Bonneville America Forums Forums Technical Forums Exhaust Non restrictive pipes = better MPG?

It sure seems that way. I have my stock pipes down to the front baffle and the one behind it. I drilled 3 additional 1/4" holes in the front baffle, and an extra 3/4" hole saw hole in the 2nd baffle*. Mine has no catalytic screens. The sound is a bit loud yet rich, but not gutted Harley loud. I can still keep it quiet, yet bark off the line if I want to.

The bike seemed to come alive by widening the exhaust smoke escape route. In that I live in a rural area with relatively little traffic, I often ride by ear, (all things considered) yet with enough RPM to be sure I'm not lugging. Prior to the drilling, the mid 3,000s seemed to sound right. Even though it's now louder, the engine seems to want to run in that 4,000 RPM sweet spot I've read about in here.

I tanked up yesterday in the morning, and then again last night around 10 pm. 143 miles at 3.02 gallons = 47.35 MPG. In 11,000 miles of a wide variety of road types, I've never come close to that figure. 42 MPG was my prior best. While I'll be the first to agree that one run on a sketchy trip odometer does not an MPG evaluation make, I do have to wonder if the pipes and running near 4,000 most of the time are the key factors.


*If I had it to do over, I may have opted to go 3/4" hole on the front baffle, and the 3, maybe 4 1/4" holes in the 2nd, to allow the greater restriction in between the two baffles, and further away from the air pump / exhaust cycle duty the pistons must perform. Anyway, there probably isn't a huge difference.

The bike does come alive with non restrictive pipes but the engine can only breathe as well as the most restrictive part and MPG has much to do with jetting. You can jet for preformance , MPG , or somewhere in between. The happy medium is usually best for your engine.

As for setup, jets have the most to do with MPG. After that, allowing the motor to make more power means the bike can accomplish the same things with less THROTTLE which is the biggest factor in the MPG equation. So yeah, indirectly, more open pipes help the MPG

but more power means using it...so lower MPG

I'm at a Uni Filter with both ends of the box opened & drilled, 130 jets, & stock needles with no shims. Home base is around 800 feet altitude, with my customary operating area at 0 to 1,000 feet. To the best of my understanding by Pat's app & some other posts, I may do well to drill the carb slides, and throw in some shims under the stock carb needles for an 05 Speedmaster.

I wonder how many or how thick of a carb needle shim to use? I'm not allergic to going over to the hardware store and buying some little stainless washers to approximate the desired thickness. From looking at the carb schematics and considering the result, a few thousandths of needle lift this way or that way may not be discernible anyway. I don't know. I didn't find what I'm asking in a search.
(Alms for the search impaired may be mailed to: me.)

I admit to some concern over running too hot by running too lean, but perhaps that's not an issue. I'm not clear on that.

I've got some shims sitting in my shed, you want em?

Sure. How many should I use?

Ahhh grashopper, one must find the way of the shim on their own PM me your address and I'll send em out.

According to Pat's calculator, with no shims you should be running 132's, and if you add one shim, that brings you down to 129's. So I'd say try adding one shim to each needle, and run it, see how she blows. Shims are WAY easier to change than jets!!

Thanks

I had a bag of shims in my truck when I was at your house a couple of weeks ago.

The louder my pipes are, the more aggressively I tend to ride, to make them obnoxiously louder... so my gas mileage goes DOWN.

Well your new shims are making the trans-sound journey today. Shmike can owe me a shim or two some other time