How do you want your cylinder filling: with or without inertia?
To take best advantage of the inertia in a fast-moving column of air flowing through an intake port, the intake valve(s) should be opened early, even before the piston has finished the exhaust stroke , and held open well after the piston reaches the bottom of the cylinder. Trouble is, in low-speed, part-throttle operation, timing that radical doesn't work. Airflow is inadequate to generate sufficient inertia , so a lot of fresh air is pumped back up the intake ports, reducing cylinder filling and disrupting flow in the whole intake tract.
Ideally, you'd like two completely different camshaft profiles - one for puttering around town and another for high speed blitzing -but how do you obtain that without changing cams?
Basically, changing cams is exactly what Honda does in its variable valve timing and lift electronic control system. Watch carefully. Directly above each valve in this four-valve-per-cylinder design is a cam lobe working through an individual rocker arm in conventional fashion. These are the "low speed" cam lobes, giving relatively modest lift and short duration. (The pairs are even timed a bit differently from one another to generate a swirl effect in the combustion chamber.)
But there's another, wider cam lobe and rocker between each valve pair. Here lives the wilder "high speed" timing, and it's brought into play by a computer actuated solenoid on the end of the cylinder head. When revs, throttle, and temperature dictate, oil pressure - via the rocker shafts - moves a split piston inside the rockers to lock the three arms together, thus transferring the center lobe's higher lift and longer duration to the valve stems. (A "lost-motion spring" keeps the center rocker from flailing about when it's not engaged.) The system provides a nice double hump in the torque curve and is already giving the new Japanese-market Integra a lusty 160 bhp from only 1.6 liters - with good flexibility. It will work similar wonders for the 3.0-liter NS-X V-6.
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