How exactly does the length/shape of exhaust pipes affect performance and/or fuel economy?

My 05 Super Glide has Vance & Hines with no mufflers (I don't know the exact name of the exhausts though), and I've considered short shots or side shots exhausts, but I've read posts where people say the shorter exhausts reduce overall MPG. How is this possible?

Here's a pic of my bike so you have an idea (and help me figure out the name of the exhaust model too)

Thanks!

 

That's sort of a loaded question. The exhaust does a number of things. The length tunes the 'pulses' of the engine to your desired RPM range. The size of the collector dictates how much air can be pushed through (large opening, lots of air, good for high hp big motors). There's tradeoffs there, if its too big for the application, you won't get the right back pressure. .giving up low RPM torque. Then there's reversion, where exhaust waves bounce back to your engine effectively sucking clean air out of the intake stroke and replacing it with diluted air from the previous exhaust stroke. That's where the loss of mpg and power comes from. The cylinder either isn't filled completely or it fills partially with bad air from the previous combustion event. Proper header design can counter reversion by having steps in the header (bends etc to act to make it harder for the reversion pulses to get back to the engine). During the exhaust stroke a pressure wave bounces from the exhaust port out your exhaust exit. Part of this pulse can and most likely will bounce back to the engine at certain RPM's. That's how it either enters back into the engine diluting the combustion chamber or can create a vacuum which draws clean air out of the cylinder.

Basically, you match the exhaust to the other engine components to maximize fuel economy and power. The shorter exhausts like you mentioned typically don't build enough back pressure to give good low end torque. There's always ways to mitigate the effects of a shorter exhaust using baffles, but if you want torque you go long exhaust. If you want high RPM horsepower you go shorter exhaust with bigger collector.

Look up some dyno sheets of the short shots. You'll most likely see a huge torque dip around 2800 RPM. Then compare that to a dyno sheet of say the D&D Fat Cat, it won't be there if tuned properly.

 

your pipes are the vance and hines Straight shots. one of my favorites.

 

Thank you for the detailed answer! I'm more of a in-city rider since my Super Glide is my daily bike, so I would benefit more from torque on the low RPM range. According to your info, then my long exhausts are the option I need to stick to.



Ok, thanks for pointing it out. I'll look for details on then now that I know their name.