(Top left) Honda’s DOHC 2018 CRF250 engine features
two cams that activate finger followers. (Bottom left)
The 2017 Honda CRF450 Uni-Cam SOHC engine.
DOHC VERSUS SOHC
What is a DOHC? How does the 2018 Honda
CRF250 DOHC engine differ from the 2017 engine?
The 2018 CRF250 is a DOHC engine and the 2018
Honda CRF250 is a SOHC engine. DOHC stands
for “double overhead cam,” while SOHC stands for
“single overhead cam.” The most obvious difference
between the two engines is the number of cams.
On the older single-overhead-cam engine, there is
only one cam to control both the intake and exhaust
valves. Honda’s Uni-Cam design activates two
intake valves by having the lobes on the cam directly push on them, while a forked rocker arm actuates
the two exhaust valves from a third lobe on the
cam. The benefit of this is less rotating mass in the
cylinder head, a more compact design and lighter
weight. The downside of the Uni-Cam engine is that
a rocker arm hampers high-rpm performance and
isn’t conducive to big valves and lots of lift. With
dedicated cams for exhaust and intake function, the
DOHC engine can spin faster, open the valves higher and be easily tuned with different lobe shapes.
The downside of a double-overhead-cam engine is
that it is heavier, bulkier and feeds more gyroscopic
effect into the chassis.
How you open and close the valves, while a very
important feature of any engine, is really less significant in action than the valves themselves. The 2018
Honda CRF250 has bigger valves than the 2017 edition. Honda bumped up the valve diameter on both
the intake and exhaust side—from 30.5mm to 33mm
on the intakes and from 25mm to 26mm for the
exhaust. Not only are the CRF250’s valves bigger,
they open up wider—from 9.2mm to 10.5mm of lift
on the intake valves and from 8.4mm to 9.5mm of
lift on the exhaust valves.
In conjunction with the two cams, bigger valves
and more lift, the 2018 CRF250 engine has a bigger bore and a shorter stroke. The short-stroke
design allows the 2018 CRF250 to rev higher,
which the bigger valves and increased lift demand
in order to process the added fuel coming into
the head. Compared to the 2017 CRF250 engine,
the stroke is reduced by 2.9mm and the bore is
bumped up 2.2mm. The rev limit was raised by 900
rpm over 2017, while peak horsepower was moved
up 2000 rpm. Equally important was the change
from a single exhaust port to dual exhaust ports
and twin exhaust pipes—one down the right side
of the bike and one down the left The final touch
was to add electric starting.
Sadly, all of these changes on the 2018 Honda
CRF250 were less than effective. The cumulative
impact was a total lack of low-end power and a
midrange that was 4 horsepower less than on the
2017 CRF250. Although there is more peak horsepower, it has a very narrow sweet spot (from 10,000
rpm to 12,000 rpm) and is followed by a flat over-rev that sounds like the engine is coming apart. It
is a very peaky four-stroke and suffers from bogging
and stalling. When you add in the fact that the 2018
CRF250 is the heaviest 250 on the showroom floor
at 228 pounds, you have to question the viability of
the 2018 CRF250’s engine R&D program.