Fuel economy

[LoHo]

On a related note...does anyone know if keeping the RPMs low for each speed is mor fuel efficient? Does running near redline use more gas than running near bog-down?

[Hmar]

I don't know about you guys, but I did not buy my XS to set fuel mileage records.

No.....But it is what I told my wife so if we could just keep this between us that would be great. Thanks.

[TopCatGr58]

Hey Loho,

I think this type of thing was hinted at with the FD750 mod thread, as to whether the engine was under more stress at the same road speed but at a lower rpm. It was surmised that it's a matter of horsepower needed at a specific road speed whether you're in a lower rpm gear or a higher one. Wind resistance is the more fuel stealing force, and as you go faster it takes more power to push the bike thru it. IMHO, the higher rpm lower gear choice I think provides the engine with more leverage to push the bike, but it IS running faster and therefore pushing/swallowing more fuel for the higher rpm, but at a higher gear/lower rpm you may have to twist/open the throttle more to provide the same level of horsepower to push in that gear.

But I wonder if the engine is burning it as efficiently when it's running close to lugging/bogging vs. at a bit higher rpm? The fuel air mixture is coming into the engine faster, possibly better atomization/mixing and so also possibly better combustion, more efficient, and therefore a little more conserving on the amount of fuel necessary to generate the required amount of horsepower at a specific road speed and engine speed!?

Sounds like a job for Mythbusters!
T.C.

[DGXSER]

Sounds like a job for Mythbusters!
T.C.

And here I wnet and shaved off my handlebar mustache after the wreck, I guess I have to much hair to be the Hyneman anyway, but I did wear a hat liek that for a while. I could never act as goofy and wound up as Adam though.

[cywelchjr]

Hey Loho,

I think this type of thing was hinted at with the FD750 mod thread, as to whether the engine was under more stress at the same road speed but at a lower rpm. It was surmised that it's a matter of horsepower needed at a specific road speed whether you're in a lower rpm gear or a higher one. Wind resistance is the more fuel stealing force, and as you go faster it takes more power to push the bike thru it. IMHO, the higher rpm lower gear choice I think provides the engine with more leverage to push the bike, but it IS running faster and therefore pushing/swallowing more fuel for the higher rpm, but at a higher gear/lower rpm you may have to twist/open the throttle more to provide the same level of horsepower to push in that gear.

But I wonder if the engine is burning it as efficiently when it's running close to lugging/bogging vs. at a bit higher rpm? The fuel air mixture is coming into the engine faster, possibly better atomization/mixing and so also possibly better combustion, more efficient, and therefore a little more conserving on the amount of fuel necessary to generate the required amount of horsepower at a specific road speed and engine speed!?

Sounds like a job for Mythbusters!
T.C.

Well, think about this though. To raise fuel economy, one of the things auto manufacturers did was to add overdrives to the cars, to lower the RPMs at cruising speed. This is to try and keep the engine in it's most efficient RPM range, which may NOT be the highest power or torque range. Where is that for these bikes, I know *I* sure don't know, but I'm guessing it's more in the middle of the range than at the top or bottom of the range, as that's where the slides get to do their magic the most. That's just a SWAG though, so I suggest you take it with a grain of salt.

[cywelchjr]

Sounds like a job for Mythbusters!
T.C.

And to do it like mythbusters, we would have to go so totally over the top, with putting gas in the tank, weigh the tank, go a fixed distance at a specific speed and RPM, then weigh the tank and calculate how much fuel used, then repeat at the same speed in a different gear.

You would of course have to do the same think with both naked and faired bikes, and with the rider sitting up and laying on the tank on the naked bike. Then you compare everything and declare the myth busted .

[crazy steve]

One thing to keep in mind is the FD swap only changes your final drive ratio by about 13%, which is far less than what a typical 'overdrive' does; those usually are at least 20%. The T56 6-speed I bought for one of my Jaguar projects has two overdriven gears; 5th is .8 (20% over), while 6th is .62 (almost 40% over). So any large gains on a XS shouldn't be expected. And the cars were 're-tuned' in most cases for better low-end performance to account for this, something that would be difficult to do on a motorcycle motor given the limitations with the exhaust and particularly the intake tracts.

My '78 never delived the economy claimed at the time by the period testers (even before the advent of mileage-robbing ethanol), and I don't know if that was because of riding style, terrain (not that much truly flat ground in western Washington), or tune on the individual bikes. I suspect the ones the magazines got may have had a little better tune on them than what you or I would get off the showroom floor. I got 40 mpg exactly once, generally I got 36-38 on extended runs, low 30s in town; high 20s if I was really beating on it, but that took some effort.

[TopCatGr58]

Hey Cy,

Yes, I agree that running the engine in the sweet spot/rpm would be the ideal. I was never one to LUG my engines, rarely ever let it drop below 3k rpm when I ride it now. And yes, at cruising rpm...4-5k, with less load on the engine, the F/A ratio leans out, the vac. adv kicks in to take advantage of that as well, again helping to provide better fuel economy.

SO..I would think if one tried to cruise at too low an rpm...below 3,500 that it wouldn't be running as efficiently and would waste fuel. Fun discussion!
T.C.

[3Phase]

When I did the 750 mod it gave the XS about 2 or 3 extra MPG on the highway and the engine was more comfortable on long trips.

A Mythbusters style investigation would be totally cool 'cause there'd be at least one wimmin' that'd help launch the XS out of a cannon, drop it off a crane, shoot it, set it on fire or some other fun combination using a diet soda, high voltage and/or explosives.

[Pat Kelly]

One thing I found using my Colortune. The carbs seem to transition from the pilot jets to the main jets around 4200 RPM.
Easy accelleration (then why have an XS?) to keep the slides down and cruising at 4000 RPM should show your best mileage.
YMMV (pun intended)

[GARTHXS]

Crappy gas definitely hurts mileage. On my run to Santa Margarita I had a full tank of crappy shell gas and I got about 35per all highway on my 78 . But on the way back got 41per .maybe its cause I was going downhill to san Diego. On the way up I was doing 70-75 on the way down 75+.

[XS1100 Newbie]

I haven't seen anybody mention at what range the cam really comes into it's working power. I know for a fact that in the cars i have built, it depended on what i was going to do with the vehicle, as to which cam was chosen. I would think that this would also apply here. We don't have the choices in cams that we do in small block Chevy's, and i have no idea at what rpm range the cams on the XS come in, but if you could match the gearing, weight, speed, and rpm that the bike would be working in, then you would have a good combo.

[motoman]

but if you could match the gearing, weight, speed, and rpm that the bike would be working in, then you would have a good combo.
__________________
Mother Yammy already did.............with the double nickels speed limit plus the occassional 5+ puts it right at the 4,000rpm.......all by design.

[LoHo]

How bizarre that I never considered a mid-point as the best MPG spot.

One XSive who attended a CaliRally reported 44+ MPG on his bike, which he attributed to low speeds and frequent oil changes. I have no idea how he decided this or figured it out, but he was one of those guys who is always right whether he was right or not.

[LoHo]

Anyone have a handle on how a windscreen alters MPG? Are fairings an aerodynamic improvement or a weight/drag liability?