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The airflow area is the smallest at the blade where the throttle shaft and blade area reduce the bore size. The larger throttle bodies will flow more even with an adapter plate behind them. An engine is a self running air pump, the more airflow you can move through it the more efficient it becomes and the more power it makes.I don't have it or a mic on me right now but it's 88.9mm at this end and you can see how it tappers down. I don't know if it's stock but I didn't put it on there. It's on my 1997 k2500 454
Because the smallest area of the throttle body is where the butterfly sits. The bigger butterfly on the aftermarket throttle body eliminates that choke point even with the same throttle body opening on the manifold. A 4" intake also flows more than 3.5" intake even if the 4" uses a reducer to 3.5" at the end of it.I fail to see how it makes much of a difference if they both narrow down to the same size which is the intake size.
The placement of the blade on the wide end vs the narrow makes that much difference?