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Fuel economy and engine RPM.

Discussion in 'The Perfect Line and Other Riding Myths' started by teeedubya, Jan 12, 2008.

  1. adam728

    adam728 Long timer

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    A Brake Specific Fuel Consumption map would help answer the question. Good luck finding any, unfortunately.

    [​IMG]

    For the example above, the red area is the highest efficiency area. Efficiency goes down as you move into the darker colors.
    • Say this car needed 30 hp to propel itself 60 mph.
    • Go to 30 on the right hand axis and follow the blue curved line.
    • You can see that something around 1700 rpm is the most efficient area for making 30 hp.
    • As you can see this is very heavy throttle, as you'll be asking ~93 ft lbs at an engine speed where ~110 ftlbs is wide open.
    • Fuel usage is about 0.411 lbs/hp-hr (or about 1.9 gallons per hour, or at 60 mph, about 31.6 mpg).
    • You could also run at 5,000 rpm and make the 30 hp necessary with much less engine load, but efficiency then falls to the 0.657 lb/hp-hr range. That means about 3.03 gal/hr, or 19.8 mpg.

    Generally most engines follow this trend, that is, best efficiency at high load, low rpm. Now tuning and volumetric efficiency come into play as well, so it can't just be said "WOT at 500 rpm is best". Many harmonics go out the window and an engine might run with poor efficiency as rpm gets to low (valve overlap, reversion, lost intake charge, thermal efficiencies, and sooooo many more variables). Plus the way the engine is tuned as well. Lug into a detonation zone and you'll often find mapping that dumps in fuel and yanks out spark to keep the thing from knocking to death. So it's back to a sort of guessing game without a BSFC map, or just knowledge of the engine (first hand, or using something that has been around a while and has lots of users and information on it).

    Good luck.
    #21
  2. TheProphet

    TheProphet Retired; Living the Dream

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    If you are really interested in fuel economy, etc. there are a few basics you can try, and one other thing to monitor your riding.

    1.) High speed = Low MPG. The faster you go, the harder the engine has to work to push you and the bike through the air. A sudden drop in MPG occurs roughly at around 55 MPH (That's why the speed limit was reduced back in the '70's - to save precious fuel), and continues downward the faster you go. Reducing the frontal area of the bike, and you can help - properly designed fairings, windscreens, and you crouching down to present less square inches facing forward.

    2.) Gradual starts, and gradual stops, butter smooth shifting of gears, along with gradual acceleration and gradual deceleration. Get in the habit of riding "smoothly". Imagine a full glass of water glued to the top of your fuel tank, and you don't want to spill a drop. (Ignore this in turns obviously).

    Lastly, try installing a temporary Vacuum Gage to a port on the intake manifold. Try to always ride in the highest Vacuum reading range. The higher the vacuum reading, the more efficient the engine is running. Once you realize what it takes to achieve the best readings, you can remove the gage if you like. In doing so, you will notice that the above, #2 , point is valid. Note how rapidly opening the throttle achieves very poor vacuum, slow and steady opening achieves the best overall Vacuum readings. Note how utilizing the proper gear ratio for the work to be done also achieves the best readings. You will also see the effect of speed, and how staying around 55 MPH or so gets the best results. Use the gage to see the cause effect of how you handle your bike.

    Now, having said all that, I'll agree this is extremely counter-intuitive to riding a motorcycle, as it DOES take 99.9% of the the fun out of it. However, if you are a daily commuter let's say, and are sincerely interested in getting the best value for your fuel investment.... there you go.

    Good Luck! :ricky
    #22
  3. tlub

    tlub Long timer Supporter

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    I actually asked a BMW factory rep about this, back in about 1989. He paused for just a second and replied "At the torque peak".
    I'm sure there are subtleties to this, as the Saturn plot shows. And for our vehicles, with horrible aerodynamics compared to almost everything else, speed will really affect mileage. But in the absence of detailed data for a particular engine, I'd pick somewhere near the torque peak. If the torque curve is fairly flat, it probably doesn't matter much, except that at higher rpms, you do have higher frictional losses.
    #23
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  4. TheProphet

    TheProphet Retired; Living the Dream

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    Peak torque is also when you are achieving maximum Volumetric Efficiency, which shows up as a higher vacuum reading on the Vacuum Gage mentioned.

    As it is somewhat difficult to determine exactly where you are on the torque curve while you are riding on the street, shifting gears, etc., keeping an eye on the gage might help. The gage is also a good learning tool regarding accelerating up to that RPM level, and illustrating at what speed your MPG's start to decrease permanently.

