Technical Insights
by Elden Carl
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2/05/10

TECHNICAL INSIGHTS

Big Single Engine Abuse Common

The KLR650 and DR650 have engines that are among the most abused in
motorcycledom. Over-gearing, over-loading, and poorly modified carburetion - among
other things - have caused short engine life, excessive fuel consumption, and oil burning
in many big singles.

As I write this I'm sitting at my favorite restaurant, La Fogata in Tecate, Baja, Mexico.  
Before I ate a great meal, I had stopped to fill my gas tank and figure my mileage on my
2009 DR650 with 6,300 miles on the clock. Running in the 4500 to 4750 rpm range on
mostly mountain roads I got 54.7 mpg. The secret is I was geared properly at 15/45 (stock
is 15/42) and did not increase my wind drag other than I am 6 feet tall and weigh 200 lbs
(the average rider is 5'9" and 160 lbs I'm told). I carry a backpack and have a small tool
bag on the rear rack so as not to increase aerodynamic drag.

My most important teacher for years has been Kevin Cameron formerly of Cycle and now
Cycle World. Mr. Cameron confirms what I have instinctively known for years; if you let
RPMs fall too far, you increase "pumping losses". Conversely, if you run the RPMs too
high you increase "friction losses". The idea is to be approximately in the middle of the
power range.

According to the late Gordon Jennings it takes approximately 15hp to drive a motorcycle
with a frontal area similar to that of the DR650 at 60 mph. Kevin Cameron, in his June
2009 Cycle World article stated that ideal cruise speed should use about 50% of the peak
horsepower of the engine in question. Since the DR650 has about 34.5 rear wheel
horsepower, the 15hp requirement at 60mph is very close to Cameron’s 50% guideline.
Every big singles rider should read this article. If you understand what Mr. Cameron is
saying, you will be sure in most cases to re-gear your motorcycle. To avoid loss of crank
momentum, pumping losses, and friction losses, try to find the middle.

By the way, another reason I get great mileage on my 2009 DR650SE is that my intake
and exhaust systems are dead stock except for iridium spark plugs and K&N filter charger.
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