Originally posted by WRX Barge: Well using the general calculation of 1.5 x hp for cfm for the engine and using corky bells thoughts that you don't want the air velocity to be great than mach .4 as since at that point, according to bell, the drag and flow losses increase greatly.
So if we do some math.. again with the 1.5x hp which will be a little low actually since you'd need more volume of a less dense air... anyway.
So if we go to the limit with 450 hp worth of air you're looking at around a 2.25" OD tube at 450 f/s (Mach .4). So to give yourself a little breathing room 2.5 should easily flow fine for 450hp.
Now say we are using this kit on a decent 2.5L and say we're making about 275 at the crank. For a velocity of 450 f/s you're looking at about a 1.75" OD tube (say 16 gauge wall).
So now let's look at a temp of 190 for outlet temp and an ambient temp of 78 for the 1.5 x hp rule (I figure that's close to what they're thinking and i'm too lazy to check). So say the density ratio of the outlet to what we'd consider ambient is 1.18. So using that let's say that instead of 1.5 x 275 we use 1.5x275x1.18 for cfm. If we do that we're looking at only about a 2" OD tube (16 gauge again).
So anyway.. rough math... with this non-intercooled system i'd say 2" OD should be fine although 2.25" would give you a little breathing room and shouldn't kill you with "lag".
Now I hope someone appreciates this... and I used 275 for crank because I didn't feel like looking up what the original post said for power and 275 seemed like a good number.
Dim be sum big werdz feller
Maybe if I understood you it would make more sense. But from a "I don't know [censored] stand point" that was a very good post and informative to us all.
