Questions about putting together a Ford 9"

[F23]

Hey guys,
I'm right at the start of modifying my 1969 Mercury Cougar as a pro-touring car, and I'm going to swap in gears and a posi into the 9in. I need some recommendations for what to do for each.

Background info:
Going to swap in G-Force T5 with either .63 or .59 overdrive
Mostly Unmodified engine makes ~315hp and ~390lbs
I will be making no engine modifications in the near future due to cost and time
It IS my daily driver
I have a VERY low budget

So, gear wise, I'm stuck between 3.89s and 4.11s. With the FMX and the 3.00 in the back, I'm turning 2500rpm at 65mph in 3rd(Top, 1.00) gear.
here are the various combinations and their rpm at 65mph with 27" tires

4.11s and .59=2040
4.11s and .63=2175
3.89s and .59=1928
3.89s and .63=2070

All of these are significantly lower (300-570rpm) than stock, so anything is an improvement. Of course, since I'm most probably overestimating the power figures, I want to get the most acceleration possible with the power I have. So I'm very tempted to go with the 4.11s and the .59 overdrive. The real problem I'm having is that the rpm drop from 4th to 5th. It's almost a 1400rpm drop! That sounds like a lot, but how much of a drop is it actually? Is it really hard to shift with that much of a drop? The drop from .63 is only marginally better, at around 1230rpm.
So the question is: is it worth it to have the acceleration benefits of 4.11s if the drop is that severe? How much of a difference would there be between the 3.89s and 4.11s? Opinions please

Onto the next, simpler question of the type of limited slip diff I want to out in. The 3 main types I've heard of are TracLoks, Truetracs, and Detroit Lockers. Keeping in mind that I am putting burnouts and quick, sharp handling infront of pure drag strip straight line speed and acceleration, but since I'm also using this car as my daily driver, in rain, sleet, rain, and everything in between, it has to be practical for everyday use, both in the parking lot and on the parkway. Keeping in mind my budget, what are your recommendations?

All RPM calculations were done on http://www.csgnetwork.com/multirpmcalc.html

Thank you,
F23

[bryant]

which gforce t5 gear set are you using? the 2.95 first or 2.42 first gear.
i have the 2.42 fist gear with 4.10s in my car. i dont remember what 5th gear i have in it right now. i think its .75 but not certain.
the gforce gears are noisy. they are made for strength not to be quiet. im planning on going swapping just because of the noise. definitely not daily driver friendly.
ive got 2 car. one being built and the one im playing with now. one has a 320hp motor the other has a 400 hp motor that will get a supercharger. for the 320 hp motor im planing on putting a z spec gear set from the gearbox.com into. for the big motor ill either go with tko or t56.

the z spec gears would be a good choice for you. i would go with the 3.89s and the stock over drive of 0.86. a lot of carbed motors wont work to efficiently below 2000 rpm because they are still fueling partially off of the idle circuit.

the tru trac will give you good all around performance.

[F23]

Thanks for the reply,
Did you get the Helical or synchronized gears?

Thanks for the info on the carbed motors and idle circuits, thinking back on it, that makes sense.

F23

[bryant]

i have the synchronized gears.

[LT1Nova]

I'm going to watch this thread because I'm interested in the rear differential opinions as well. I'm going from 28 spline to 31 or 35 and trying to decide which style to go with.

I had a detroit locker before and loved the fact when going around a corner I could mash the gas and kick the rearend out. What I didn't love was the "sway" the vehicle had when lightly accelerating or getting on and off the gas in town or traffic. It would slightly shift one way or the other a bit. This quirkiness kept my wife from driving the car, which is not a goal of mine. I swapped in a used stock tracloc and the sway went away, sadly so did the ability to powerslide. Now it only lights up that inside tire until things level out.

I've been looking at the same options as F23 but also the Auburn Pro Series limited slip. I'm leaning towards the Powertrax Grip Pro helical style unit but am curious to hear from someone that's powered a car through a corner with one.

[bryant]

i hadnt seen those power trax units yet. they look to be a torsen type design like the trutrac, s track, wave loc, and platinum track. the pricing seems really good. it doesnt have any indication of having any sort of preload like some of the torsens have to give it an initial resistance to differentiation of the axles.
i can tell you it wont kick your rear end out like the detroit but it also wont let the inside tire just spin like the used trac lock does.
you can rebuild and tune a track lock with shims on the clutch pack. a auburn pro will have a much higher friction level than your track lock has and should last longer. a tight track lock and an auburn pro should allow you to kick the rear end out in turns when you hit the power. they will both wear out over time. the track loc is rebuildable and auburn offers a pro rated exchange program.