    A good video:



    FWIW, they also have very sophisticated digital Vacuum/MPG gages that hook up to OBD IIs or your ECM, but I'm talkin' low bucks here.
    #24
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  5. markk53

    markk53 jack of all trades...

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    The point of running in the fat part of the torque curve is the key. Two things, personal experience and Honda Service, qualify my comments.

    We had Gold Wing riders complain about gas mileage. They did the "lowest possible rpm" deal and got crappy mpg. We had one rider complaining only to learn he was riding no higher than 2700 rpm and would be lugging the bike through a parking lot in fourth gear! Honda Service department's response was that the GL1000/1100/1200 engines were designed with the meat of the torque between 3000-4500 rpm. The riders are riding out of the best performance zone, they should be at least at 3200-3500 rpm for most efficient performance, even if it means downshifting.

    I happened to have a standard 1100 and a friend a standard 1200 both with windshields and at the time we did some comparison, riding 2 up in west New York and Pennsylvania on our way home from Niagra Falls. We went on the 2 lane back roads winding through the Appalachians and the foot hills. We both were easily getting 50 mpg holding around 3200-4000 rpm, regardless of the amount of time in 4th and even 3rd on some of the hills.

    Then there is the riding I do on my KLX650 single. It seems like no matter whether we are riding dirt/gravel most of the day, running hard playing supermoto on the back roads, or doing a general mix of town and country riding I am within a couple miles of 60 mpg on virtually every tank of gas. I keep the engine at around 3500-4500 rpm with the occasional higher rpm when running harder or lower rpm in urban settings.

    The key is to know what rpm the torque peaks at and then run a range a couple thou below up to that peak for best performance. The GL1100 has its peak at 5500, so the 3000-5000 range is the beef. The KLX650 is, oddly enough also at 5500 rpm, so my running 3500-4500 is in the strong part. Now the KLX250 has claimed peak torque at 7100 rpm and it shows, the bike pulls strongest from around 4500 up. Running too far outside the strongest part of the torque curve will actually decrease mpg. In the four cylinder Gold Wing case running below around 3200 rpm would have mpg drop more than running 3500 rpm. The engine has to work harder to lug along than it does to sing happily in the good zone... more gas eaten up.

    Here's a real world dyno chart from a rider's KLX250, showing the range of torque, which conforms to my experience, the strength actually runs from around 4500-7500 with a good practical shift point of around 5000-5500 rpm to take advantage of it for best mpg.

    [​IMG]

    Summary - find out where the torque peaks and run no lower than say 3500 rpm lower up to the peak rpm for best performance, including MPG. If you can actually see a dyno chart from some publication you can even get a better idea.

    Here's one from a big bore KLX351 Look at how flat the torque curve is, the pull starts at 3500 instead of 4500

    [​IMG]

    And here is Honda's "tractor" CB500:

    [​IMG]

    That engine WILL pull down around 3000 rpm cleanly, no problem. So you see how knowing peak torque helps and a dyno chart that has more information can help you get where you want to be with proper shift points for best mpg.

    Just start looking for your model's horsepower and torque in a google search, using images instead of all.
    #25
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  6. The_Precious_Juice

    The_Precious_Juice The Virginian

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    Holy Sh`t!

    Thanks for dropping the knowledge!.

    I have a picture of the Versys 650 Dyno and it's encouraging!
    The horizon starts around 3,800rpm or so.

    The +1 sprocket in the front would help out when I need to go about 70mph. That moving for my riding, and speeding in Canada.
    #26
  7. RickB1975

    RickB1975 Long timer Supporter

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    Seriously, the worst thing you can do for gas mileage is braking. Riding a nice smooth steady speed and coasting into curves and accelerating out of them smoothly will get you the greatest increase in fuel economy. of course the gearing of the bike matters, but I bet just learning to ride smooth will net you greater increases in economy over a sprocket change. My rule on long trips if I want to squeeze the mileage. Ride in the tallest gear and at the RPM the engine seems happiest. Try to not have to brake at all. Don't tailgate, other drivers seem to want to increase your fuel usage by driving at sporadic speeds, so leave some room to coast to slower speeds. Don't accelerate up a hill. try to get a bit of a run at a hill prior to climbing. Accelerate smoothly towards the hill and let the bike decelerate going up the hill, and accelerate back to speed when going down again. It's not about maintaining a steady speed, or RPM, it's about maintaining the work load of the engine, and keeping it where it sounds, and feels happy. I've gotten 260 miles out of a single tank of gas while traveling the Trans Lab Highway with my 2014 V-Strom 1000 by following these rules.
    #27
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  8. shupe

    shupe Been here awhile

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    Disagree.
    The torque peaks you mention are off a dyno reading taken at WOT.
    There is nothing to suggest that the WOT torque peak RPM is the most efficient at lower throttle settings.
    The graph Adam728 posted tells the story, as it relates to the horsepower required (and hence throttle setting) for a given speed.
    #28
  9. markk53

    markk53 jack of all trades...

