Hio Guys..
Simon, the throttle body size on the single unit is often connsidered a weak point or restriction.
Early golf gti tuners (VW Rabbit in US) had a craze for increasing the tb size..
when actually tested it didnt do anything apart from move the torque curve up slightly, hence reducing low end pulling power.
There is a maze of theory regarding pusle tuning, inlet tract length etc..
a lot depends on the design of the plenum chamber (the box with inlet pipes going to your cylinder head)
the bottom line is that bigger is not always better, and as Ben rightly says, it needs to be dynoed to see the results.
imagine HOW the intake charge air is induced into the cylinder, the cylinder goes down due to the expansion of the ignited mixture, as the piston passes
C (bottom dead centre) the exhaust valve opens and the whole lot exits at high speed into the exhaust manifold.
now the inlet valve opens as the piston starts its downward cycle 1 rev later.
We now have a dual effect, vaccum created by the downward travellinmg piston and also a little factor called overlap of the valve timing meaning that at a certain point the exhaust valve and inlet are still effectively open.. the high pressure exhaust pulse exiting the engine causes an even greater vacuum or pressure differential in the cylinder as compare to manifold air... the inlet charge is drawn in. yep ?
now, if the inlet system consists of an air holder device.. ie - inlet tract tube / plenum or throttle body unit that is sized correctly for the volume of air, then not only do we have a charge ready to inhale, but the SIZE of the orafice that leads to outside air becomes critical for power delivery.. why ??.
Well, wouldnt it be nice to NOT have a huge opening to atmospheric air.. have a slight restrictor, the reason for this is to allow still another pressure differential to happen between the intake valve when sh*t and the intake from the air outside... this causes a high gas velocity / flow rate and the charge holder or inlet tract / pipe is filled ready for the next intake.
air is a fluid to all intents, can you picture the turbulence at the intakes if no pre charge length was used ??.. ) it would be chaos with each induction interfering with the next.. as air is induced rapidly, and stops being induced rapidly, a layer of distured air exists at the intake that will reduce power by making the gas flow uneven. Providing the length and volume of the pipes is designed correctly, then you can have enough charge in the intake pipe ready for the next cycle, and keep the disturbed boundry layer out of the system. The pipe will be effectively recharched ready for the next induction.
AND this is only effective a t one point in the rpm / load range. so all other areas have to be a compromise.
bottom line.. it aint that simple lol...
we could start discussing why a free flow exhaust is more often than not BAD for power in another thread.
Joe