[Rick Parker]

The valve sizes in the modified Shelby heads as I recall were 1.6/ 1.875. Any larger and the intake seat was compromised by the 18mm sparkplug. I modified a set of Hipo 289 heads and fit these valves and they worked well. Later when development of aftermarket Ford heads was in its infancy there weren't a lot of choices. World Products was one of the first and their Winsor Jr head had 1.94 / 1.6. it is an ideal fit for a smallblock as the velocity through the ports is maintained regarless of what carburetor is being used.

ERA Chas: The throttle plate on the 48 IDA is 48 MM but the venturi size is standardized at 37 mm but is upsized based on cubic inch displacement or usage.
Go to large and throttle response gets real soft.

There are a couple of specialists that increase the throttle bore to 51-52 mm and install new throttle plates.