There have been some serious debates concerning combustion efficiency going on in serious hi-tech forums and from these it seems that it is well understood that there is more to producing torque and power than just filling cylinders with air and fuel. At low speed the engines ability to breath is hardly a problem as relatively speaking, there is plenty of time to fill the cylinder as full as it is ever going to get. The key to making low speed torque is two-fold a) how much charge is trapped at valve closure and b) how effectively the charge is burned.

Aspect a) is governed by the reverse flow properties of the intake valve seats and to a lesser extent the port area and the cam event timing. Better anti-reversion at low lift and higher port velocities are good here but at the end of the day it's still largely down to the intake valves closing soon after BDC.

AS for point b) we find that how effectively a charge burns is down to mixture preparation and in-cylinder motion at the time of ignition. Assuming that mixture preparation is good then, at this point, best results would be achieved by the cylinder with the best combustion dynamics stemming from appropriate in-cylinder charge motion. The bottom line here is that a parallel valve two-valve bath-tub or wedge chamber design has inherent swirl properties. This means a head designer would have to work hard to eliminate the heads natural advantageous tendency to swirl the mixture (and believe me some have managed to do that). For a two valve head then the natural tendency is to swirl the mixture and this, in part, contributes to a two valve heads normally superior low speed torque.

On the other hand a conventional four valve head has no natural tendency to swirl - but it does have a tendency to 'tumble' the charge. The following illustration shows the difference between the two.
Figure_1
Not unreasonably a first inclination would be to assume that both types of mixture motion would work equally well. Up to a point they do but as the piston nears the top of it's stroke these two forms of mixture motion diverge in characteristics. The tumbling charge motion breaks down to small tumbling eddies where as the swirl, as it gets compacted in the chamber smaller than the cylinder bore can speed up (conservation of angular momentum) while still retaining it's swirling motion form. This swirling motion in conjunction with a well placed spark does a good job of effectively and rapidly burning the charge. On the other hand the tumbling action seen most often in four valve engines tends not to produce enough charge motion at low speed to produce as good results as the two valve engine swirl. Also tumble can be almost cancelled out when compression ratio's much above 11/1 are used.

In addition to the lower combustion speed the inclined valve four valve heads tendency to allow the fresh incoming charge to cross flow out of the exhaust during overlap also cuts the low speed torque as well as increasing fuel consumption and emissions. With the appropriate proportioning of valves and bore/stroke, a two valve engine, with a well designed cylinder head, can out-perform a four valve engine to about 4000 rpm.