust gasses out of the way than we thought. Here’s what engines need, and what they don’t.
Usually, when we see a guy in a YouTube video start out in front of a whiteboard, we move along. But when Jason Fenske of Engineering Explained is doing the talking, we’ll stick it out. This time he takes on the oft-touted “engines need some back-pressure to work properly” claim. Long story short, they don’t, and back-pressure is bad.
Fenske begins by going over the basics of engine operation. It’s a good watch if you’re just getting into the inner workings of your Dodge, and not a bad refresher even if you’re an old pro. The simple example of an air pump works: air in, air out. Though, with an explosion there in the middle, the “out” part becomes critically important.
“The whole purpose of an exhaust is to evacuate [from the cylinder] those spent gasses as quickly and as efficiently as possible,” he said. Fenske illustrates his point with an example. If the pressure outside were quite low, the pressure difference would be even greater. This allows the exhaust gasses to escape even faster. The converse is also true if the pressure in the exhaust were higher (back pressure) the gasses would not be able to escape as quickly, thus robbing the engine of power.
He then explains the difference between straight back pressure and what vehicles actually need: effective exhaust gas scavenging. Basically, he explains, the balance of velocity and restriction. He explains that you need some resistance because it creates velocity and thus better scavenging. Too much restriction and you get back pressure, and not enough gasses can flow out. Not enough, and you have almost no velocity and not enough gasses flow out.
Things then take a turn for the more complex. Stick it out, he explains why (albeit, indirectly) you want to go with a name brand header instead of a cheap one. Header length is of vital importance and could actually cost you horsepower if designed poorly.
“So, the pressure wave [from the combustion process] is traveling along. Anytime a pressure wave reaches a point where it changes diameter it’s going to reflect back a pressure wave,” he said. Then he explains that when the diameter increases, it’s a negative pressure wave. This means that negative pressure can be used to lower the pressure in the cylinder, which not only helps evacuate the combustion gasses, but it sets the engine up to more easily draw in fresh air on the next stroke. See, pretty important piece.
“Of course, if you tune this incorrectly, if you were to have the wrong length, you can hurt the performance of [the engine] if you have this (pressure) wave arrive at the incorrect time… you’d have even less power.”
Now, how do you explain that the next time you meet up with your friends and someone says your truck needs back pressure? Tell us your plan in the Forums.
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