NoReins

Hey guys, we are starting to think about rebuilding our 8" rear end. We will be running a mild 351w (350-375hp) and T5 trans.

We want to retain the 8" and add a "posi" or traction lock and new gears. What are good donor cars to find 8" posi units to install in our 8"? Any recommendations from experience of what gear ratio to use? The car will be 100% street driven, mostly highway miles. I'd like some pep, but I won't be racing anyone.

Any thoughts, comments, suggestions are welcome.

frdnut

The 8 inch posi units are very rare...You will be hard pressed to find one in a junkyard..You will probably have to go aftermarket..If so you will have to wiegh your options against going with a 9 inch..The 9 inch is much easier to get parts for and trac loc units were much more common...As for a gear ratio I would think you would want something around 3.50-3.70 depending on whether you are more interested in performance or gas mileage?

Oxnard Montalvo

Tracloc units are readily available for 8 inch rears.
Even so, that's a bit more horsepower than most people would be comfortable with for an 8 inch, a 9 would let you sleep better. The general rule is 300hp or less for the 8.
The problem is that although the design of the 8 is almost identical to the 9, it's on a smaller scale so in every area it's not as strong.

FORD 8 +9 INCH REARENDS

Gun Jam

Well if those are RWHP numbers then its going to be at or past the maximum rated specs for a T5 (A cobra to Z-spec t5) A standard world class t5 is ****ing out!

So assuming you use one of the beefed up t5s say the Z-spec then a 3.80 gear would unload the demand on the transmission and improve life span and would allow 90 mph at 3,000 RPM in 5th gear with a 25" tall rear tire. However with that much torque you may find yourself better suited with a 3.50 gear which I think maybe better aligned with the 350 Hp rating. But this will increase shock load and peak torque on the t5...will it hold up? It should for a while maybe you wont get that 100,000 miles out of it If it grenades there's always the TKOs to move on to.

As for the rearend I recommend the Detroit true trac lsd why because I use one. Nuff said...well okay also because it functions just like an open differential you dont even know its there until you use it and it leaves two nice black stripes on the ground. It doesnt pop or clank or wear out tires prematurely. It also functions safely in the rain as long as you are not an idiot. Further more it is gear driven and does not require service as it has no clutch pack.

However as always if you choose this type of differential I must recommend you practice with it on some type of fairly safe open area like sandy or wet pavement at speeds of 0 to 30 mph (so no close by poles) when a car with no traction control and a torque sensing diff / LSD breaks loose above 10 mph you have about a half second maximum to react with throttle management and counter steer before the situation becomes so degraded and the vehicle is yawed so far out of path alignment that it can not be saved (assuming there was something to hit...which of course there is not in this case) Because both rear wheels have near zero traction as they are both broke loose by the LSD (no peg leg here to maintain traction) the *** of the car does the only thing it can ...pivot around the nose based on torque or g force direction and best believe it can do so with amazing speed. So practice is necessary.

And of course the true trac is available in 9" as well should you choose to go that route.

-Gun

drewsky

i built my 8" into a track lock 3:55.. with c4 auto and a 351w... wish i ould have done research first!! the ratio is waaaayyyyy too high!! cruise at 40mph.. need bigger tires in the back. really fast off the line tho

Iskwezm

I have Currie TSD with 3.80's and T5 out of a 85 GT.This combo has lasted for about 11 years to date with a nitrous, then blower, then the turbo

Gun Jam

You are forgetting the t5 has a .63 overdrive...Even with stock height 25" tall tiers a 3.80 (which I daily drive around town and often over 100+ miles on freeway) is a cake walk and provides 2400 RPM at 75 ish MPH. Even a 4.11 ratio with t5 is totally doable even if you drove it on the freeway sometimes. I would say go 3.80 to get that kick out of the hole but with 350Hp Im thinking it would simply be a constant battle to maintain traction. The 3.50 would allow longer pulls per gear especially in 1st and still have plenty of power to get that 0-60 time.

-Gun

andrewmp6

The maverick/comet is the only car you might find in a junkyard with a 8 inch but most never had a posi so its easier to buy it new.

2+2GT

The 75-78 Mustang II V8 came with an 8" Traction-Loc standard.

Currie sells an 8" 4-pinion Traction-Loc that's probably stronger than the average 9".

Iskwezm

It has a billet side cover and has lasted a while, but the torque sensing aspect is a little iffy to me