A question, potential horsepower versus required horsepower.
 

I've been looking at a variety of bikes from the perspective of building an aero tourer and it seems that there is a significant advantage in fuel injection. The trouble is that here in Australia the only fuel injected bikes are quite big, usually 750 cc and up (especially at a price I can afford).

I could go to a smaller carbed bike but a question first about horsepower potential and used horse power.

With a cd of 1.0 a K75 BMW gets 43 mpg US at 70 mph as the spec sheets say. But using the calculator at the top of this page if I feed in a cd of .4 (as in a full Vetter fairing ) it says I will get 99 mpg US.

Is this believable?

Firstly this is a large engine , a 3 cylinder 750 but also a very good one that has an early fuel injection system. If the reduced drag only requires 8 hp versus 17.5 hp with a cd of 1( per the calculator ) is that all the engine will develop ? Meaning only using that much fuel .

Any input is welcome !

Peter
The calculation of the needed horsepower is probably correct.
What you need to find the fuel consumption is the BSFC map. At low engine load the fuel consumption per horsepower will be a lot worse than at higher loads.

That makes a lot of sense, the larger swept volume of a larger engine needs to be filled and at minimal levels of power production things are probably pretty inefficient. A smaller engine working in it's optimal range would be preferable.

Another question .... what is a BSFC map and where do I look for one ?

Peter
That is a map that shows the fuel consumption per horsepower as a function of engine speed and torque. I think somewhere in the wiki part there are examples .But only of some cars.

Thanks Janvos, I found this and it explains it pretty well.

http://pic40.picturetrail.com/VOL282.../407396162.jpg

Peter
At part throttle the consumption per horsepower is even worse than is given in your graph, which is a WOT (wide open throttle) curve.

This all get rather interesting because with a Vetter fairing, assuming it's a good one and the cd is around .4 most bikes will only need around 5 hp to travel at 60 mph. Almost anything will be operating at a small throttle opening, even a 250 will be putting out 25 hp at full power .

Will you carry one person or two? Fuel injection is more consistent and doesn't have the cold weather issues that are common with carbs. But you have to get into a newer bike to get it. I don't think the K75 will make it to 90 mpgUS with streamlining. The engine is too big. You will be operating it at small throttle openings which are far from the efficiency range. The G 650 GS is a lower power single and can do great numbers. One person here gets 70's with stock body work. The CBR250R is about the best starting point if you want to build a solo streamliner. It has the perfect power of 22hp and much lower rpm stroke and cams than a used Ninja250 which is the cheapest place to start. For a two up machine, The new Honda CTX700/ NC700 has one of the worlds most fuel efficient motorcycle engines that makes a plentiful 48hp. Stock bikes ridden normally are getting 68 mpgUS from the ultra long stroke, 6,500 rpm redline engine. This is only 2 mpg less than the CBR250R. The Ninja650 can be surprisingly efficient at low rpm's with a long gearing change despite it's 68hp peak.

The CTX 700 isn't for sale here and is out of my price range anyway so it's really the Honda 250 or for me, the BMW G650 Cross Country, a slightly lighter version of the GS (a secondhand one and quite a bit cheaper than the GS). I have to admit a preference for the BMW, it delivers almost the same MPG as the 250 but has room for pillion.

How much is a used G 650?