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Discussion Starter #1
Considering ideal conditions such as cruising on a flat highway, is there a certain rev range that is more fuel efficient than another? Or is the difference negligible?
 

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Discussion Starter #2
I think I've just answered my own question. Maintaining a constant speed requires a certain amount of fuel energy. If the engine was spinning slower more fuel would be used in one cycle to maintain that speed. If the engine was spinning faster, less fuel would be used in a cycle (but more cycles per second) so the amount of fuel consumed over a period time would more or less stay the same. However, consumption at high revs may be a little higher to compensate for the various forms of friction throughout the engine and transmission such as mechanical, fluid, etc, which would probably be apparent at higher revs thus lowering efficiency slightly. I guess the difference is negligible afterall... but it should still be on Mythbusters.
 

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I had always had the belief (pulled from thin air) that the lower the RPMs the better the mileage. This definitely holds true for our Grand Prix and I've noted over the years that the top gear of most of our cars has close to its low RPM point at about 50-60mph, which coicides with the average speed on highways (for most people anyway ;) ).

My theory goes like this: if the engine spins slower, less energy is needed to achieve and maintain that spin. However, I've ridden my bike hard for a tank or two and then ridden it like a puss and found the mileage to be roughly the same. That seems to conclude what you have: it doesn't matter.
 

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you have answered your own question

Not only does friction increase with speed, but so does momentum (mass*speed squared) since each piston has to stop and change direction, the less momentum you are fighting with the better. But considering how light the engine internals are the difference is pretty negligable.
The key factor is going to end up being the valve overlap (the time in the cycle when both exhaust and intake valves are open). Over lap depends on certain gas (intake and exhaust) velocity to work right, so that fuel enriched air doesnt end up traveling directly to the exhaust or vice versa,
 

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I believe that it is due in large part to the fact that when our bikes are at higher revs it doesn't necessarily mean the engine is working a lot harder. Considering that most liter sportbikes weigh around 400lbs, our engines don't carry the same kind of load cars/trucks do. I think that is mainly why running the bike harder doesn't take much affect on mileage.

As in running a car harder the engine has a lot more strain on it when the car wieghs like 3,000lbs or more. And getting the best mileage doesn't necessarily mean keeping the lowest rpms while driving. If your motor is lugging, like it has no power in a higher gear while trying to keep your vehicle moving at a certain speed then it is putting more strain on it then. The key to getting the best mpg at a certain speed is to keep the car in a gear that will give it enough power to not put strain on the motor at that speed while keeping as low an rpm as possible.
 

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It's a little more complicated than that.

1. Drag losses increase with speed, exponentially. The slower you go, the less drag you have, and less energy is required to go the same distance. That is of course, obvious.

2. Engines are most efficient not at lower RPM's, but rather at specific RPM's, AND loads. If you run an engine at a higher, or lower load, or at a higher, or lower RPM than that which results in the highest BSFC (brake specific fuel consumption) then you are sacrificing efficiency. For most engines that means a nearly fully loaded engine running at somewhere around its peak torque curve.

3. Obviously, a fully loaded sportbike engine running at its peak torque curve would be driving that bike at a very high speed, resulting in very inefficient running conditions, even though the engine itself may be operating at its optimal BSFC. Conversely, cruising around in third gear, slightly off idle going 25 mph results in another inefficient running condition, since engine efficiency is very low, even though minimal power is required to propel the bike.

4. So there is a tradeoff, and an optimal speed for the most efficient running conditions. For most cars and light trucks, that speed is by design somewhere around 50-60 mph. This translates in to running the vehicle in the highest gear without lugging the engine, and then a slight amount. Again, that's still an inefficient running condition with respect to the engine, but the optimal tradeoff between engine efficiency and vehicle drag.

5. I notice myself, that there is surprisingly little difference in mileage on my bike, whether I cruise at 60 mph, or 90 mph. Obviously, there is a whole lot more incremental drag at 90 mph, but on the other hand the engine is running much closer to peak efficiency, which provides some degree of offset.
 

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i have always heard thats what 5th gear was for....there no real powerband in it. heard that you'll get better mileage in it opposed to 4th. but thats pure heresay.
 

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Meat_Shield said:
i have always heard thats what 5th gear was for....there no real powerband in it. heard that you'll get better mileage in it opposed to 4th. but thats pure heresay.
Very possible, depending on your bike, cruising speed and sprocket gearing.
 

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ya i always use 5th when im on the hiway. unless there is traffic in that case im in no higher than 4th just in case i need the instant power due to a jackass cager. but i do notice better mileage in 5th....of course that could be just because they are hiway miles.
 

