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View Full Version : The Backpressure myth


Joe@Trick Tuners
03-03-2010, 07:28 PM
This must be the most common myth on the internet. You can google "back pressure myth" and read for days, but i'll sum it up for you.

Its not the lack of back pressure that can cause a reduction in torque or horsepower, its the lack of velocity. High exhaust velocity is good, back pressure is BAD! This is why you never hear about a exhaust tip restrictor to boost low end torque.:eek: The origin of the myth comes from people that install too large of an exhaust system for their hp goals. If you take a honda civic with less than 200hp and stick a 3" exhaust system on you will loose some low rpm TQ (part throttle response) due to the loss of velocity. Velocity in the exhaust helps the headers out with their scavenging effect.

So removing the cats cannot cause a reduction in power because you did not alter the exhaust velocity, only the back pressure. If you were going for max hp and had lots of $$$ then you could try different size pipes and see which one gets you around 1-2 psi of exhaust pressure. This will ensure that you have adequate velocity. Or if you have even more $$$$ then you could measure the exhaust velocity. If you have 0 psi (like a 8" exhaust:() You will loose low end and midrange torque.

Now once you throw rpms into the mix it gets more complicated. The exhaust system needs to be designed for your target rpm. If you make the exhaust work best at 1000 rpm you are going to choke it at WOT, bad! If you size it for 6500rpm then you are going to take a hit on the low throttle, or low rpm wot areas. If its a drag car, size it near peak rpms. If its a road racing car you might want to reduce the size a bit so that you can get a little more midrange tq at the expense of high rpm tq (hp).

I have not done any type of testing like this, but my general rule is that you don't need dual 3" pipes on a mustang until you are over the 600whp mark.

Remember that the exhaust works as a complete system, from the heads, to the tailpipes, but the headers play the biggest part in the equation.

limited
03-26-2010, 04:40 PM
good post. I always laugh when people blame backpressure(lack of) for loss of power when it is the scavenging that is important.