Just wondering what you guys think about this comment regarding backpressure from someone on another forum:

If you do a search for posts about backpressure, gutting cats, and exhaust sizing, you'll see lots of quotes i've thrown up and crap like that.
But the basic gist of it is that people always say that you don't want a big-ass exhaust on your car because you'll lose backpressure, and you need backpressure to make low-end power. *shudder* I can't believe i just typed that.

It's not true. What you need is good exhaust flow to make power in any rev range. Having backpressure is a side-effect of having good flow at lower RPM.

The problem is if you have a small exhaust port that feeds into a massive cavity of a pipe, the exhaust gasses expand to fill the piping. Expanding gasses cool down, cooler gasses move slower--simple thermodynamics.

When the gas slows down, it prevents the cylinders from emptying as fast as when the gasses are hot. You don't want to lose exhaust gas velocity because you'll lose the cylinder scavenging effect, whereby exhaust gasses are being pulled out of the cylinders during the exhaust stroke. Any gasses that are left in the cylinder after the exhaust valves close is that much less space available for incoming air during the intake stroke.

So, there's more backpressure with a stock exhaust, but that's a tradeoff you make to get proper cylinder evacuation.

So this might not make sense so far--why is cylinder scavenging good if you're having more backpressure along with it? Backpressure is bad because the engine has to fight against it, but realize that the piston is not completely pushing all of the air out of the cylinder on each stroke--there's always room at the top of the cylinder. If the piston isn't pushing all the exhaust gasses out of the cylinder, then how is it getting out? Well, that's thermodynamics for you once again--the gasses move from the high pressure area to the low pressure area in the exhaust pipe. But it won't be drawn out as well if the gasses are expanding on the other side of the port.

The story changes when you've got a turbo on there of course. You're muscling the gasses around by sheer force in that case. Stick a phattie exhaust on there and don't worry about cylinder scavenging because you'll more than compensate for it with valve overlap.

So, that's that. Backpressure = BAD, ALWAYS. Exhaust flow = IMPORTANT, ALWAYS.

Keep in mind I'm no expert, so cut me some slack on the details. But what I just wrote should be enough for most of us to believe... I hope.



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i believe that is from WheatKing, ASOG administrator. Contrary to popular and uneducated belief, that is the way things work. Backpressure is bad, period. Exhaust gas velocity is the most important aspect.

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yeah... there was an article in a recent import magazine (import tuner...maybe???) about exhaust flow and backpressure. There was an automotive engineer who confirmed...

BACKPRESSURE = BAD
VELOCITY IS WHAT MAKES THE DIFFERENCE.

Tim
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Yeah, backpressure is a bad choice of words, but it's probably a little easier than explaining thermodynamic principals.

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