Ford nine inch help

what's the best posi rear end ratio for a ford nine inch rear axle that would bolt up to a t-5 ****** and is powered by a 239 ford flathead?

 

Depends on tire size and trans gear ratio. Type the words RPM calculator into your computer search box and info will appear. For instance:
http://www.csgnetwork.com/multirpmcalc.html

 

something in the 3.50 to 4.56 range, probably.

might help to know tire size, engine modifications, driving plans, etc

 

For cruising?

For racing?

For pulling????

Just a shot in the dark I would say 3.08-3.23 for cruiser and 3.55-4.11 for a peppy street engine.

And posi has nothing to do with the gear ratio.

 

There's boatloads of lockers and posi units available for the 9". You'll need to know the axle spline count as well. 28 and 31 spline are the most common. 35 and 40 spline setups are available aftermarket. There is no carrier break like in a Dana axle.

'drop in' traction aids like mini spool, Tru Trac, Lock Rite and others are relatively simple to install.

Full carrier lockers like Detroit and ARB (air locker) are also available.

There are no electric lockers available for the 9" at this time that I'm aware of.

Posi's are kind of a pain, especially if you have much HP, and are running larger than stock tires. They have to be rebuilt every few months, and don't really offer much more traction than a standard open carrier.

Personally, I like the Detroit. It's always there, and tough as nails.

If you have on board air, or just want a selectable locker for the strip, or running around, the ARB is tough to beat.

 

We run a 3.00 ratio in our flatty roadster, the T-5 is going in soon. For our little lightweight car the flatty pulls the 3.00 just fine for cruising.

 

With a flattie you want something that will keep the rpms lower. back when we built one with an auto backing her but we used a 3.10 gear. The gear came with the rear but it word great

 

Ratio? Pick your poison. Depending on the weight of the car anything from 3.00-4.11 would be OK with an OD. A posi? Forget it, you'll never need it.

 

I have to disagree with Gummi Bear as I have a 9" posi with 33" tall and 19 1/2" wide Mickey Thompsons with a big block and a 400 trans with a B&M hole shot convertor in it. It has a 4:11 gear and I have done so many hole shots and 1st thru 2nd gear patches that I am on the second set of tires and no problem at all with the posi unit. Knock on Wood!! It is several years old now too. It is not a Detroit Locker eather. Maybe because the car only weighs 2150 lbs with out me?
My freind says you do not want a posi in that light car as it will be too squirrely, so I took him for a ride and laid 2 perfectly matched tire marks all the way through 1st and half of 2nd gear and then went back and showed him the 2 parallel marks. He still does not want any more rides in my car!!
Posi is the only way to go, who wants to only smoke one tire? Gary M.

 

That's amazing that it's held together for so long and so well. My luck was never that good with them.

I was killing them in my Bronc before I got the Detroit. Keep in mind that this is a pig of a vehicle, ~4500lbs give or take a few. I was 'stripping' them out on 31's, and that was with a Jasper 302, RV cam, Qjet, C4 trans and 3.50 gears.
Of course with that same setup, I broke a couple of shafts and grenaded a couple of sets of spider gears in open carriers too.

My upgrade to the Detroit was a dumb luck deal, I stumbled on a Sunray 9" already built (Detroit, 4.88 gears, Moser shafts, Sunray housing) for basically the cost of the locker itself. I'll never look back.

www.sunrayengineering.com They got started building rearends for drag racers and hot rodders, and have gotten into the rock crawling gig, and they are a very revolutionary shop.

 

I agree. Gummy is looking at this from a rock-climber/extreme off-roader type situation where you've got one tire with very little traction and the other tire with LOTS of traction......a situation meant for a true locking differential, not a clutch type diff. You don't just kill a 9" Traction-loc (limited slip) differential under normal strip-strip use. On the street with anything this side of 500HP your "OK" with a Traction-loc if it sees limited duty with slicks. Anything that sees more strip than street and plenty of time on slicks should step up to a Detroit Locker.

-Bigchief.