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    What do you disagree with specifically?

    Torque is the ability to do work, all that chart shows is the capability to do the work at a given rpm - any thing under that curve just takes less throttle to reach and maintain. The engine is not working as hard. Now look at the lowest torque levels down around 2000-2500 rpm that low level in the line is how much power you have to do work, at full throttle, meaning the engine has to work harder to reach the same level of work as at a higher rpm where the engine's capability is much higher at full throttle, which means it can do the equivalent work to that lower setting with far less throttle. The reason lugging down in top gear isn't as good as letting the engine spin a bit. The work is overcoming internal and external friction, carrying the weight load, accelerating rotating masses, pushing the motorcycle through the wind, and deal with road elevation changes. If you only have say 10 ft/lb work capability at 2500 rpm in fifth gear and the work load is calling for 10 ft/lb it will take everything the bike has at full throttle to maintain the work, but in one gear lower the engine has the potential for 30 ft/lb work at 3000 rpm now it's only using 30% of the available torque to do the 10 ft/lb work and will only need enough throttle to maintain the 3000 rpm.

    We experience that when going over a hill. In a high gear you may need to open the throttle a lot to try to get it to hold speed, drop a gear and have a 10% drop in rpm, but it only takes 1/2 throttle to now maintain that speed. I'm pretty sure it is about momentum of moving parts in the engine as much as anything else. Oddly enough the same is somewhat true in bicycling, it is more efficient to spin a higher cadence in a lower gear to go over rises than to try to pull the higher gear with a low cadence.

    It is hard to explain this, but even if I cannot have it make sense, does not make it incorrect. It is proven. That was why Honda recommended keeping the RPM over 3200 rpm for best performance. Anything less was no longer as efficient. And I'm taking Honda's word over any one individual lacking the credentials any day. They know their stuff. The reason I refer to the Honda comments on the topic.
    #29
  10. sloKRD

    sloKRD Safety Third

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    People that think they know everything really annoy those of us that really do.
    #30
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  11. markk53

    markk53 jack of all trades...

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    That's why I noted the support for my comments with those we got from Honda. It isn't my opinion. it is their expertise.

    As one rider put it the rpm where the engine is happiest.
    #31
  12. sloKRD

    sloKRD Safety Third

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    @adam728
    I have never seen a torque, rpm, hp map like that. This is fascinating and I think this is very close to the real answer to this question. The double hump in the torque curve is also interesting.
    #32
  13. ibafran

    ibafran villagidiot

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    MHO: I have been fooling/testing MPG with my own bikes and cages for about 40yrs. The following are my observations.
    It is worth it and fun to me to see what I can get out of my equipment.
    MPG savings is only worth it if one can accumulate the savings over a long period of time/distance. Ergo: riding to save fuel up a long hill is only worth it if one does it on a LOT of hills.
    Having a decent test loop and taking the time to ride it in a consistent manner many times while collecting data gives reliable and understandable results.
    Aero drag is a very big thing. Riding at 40mph or less all the time is boring. But one learns a lot if one has the patience. A bike traveling with 65mph of windload will only give a best mpg for that speed. Thus, reliable slab mpg at 65mph will be worse than remote back roads at a constant 40mph. My 05 Tri 1050 Sprint ST (open class sportbike) reliably gave 48mpg when ridden 'sensibly' and 60mpg in econo mode. The vacuum gauge is a very good idea if one wants to learn quickly what really works.
    As posted by another inmate, riding smoothly pays huge dividends across the whole range of operation including safety issues. The slower one rides and the smoother one rides and the longer one can continue in that mode the faster the mpg climbs. Said Triumph gave 60mpg over a 200 mile loop at 25mph (just above stall speed and engine lug) in top gear with the throttle nearly shut the whole way. A constant use of a throttle lock to dampen accelerations on one's daily commute might give +5mpg improvement for a tankful if done with no other changes.
    Adding a tooth to the CS sprocket is so small of an advantage that it takes a LOT of econo mileage to see + results. I tried it and left the gear in place after my testing as it didn't seem to hurt any acceleration that I ever needed on that sportbike.
    In the Far North, I had a trip leg of 385 miles with no fuel. I needed to know how much extra fuel I had to pack to make that distance and how to ride the bike to make it count plus some 'reserve' for unknown fun.

