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05/05/2006 - Tech Spot: VTEC

ENGINES ARE ALWAYS a compromise. What helps an engine produce strong torque at low revs may mean that it’s harder to produce good power high up in the rev-range, but there are ways around this. Honda’s VTEC system is a good example. For more than 15 years Honda’s VTEC system has been used successfully on cars and over the last five years on the VFR motorcycle. VTEC basically is a variable valve timing system which achieves a compromise between the ideal valve area needed to achieve optimum power and throttle response at low revs and do the same job at high revs, too.

When an engine is operating at low speeds, gas velocity in the inlet tracks and the valve throats is low, so less valve area produces more power per engine revolution where smaller amounts of fresh charge can be burned more completely when the cylinder’s ability to fill itself is low.

Then, when the engine is spinning at high revs, gas velocity is high and the cylinder can take all the fresh charge you can throw at it. That is when larger valve area is needed with all four valves working. On the Honda VFR you have four valves per cylinder and the VTEC system. This means that at lower revs only one pair of inlet and exhaust valves operate, with the second pair coming in at 6600rpm, giving top-end performance a healthy boost.

For more on Honda’s technological achievements, go to: www.world.honda.com/motorcycle-technology/vtec/