I know completely nothing on this, but I would tend to think smaller...As the diameter lessens, the pressures should build up and increase the velocity of the exhaust gas. Higher velocities = faster spool (I would expect). However, too small would mean excess backpressure on the engine itself, and may not allow for complete evacuation on the exhaust stroke. Combining the two, I would venture to say larger primaries and smaller components (assuming the primaries are the 4 tubes dumping directly from the exhaust ports, the component is the mated section connecting to the uppipe :dunno), but that's only if you can taper sizes down. If you stay equal ID until the component, you would want smaller primaries. Makes sense in my head... Edit - while on the subject of completely hypothetical guessing, what would be the advantage / disadvantage of equal vs unequal length headers? I'm guessing unequal length would cause more turbulence thus restricting flow, but equal length will have more bends thus increasing the static pressure. Any experts care to chime in on that?
I tend to agree. It's a combination of balance. My headers are 1.6" primaries. In the uppipe there is a 45mm velocity stack. However when I go rotated I will lose the uppipe so I need to figure out if the 1.6" headers I have are gonna work great. SS thinks so, so I'm gonna stick with them. I just want some healthy information, discussion and such so I can do research as always.
They are. Came straight from the horse's mouth. Here is Rich's email. The 4 runners have an inner diameter of 40.7mm. Where they merge into 2, each runner is 43mm ID, and finally the 2 merge into the collector and uppipe at 58mm ID. The sleeve in the uppipe goes down to 45mm but opens back up again.
Doug we are talking about the Equal length's and not the non-equal length's too. I've held my EL next to MadDad's NEL and mine were noticably larger.
Are you looking for spool or top end? Whats done to the motor? what size turbine housing? I would venture to say tapered would be the best. The smaller the tube the more sacrifice of top end flow you get but you keep the velocity high which will spool the turbo quicker. There is always a tradeoff. If someone would make a cfd of the header you could play with the piping size and see how it changes the flow.
I'm looking for the best of both worlds. I want the most feasible and widest power band I can achieve. Car will be a 2.5L with forged pistons, WRX heads with 272 cams. 8.8:1 CR.