BSFC for fuel injected BMW F650GS?

[TedV]

Out of curiosity I've been trying to find fuel consumption chart for the single cylinder twin spark F650. No one at F650.com had an answer which sucks since that's where the most knowledge on the bike is found on the web.

However, folks here are more in tune with fuel economy.

[sendler]

http://www.fuelly.com/motorcycle/bmw/f650%20gs

[TedV]

Thanks, but I already know the general fuel economy of the single cylinder. The link you posted is for the twin cylinder detuned 800cc motor that BMW calls the F650 now. Thanks to BMW for being confusing.

I'm more interested in the g/HP or g/kW chart to compare the thumper motor against other small gas or diesel motor. As it sits it looks the best motorcycle stock for carrying lots of stuff at highway speeds which is just a generalization. Almost 74mpg at the Vetter challenge conditions, 54 at beyond highway speeds. Both with 4 days worth of camping gear. I'd love to compare a CBR250 loaded the same, riding the same.

My cashflow and free time demand I figure as much out on paper before I blindly build something.

[sendler]

If you can get by with 23 hp and fuel economy is the goal, the CBR250R will beat the BMW carrying any identical solo rider and gear up to it's top speed of 93 mph. Many people have toured cross continent on it already. It is a dream to ride in even in bad weather. If you need to carry two people, it can be done but you will probably want more power. The BMWs are great on gas for the power they put out if you need more speed. There is talk of a new CBR400R from Thailand.
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Honda CBR250R Forum : Honda CBR 250 Forums

[Ken Fry]

Originally Posted by TedV

I'm more interested in the g/HP or g/kW chart to compare the thumper motor against other small gas or diesel motor.

For full throttle, the BSFC curve looks like the torque curve flipped upside down.

You could back into a number by comparing fuel efficiency at 120 k/h vs 90k/h, using the BMW specs. Rolling resistance remains essentially identical at those two speeds. Aero drag goes up with the square of speed, etc. (This is the same idea as a coast down test.) If you get where I am going, you can see how this can be done.

But I am not sure that you would get closer than just assuming 30% efficiency at peak for both the CBR and the BMW, and then looking at the shape of the torque curves to assess BSFC at lower loads. Comparing the published FE figures for each would let you fine tune this a little, and I would not be surprised if you found the BMW to be a little more efficient at peak (32% vs 30% maybe). But then accounting for the greater distance from peak with the BMW for a given (reduced) power requirement, should make the CBR a better bet. (I think you would need European test numbers for each -- I'm sure the US numbers are not obtained in the same way. )

This would be ever so much easier with a real BSFC map -- but they are very hard to find for motorcycle engines. The map could reveal a part throttle anomaly for either engine, that you could only guess at... at best.

In a previous life, I built and ran a motorcycle dyno at a motorcycle school. If you could find such a school, maybe they could put the bikes side-by-side. I used to use a pipette for fuel flow. You'd calculate the power you think you will need based upon your aero design, and run the dyno test at that hp. A DynoJet would be useless -- has to be a constant load type.

So... I'm not being much help here... wish I had the maps in front of me.

[sendler]

I think the CBR250R engine could be easily detuned somewhat by reducing the valve overlap as the cam gears are friction fit with no key. They would be infinitely variable without any machine work. That engine would also benefit from shaving the head to raise the lowish compression ratio up from 10.7. The power band is already quite tame compared to the Ninja but could no doubt be better for fuel economy if some peak horsepower could be given up by changing the cam timing. The Honda also features a nice tuned runner leading into the throttle body which could also be tweaked longer once you find out what rpm you want to optimize for. The cat and quiet, stock exhaust are also holding back FE. A loud slip on muffler increases power 8-10% all across the bottom and midrange but I have thus far decided to leave the cat in place. I would love to borrow a portable emissions logger for an hour to get some idea how effective the stock cat is. Anyone starting from scratch with a ground up new vehicle could find a better, more exotic engine but as a turn key platform for light aero mods (or stock) the CBR250R is doing some great numbers at a give away price.

[TedV]

Thanks, No problem going off on a tangent, or giving too much info. It might help someone else in the future.

Reason I’m wondering about the BSFC map is 2 fold. I have one I ride now and I keep thinking of a project that would need a motor in the 50 to 75 Hp range. Not exactly a motorcycle use for the engine. I’ve heard folks talk about different fuel maps programed into different bikes. If that is the case then there are many BSFC maps depending on tune, so no easy button on this one. Never hurts to ask tho. Bike motor is lighter than auto based motors but way less torque than a 3 cylinder VW Tdi. There are ways to get the little VW motors in the US, but not as easy to get as the Japanese used JDM. I have plenty of experience with VW diesel.

I do like the CBR250. Wish they were available a few years ago when I purchased my CB250 Nighthawk. I ran a Saddle Sore 1000 on the CB250. 19.6hp, 5 speed, experienced almost every hill on I-40 in the state of TN. Top speed 76mph on flat ground if you positioned your body juuuuuusssst right. I definitely say the CBR would work better in the same role as the CB around town, and much better highway, but I say the F650GS Dakar will handle non pavement a lot better. There is no need to slow down for speed bumps, pot holes, creeks, rocks,.. The Dakar does everything except not be top heavy.

Too many ideas for projects, not enough time.

[alvaro84]

Originally Posted by Ken Fry

...I would not be surprised if you found the BMW to be a little more efficient at peak (32% vs 30% maybe). But then accounting for the greater distance from peak with the BMW for a given (reduced) power requirement, should make the CBR a better bet.

This is why Pulse&Glide works quite well on my BMW F650CS, even without instant feedback (the earliest feedback I can get is at the pump).

OTOH if you look at my log (Teresa is my bimmer) and sendler's (Honda CBR250R FI Single is his CBR, not a big surprise ) you can see some difference in their FE, he can do 80mpg with the CBR around freeze point, while I'm in the low 70s even at the low speeds I can hold in cold. I often ride at 50-60km/h (~31-37mph), in 4th (because I have to) at those temps, so the engine efficiency must be pretty bad, but at least the aero drag is lower...

[sendler]

Originally Posted by TedV

I’ve heard folks talk about different fuel maps programed into different bikes. If that is the case then there are many BSFC maps depending on tune, so no easy button on this one.

Changing the fuel map is only one aspect fuel efficiency and wouldn't be the major one assuming the maps are around 14.7 to begin with. I think cam timing and gas flow rates/ pulse tuning are the major considerations. Much of which is fixed in the engine design. Along with heat loss from too much combustion chamber area from an engine that is unnecessarily large in displacement. Which also runs at higher intake vacuum due to having too much peak power for the job.