"Phil Irving states that the overall inlet tract should be 90" divided by the RPM".
Can't say I've heard that one before, or seen it in print.
Where did ye find ?
And how would that formula work, 90" divided by say 8,000 or 10,000 rpm gives a rather small number !
And if it was that simple (ignoring the 3 zeros) inlet tracts would all be MUCH longer... ??
Over the decades, carby placement has varied all over the place, seemingly with very little difference.
Veteran machines sometimes had the carby at arms length on a v-e-r-y long manifold,
whereas some more recent machines had the carby right on the inlet valves doorstep.
Heat insulation is one reason for a bit of distance...
Concentric and monobloc Atlas's had different lengths of spacer, purely due to these 2 carbs being different lengths themselves, and very little overall spare room to fit them into.
Fast race bikes almost always had more length to the inlet tract, if they could manage it.
Dunno that it makes much difference for road bike use pottering down to the cafe though.
hth
