Ok, I've been searching for a few hours and I can't seem to find a answer. Or a equation to help but basically I have a tri-y exhaust that's 2" off head, 2.25" from bottom of header to 3rd y. To 2.5" pipe the rest of the way to muffler which is 4" outside diameter and a outlet of 1.5". Why is the outlet so much smaller than the exhaust pipe?

Second question is right now I have 3 in outside diameter throttle body, with 3.25" short ram intake. My avg intake temp is 140+! So I'm going to get a CAI but can't decide which diameter to go with. Suggestions? Not the kind " oh well my buddy's friend said this so that's the ****" I mean a real answer. I read some about the ventrelli effect? Spelling is terrible lol but you get the point.

Also it's N/A H23a VTEC. All stock for now except exhaust and intake.