Jump to content
IGNORED

Back pressure and gas mileage


gasser

Recommended Posts

How does back pressure and free flowing exhaust pipes affect gas mileage? Do open pipes, like drag pipes, lower mileage and would adding something to increase back pressure increase mileage?

 

The reason I'm askling this is that after riding my RT I'm considering going back to quiter pipes (baffles) on my HD. Before any fellow HD riders freak out, I still like the sound, but maybe a little less of it. I especially like the MPG on my RT--52+ mpg at 70 or less. Whew!

Link to comment
Lone_RT_rider
..... Before any fellow HD riders freak out, I still like the sound, but maybe a little less of it. I especially like the MPG on my RT--52+ mpg at 70 or less. Whew!

 

Is your HD fuel injected? If not, that and aerodynamics probably have a bit more to do with the gas mileage. Sure reduced back pressure can affect the low end torque and therefore gas mileage, but I am going to guess that there are other factors that contribute more.

 

Shawn

Link to comment

Generally less backpressure is good (although some amount is required for proper cylinder scavenging), but only if the fuel/air mixture and intake system are properly optimized. Where many run into a problem is throwing a free-flowing exhaust on an engine and making no compensating changes in the induction system. This can easily make things worse instead of better.

 

Don't expect enormous differences for street use, though, in which application a quiet exhaust system can be just as efficient as an obnoxiously noisy one. Vehicle manufacturers generally do a very good job of matching intake and exhaust flow and and uninformed owner messes with this at his peril.

Link to comment

Shawn

 

Yes, it is fuel injected. I'm always trying to find the right mix without going to the dyno. I may tinker a bit with some type of baffle to see what it does. I just don't know the impact of exhaust gases on fuel injection.

Link to comment
ShovelStrokeEd

To be facetious, the exhaust gases are at the other end of the breathing system and traveling in a different direction than the spray from the injectors.

 

To be a little less so, if the engine is flow restricted by the exhaust system, hardly ever the case, except at wide open throttle and maximum RPM, even with stock pipes, a more open system might, and it is a big might, gain one a bit of power, provided the mixture is adjusted for the change in flow.

 

Most of the big HP gains come from tuning of the lengths of the head pipes and collector to take advantage of the sound wave that is reflected back up the pipe. When done properly, this can be made to coincide with the overlap valve event, over a fairly narrow range of RPM, and, again when carefully designed, allow first a negative pressure wave to arrive at the exhaust valve to aid in pulling mixture through the combustion chamber and then a positive pressure wave to push excess mixture drawn into the exhaust pipe back into the cylinder right before the exhaust valve closes. It's all very tricky and requires than cam timing be matched to exhaust system tuned length. It gets even more complex as these pressure waves move at the speed of sound up and down the pipes and there can be multiple waves in the same pipe at the same time, even moving in opposite directions.

 

None of the above answers your question, I just was feeling wordy.

 

The short answer to your question is that yes, you can go to a quieter exhaust and not even lose any power. It will require that you do some tuning to gain the maximum benefit and, in aid of that, you're gonna have to spend some money. Since your bike is EFI, the good news is that tuning is pretty easy and, if you buy the right stuff, you can even do it yourself. What you need is a Power Commander PCIII, a laptop computer and a wide band O2 sensor installed in your exhaust system. You can then observe the actions of your tuning efforts and, with a little experience and some experiments, optimize you system for economy and/or power. You usually can't have both but you would be surprised at how well you can get an engine to run with a couple of weekends of tinkering.

 

Quite a few folks I know have gone well beyond the PCIII thing and installed a fully tunable EFI system. I think Holley makes one specifically for the HD and it even comes with a couple of base maps that pretty much make it easy to get started. So, for a grand or two, you can amaze your friends with your newfound ability to tune your bike, learn a whole bunch and, if you go nuts about this stuff, acquire a whole garage full of broken parts. The latter is also known as racing. eek.gif

Link to comment

Wow! That's a great response. Thanks for the info. There are some knowlegeable people on this site; I'm glad I found it.

Link to comment
if you go nuts about this stuff, acquire a whole garage full of broken parts. The latter is also known as racing.

 

Hey, look at me! I've been racing and didn't even know it! Did I win?

Link to comment
NoLongeraK1200RSRider

One other thing that I didn't see mentioned is "when doing this tuning.. the bike needs to be moving to get the absolute best readings due to the impact of air flowing as opposed to just drawing it in" just a thought.

Link to comment

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...