I have a 76 Cosmo that I'm going to put a ported NA 13B 6 port in from a 87 FC.
I don't know much about mazda axles, so I don't know which ones are good andt what they can take.
The Cosmo has a 5 link live rear axle with a 4 luge bolt pattern with a 3.636 ring and pinion stock.

Will I need to upgrade my rear axle?
And is this axle very good?

 

I cant tell you for sure how strong the rear axle is in the car but can tell you the 2nd gen rear axles from the Turbos are strong and posi - and even the GXL model N/As - are strong and posi rears (posi standard 86-88, optional on 89-91 GXL's) . The ones from the turbos are really good. Both the 2nd gen GXL and Turbo rears have 4:10 gears (definitely way quicker acceleration than your gear ratio, but would be less fuel economy. Make sure you know what u want from the car and choose).

The drivetrains for N/A and Turbos are are different (starters, etc too) so the driveshafts for N/A and Turbo wont mix with the opposite rear end (unless you drill the flanges if your careful and make them tho). You are going to have to figure out the length of the driveshaft needed for your swap anyway.

Also, I dont know what the Cosmo has for brakes but the GXL model and the Turbo model both had awesome brakes (5 bolt pattern, 4 piston front caliper, 2 piston rear). I know people have put these on 1st gen Rx7s. If your gonna do the swap to the 5bolt, changing the rear axle means your half way done if you use a GXL or Turbo rear. If you cant change the front to the 5bolt for some some reason and still need to do the rear end swap here is a link with info on models and rear ends from Grannys speed shop - http://members.tripod.com/~grannys/rx7info.html - may be of some help.

Good luck, hope the swap goes well.

 

Installing in a IRS system in a car with a 5 link solid rear axle sounds like a lot of work.

Are the IRS systems from rx7s easily retro-fitted to cars with live axle rear suspension systems?