Identifying 5 Speeds.1)

[Zed Head]

I typically go back to the beginning and rethink what I'm trying to do after I collect a bunch of information.  Often I find that my first plan wasn't really feasible.  Some things that might help your thinking are the fact that most transmission/diff combinations are chosen by the factory to give about 12:1 first gear ratio, for performance-oriented cars.  Below 12 and standing starts start to need more attention.  The GL's and other odd options maybe not, but the basic cars are always at about 12.  4th gear is always 1:1 unless you get a non-typical racing old 5 speed.  The spread between 1st and 4th is what will give the "close ratio" feel.  Racing setups start up in the 2's for 1st gear, for example.  Put a chart together of combinations and you'll be able to see right away what the whole project will take.

That's just numbers.  On the other side, you have availability, difficulty of the work, cost, and quality of the parts.  5 speeds, transmission modifications, rear diff swaps, and new aftermarket axles, as a few things to consider.

Hard to recommend.  I think that you said you really just wanted a 5th gear.  The simple way to get there is a Z or ZX 5 speed and a 3.54 or 3.9 R200 diff.  The fact that you've spent so much time on the CA 71C swap though, shows you might really want a more complicated project to work on.  That's why my garage is full of parts.

[Matthew Abate]

Honestly, I thought the ca was a 71b and it would just be a bellhousing swap and bolt in.

The chart is what got me fixated on the 3.59 to 0.813 ratio spread. With a 25" tire at 4,000 rpm and a 3.545 rest end you get 23 mph in first and 103 in 5th; the widest difference available.

Maybe a ZX is the way to go since I want the WRX STI rear end

Sent from my iPhone using Tapatalk

[Zed Head]

I've had my car for a few years and each change is just a new opportunity to optimize whatever I'm working with, which is whatever is available at the time.  I had a 76 4 speed with a 3.54, then a 78 5 speed with a 3.54, then an 83 5 speed with a 3.54, the the 83 with a 3.9, then a late 80's with a 3.9.   The factory pretty much made the right decisions, except for making the post-80 5th gear taller.  

If you want a 5th gear for highway cruising, just pick a set of parts that you can deal with and get it done.  You'll probably be back in the future, looking for something else.  But if you know that you want the STi for limited slip, plan around that.  Looks like they only come in 3.9 or 4.11 so a 3.592 might  be a wheel stander. 14:1 first gear.

http://www.gearhack.com/myink/ViewPage.php?file=docs/Subaru+Transmission+Chart

[Zed Head]

My mistake.  Looks like you can get a 3.54 Subaru diff also.  One more variable.  Where's the calculator?

http://forums.hybridz.org/topic/105868-subaru-diff-clarification/