Thank you Frank and Doug for helping out on this one.  That covers everything and more that I would have brought up.

Lorena, Texas (South of Waco)

Back pressure is the pressure in exhaust manifold caused by the worst flow restriction which is usually the muffler.  As the piston accends the cylinder and the exhaust valve is open stuff will flow out of the ex port as long as the pressure in the cylinder is higher than the back pressure in the exhaust manifold.  This is the simplest view of the situation and might be called the static case.  It is realistic for mild street engines with stock or nearly stock exhaust components that don't rev too high.  The point here is that there is very little chance that the stock type system will ever develop negative pressure, but reducing the positive pressure will usually help power and milage.

However cars with open or closed headers tuned to length can easily develop negative pressure under the exhaust valve. This happens on purpose so that the intake and exhaust gases will develop speed and therefore inertia which will allow the cylinder to be "packed" with a denser mixture of fuel/air. The exhaust system aids this packing by removing the spent exhaust gases faster and in the process speeding the intake flow, increasing its inertia. A long overlap cam helps this at the right speed. In addition a well designed set of headers will send a reverse pulse back up the pipe just as the ex valve is closing ramming some good mixture back into the cylinder.  All this is intended to take place at WOT where the goal is the worst milage possible which is also most power!

This slippage of air/fuel mix into the exhaust is not what is needed for good milage at part throttle operation. The design of the system to reduce the slippage into the exhaust is what gives higher back pressure.

Doug T

The Highlands, Louisville, Ky.

      Overly large has more to do with velocity than "Back Pressure". You guys are trying to make me think, and I resent it! If the velocity in the pipe is too slow, especially headers, there will be no scavenging effect at lower RPM. Read Phillip Smith, "The Scientific design of intake and exhaust systems"  The effects of pipe diameter are lessened by mufflers and the bends required to fit the exhaust system into the car. While back pressure will affect jetting, it mostly can be said that it only hurts when you over scavenge a cylinder, doesn't often happen with mufflers.

 

Frank/Rebop

Bristol, In ( by Elkhart) 

Simplified, an overly large exhaust system can promote too much scavenging at the lower rpms which will allow raw fuel to be drawn into the exhaust system instead of permitting it to go through the combustion process and be burned where it can make additional power with a reduced throttle opening.  The camshaft is a large player in all this and the reduction of the lobe centerline along with an increase in intake duration automatically puts the engine in a position to ‘waste’ more fuel at the lower rpms especially when done in conjunction with an increase in exhaust system sizing.  Backpressure in this instance helps to keep the raw fuel in the cylinder rather than letting it be pulled into the exhaust system prematurely.  Selecting the ideal exhaust system is difficult in that it becomes a compromise between peak fuel efficiency and peak performance.  A given exhaust size for any combination will unvariably have it’s own sweet spot for peak efficiency but any rpm range above or below this will be out of the ideal rpm range for fuel economy.

It’s obviously more complicated than this but this gets the basic jist across.

Lorena, Texas (South of Waco)