2.25od back to the throttle body. I got the calipers out and measured the id of the throttle body. Guess what? 2.25" on the money. The other plumbing? Is metric but comes in at 1.95" od with a beaded end=2.05". It makes aerodynamic sense to me to be using 2.25" with a 2.25" to 2.38" reducer at the throttle body and fit a flexilbe machined o ring internally to close the gap in the reducer to the throttle body...