An old hot up trick I read in here pre the 2006 blackout.
The ID of the genuine Brumby Y pipe at the engine end ~38mm , the ID of the 89 mpfi EA82 L Series Y pipe complete with cat and O2 sensor to play with later - ID at head ~44mm
Bolts up nice to later EA81 heads that HAVE - note this is an edit !! the cast iron pollution EGR fitting. The Y pipe is about 20mm longer in total yet bolts up at the tapered flange end, just pushing rest of exhaust along a bit, so some adjustment is needed, and also need to custom make a mounting bracket to sway bar or box.
I can notice improved light throttle response


Cheap and cheerful mod !
Now I have an EA81 cat Y pipe to kick about !