Anyone know what the differences in the Rs1600i inlet plenum over the XR3i MFi

I know what externally but were the internals opened up slighty?

they also have the 4 bolt throttle housing same as early XR3i's

I'm thinkig of uprating my MFi Xr3i racecar engine wise and want to keep the standard MFi set up if possible

or it will be Jenvey time

Cheers!