Use FC trans in FB
 

Question is how different are the ratios in these manual gearboxes? Any advantages or disadvantages?

Look here yet?

https://www.mazdatrix.com/faq/ratios.htm

I have an FC tranny in my car and I don't even think about the ratios. The part I needed to pay attention to was swapping the tailhousing from an FB trans to this one. The shifter housing etc too. I also swapped in a new return spring and did the front and rear seal. Competition trans mount as well. Of course I use hockey puck motor mounts so the whole system drives really nice with only a bit of vibes. I'm not interested in driving a rubber band which is what you get with stock duty stuff, in my opinion.

My best transmission is built from 3 different donors. The tailshaft housing is from an FB to fit the crossmember, the middle section with all the gears and the shifter housing are from a Miata to get close ratios, and a better shifter detent system, and the bell housing is from an FC, the FC bell housing has some strengthening ribs and some cooling ducts for the clutch.

mhr650, that sounds like a perfect tranny for these cars. :icon_tup:

I have to put up with the FC ratios but they don't seem to be that bad. I also have the ribbing around the bellhousing so I think it might be S5 or late S4.

Was this all bascially just a bolt togther thing, or was some fabrication
required? I'd like to see a thread about how this was done.

The only modification is that you have to shorten the Miata input shaft, the directions are on the Mazdatrix site. to get the input shaft out you have to completely disassemble the gear clusters which requires a few special tools to pull the gears and synchro hubs.