71 Mustang Mach 1 - 351C automatic transmission question

[phil2273]

I will be re-building my 351C to roughly 450-500hp. It will be a street cruiser, Sunday driver. I want to be able to go down the highway also. Currently has a FMX transmission. From everything I've been able to read the FMX tranny is not the way to go - too weak. What's the best automatic transmission for this build? I'm leaning toward AOD, with 3.25 to 1 rear end. Also considering a C6, with 3.00 to 1 rear end. Current rear end is 2.75 to 1, non-locking. Upgrading the rear end will likely also be part of the upgrade.

 

[71coop]

Nothing wrong with the FMX, it's actually pretty durable. The only down side is parts availability, but I believe it could be built to handle the power.

Having said that, anything approaching 500 HP deserves a second look, and should consider maintenance down the road. An appropriately built C4 or C6 will be easier to deal with if you need any repairs down the road. Not sure how durable the AODs are, at that power level it might need some work too.

 

[MAD IRISHMAN]

Not familiar with the AOD. But the C6 is like an anvil. Rebuild parts are still available.

 

[machattack]

I will be re-building my 351C to roughly 450-500hp. It will be a street cruiser, Sunday driver. I want to be able to go down the highway also. Currently has a FMX transmission. From everything I've been able to read the FMX tranny is not the way to go - too weak. What's the best automatic transmission for this build? I'm leaning toward AOD, with 3.25 to 1 rear end. Also considering a C6, with 3.00 to 1 rear end. Current rear end is 2.75 to 1, non-locking. Upgrading the rear end will likely also be part of the upgrade.

Not sure where you read that the FMX is a weak transmission? A friend of mine uses the fmx solely for strip racing. The trannys are built up tho, to handle the H.P. You can get shift kits for your FMX. I rebuilt my FMX and from what I see in the inner workings..........its built tough,tough! including the center cast iron carrier. I'll post a pic to show just how large its body is. Yup its heavy.

Is your tranny failing now? Why would you want to change?

 

[Hemikiller]

Nothing wrong with an FMX and according to Car Craft magazine, it uses even less power than a C4 and is more durable. The C6 is more durable still, but at the cost of about another 30hp. That said, you need to factor the cost of acquiring, rebuilding and retrofitting the C6 with the proper parts for a floor shift car. The column shift transmissions are very common, but you need a floor shift linkage setup to work properly with the factory shifter. You're also looking at a modified driveshaft. At the 450hp mark, I think you'll be fine with an FMX and a stall converter.

That said, my newly acquired 71 Cougar has an AOD behind the 351C with 3.50 gears. It's a nice highway ride for sure. Again, you're looking at a fair amount of cost to retrofit.

Powerglide_____18 hp
TH-350________36 hp
TH-400________44 hp
Ford_C-6______55-60 hp
Ford_C-4______28 hp
Ford_FMX______25 hp
Chrysler_A904__25 hp
Chrysler_727___45 hp

 

[Paul M]

The FMX is easily buildable for that power range, just a little heavier. The C6 however, is the most capable, parts friendly & versatile..

 

[AcesArneson]

I will be re-building my 351C to roughly 450-500hp. It will be a street cruiser, Sunday driver. I want to be able to go down the highway also. Currently has a FMX transmission. From everything I've been able to read the FMX tranny is not the way to go - too weak. What's the best automatic transmission for this build? I'm leaning toward AOD, with 3.25 to 1 rear end. Also considering a C6, with 3.00 to 1 rear end. Current rear end is 2.75 to 1, non-locking. Upgrading the rear end will likely also be part of the upgrade.

