the series 4 rx7 compressor wheelis 51 mm the series 5 one is 54 or 55 mm and they are steel I ahve driven an r32 with this turbo set up with a t4 front wheel worht some though.
btw still ahvent fittted that turbo waiting for the ceffy to come back. also detonation could be because of a lack of fuel have you upgraded injecotrs and afm?
cheers
meggala