Muffler Placement, RPM Scavenging, and Backpressure

There is a lot of misconception about backpressure. There are two pressures in the exhaust. One is the gas (315+ fps) and the other is the sound pulse (1680+ fps).

The gas needs to move through the pipe and get out of the exhaust as fast as possible. This helps let the cylinder fill up with fresh air and fuel for the next firing. Getting these gases out of the way quickly help the engine breath better.

The sound pulse travels away from the explosion and gets reflected back by the muffler to (ideally) help time the scavenging of the exhaust gas.

This back and forth movement is called reflection. Reflection should be timed with the closing of the exhaust valve. You want the reflected pulse of the sound wave to travel back and reach the cylinder before the exhaust valve closes. This helps to prevent the new air and fuel from entering the exhaust pipe with the old spent gases.

Based on desired RPM range, you place the muffler at a desired location to time these pulses. You cannot set it up for the complete rpm range. The general rule is: move the muffler closer to the engine to increase low and mid range torque, move it away to create torque at higher rpm.

Torque gets you up to speed quicker. Horsepower gets you faster top end speeds. The flatter you can make the torque curve the more pull you’ll have throughout the rpm range. This is what you feel when you are accelerating and then all of a sudden runs out of juice so you shift to another gear or let the torque converter do it for you. The rpm's drop back into the area where the vehicle is making good torque and it starts to pull again.

The horsepower is created and will continue to be created until the gases cannot leave the cylinder fast enough (or the engine blows). Horsepower falls off due to the engine working against the backpressure in the exhaust. Since these are the hot burnt fuel gases and they only move at 350 feet per second a larger opening in the exhaust would logically reduce the backpressure and let the engine make horsepower up to a higher rpm.

The caution in this area is that if you make the pipe size too large the gases begin to cool, becoming heavier. Heavier gases slow down and may only move at 200 feet per second. This is where the confusion lies in back pressure.

If you make the exhaust openings too big, there’s no gain in horsepower. People started saying “you need some backpressure to make horsepower”. Wrong. What you need is an opening in the exhaust that will let the gases escape before they cool too much. This is where the development of exhaust wrap came in. Over sized exhaust pipes required insulation to keep the gases from cooling too quickly. Overly long pipes can do the same thing.

If you experience low end torque loss with a new muffler, you may need to move the muffler closer to the engine, but remember, the closer the muffler is to the engine, the hotter and noisier the muffler becomes. It can make the interior space unlivable.

If you don’t gain HP at the top end, look at your pipe size. Too large, either wrap it or downsize it. Too long, cut them and dump the exhaust gas out the side.

 

True. On my Silverado 2500HD which had a 6.0L, and 3" H piped true duals with Flowmaster Super 44s, the mufflers were 2"s under the cab, and the rest of the muffler was under the bed. I heard it more than enough in there The back pressure was actually still quite good, even with dual 3" pipes. Low end still pulled you back in the seat a bit.

http://youtube.com/watch?v=ACqPGCJz29I

 

Very interesting article I've been asking myself some of the topics appointed in the article. Am I losing horsepower or torque with my setup? I have a 97" w/ the 5.4 running 2.25" true duals straight out w/ 3" tips w/ no mufflers and high flow cats. I sometimes think I might have lost low end but really can't tell because the larger tires may have some effect also. I have 200,300 miles on her so take that in consideration.

 

Throw in a crossover of some sort (an x-pipe preferrably). Your exhaust is very free flowing and whats keeping your exhaust gas velocites acceptable now is your small pipe diameter.

But like you said, on a truck with that many miles and oversized tires, I doubt you'll feel much difference, especially if you don't do any towing.



I don't care how many times you tell people, it's not about back pressure, it's about exhaust gas velocity and exhaust scavenging. They just don't get it....

 

Is this a good x pipe? http://www.performancepeddler.com/de...T_ID=CHE429834 I've seen the Magnaflow and this one is a good bit cheaper?