Bigger is not always better when it comes to bore diameter. A larger bore diameter can ultimately flow more air, but at lower air mass flows (lower rpm) the air speed is reduced compared to a that of a smaller bore, reducing lower rpm performance. Correctly sized bores idle better and have more controllable torque changes as the throttle is opened at low throttle openings, improving the “driveability”.

If you were previously running sidedraft carburettors remember that the carburettor bore size is not a direct comparison to fuel injection throttle bodies. Carburettors have chokes (required to make them work), making the effective bore diameter much smaller than the number on the casting. 45mm carburettors have choke sizes between 28mm and 40mm. Fuel injection throttle bodies do not have chokes; a 45mm fuel injection throttle body is 45mm throughout.

If 48mm throttle bodies are chosen the inlet manifold is taper cut at the throttle body end to 48mm for just the first 5mm leaving the rest of the port for your engine builder to shape as desired.