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Introduction
The advantages of overdrive need no introduction. These days, driving your Ford Muscle machine on the freeway without overdrive means revving the heck out of the motor or running late 70's smog era low numerical rear end gears. Neither choice is attractive for gas mileage or performance, or what we really want - both.

For this car, a 1971 Mustang originally a 302 car and now packing a 572 stroked 460, the existing C6 was spinning at well over 3000 rpm to stay with freeway traffic, even with compromise 3.50 gearing. Add the fact that the streetable 2400 stall speed was well below optimum for a drag strip launch, something more flexible was called for. The preferable OD manual transmission rated to handle the torque of a stroked 460 was the six speed, Viper spec, Tremec T56. A couple quick calculations showed that the ideal rear end gear with a 28" tall tire and a 6200 rpm shift point would be 4:11 to 1. With the T56's .50 6th gear the final drive is close to 2:1, meaning a little over 2000 rpm at 75 MPH. Talk about having your cake and eating it too.

The only problem with the T56 swap is that Ford only offered the 429 engine in one model year of Mustang, 1971. Therefore there is a lack of instruction and guidance on performing the T56 graft into this year Mustang. That is, until this article.

Myth Busting

Myth #1 - A manual transmission means weight savings.
We all know the Ford C6 is tough but heavy. How heavy? On the authors' handy physician's scale the C6 with converter, flexplate and fluid weighed 218 lbs. The T56 with fluid weighs only 128, but add the NHRA mandated scatter shield (55 lbs.), steel flywheel and twin disk clutch assembly (48 lbs.) and the stick tranny is actually heavier than the C6 by 13 lbs. You can save a little weight during the conversion using an aluminum flywheel.

Myth #2 - Mounting a T56 behind a 460 requires a custom adapter.
For this project, the turning point was the introduction of the McLeod modular steel bellhousing. This meant a straight forward bolt up of the T56 to the back of the 460 and the avoidance of custom machined adapters.

Myth #3 - The C6 is huge, if it fits so will anything else.
While it's true the high bellhousing mounting pattern on the 429-460 and the M family of Ford engine takes up a lot of space, but the automatic

While comparable in length, the T56 case has greater bulk, requiring transmission tunnel modifications.
transmission tapers off quickly. The manual's six beefy forward gears and the internal shifter rail takes up more room front to back and adds height back where the vintage transmission tunnel really begins to narrow. Also complicating the issue is that the transmission mount is six inches further back than the location used by the older autos and sticks in vintage iron. The case size stays large back through the sheet metal tunnel brace common to earlier Fords.

The easy fix is to tilt the engine a bit more to get the T56 under the tunnel, but this would mean reduced ground clearance for the headers and transmission, as well as bad driveshaft angles resulting in vibrations. The other issue is the location of the internal shifter. There are two shifter pads on the T56 but both tend to be forward of the typical pony car or mid size Ford shifter location. There are now shifter extensions which add a lever on the tunnel with a link to the actual shift lever, but these would look funky in a vintage muscle car and preclude using a factory console. The other choice is to cut a new hole for the shifter, but again this leaves no room for the factory console and a knuckle busting long stretch for third and fifth gear.

The ideal solution was to set the engine back 1.75" and down .75". This sounds worse than it is as in a '71 Mustang the engine has plenty of room to move back without touching the firewall or moving vital components. It does require some surgery on the tunnel brace but it puts the shifter in the original location and made the drive shaft angles almost nonexistent.

One step at a time
After putting the wheels in motion and waiting for the first production run of the new modular bellhousing to go through quality control I was faced with the decision to do the job of fitting an untried combination from under the car or going through the hassle of pulling the engine and mocking up the install out of the car.


The transmission was mocked up on a stand with a spare 460 motor. Slight grinding of the bellhousing flanges was required to clear the headers.
Fate intervened in the form of three spun cam bearings, the assorted bent valves, sheared timing gear pin and other fun stuff that comes with parts that suddenly stop rotating at freeway speeds. With the engine out and off to the machine shop, a borrowed block and heads stood in and allowed the transmission/bellhousing assembly to be test fit on a stand and then in the car. This scenario also worked to my advantage in that I had plent of space to make all the necessary sheet metal, mounting, and clutch modifications.



Clutch and Pedals

The '71 clutch pedal assembly was sourced from Perogie Enterprises.
The odds of finding a '71 Mustang big-block bell crank linkage and pedal assembly in the wrecking yard are less than winning the lotto. Rather than waste time and money searching for an out-dated clutch actuation mechanism I took the opportunity to install a Mcleod hydraulic clutch setup. All that remained was sourcing a pedal assembly. Fortunately shops like Perogie Enterprises usually have complete assemblies for sale.


