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I’m looking to replace the tranny on my 73 mustang and I’m looking for one that’s gonna really help increase my top speed any suggestions would be great
 

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Need a lot more information than that. Detail information.

But the short version is that as long as you stay with a transmission that goes to direct drive (1:1) in top gear, chances are that none of them will give you any more top speed than you have right now. In this case, top speed is going to be mainly a function of your axle ratio, rear tire size, and how much power your engine is making (and at what rpm).

An overdrive transmission may or may not give you a higher speed (it'll depend on how closely the overdrive gear is spaced away from 1:1).

If your current transmission is an automatic, swapping to a stick might get you a few more mph because you wouldn't be losing any engine rpm to torque converter slippage any longer. I say 'might' because details like the above-mentioned things matter and I don't know what any of them are at this point.

I have a way of estimating fairly closely what should work but I need to know what I'd be working with.


Norm
 
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Not enough info for me to generate a reply...
 

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Are you being RPM limited, or is the engine just "done" flat out in high gear? Most cars in '73 would probably poop out around 110-120 (even when new), not because they needed a new transmission, but because they simply don't have the torque to push the car down the road any faster. Changing gearing isn't necessarily going to fix that.

I'm also curious about where you feel a burning need to go this fast, because 1973 cars (even Mustangs) are pretty heavy, and not typically track cars. Ethical and safety concerns aside though, you can make these old sleds really get up and move, with a bit of work. Your stock heads, cam, intake, and exhaust are probably the parts most responsible for keeping you from high speed glory.
 

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Discussion Starter #5
Are you being RPM limited, or is the engine just "done" flat out in high gear? Most cars in '73 would probably poop out around 110-120 (even when new), not because they needed a new transmission, but because they simply don't have the torque to push the car down the road any faster. Changing gearing isn't necessarily going to fix that.

I'm also curious about where you feel a burning need to go this fast, because 1973 cars (even Mustangs) are pretty heavy, and not typically track cars. Ethical and safety concerns aside though, you can make these old sleds really get up and move, with a bit of work. Your stock heads, cam, intake, and exhaust are probably the parts most responsible for keeping you from high speed glory.
The entire motor is being being redone with aftermarket performance heads and can with an edelbrock rpm performer intake 4 barrel Holley carb and the exhaust manifold is being replaced with headers I have an aftermarket 5 speed manual for it as of right I’ll be putting a stage 3 clutch in and replacing the rear diff here soon it does have oversized tires on it that the previous owner had put on that I’m going back down to stock size the motor will be bored 40 over if the exact specs on what is being done to the motor would help I do have the paperwork for it the machine shop I took it to is well known where I live for building dirt track motors and the machinist said his best estimate before he dyno’s the motor is around 450 hp
 

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OP: I suggest you pull up a various on-line Gear Ratio Calculators and see what info is needed to answer your question.

Your answer involves rpm, axle ratio, tire height, transmission gear ratios, etc.

Using data on what you currently have and using data relative to what the gear ratios are known in a given brand transmission, a calculated comparison can be made. It's a model. All models are wrong and all are useful. The model can calculate top speed , but whether you actually achieve it depends on other factors, for example car weight, wind resistance and, speed limiters on modern cars.
 

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450 hp in a '73 ought to get you some respectable MPH on the top end if you're running highway gears, but there's an awful lot besides peak horsepower involved here! Putting your car somewhere between peak torque and peak horsepower at the speed you guesstimate you're going to be doing is the way the Bonneville guys plan it. In the 'real world', building for high average horsepower instead of just a high-winding peak usually works best, because it allows you to actually get up to speed. Without knowing what the car's intended for, it's kind of hard to make recommendations for you!

As an engine builder, I am sure that you've seen a lot of guys with well-built low-buck combos that absolutely smash the guys bragging about big horsepower numbers. Trying to plan this with parts that make sense for your application, in a reasonable cost range is going to be the trick. I mean, you could spend insane amounts on a Lenco and 9" rear end, lightened driveshaft, and maybe a blown big-block up front, and go just as fast as your tires would let you. But there are real world considerations here! So "which transmission is right" has a lot more questions involved before I could come up with a good answer!

So TL;DR: 1) what's the car going to be used for, 2) does it need to accelerate quickly, 3) what engine is making the 450 HP (351? 302? FE? 460?) and 4) would it be helpful to save money, or are you just looking for the 'ultimate solution' here?

Also, do you intend to stay with a stick shift?
 

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The entire motor is being being redone with aftermarket performance heads and can with an edelbrock rpm performer intake 4 barrel Holley carb and the exhaust manifold is being replaced with headers I have an aftermarket 5 speed manual for it as of right I’ll be putting a stage 3 clutch in and replacing the rear diff here soon it does have oversized tires on it that the previous owner had put on that I’m going back down to stock size the motor will be bored 40 over if the exact specs on what is being done to the motor would help I do have the paperwork for it the machine shop I took it to is well known where I live for building dirt track motors and the machinist said his best estimate before he dyno’s the motor is around 450 hp
450 HP at what rpm? Peak torque and its rpm if at all possible.
Which aftermarket transmission (gear ratios vary a bit, and I have the individual gear ratios for about a hundred different transmissions)? What's in the car now?
What tire size are you going to? What's on there now?
What's your axle ratio now? Are you going to be changing it?

Those numbers go into a spreadsheet acceleration simulation that can also provide a pretty good estimate of top speed (you've reached your top speed when acceleration drops to zero). This spreadsheet does consider aero and rolling drag and quite a few other things not yet mentioned by anybody. Reaching the last mph can take well over half a mile, and this spreadsheet can find that.

776969



Norm
 

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I think I would choose the transmission for other reasons -- basically, what will hold up to the torque levels and fit and has the number of gears that you want and shifts well . . . and then tweak the final drive ratio using the rear end gearing if it turns out that you are RPM limited not drag/HP limited
 

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I would go for the latest model AOD. Monster transmission has a good website that describes what to expect here. I don't recommend buying from them, better get hooked up with a good local transmission shop who can rebuild it with a high horsepower kit, and choose the shift kit that you want. Ford made a lot of them so salvage parts deals are plentiful.

I have one in my big block 4-seat TBird built to handle the torque, and it's a great transmission. It works with a throttle valve (TV) link, which are tricky to set up initially (let your shop do this), but once dialed in lets you downshift by using the accelerator pedal and is very predictable. Gears 1-3 are identical to the FX, MX and and FMX transmissions of the 60's and 70's. The OD ratio is 2/3 so you can choose a numerically high rear end ratio and still get relatively low RPMs down on the highway.
 

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^^^ I don't think that's what OP is looking for . . .


Norm
 
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With that much power, an AOD is just going to snap the 2-piece input shaft like a stick, and shrapnel itself. Unless you spend $2500 or so for one that hasn't got a torque converter lockup. They shift by committee too, unless you change a lot of parts. Not bad for a cheap cruiser, but not good for something performance oriented either. (sorry Yadkin! I don't want to step on your toes here.)

For the money, a 4R70W is a much better OD automatic, even when you include the controller for it. Stock, it'll handle more horsepower than most built AODs, and it shifts much cleaner.

But none of that matters if you want to row the gears. =)
 

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If he's running it behind a stout 351 though, the first time he powershifts it, even a TKO will be spitting out teeth like an 80 year old wino in a boxing ring. The T5 does not take kindly to big torque, if his tires are sticky.
 

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Maybe he'll come back and tell us what else he's got planned for this car besides top speed.


Norm
 
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The T5 in my '91 Mustang went south on the first 2-3 power shift behind a 351 - prior to adding a supercharger.


Chas

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