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Meat_Shield said:
ya i always use 5th when im on the hiway. unless there is traffic in that case im in no higher than 4th just in case i need the instant power due to a jackass cager. but i do notice better mileage in 5th....of course that could be just because they are hiway miles.
Actually I had a brain fart - I re-read your post. What I meant was that it's possible but not likely that 5th may give you better mileage than 6th, if you have a six speed transmission. I suppose the same could be said of using 4th as opposed to 5th, but that's more unlikely.

I never drop a gear for cruise. I might if I'm lane splitting, or in really heavy traffic.
 

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Mind you back in the late 40s we would have some fun at a road raciing circuit at the end of the day. Most circuits were small WWII RCAF training strips.

Idea was who could go the fartherst on 1/ pint of petrol. I thought about this & hauled out an bashed up 350cc OHV AJS single. Strip it of all lighting or you name it, find a pair of almost woren out tyres, slap them on with 60psi air pressure, choped the handlebars shorter, (I would have put on a much smaller fuel tank only that was not allowed or even flat handlebars, & had to have a LP on it, even if it was outdated & I also wanted to haul off the engine mount plates to drill them full of holes, to sprocket & such, but it had to be "as is").

So I would take off slowly & soon be flat out in 4th (the bikes only have 4 speeds in those days) on the tank with right hand on throttle while left would be on the tele fork leg. Droaning away doing 22mph. Of the three times I entered they had to stop me as being the last still running.

Mind you I felt the pain the next day or two from t he above & NOT the actual road race of the day. But it was a challenge.
 

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I don't know all the tech info but on my zx-14 (has a constant MPG reading) if I go 70 in 5th gear I get better mileage than in 6th but if I speed up to 80 in 6th I get the same mileage as 70 in 5th (matching RPMs). I think the difference is 3500 rpm vs 4000 rpm with 4000 getting better mileage but I haven't ridding in over a month :( and I don't remember the exact RPM. That's also just my bike and I have no clue why it's like that.
 

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Seems that the biggest factor in bike engines is volumetric efficiency, i.e. how well the bike fills its cylinders. High-powered engines (such as the ZX14) are designed to fill their cylinders best at higher RPMs, ensuring that they produce the most power possible. The consequence of this is that the cylinders do not fill as well at lower rpms. This is why many times you will get better mpgs in 5th than in 6th. As the rpms rise to the level of highest efficiency, more power is used to overcome internal friction, so not only is there is not much of a mpg gain from running at higher rpms, it peaks very quickly and then begins to drop off.

Volumetric efficiency explains why an unfaired 1340cc Harley will often get better gas mileage than a literbike, or even a 600 in some situations. The Harley engine is spinning at a speed that gives it its best cylinder filling at cruising speed - the sportbike is way below its best rpm for max efficiency.

Many more variables enter into the equation, but these are the basics.
 

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...

I have read all of your ideas and if I am not mistaken, there is this little thing the you hold with your right hand called a throttle. The farther it is pulled the more fuel going into the carb. Now with newer fuel injected cars and bikes with their fancy ECU controled throttles, its a lil more complex but in the end the more you pull the throttle the more gas your are using.

To put it bluntly.
 

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Discussion Starter #15
Not exactly. My second comment was putting it 'bluntly' without sourcing any physics/engineering textbooks.
 

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I'd also say that throttle position would need to be factored in as that also controls the amount of fuel usage. Most people don't realize that sport bikes only put out between 20-40hp at the low end of the power band and finally spikes up around the 6-8k range (for 600's at least). You really don't need that much power to propel a bike.
 

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Guys, you are forgetting that we are assuming constant traveling speed. So the throttle position is whatever is required to get enough horsepower to overcome drag and rolling resistance (I donno, 7-10?).

On a bench, an engine is most efficient with the throttle wide open.
 

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Discussion Starter #19
According to my F4i's sometimes obscure interpretation of speed and distance I get exactly 40MPG on the highway. The manual states the fuel tank has about a 5 gal. capacity including about a 1 gal. reserve. However, when I ride a lot of highway miles and I hit reserve, I only fill up with just over 3 gallons, not 4.something. Trying to determine when to refill the tank has been the biggest mystery... but it seems to go for about 140 miles before hitting reserve, 120 if some city is involved. Is there anyone else with an F4i to compare my numbers to?
 

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ya the tanke size it like 4.something which includes the reserve. if you fill up right when your reserve bars show up then ya it should only take 3.x gallons. those mileage numbers sound exactly right. that right where im at as well. about 120 city and 10-20 more if its mostly hiway.
 
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