You have the same pre-rebuild condition with the same goals. I am still in the middle of my rebuild and have a way to go yet before putting it all back together but this is where I am;
-hired a reputable mechanic to rebuild to 450hp, the motor is finished and I will pick it up next week from The Block Shop in Edmonton, Ab.
-found a 4R70W from 2004 Mustang that I had rebuilt at Can-West transmission in Edmonton, Ab. You need to ensure it comes from - V6 so it has the Essex Bellhouse Pattern, the 4R70-75W’s from a panther don’t fit unless it is from the first year before they change to the Mod Motor Bellhousing (made this mistake and have a 4R75E for a Mid Motor if anyone wants it. ). This is an electronic transmission and will either need a transmission control module or will need to be controlled by an ECM like an EEC4 or 5. This will also require modified shift linkages and gives you the ability to go without a cable on the Speedo. This tranny will allow for lots of power and decent cruising speed~2220 rpm for 105kph.
-The EEC5 ECM will also control the spark on the engine and allows for SEFI conversion if you are interested, if not you can just read the rpm and let it shift for you. The 4 and 5 allow for reprogramming which is a one time hit and pretty pricy but hopefully worth it. 450hp is just my first target, I bought a crank that will handle more if this isn’t enough to move my car the way I want it.
-I swapped out the diff for a Detroit locker with 3.5:1.
-Still have to get the headers and exhaust replaced when the engine is.

Have fun, this has been a great experience for me.

 

[phil2273]

I want to thank everyone that has responded. This is a great forum. I've owned this car since 1998 and never had issues with the FMX. It has a 351C 2V and I don't drive it hard. To answer the question about why and where I heard the FMX is too leak, this article https://www.motortrend.com/how-to/0911rc-ford-automatic-transmission/ and my mechanic recommended I replace the FMX. That said, I've also seen similar posts that FMX can handle the power setup I'm planning. Based on these responses, I called my local tranny shop, and they agree they can build my existing FMX to handle this setup. They confirmed they can find the parts. With all of this information, I plan to keep the FMX and rebuild it. As mentioned previously, I do not plan to drive it hard - but, we'll see once I have 450hp at my disposal.

Right now she's getting suspension/brake make-over. Global West w/ QA1 front coilover, new sway bar, new rear leaf springs, QA1 rear shocks, Wilwood on all 4 corners, new 17" Legendary wheels. At some point, someone replaced the 3.00 to 1 locking differential with a 2.75 to 1 non-locking. I'm replacing and putting a 3.00 to 1 trac loc back. The engine will get a 408 stroker, aluminum heads and intake and a Holly Sniper EFI. In return, my wife gets a new kitchen make-over.

 

[cjstaci]

IMO, it seems like you have lots of options. Any of these transmissions can be built to handle 400-500hp. If cost is a concern, then make a spreadsheet and compare the cost difference of each transmission and the additional cost of a core and related parts if you switch to a different trans. Talk to a few trans Builder’s and see what they recommend. You will spend more for internal upgrades for the FMX and C4, the C6 just a good rebuild will do.
The biggest enemy of automatic transmission’s is heat. Use a good oil cooler and a temp gauge.

 

[Sheriff41]

Can you explain your reasoning for the built engine and going with 3.00:1 gears? It seems an odd combination.

Be sure to match your cam, gear ratio, and torque converter to get the best out of your build!

 

[mrgmhale]

FWIW, I 2nd the upvote in favor of using an FMX. Sure, being a cast iron case they are a bit on the heavy side. But, they are also very durable. I had one on my old 1969 Mach 1 back in 73-76. The 351 engine was built for street strip., I never put it on a dyno, but it ran 14s in street trim, corked headers, and with a very steep 3.0:1 conventional (non-TractionLok) rear axle and 70 series bias ply tires. I rebuilt the FMX, not because it needed a rebuild. Rather it was to use a higher stall speed B&M torque converter (can't recall the RPM rating for stall speed), and install a B&M Sfift Kit and some other B&M beefier than stock internals. I used to take some ribbing from other folks about it being a "truck" tranny, and being an automatic in general. All I know is it held together despite the abuse I sent to it, and shifted firmly - not one issue. I felt really good I had an FMX vs a lighter and lighter duty C4. I would have been fine with a C6, but I had heard the parasitical loss through the FMX made it a preferable transmission. All in all, I am a believerin using FMX, with upgrades, for moderately built engines.

