Which Posi

[po51 21]

I'm getting ready to replace my 3.00 9 inch with either a 3.25 or 3.50 I want to install a posi at the same time. Which posi would be best? I plan on local driving ans maybe a twice a year 100 mile trip to San Diego. my Mach1 has a 408 stroker 525 hp with a built up FMX tranny. Any thoughts or ideas would be great!

                                                                                                 Steve

[Rsanter 152]

You can use a stock posi unit which is still available new or,you can go with the more modern torque worm type that is popular with the late model crowd

 

Bob

[barnett468 418]

.

You can also put a 3.43 in it.

[SA69mach 39]

A true-trac LSD would be my choice for your car and for your driving needs.  3.50 gear set, forged 31 spline axles, new bearings and seals.  That is a good package that should last and give you a quiet and solid drive.

 

If you are really punching the car a lot, 525 hp is going to push any LSD.  Any true locker is stronger than an LSD, but comes with noise and brutal characteristics, and will chew up your tires and scare your passengers. lol

[po51 21]

Thanks for the feedback, Never heard of a 3.43 for a 9 inch will have to look into it!

[RPM 1,190]

I don't think a "Posi" will fit in a Ford, unless you have a Chebby rear axle :)

[po51 21]

OK, I mean a limited slip

[Brian Conway 264]

I run a stock Traction-Loc with 3.25 gears behind my 428/4 speed.  Freeway 75 mph at 3300 rpm.  Those gears are little bit of a handicap at the track but fine for the street and plenty of punch in all gears.  Brian  

[Ridge Runner 1,112]

I have usually used a Trac lock ,but make sure to run the correct gear oil for limited slips .

[SA69mach 39]

Is your 525 hp at flywheel or rear wheels?

[po51 21]

Flywheel, the car is just for fun nothing for track just to take to a few meets and around town. Had to rebuild the 351W so decided to ann a little extra!

[DRASTiK 22]

I just installed a ford trac lok, 3.25, and 31 spline axles a couple of weeks ago.  I originally had a 2.79 open diff.  Big difference, and I'm very happy with it.  I originally was going to go with 3.50, but like you - I want to be able to take it on the highway sometimes, and 3.25 was a good compromise.  So far, I have no regrets.  I'm running a 347, Trick flow heads, custom grind cam, ProSystems carb, etc. with a top loader.

 

I may see the drag strip once or twice a year, but my priority is to enjoy driving it.

[SA69mach 39]

Your rear wheel hp is under 450, and the truetrac will handle that with ridiculous ease.  Smooth and maintenance free. Very strong. 

 

If you are not racing, then they are, by most standards, the perfect LSD for a streeter with solid hp and live axle rear. 

 

You wont be unhappy with the package I spotted above - 3.50, 31 spline, forged axles, truetrac. Great 9 inch combination for any 3 speed automatic Mustang.

 

It is a lot of work and $$$'s and I would not just throw suggestions from my 'wish list' for you to go do.    I have two cars with trutrac Ford 9 inch rears, 31 spline, forged axles.  The 3 speed auto has 3.50 and the 5 speed has 3.73 gears.   

My 500 rwhp car has the Detroit Locker. 4.30 gears, 6 speed T56 Magnum, lots of go fast bits.  Brutal to drive on the street, but absolutely bulletproof track car for nearly 10 years.

My Mach 1 has a 3.00 peg leg 9 inch rear, but this season it will be getting,....  drum roll,....3.50, truetrac 31 spline forged Dutchman axles.  In my view it is a proven combination for mid/high hp street car.  They can take a beating.  they run hard light to light.  They are acceptable for highway cruising.  You don't give up much with that set up. Maybe long long hauls, that gear would be better at 3.25, but unless you are doing interstate trips 10 times a year, forget it.

[Vicfreg 771]

I vote for the 3:50 9" traction lok with 31 spline axles. That is my set up I am currently building for my 393 stroker C-6 combo for my 1970 covertible. (I am interested to see how the FMX will handle your HP, that's why I went with the C6, so look forward to hearing more about that.)

 

Make sure you consider your converter stall speed when picking your gear ratio. In particular with a non overdrive auto trans, you don't want to be below the top end of your stall rpm while cruising down the highway, you will get some "slip".

[Vicfreg 771]

If you go to the TCI website, that explains the stall speed criteria. Has a chart that shows crushing speed RPM versus brake stall RPM.

[Rsmach1 71]

I am interested to see how the FMX will handle your HP, that's why I went with the C6, so look forward to hearing more about that.

If properly rebuilt his FMX should be OK, as you stated just need to choose a converter/rear gears wisely, and not be cruising down the hwy at 2200 rpm with a 2500 stall converter. It's also advisable to get a large trans cooler (preferably with it's own fan) to keep temps in check. Being cast iron the FMX tends to run hotter than it's aluminum counterparts.

[SA69mach 39]

FMX is heavy, and that is about the only downside when compared to C4, C6.  FMX is very strong, very durable, and does not rob power anywhere near the C6, and still less than the C4.

 

I don't see your FMX a problem, so long as it has had a quality rebuild.

 

As far as transmission coolers, be aware that rubber hoses have no business in the entire array.  Only use metal lines and braided lines for flex joints.  If your rubber lines break you are guaranteed a blown trans, wherever that happens to occur.  Overheating transmissions are due to worn internals, bad seals and low fluid.  If you don't have those issues, then probably don't bother with the cooler, unless you are racing or running 6500 revs or above.

If you don't know the condition of your transmission, then for peace of mind you could install a trans temp gauge and monitor it  for a while.  Like 65 bucks. 

 

Any trans will wear out, it just wears out a bit faster with high hp loads.  OH, and also agree with the stall speed.  Don't get sucked into the hype of getting a "high stall" converter unless you are seriously running dragstrips every weekend.  High stall is a pig around town.  Pick a stall that is under your revs at highway cruising speed at 70 mph, as suggested by RSMach1

[RPM 1,190]

Good info here for us FMX owners. Thanks guys.

[70769 18]

I went to a true track in mine, I did an entire new third member from Currie, upgraded to 3.30 gears and a 21 tooth speedo gear. I had to send mine back due to the pinion depth and lash being set too tight and the break away torque was too tight as well, got it back and I'm quite happy with it.