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Drivetrain advice
Recently purchased a very clean 72 Mach 1 and need to tame it's drinking problem a little; looking to make a cruiser with go power. It's coming with a C6 with a 4.11 in the rear end; and I'm looking to change them to a 4R70W trans with 3.50 gears. The engine is a 315C 4V and according to the rebulider pushing 380 horses. Sound off folks.....good choice or bad.

I know very little, but you could keep the 4.11's with the 5 speed AOD and you will have the best of both worlds, I would think.


[Image: 25yvyp3.jpg]
Tko 5 speed would be an option

He has all the vices I admire and none of the virtues I despise
That sounds like a perfect combination to me. I did a wide ratio AOD (which uses thev4R70W gears) in my '73 and put in a 3.89 rear end. My engine is 2v, but I have upgraded the intake manifold and carb to Edelbrock combination with dual exhaust. So I figure I have around 200hp.

I cam smoke the tires whenever I want from a stop, and get a great launch. And highway speeds are still good RPMs (high 2,000s) with 26.4" tires.

In hind sight, I would have been just as happy with 3.7 gears. With the amount of power you are looking at, 3.5 will be amazing with the 4r70w, and you will get really good mileage at freeway speeds.

sm3570, Luke, CZ-75 have a valid point. According the 4R70W tech specs, the gear ratios are: 1st: 2.84 :1, 2nd: 1.55 :1, 3rd: 1.00 :1, 4th: 0.70 :1, Rev: 2.23 :1
According to the gear ratio calculator I found, 15 inch tires and 2200 RPM’s, you are running a speed of 55 MPH.


[Image: 176dtd.jpg]

C6: 1st: 2.46 :1, 2nd: 1.46 :1, 3rd: 1.00 :1 Rev: 2.18 :1
[Image: t96wdx.jpg]

According to the C6 gear ratio, running 52 MPH, you are turning 3000 RPM's.

Not a Bad combo setup here. Please take a look at the calculator and test the numbers.

mustang7173 Big Grin
You need to put your tire diameter instead of wheel size then figure it.

73 mach1
Nice looking car! Welcome to the forum. I run a c6 and 3.5 gears also. My car has been that way for years and it too is thirsty. I just finished installing a gear vendor overdrive. I only have 12 miles on it so far so I can't give you too many details on it. If your c6 is running pretty good this might be an option for you. They are not cheap. It's a fairly straight forward install.

Price wise a 4r70w vs a gear vendors is going to be a push. A lot depends on how you source the transmission and how much of the wrenching you do yourself. You could do the 4r70w for half as much as the gv. The 4r70w has slightly lower first gear which helps off the line and a taller OD which would help on the freeway. (.7 for the 4r70w vs a gear vendor .78)

If I hadn't had the c6 already built I probably would have leaned more towards the 4r70w. One of the deciding factors for me was the passing gear. With the 3.50 and the OD on I can shift down into second on the transmission for a really nice passing gear that closely matches how the car originally was when I bought it with a 2.78 rear.

Starting over I would seriously consider a 5 or 6 speed manual.

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:

yep- I made an error here. Here is the update with a 205/75-R15 = 27.106 in.

4R70W: Gear ratios are: 1st: 2.84 :1, 2nd: 1.55 :1, 3rd: 1.00 :1, 4th: 0.70 :1, Rev: 2.23 :1

[Image: 2wdr6s5.jpg]

C6: 1st: 2.46 :1, 2nd: 1.46 :1, 3rd: 1.00 :1 Rev: 2.18 :1

[Image: 2cyp3qr.jpg]

73mach1 -Thanks you for pointing my error out.

mustang7173 Big Grin
I couldn't be happier with the wide ratio AOD and 3.89 gears. Even with the neutered '73 motor, I can smoke teh tires, yet still get reasonable RPMs on the highway.

If you're going to have 27" tires, I would recommend 3.7 rear versus 3.5.
If you're thinking of keeping the slushbox, the only feasible overdrive units are the AOD and AOD-E/4R70W. Keep in mind that an issue with the earlier controller-less AODs is that a) they are considerably weaker in torque output and b) they can burn up very quickly with incorrect TV linkage adjustment. A stock late model 4R70W is good for 500 hp, but requires an electronic controller; either a Baumann standalone unit or an EEC-V computer with the integral controller is required. Shift points are also adjustable with the 4R70W.

Personally, I'm going with the 4R70W since I just converted my engine to EEC-V and the computer has the controller.

1971 Mustang fastback: 10.3:1 C90E 408W hydroller - CDAN4 EEC-V w/EDIS8, girdled, lowered and caged
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