Another McLeod sourced #4 hose connects the clutch master cylinder to the slave cylinder mounted around the input shaft. The constant engagement throw-out bearing presses against the McLeod Street Twin clutch ensuring almost instant disengagement when the pedal is pressed. (T56 slave cylinder installed ) The Street Twin is a two disk system designed to hold big block torque without requiring a power lifters leg muscles. The small diameter diaphragm clutch only needs a half inch of travel which is supplied by a one inch stroke at the clutch master cylinder or about three inches of pedal travel.


Rather then search for factory mechanical clutch linkage a McLeod hydraulic assembly was selected. The kit included a Wilwood clutch master cylinder. Shown in image #8 are the slave cylinder, throwout bearing and hoses.

The clutch master cylinder will be mounted in the engine bay. Some precut ProFab tabs, a piece of tube, a few rod ends and hardware store bronze bushings pivoting on an axle bolt made for a compact bell crank. A piece of scrap stainless tube, left and right hand .375” fine thread nuts and two surplus heim ends make up the clutch pull rod.

The upper end of the clutch pull rod attaches to the top of the clutch pedal on a welded stud. Stepping on the pedal pulls the rod up which pushes the master cylinder rod inwards, forcing fluid to the throwout bearing, in turn depressing the clutch fingers and releasing the disc.

The clutch pull rod passes through a hole cut in the steering shaft mount plate. That's the steering shaft boot on the right. An additional piece of aluminum is used to add rigidity to the plate.

In order for Wilwood master cylinder to fit between the steering box and the side wall of the engine compartment, the bulky factory steering rag-joint was replaced with a Borgeson universal assembly.

The hydraulic clutch master cylinder is a tight fit and power steering lines had to be rerouted to make clearance.

The hydraulic fluid for the clutch master cylinder is mounted next to the brake master cylinder.

The slave cylinder has two #4 braided steel hydraulic lines. The pressure line connects to the master cylinder and the bleed line is terminated with a bleeder scew.

A brake pressure switch is T'd into the pressure line and acts as the "neutral" safety switch. The clutch must be depressed for the car to start. This approach will pass tech inspection at NHRA and IHRA tracks.

McLeod's Red Roberts helped select the right clutch, their Street Twin clutch and 30lb flywheel. ensuring almost instant disengagement when the pedal is pressed.

 

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In This Article:
The advantage of overdrive transmissions are so great that no vintage Ford should go without one. You have choices, be it automatic, manual, or an add-on unit. We show you how to go for broke by installing the big daddy of sticks, the Viper-spec T56 six speed, in a 1971 Mustang. We'll also detail how to convert to a hydraulic clutch and move the motor back.

   
 
The Tremec T-56 is a six-speed manual transmission with 5th and 6th gears being overdriven at 0.74 and 0.50 respectively. Originally equipment for Dodge Vipers, this unit is rated to handle 650 lb-ft. of torque. In reality it probably handles a good deal more, and is the perfect choice for big cubic-inch Fords. Photo: D&D Transmissions
   
   
   
 
So you want to OD?
You've got choices. You can go automatic overdrive such as AOD, E4OD, or be a sacrilege and use a GM 200R4. If you want to row gears there's the T5, TKO and T56, not to mention Richmond and others. Finally there are add on units from Gear Vendors and US Gear. Each has positives and negatives to consider.

Automatics
With a three speed automatic faster quarter mile times and that fast launch means a high stall converter, more slippage, more heat and real compromise for any car that's supposed to be a do it all driver. Overdrive automatics add an additional gear, but still need a higher stall converter for effective drag strip launches and the lock up feature that holds down converter slippage and increases highway efficiency is often the first to go to keep a serious AOD or other overdrive auto in one piece.

Sticks
With a stick shift you have almost infinite control over launch rpm and with a clutch there's no slippage. The mechanical coupling from engine to transmission means reduced parasitic power loss. For bracket racers a stick is a real challenge to consistency, and there's the fun of driving in bad traffic.

Add-on Units
The add on units give another set of gearing to an automatic or stick and are very reliable, but they can be hard to package in a tight transmission tunnel and aren't inexpensive either. They allow you to preserve an existing set up with adjustments to mounts and driveshaft length. They are generally not used in racing due to the challenges of shifting two units.
   
 

 


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