Quote:
Originally Posted by Formula413
As has already been said, a low restriction exhaust will really only provide benefit at high RPMs, which is basically irrelevant to FE. A simple explanation would be that under light loads the engine does not have to push very much exhaust gas out the exhaust, so whatever restriction exists in the exhaust is likely not hurting efficiency, while under heavy throttle at high RPMs there is much more exhaust gas being produced, and a low restriction exhaust means the engine will not have to work as hard to push all this exhaust gas out. There are other factors that come into play but that is the simplest explanation. This is why I have left my Escort's exhaust unchanged, but have added a very low restriction muffler to my Firebird. Doing this produced no noticeable change in FE but did improve power. Plus it sounds cool. 
I have heard it suggested that running an exhaust with too large a diameter for a given application will actually allow atmospheric pressure to work against the engine. I have no idea whether this is true or not. |
Very nice explaination.
A different way to look at it is that mufflers are rated (big rig ones are anyway) for flow rate at a specific pressure drop across the muffler. More flow and less drop = less energy wasted pushing the piston UP the cylinder on the exhaust stroke. If you have 5 psi in the exhaust pipe for instance and a 4" diameter piston you have 12.5" of area and 5 psi so basically 60 pounds of force holding the piston back. That means you need to burn fuel equal to 60 pounds force extra (times the number of cylinders) to overcome the exhaust back pressure.
But as Formula413 correctly pointed out, running for high mpg's means you are only using the minimum fuel and generating a minimum amount of exhaust gas. So the pressure is very close to zero and the force is very close to zero. So the gains from the exhaust are also close to zero.
High HP and High rpm engines generate more exhaust, so the pressure is higher and the gains can be significant.