Topic: Overdrive in a Classic Mustang

[428CJ70]

I’ve got 3.91 gears in my 428CJ powered ’70 Mach 1 w/ a C6 automatic transmission. The car was rebuilt with street/strip duty in mind. I love the performance of the 3.91 gears but I hate the resulting high RPMS when cruising on the interstate. I’ve decided that I need an overdrive solution, and I am looking for anyone with experience in any of these solutions or perhaps some others. The 3 solutions I’m considering include:

(1) Bolt a Gear Vendor’s over/under drive unit to my C6. Cost is around $3000 with shipping and requires a shortened drive shaft. This unit supposedly doubles the number of forward gears, allowing you to shift to 1st (C6 in 1st, GV in low), 1st high (C6 in 1st, GV in high), 2nd (C6 in 2nd, GV in low), etc. They don’t lower 1st gear like some of the OD transmission options, but they do provide half gears allowing the motor to remain closer to peak RPMs when drag racing. I don’t follow how the shifting is accomplished but apparently pushing a button on the shifter allows you to control the GV unit. Apparently they make this process much easier than it sounds. Final drive ratio is 3.05 instead of 3.91 so the motor is turning 22% lower RPMs on the highway. I can keep my converter, existing shifter, speedometer, etc. I understand that the speedometer setup can be modified so that it remains accurate even when the GV unit is in high gear. I may or may not have trouble getting the unit to fit in the tunnel.
 
(2) Retrofit an OD type transmission using a conversion kit to allow the new transmission to bolt to my 428CJ block. Costs vary. The conversion kits are $600 to $800 and I can find the transmissions from $650 (good used 700R4 a friend has) to about $2200 for a hardened E4OD. The 700R4 has very nice gear ratio (3.06, 1.63, 1.0, 0.7). Its first two gears are lower than any other OD tranmission I found. The Ford 4R70W transmission can be also obtain with a wide gear ratio (2.84, 1.55, 1.0, 0.7). The advantage of using one of these transmissions: lower first gear is 3.07 or 2.84 with a new tranmission, 2.46 using the C6 + GV; a new transmission does not require an additional mechanism to shift the GV (may or may not be an issue); depenidng on the transmission, replacing my transmission can be cheaper than the GV unit; replacing the transmission lowers highway RPMS by 30% compared to 22% for the GV unit (2.74 final drive compared to 3.05); the 700R4 supposedly has lower parasitic horsepower losses and weighs a lot less than a C6+CV unit. The disadvantage is that I may still have to use a button to enable overdrive because I want to retain my stock shifter, it will require more labor - especially if I go with the 700R4, and the result is 4 foward gears while the GV option results in 6 gears.
 
(3) Buy a PA built Ford-type overdrive transmission that (if I read correctly) has an FE compatible bell housing eliminating the need for the $600 to $800 bell housing adaptor kit. The problem is that the PA transmissions are expensive making this the most expensive option ($3500 to $6500). It also elminates the most attactive transmission option: the GM 700R4.

Has anyone added some type of overdrive (GV or OD tranny), especially in a classic mustang? What was your experience like? Can anyone shed light on how what the driver has to do to shift through all "6" gears when a GV unit is installed? How seemless is it really? How does the GV solution integrate with the console mounted automatic shifter?

Currently, I am looking hardest at either buying the GV unit or installing the 700R4 with an adaptor kit. If I go with the GV kit, then I may also look at the cost of lowering my 1st gear and 1st and 2nd gearin my C6.

[jethat]

I changed from a 4 speed top loader to a TK0600 tremec 5 speed so I dunno about the auto overdrive but I can tell you having an over drive makes the car totally different and its almost worth the money.. Quieter less vibration better gas mileage my wife actually enjoys riding in my car now..
I dont have experience that way with autos.. but I would spend enough to do it right. I did  on mine the conversion was close to 4 grand. Like a totally different car..

[Jeff73Mach1]

Like Jethat I upgraded my 4 speed to a 5 speed and have been very happy.  I went a cheaper route and used a Keisler modifed T-45 that has a low 1st gear, my mechanical clutch and original bellhousing and a deep overdrive.  As has been said, the car is totally different and mine has both a tire shredding personality and a civilized personality.

Of your 3 choices I would be inclined to go with the 3rd option and use a PA AOD built for your power levels.

[ol dirty doug]

i had one on myfairmont wagon, 302 with a c4 and 4.56 gears.  i now have a fully built aod with the close ratio gear set and a tall overdrive ratio.   i also have my 1927 roadster chevy 377 turbo 400 trans with the same gearvendors unit just had to get a different adaptor.  love them both but if you have the money build a aod. its less hassle and easier to use.

[428CJ70]

The only fear with the AOD is my console shifter floor shifter.  It only has 1st, 2nd and Drive.  I would like to keep the floor shifter.  I understand you can control the overdrive with a button just like the GV unit.   But I was told that you have to remember to switch it out of OD or that the transmission will not downshift when you slow down. 

What options are there to use my original 3 speed shifter with a 4 speed OD type automatic transmission?