    To answer the OP's question for the 'real world' that I live in, Ride up the hill at less than 40mph(30 would be better) in top gear if the bike will tolerate that and with as constant/smallest a throttle as possible. Enjoy coasting down the other side with the engine switched off to save even more fuel. No roosting. Wheel spin eats fuel as bad as anything. Knowing what one is doing with one's fuel all the time allows one to anticipate when it might be best to quit playing around and ride in econo mode to get some more fuel. Riding with some care in Moab with a buddy, one bike got back to town and fuel (mine) and the other bike was a mile short of the station. Knowing how far one can ride on a 12oz pop bottle's worth of fuel could make all the difference?

    Storytime: In CO many moons ago, I had to ride about 50 miles to the next town with something significantly less than a half gal of fuel from a town that had its power knocked out and no pumps were working. I rode (slowly) up a 25 mile grade to get over a pass. And switched off the engine to coast down the 25 miles on the other side all the way and right up to the fuel pump in that town. Another time (Nebraska?) I was easily going to be about 10 miles short of fuel with about 40 miles to go. I went to super-econo mode and switched off for every downgrade. There were enough downgrades that I made my fuel destination. Coasted into that one too. And again, in the Far North, my trip budd ran out of fuel about 8 miles from the pump. But we had a downgrade... that went all the way to the pump. He was screaming with elation as he coasted up to the pump.
    #33
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  14. adam728

    adam728 Long timer

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    Cleaning up some folders on the computer this morning and stumbled across these from a few years back. I was trying to see how much speed effected mpg on the cars I had then (still have the Terrain). There are lots of these sorts of plots out there, and almost all of them show slower is better (within reason, not 1 mph).

    I should try something like this with the DR someday, but I already know slower is better with that bike. Normal commuting (including gravel roads and WOT blasts) is about 42 mpg. Taking it easy, but still cruising >60 mph with traffic gives about 46-48 mpg. Keeping to back roads and 50-55 mph gives 50-52 mpg, but bores me to death.

    2008 Mazda3 S Speed vs MPG.png

    2012 GMC Terrain Speed vs MPG.png
    #34
  15. markk53

    markk53 jack of all trades...

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    Key phrases in bold. When the engine is out of the optimum range trying to accelerate will result in extremely slow acceleration, no acceleration (hills), or even the engine/bike bucking shuddering. Big singles will frequently be out of the good range when below 3000 rpm. It also can depend on load and steepness of grade. Big inch multi cylinder engines will pull stronger from lower rpm, but that may be entirely in the design. The pull of my old 1100 Wing versus a GSXR1100 at 3500 rpm favors the Wing. Now hit 4000-5000 and they will shoot past like a rocket. Same size engine, different state of tune.

    Funny part to me is that I was able to run around on a 700cc Nighthawk S and on an 1100 Gold Wing pulling 50 mpg regularly riding as I pointed out, with the power band in mind. The Wing was around 3500 rpm and the Nighthawk was around 4300 at 60 mph. I rode the Wing between about 3200-4200 rpm and the S between 4000-5000 rpm regardless of gear. Then I run around in general riding with my KLX650C with a 678cc big bore with a full aftermarket exhaust and get 60 mpg varying by a couple one way or the other, without any problem. Doesn't matter what kind of riding I'm doing, I just keep it around 3500-4500 rpm in whatever gear necessary. The only time I was under those general rpm ranges would be in town puttering in traffic.

    I do nothing to try to get better mpg, just ride like Honda said to do, yet it seems I get good performance for the models of the bikes. Seems a lot of the riders are in the low 40s or worse. My bet is they're lugging the bikes around in high gear. I followed the same thought process with the S and the KLX, keep it in the good zone.
    #35
  16. ibafran

    ibafran villagidiot

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    Nice graphs, Adam728. Illustrates my points exactly. If one is aiming for the best possible MPG, slowing to something under 40mph to minimize aero drag and riding in top gear while chopping the throttle gets the job done. Why the top graph didn't leave the cage in 5th gear escapes me? Sure, being able to accelerate briskly while in econo mode suffers. Operators need to make a damn decision and commit to either save fuel or not. It's not rocket science. The OP wants to know how to save fuel. Performance (speed) is a conflicting issue. One cannot have it both ways.
    That is not to say that the manufacturers cannot give a desirable compromise. Making the aeros as slippery as possible and providing the tallest top gear (overdrive?) that the market will bear will generate some nice mpg at hwy mph. No matter what, any given vehicle will only perform so well in each category, mpg vs mph. And it comes down to the operator knowing how to get the best numbers for the situation.
    #36
  17. nk14zp

    nk14zp Long timer

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    Grade? Nebraska? Unpossible.
    #37
  18. adam728

    adam728 Long timer

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    Neither was in top gear below 40-ish mph. Both were automatics and that's just how it was.
    #38