Thanks, but neither of my browsers will do this.
Your browsers are probably configured to block all pop-ups. The pics are displayed in a photo-gallery-focus pop up.
IMHO masking the port anywhere near the port face is an obstruction of greater effect than raising the gas velocity for better extraction. There is a well respected maximum taper (for megaphones) of 14° which should be used to minimize flow loss (anything sharper is restrictive), which would make the primary a baffle cone tapering from 1-5/8" down to 1".
I can’t agree more. The Vizard calculations speak to the primary tube diameters for the majority of the running length. Your points on transitioning to that diameter are spot on and are echoed by Vizard himself when discussing some tuners' obsessions with ‘equal length’ length primary tubes: “A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays”.
What I like most about Vizard’s insights is that he asks us to look at the volume of exhaust gas at hand. Swept cylinder volume in cubic inches can be converted to cubic feet by dividing by 1728. Multiplying that by the target RPM range yields the volume of gas to be expelled: 261/6 = 43.5 / 1728 = 0.025173611111 * 3000 RPM = 75.5 CFM
@4000 = 100.69 CFM
@5000 = 125.86 CFM.
Your point on the impact of volumetric efficiency echoes Vizards key take away on exhaust system design – when in doubt go smaller.
As for the question posed in the OP – the Vizard calculation suggest primary pipes sized from the lower left side of the range (for the single valve ports) and in the middle of the range for the siamesed ports.
