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A SOHC setup is perfectly capable of handling 4 valves per cylinder. DOHC is only *needed* if the valves/cylinder are more than 4 (e.g. 5 valves per cylinder as on Skoda Octavia vRS / Yamaha FZ1). Though it is possible to use SOHC for 4-valve engines, but this translates into a complicated combination of rocker arms and cam lobe shapes. With SOHC, the camshaft is usually in the middle of the head because it has to drive both the intake and exhaust valves, robbing the sparkplug of it’s optimal location (Hence the requirement of multiple spark plugs/cylinder).

On the other hand in case of DOHC, it allows the manufacturer to *easily* implement a 4 valve per cylinder setup. Most of the time it also allows the engine to rev higher. It also allows better placement of the valves in an optimized setup that gives you maximum performance. But the disadvantage of such a setup is more weight, more cost and more complexity. It takes more stuff to drive two camshafts.

In the end, a SOHC 4 valve engine would have better torque on the low end where the DOHC valvetrain’s weight results in comparatively lower torque. But at high engine speeds, the 4 valve DOHC engine’s peak torque and horsepower would be greater. That’s the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power. (The reason you *feel* that Duke has a better bottom end probably comes from the fact that Duke is shorter geared as compared to 200NS. Such behavior is not due to the engine itself.)

@Thread starter: R15 has an SOHC setup.