Rear end gearing impact?
 

So my next question is about rear gear ratios.

For this Frankenstein project of mine that I'm working on, I need to find a rear career. I've been looking for a T-bird 7.5 IRS unit as I know I can get 4.10 gearing for it, but so far I have had little luck finding a unit with a LSD diff installed and they are very expensive to buy. What I did find was a Lincoln LS 8.0 career with a 3.56 ratio and it is an easy cheap swap to a LSD diff.

Having read up on what an RX7 comes with, I'm seeing that Mazda put some short gears behind the rotary and I'm wondering just how big of an impact it would have on the performance of the car going to a taller gear like the one in the LS?

I do intend to have a turbo on the engine, which will help with torque, but if Mazda has a shorter gear I'm guessing there is a reason.

Would a custom geared trans or maybe an RX8 6-spd be needed to make up for the gearing change?

Going to a taller rear will slow off line acceleration but, give you better top end as you will be in the gear longer. It is a fine balance. The lower, the faster off the line and more power to the wheels but, will have to change gears and that takes precious time, even for automatics but, depends on what you are building the car for.

For drag racing, best combo is taller gears for manuals and shorter gears for autos. You can 'pop' the clutch at a given rpms for max power at start but, you want to stay in the gear for as long as possible between shifts. For and auto, you can put a high stall on it but, if you have THAT kind of power, a powerglide is the best option with gearing that will hit max rpm power in high at the 1/4 mile mark.

If you are going to drag then, slicks with variable diameter will create gearing as they get faster and sidewall gets taller as well.

The only reason mazda put lower gearing on the rotary is due to its lower amount of tq available. Since the rotary can rev, you can balance the tq through mechanical leverage by shortening the gearing to increase off line acceleration and less stress on the motor.

Ex. stock rx8 with 130 tq to the wheels is multiplied by 3.76 in the S1 so that you now have 488.8 lbs of tq at the wheels in first with a 4.444 rear. if you change the rear to a 4.1, you would have about 451 lbs of tq but, your speed in that gear would be higher.

In your situation, I would attempt to get an entire rear assembly that has a rough match on the wheelbase. I wouldn't go any higher than 4.2, but that's my personal preference.

There's a lot available (with some really solid aftermarket support) that you could manage. I drove up to Canada in August and walked away with a full r32 skyline rear sub for 500. 240s, older celicas/corollas, supra 1st gens, and a whole bunch more I probably haven't thought of could work.