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The reason is because you want to keep the right balance of air pressure and air flow. If the intake piping is too small, it doesn't flow enough air. If it is too large, it drops the air density and you end up with less than adequate pressure on the butterfly. Ever wonder why the AEM makes more power at 2 3/4" than the Injen at 3"? Now you know.
It's all about the ratio of the diameter of the intake pipe to the diameter of the throttle plate. This depends on the amount of A/F the car can move, which varies depending on redline and cam grind (among other things).
For a B18C with a stock redline and stock cams, you want the ratio to be somewhere between 1.15 and 1.2.
For the AEM Intake:
Intake: 2.75" = 70 mm
Throttle Body: 60 mm
70/60= 1.17
So if you want to go with a 3" intake (Injen or AEM Type R), you would need:
Intake: 3" = 76.5 mm
Throttle Body: 63.75 - 66.5 mm
76.5/63.75= 1.2; 76.5/66.5= 1.15
If it were me, I wouldn't worry about a larger throttle body, intake, and intake manifold until after I had the cams to utilize them. Your engine as it sits with the stock cams doesn't breathe enough air to necessitate opening it all up. I would take this money and invest in some camshafts, cam gears, and some dyno time to have it tuned. Then it would be worth it.
If this is something you absolutely have to do right now, I would forget getting a new throttle body. OEM throttle bodies can be bored out up to 4 mm. Have it bored to 64 mm. Whatever manifold you decide on should be ported, as well as the intake ports to remove any bottlenecks. Then you'll have some really good groundwork to put in camshafts to utilize the breathing capacity you've created.
Catch my drift?
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