[nailhead425]

The main benefit of DOHC engines is the abiilty for small-bore engines to breathe better and rev higher. The Ford Mod motors are an excellent example of how 3v and 4v heads will improve flow and make more power. The reason is the really small 90mm (3.54") bore. Larger valves are significantly shrouded by the cylinder walls and thus flow is limited.

On the Gen III/IV small blocks the head flow is amazing. There is a diminishing return to 3v and 4v heads with these engines. First, additional valves will flow too much at low rpm, decreasing efficiency, power and economy. Secondly, the horsepower gains on 6.0 and 6.2L engines would be met with a reduction in low-end torque. On high-end cars, 7.0L and up there maybe enough gains to develop a 3v head, but on smaller displacement engines current head designs support far more horsepower than most folks will ever utilize.

The DI, VVT, and 6-speed transmissions are the next step.