Hi - what are the chances of the 121 diff coping with 200 bhp non turbo 13b set up. I've been told that it's a 24 spline (12A and piston) rather than 28 (RX5). Cheers for any thoughts.
24 spline centre/axles will just cope but don't thrash it because you will very quickly find the limitations of it. If you weld up the centre expect to break an axle.
28 spline centre/axles will easily cope.
The issue with these diffs is more about finding the right gearing. The smaller 24 spline centres have the better gearing (4.1 and 4.4 available) while the 28 spline centres are limited to 3.36 and 3.9.
I have now seen that you can machine the 28 spline hemisphere to take the smaller crownwheel of the 24 spline centre and package that into either carrier and then run the 28 spline side gears etc for the best combo of strength and gearing.
However its alot of mucking around with a standard diff when the trend toward hilux diffs is well known.