Change driveshaft length going from FMX to C4?

[1973mach1]

Does anyone know if I can use the same driveshaft for both of these? I know the C6 is a longer tranny then the c4 and needs to have the driveshaft lenghtened but are the FMX and the C4 the same size? Has anyone done this swap, is there alot involved?

Thanks,
Jim

 

[1970 slantroof]

I had a 1979 Ranchero with a 302 and FMX. When it passed 252,000 miles I replaced it with a 302 and C-4 from a 1977 Comet. I had to locate the crossmember to another set of holes in the frame but the driveshaft fit OK. I assume it's the same with a Mustang.
Howard

 

[69stang351]

First of all, why are you taking out the FMX and swapping it for a C-4?? sure it may be stronger than the C-6 but the FMX was built to handle power, and they can take it. Unless your tranny is dead I would recommend keeping the FMX and not making the swap at all. Just my .03 though.

 

[1973mach1]

First of all, why are you taking out the FMX and swapping it for a C-4?? sure it may be stronger than the C-6 but the FMX was built to handle power, and they can take it. Unless your tranny is dead I would recommend keeping the FMX and not making the swap at all. Just my .03 though.

I heard their heavy and cause alot of power loss to the rear wheels. I'd rather stay with it but seems everyones been telling me to go to a c4. Maybe i'll just install a shift kit and higher stall on the FMX. It's 30 years old I dont know how it will handle 375HP or so. When I pull the trans what should I replace on it to handle the power and clean it up some?

Thanks,
Jim

 

[GaPonyFarm]

I heard their heavy and cause alot of power loss to the rear wheels. I'd rather stay with it but seems everyones been telling me to go to a c4. Maybe i'll just install a shift kit and higher stall on the FMX. It's 30 years old I dont know how it will handle 375HP or so. When I pull the trans what should I replace on it to handle the power and clean it up some?

Thanks,
Jim

I have a 70 FB with a 351C, pushing around 450hp. The FMX seems to handle it just fine, with no special hardening. One thing is for certain, the FMX is much tougher than the C4 or C6 and will handle all the heat you can send it.

It might be a good idea to have a fresh rebuild, if you don't know how long its been since it was gone through, but I's definitely stay with the FMX.

 

[69stang351]

Good, Good, Good I'm glad someone is actually backing me up on this, most of the time its 'take it out, its junk' or 'it wont stand up to the power' or they dont last' Maybe we should start a FMX forum to teach everyone the strengths of the FMX and how it is better than the lighter duty trannys. lol

 

[D.Hearne]

I've got a 63 Merc in the back 40 that's got an FMX behind the 390 under the hood. A small block ain't a problem for an FMX. Back in the 70's, nobody wanted either a C4 or a 351W, the hot things to have were 351C's and FMX's, what a difference 30 years makes. If the C4 was so good a transmission,why did Ford base the AOD on the FMX? I had a C4 in a 69 E330 van behind a 302 and in 125,000 miles, it went thru 5 C4's, all were the heavy duty ones. The only drawback to an FMX is the lack of aftermarket support. I did notice that Summit listed a stall converter for an FMX though, that was a surprise.

 

[GaPonyFarm]

D.,
I'm not saying that the C4 is junk. In fact you can build a nearly bulletproof C4 to handle 1200+ hp. If he already has an FMX, that is definitely the way to go, its simply a better transmission. I think the reason for lack of aftermarket support is that there aren't many of them left... and in my opinion, few improvements that need to be made on the FMX. Most people toss them because they're so heavy and that weight costs horsepower. Everything is a trade off...

69stang351,
An FMX forum or an auto tranny forum in general might be a good idea. I don't think any of the automatics get the respect they deserve. I for one am tired of hearing that the only smart swap that should be made is to a T-5 or other manual.

 

[69stang351]

I'm not one to bash transmissions either, I was just saying that he doesn't need to swap out his good FMX for a C-4, if he did have the C-4 already I would have recommended switching it to a FMX or just building the C-4 up to handle the power. Sorry if I came across this way, but sometimes it just annoys me when someone says the FMX is crap and should be switched out for anything except for a 3-speed manual. My regards to anyone who took my post wrong and thought it was bashing the C-4.