 

[AcesArneson]

I want to thank everyone that has responded. This is a great forum. I've owned this car since 1998 and never had issues with the FMX. It has a 351C 2V and I don't drive it hard. To answer the question about why and where I heard the FMX is too leak, this article https://www.motortrend.com/how-to/0911rc-ford-automatic-transmission/ and my mechanic recommended I replace the FMX. That said, I've also seen similar posts that FMX can handle the power setup I'm planning. Based on these responses, I called my local tranny shop, and they agree they can build my existing FMX to handle this setup. They confirmed they can find the parts. With all of this information, I plan to keep the FMX and rebuild it. As mentioned previously, I do not plan to drive it hard - but, we'll see once I have 450hp at my disposal.

Right now she's getting suspension/brake make-over. Global West w/ QA1 front coilover, new sway bar, new rear leaf springs, QA1 rear shocks, Wilwood on all 4 corners, new 17" Legendary wheels. At some point, someone replaced the 3.00 to 1 locking differential with a 2.75 to 1 non-locking. I'm replacing and putting a 3.00 to 1 trac loc back. The engine will get a 408 stroker, aluminum heads and intake and a Holly Sniper EFI. In return, my wife gets a new kitchen make-over.

Sounds like she is getting a good work over. Let us know if you think the stroker paired with the 3:1 rear gears feels torquey enough for you when you have it on the road.

 

[phil2273]

Can you explain your reasoning for the built engine and going with 3.00:1 gears? It seems an odd combination.

Be sure to match your cam, gear ratio, and torque converter to get the best out of your build!

The main reason for my 3.00 to 1 decision is highway drive-ability. Since I'm sticking with the FMX w/ no overdrive, I want to be able to drive 70mph on the highway at a decent (2500ish) RPM. My mechanic had a on-line RPM/MPH calculator to help with the decision. I'm willing to sacrifice some low end performance so I can enjoy day trips with it. I'm by no mean an drivetrain expert. Open to suggestions.

 

[MaddMike]

I went with an FMX when I restored mine, paired with 351C. No issues at all and the confidence of knowing it is a heavy duty trans for when I do step on it. I also added the B&M shift kit. It SNAPS into low gear and has a pretty hi rev before 2nd and eases into 3rd seamlessly.

 

[mrgmhale]

The main reason for my 3.00 to 1 decision is highway drive-ability. Since I'm sticking with the FMX w/ no overdrive, I want to be able to drive 70mph on the highway at a decent (2500ish) RPM. My mechanic had a on-line RPM/MPH calculator to help with the decision. I'm willing to sacrifice some low end performance so I can enjoy day trips with it. I'm by no mean an drivetrain expert. Open to suggestions.

Our 1969 Shelby gT50909, with its CobraJet 428, came with factory air conditioning, Because of the A/C Ford felt they had to prevent over-revving the engine and damaging the A/C compressor. So they required A/C
shelbys to have an auto tranny, and I AC Shelbys also got either a 3.25:1 or 3.0:1 rear axle gear. Sadly, the rear axle ratio being that step means a loss of otherwise very spirited performance at the low end of performance. But, like Phil2273 says, it makes highway speed driving a lot more enjoyable. Besides, those 428 CJ engine have a low of low end torque, and our GT500 is plenty fast off the line. Once I hit just over 25 MPH in 1st gear you can color the Shelby way gone. If I did not know any better I might be tempted to swap in a 3.5:1 rear axle gear set. But, I do know better, and have a 73 Mach 1 with a 3.5:1TractionLok rear axle gear set. The low end performance, but the highway cruising was not all that fun ith the engine running at higher RPM that I am used to. I "fixed" the Mach 1 by swapping its C4 with an AOD (4 forward gears with 4th gear being an OverDrive ratio). We now have the best of both worlds, but no way will I be modifying the Shelby.

 

[AcesArneson]

I would really suggest looking at a 4R70W instead of the FMX, especially since you are going with EFI. You can use the EEC-V from a 2003/4 Crown Vic with a 4.6l engine to run the EFI. If you find a donor car you might be convinced to go with SEFI instead of throttle body injection, the only real pain is adding ports with bungs to your intake manifold. The EEC-V is programmable and you can use it to run the 4R70W. It really is the best of both worlds, you get low end speed without high rpm on the hiway, the extra gear will make the car because now you can get 3.5:1 rear gears. Just sayin’.