This is what you get when you have a lot of time and get a bit OCD about pipes.
Again only modelled in EngMod4T not dyno tested. Data has a good correlation to dyno figures for the baseline. Total runtime for these graphs is about 14 to 15 hours.
Same race spec engine (Custom race cam, good head all dancing all singing) only change is the diameter of the centre branch on two variants of manifold.
One is a 3-1 manifold from down under (which was pretty optimised) the other a LCB manifold based on the lengths Vizard has published, starting at the diameters that where iteratively found to give the best curves for smaller and less racy engines using a high end simulation package. Diameters where varied using small steps (1 or 2 mm).
Graphs are split into inner and outer cylinder pairs. Absolute values are more or less wheel KW per cylinder but not really important. RPM plots are to a heady 10k rpm which is not recommended with a 3.2 inch stroke ! The model does not take into account valve train instability or crank harmonics.
What diameters ? I’m not going to tell as this is a running project and I just wanted to illustrate the general behaviour of both the 3-1 and the LCB. Not in the graphs are the effects of length. Suffice it to say that it does not seem to do much. For an LCB it is very hard to improve on the lengths as published by Vizard 40 odd years ago. Primairy length for a 3-1 (where the centre is actually a secondary length) is not very critical.