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Why does VTEC vary lift as well as valve timing?

2 March 2005
Redmond WA USA
Why does VTEC vary lift as well as valve timing? It seems to me that more lift is always a good thing at any RPMs.


With the standard set of lobes engaged on the cams, the engine is geared toward maximum efficiency. This is acheived with moderate lift and staggering of the valves to promote fuel charge swirling which helps to completely burn fuel and maintain the fuel in suspension at low velocity. This is why it is common to get 28 MPG on the hiway. When the VTEC is engaged, both valves open at the same time on a single, higher lift and longer duration lobe with more overlap between intake and exhaust. This will allow more fuel charge to enter the cylinder, and at the higher RPM, the velocity is high enough there is less concern about the swirl pattern, just getting as much fuel charge in as possible. Mileage goes to hell, power goes way up.

This is why we get the best of both worlds with the VTEC motor.