MORE MPG – THE HOLY GRAIL

More mpg is a common quest for Transalp and Africa Twin owners here in Turkey, myself included, after weeks of research one comes across so many different options, while 50 mpg is OK for a standard Transalp it is not fantastic. You can pay out hundreds of Pounds for promised improvements by company’s selling exhaust systems, from reading many on-line forums these promises don’t materialise for some people who bought them.

petrol_pump_mp3h0353

Mpg is a formula between engine revs, horse power,  gear ratios, weight, and wind resistance. Below 20 HP it is difficult to squeeze anything out but the Transalp 650 has 55 HP, plenty to play around with. The most cost effective way to increase mpg is gear ratios, changing the sprocket sizes is cheap and easy to do.

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Standard Transalp ratio is 15 teeth on the counter shaft sprocket and 48 teeth on the rear sprocket (15/48) you divide 48 by 15 = 3.2 the ratio means the small sprocket turns 3.2 times to turn the rear wheel once.

45     46      47      48      49      50      51

14       3.21   3.29   3.36   3.43  3.50  3.57   3.64

15       3.00   3.07 3.13   3.20 3.27   3.33   3.40

16       2.81    2.88   2.94  3.00  3.06  3.13    3.19

With a gear ratio of 3.2 the standard 15/48 set up gives you 100 kph. at 4500 rpm. The rear sprocket is the easier to change so by dropping 2 teeth to 15/46 the ratio changes to 3.07 which means at 100 kph the engine revs drop to 4302 thus saving petrol over distance, approximately 4.4%  less consumption, all things being equal.

If the small 15 tooth sprocket is changed for a 16 tooth the difference is greater, 16/48 gives a ratio of 3.00 which means at 100 kph. the engine revs are reduced further to 4185 a 7% difference which will have a noticeable effect on acceleration, much slower.

Max. torque on a Transalp is at 5000 rpm and max. horse power is at 7000 so you are within the power band and it should pull the higher gearing but 7% is to much for my thinking and I will stay with the much less noticeable 4.4%

My tank average (not including 4litre reserve) is 270km for 15 litres, this equals 18km per litre, so with these changes next week I should gain 4.4% tank range (all things being equal) 270 x 4.4 = 11.88. Tank range should improve to 282km which equals 18.8km per litre, on a 6000km European journey this is a significant fuel saving.

The next items to research are changing the air filter and removing the catalytic converters, these changes make engine breathing easier which in theory improve acceleration and mpg, I have done this on a previous bike, a 250cc and there was some slight improvement, would the difference be greater on a 650 ?

dcp_3126

The photo above is the black bike I played around with, it was a 250cc copy of a Yamaha Virago, 19 HP. I bought her because she was so pretty and being sold so cheap, I read extensively about mpg, Torque and HP. Engine ”breathing” has an effect on all of these, I drilled holes in the exhaust baffle plates and completely removed the air filter and intake box then we went for a test run, two up for a hill climb with naked carburettors.

Having no access to dyno testing I used familiar hills, long and straight, in standard form the bike would only pull to 65kph in 4th gear two up. Now she pulled up to 72kph still in 4th gear, on the second long hill climb where the gradient was less, standard the bike pulled up to 70kph in 4th but now the speed had increased to 83kph.

This means air filter and exhaust have an effect on :-

A. Torque

B. Horse power

C. A little bit of both

Without dyno testing it is impossible to know where the change lies, I replaced the air filter but drilled large holes in the airbox and repeated the hill climbs again, on the steeper hill the speed dropped back to 70kph and the speed on the second hill dropped back to 78kph both still in 4th gear, this is approximately a 7.7%  increase in something ?

PETROL PRICES

UK  …………………………………………..1.00 Euro………………..1 Litre

France……………………………………….1.20 Euro………………..1 Litre

Greece……………………………………….0.96 Euro………………..1 Litre

Turkey……………………………………….1.44 Euro………………..1 Litre

USA…………………………………………..0.35 Euro………………..1 Litre

Drilling holes in the top of my Transalp airbox and changing to a K&N air filter will be a good place to start for mpg, more air means leaner burning. So with this modification plus the smaller sprocket 60 mpg should be attainable.

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~ by travelswithmymotorbike on December 8, 2008.

5 Responses to “MORE MPG – THE HOLY GRAIL”

  1. Interesting maths Dave, thanks for sharing it with us, I was thinking more about mpg myself.

    J

    • Hi James,

      I think more people think about it only when the petrol prices rise, here in Turkey we have the Worlds highest prices so we think about it all the time !!

      Cheers

      Dave..

  2. I’ve known some friends who’ve changed to a smaller back sprocket to save petrol, it does work because it lowers the engine revs

    • Hi Owen,

      İt works all right but there are not places on the Net to find out the info, so thought I would stick on here.

      Cheers,

      Dave…..

  3. Hey Dave- I am the only American motorcycle rider (that I am aware of) living in eastern Turkey (Elaziğ specifically) I have a ’04 TA that I have gone through and updated just about everything. I’ve been traveling around the south and north eastern parts of this country this summer. I noticed that if I held the bike under 100km (not easy) I can get a good 350km per tank, above 100km I get around 250km. I plan to drop the rear sproket to a 46 this winter. I put in the K&N, iridium spark plugs, arrow exhaust MIVV muffler and synthetic oil. I also changed the main jets from the stock 132 to 125. So I’ve done just about everything except take the heads down for porting and polishing, which I really do not want to do! I’ve debated drilling holes in the front of the air cleaner but some of the riders on the owner’s sites said their bikes ran worse afterwards.
    I am extremly happy with the bike and plan a serious European trip next summer. Is there any advantage to changing the back sprocket to a 46 then raising the front to a 16? Thanks- Harry

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