Yamaha’s YZ450F reversed head

Welcome to the first installment of Technobabble, featuring Yamaha’s new reversed cylinder head YZ450F …


Since most of us here at CMG are somewhat tech headish by nature we figured that it might be a good idea to set aside a feature section dedicated to analyzing and promoting interesting technical developments that occur in the industry.

Welcome to the first instalment of Technobabble and Yamaha’s new reversed cylinder head YZ450F …

title.jpgWords: Costa Mouzouris. Pics: Yamaha

You don’t need to be a motocross buff to appreciate the technology behind the 2010 Yamaha YZ450F.

I recently attended the press launch of the new YZ450F, held in Montreal’s Olympic stadium in conjunction with the Montreal Supercross.

We were encouraged to bring riding gear to sample the new machine on the supercross circuit, but since the only doubles I can land involve scotch, I declined the offer to ride the bike.

I was, nonetheless, impressed with the technology of the new bike, particularly with its engine.


Recently, Yamaha has been pushing the boundaries of engine design, as evidenced by the latest R1 with its crossplane crankshaft, which completely changed the power characteristics of an inline four. The new YZ450F uses similarly innovative technology, as it uses a “reversed” liquid-cooled, four-stroke single.


Firing the exhaust directly rearwards involves some pipe trickery.

At first glance, the most noticeable feature of this new design is the oddly angled cylinder, which slants 7.5 degrees rearward. A closer inspection reveals a cylinder head that’s bolted on backwards: the intake faces forward, the exhaust towards the rear.

The unusual cylinder angle and reversed head are the latest tricks Yamaha uses to further fulfil the never-ending quest to centralise mass, which improves handling. With the airbox now placed ahead of the engine, the fuel tank can sit lower and closer to the bike’s centre of gravity. Another benefit of the airbox placement is that intake air has a straighter shot at the ports.

To achieve the appropriate exhaust-pipe length with the exhaust port at the rear of the engine, the header makes a loop just aft of the single shock, before emptying into the muffler.


Intake at the front allows for more direct shot of air induction,

Inside the engine, Yamaha has moved to a four-valve head, breaking from its traditional use of five poppers per cylinder. As Rossi experienced with the YZR-M1 MotoGP bike, four valves provide better driveability and engine control than five, as well as reducing frictional losses and providing a shorter cylinder head. That last bit also contributes to centralising mass.

Yamaha claims this new design moves weight directly to the centre of the bike and lightens the front end, factors that combine with a new chassis to greatly improve handling.

Another M1 trick used on Rossi’s race bike beginning last season and incorporated into the 2010 YZ450F, is an offset cylinder design. The cylinder is not perfectly aligned with the centreline of the crankshaft, a feature that reduces piston-to-cylinder friction on the power stroke. The benefits of this simple change are twofold: reduced piston wear and increased power.


Power Tuner comes with all Canadian YZ450Fs bought before Dec. 31.

The new YZ uses a battery-less electronic fuel-injection system. The system is tuneable using a hand-held “YZ Power Tuner”. To change power characteristics, you simply plug this device into the bike to alter the fuel and ignition mapping.

Nine setting points are available for fuel and nine for ignition timing. Maps can be stored for future use, like returning to a setup that won you a race.

Yamaha is giving away this $350 tool – which will prove valuable for race tuners – to anyone buying a 2010 YZ450F before December 31, 2009. Consider yourself lucky to be Canadian; U.S. YZ buyers have to pay.



How would you do this with four pipes?

I didn’t ride the new YZ450F, so I can’t comment if it does what Yamaha claims. But I can appreciate the application of this new technology and certainly acknowledge its possible benefits. I asked Yamaha’s technical specialist, Dave Shepherd, if this new engine would eventually be available in the WR enduro bikes, and he said yes.

As the current trend in sport bikes is to centralise mass, I then suggested the possibility that this reverse design could be applied to multi-cylinder engines, like parallel twins and inline fours. John said the limiting factor was the complex exhaust system, which would make such an application improbable.

Hmmm, I’m not so sure. I think the technology exists to design a compatible exhaust system, at least for a twin, and I wouldn’t be surprised to see a reverse-engined street bike from Yamaha sometime in the future, particularly since the company has proven it doesn’t shy away from introducing innovative designs.

The new YZ450F retails for $9,499 (white is $9,599). More info at yamaha-motor.ca.


For the motocrossers out there, here are a few details on the new YZ450F.

Bore is increased by 2 mm, stroke decreased by 2.6 mm.

Oil tank is now integrated into the crankcase: no external oil lines, lower C of G.

Revised five-speed gearbox

Revised counterbalancer for reduced vibes.

Quick-access magnesium clutch cover.

Re-packable muffler now uses Allen bolts for easier serviceability.

Works-style “barrel adjuster” for on-the-fly clutch lever adjustment.

New bilateral beam aluminum frame optimized for high lateral and tortional rigidity and incorporating “tuned flex” longitudinally.

New cast aluminum tripleclamps feature reduced fork offset (25 to 22 mm).

Four-position handlebar mounting is retained.

New aluminum swingarm now positions shock in line with the bike’s central axis, used to be slightly offset.

48 mm inverted fork has 10 mm more travel.

New fully adjustable rear shock uses a smaller-diameter piston rod (18 mm) for reduced friction.

New bodywork that uses a 6-litre fuel tank (one less than last year due to the new engine’s improved fuel consumption).


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