[jethat]

My experience with AOD trannys has been you have to push the button to turn overdrive off or it automatically shifts into it and it does shift down no problem when you slow down and if you dont push the button you can still shift into 2nd. The AOD would work with your shifter you would just have to put a button somewhere. The only time, in my Fords with the button I use it is when coming down steep grades. Other wise it works just like any other Ford automatic (put it in 2nd and it stays there)

[ol dirty doug]

stock 87-93 mustang shift pattern for aod is as follows.....     1 gear....then 2nd/3rd(it automatically shifts from 2nd gear to 3rd)...... then overdrive.   so you have only 3 shifter points.   so you can either run 1, 2, 3, with push button overdrive.   or you run 1, 2-3, od  (when racing you launch in 1st then shift into 2/3 (which is the 2nd position on the shifter), as soon as 2nd gear holds you put the shifter back into first gear (the 1st position ) to hold until the rpm you want to shift into 3rd at, then you place the shfter back into 2/3(which is the 2nd position) and it shifts into 3rd, then for od you shift it to the 3rd position

[petestock]

fantastic - i'd be arrested within seconds! There's so many cameras here in the UK that you have to be careful. I'm planning on buying a mustang (whisper this - I currently have a Porsche 911) and most likely I'll be putting it up on bricks then revving the engine on sundays. How do you think it would be in the UK? We have no interstate, just the M6 toll road where you get about a mile of freedom. The rest of the time I'll have to behave - and do track days (I assume you have track days in the US?)

[jethat]

I had a British dude tell me one time the main differences between Americans and Britons are Britons think 100 miles is along ways and Americans think 100 years is along time..

[petestock]

Nice one fella! We are divided by a common language!
What do you think then? I'm thinking V8 just so I can listen to it, but 6 cyl might be more usable.I would be happy either way - big beast track day car or day-to-day usage, but I think day-to-day works best. What do you recommend for everyday?  Do you guys do track days?

[jethat]

If your going to have one shipped get the V8. It will have more value and right now there are deals on collector cars to be had..

[petestock]

Thanks, that's definitely the answer I was looking for! I feel guilty for hijacking  this discussion so I'd better start another one about the hunt for my new car.

[428CJ70]

My experience with AOD trannys has been you have to push the button to turn overdrive off or it automatically shifts into it and it does shift down no problem when you slow down and if you dont push the button you can still shift into 2nd. The AOD would work with your shifter you would just have to put a button somewhere. The only time, in my Fords with the button I use it is when coming down steep grades. Other wise it works just like any other Ford automatic (put it in 2nd and it stays there)

Apparently the behavior of the shifter depends on who built the AOD.  After researching it, stock AOD tranny's have LOW, Automatic w/o overdrive, and Automatic w/ overdrive.  But folks like Lentech build them differently and I have seen conflicting information.  Based on their web site, Ithought it was 1, 2, full automatic and there is a button that locks out overdrive.  But their comment to not turn on the button until you are on the highway puzzels me if the D position will switch through all 4 gears and back down automatically when OD is enabled?  I would expect you would only disable overdrive when racing.  In your scenario, does it go through all gears when OD is enabled?

[jethat]

In stock form you simply put it in drive and it goes up through the grears and back down. The only time it does not go in and out of OD is when it disabled by pushing the button. In my f-150 (4r70w that shifts the same as an AOD) I just put it in drive unless I'm comming down a grade or towing.. The difference with Ford auto trannys is you can start out in 2nd gear if you put it in 2nd it stays there. GM or Dodges dont do that.. Your C6 should do that to.

[ol dirty doug]

the button you speak of is for either full lock up converter or only to engage od.  in the one i have that has the lentec valvebody it orignally was 1...2....3...  but when you let off the gas in 3rd the od kicks on then you can lock the converter.   i converted it to 1...2...3... push button od  and either way you only want to use the button on the freeway as so you dont lug down the engine in od or you dont shift into any other gear while the converter is locked up.

[428CJ70]

In stock form you simply put it in drive and it goes up through the grears and back down. The only time it does not go in and out of OD is when it disabled by pushing the button. In my f-150 (4r70w that shifts the same as an AOD) I just put it in drive unless I'm comming down a grade or towing.. The difference with Ford auto trannys is you can start out in 2nd gear if you put it in 2nd it stays there. GM or Dodges dont do that.. Your C6 should do that to.

The 4R70W might work that way but I have confirmed that the AOD does not.  You cannot start out in 2nd in an ADO because there is no "2nd gear" position on a stock AOD.  The ADO's forward shift positions are: low, D and OD, and no lockout buttons.  The 4R70W does have an electrically controlled lock out feature.  I read about it in a controller manual today.  The scoop on the AOD is that when it came out in 1980 (or very early 80's), Ford was still using the C4 and C6, and their cars still had 3 position shifters.  The AOD was not electrically controlled so a lockout button was not an option.  So instead, Ford played with shifter patterns to design a way to make a 3 shifter work with a 4 speed transmission.  The 2nd position is the same as the 3rd postion except it locks out OD.  You can still manually shift an ADO, but to shift to 2nd, you "double tap" meaning you go from low (position 1) to position 2, let the tranny  shift, and pull the shifter back into position 1 hold the tranny in 2nd gear.  I also read that this behavior, while it works, was apparently not intended because it eventually causes wear to the overdrive band and direct clutch.  The ADOE came later (basically, an electrically controlled ADO) and then the ADOE/4R70W (basically, an electrically controlled ADO with wide gear ratio). I don't know if a standard ADOE has a lock out feature and I didn't see it mentioned in vehicles with this transmission but who knows?   its possible because its electrically controlled.  It appears Ford used a 1,2 D/OD shifter operation for the 4R70W with a button that determines if OD is available in the 3d postion.  I can see the need for this in the wide ratios 4R70W transmission because it is commonly used for towing and later in performance vehicles where the driver wants to manually shift w/o damaging his transmission.  I think I saw where the ADOE follows the 1,2,D/OD pattern but the poster could have been talking about an AODE-W/4R70W.

And yes, a C6 can be forced into 1st or 2nd.  You can start off in 2nd if you like.

What I have not found is, what happens if somene driving along in an ADO vehicle shifts down to low?  Does it go to 1st?  To 2nd?  Does it depend on speed?  Or what?