I was reading Vizard last night about manifold port runner diameters and his recommendation was for up to 1-1/4" 'at and near to the cylinder head face' for 850s to 1100s and 1-3/8" or larger for engines up to hot 1400s. So I measured my 1-3/4" SU no-name water heated alloy manifold and found the runners to constrict down to about 1.1" at their narrowest which seems to be a little restrictive for a modified 1275 engine. Comparing to a Weber DCOE manifold or likely even twin 1-1/2" SU manifolds, it's a whole lot smaller.
Is this typical? Wouldn't it be of great advantage to open this out to minimum 1-1/4" all the way through? Otherwise I can't see how a 1-3/4" carb can have any advantage over a 1-1/2" on this manifold.
Edited by hhhh, 23 December 2017 - 05:16 PM.