Hello John,

For race engines you should use the big tube header, low backpressure will garantee highest top end power and additionally it will shift the point where a beneficial pulse effect occurs (is also function of intake runner length, diameter,cam profile ...) upwards in the rpm band (for example 5500rpm).
A friend of mine runs a B20 with S-cam and DCOE45's and uses a big header, torque peaks @5500rpm while this same engine with twin pipe stock manifold makes torque peak at 4800 rpm.

If you use this big tube header on a stock engine which has peak power @4800 rpm you will not reach the point where the pulse effect occurs and actually be of worse than with a smaller tube header which offers this effect at 3500 rpm.

CU

Stefan