dbgrigsby93

I'm going to be changing out the 2v heads for 4v heads sometime in the near future on my '73 mach 1 and after reading a thread about FMX trannies I'm being lea to believe that the 351c 2v had the FMX but the 351c 4v had something else. Is that true? What transmission did they put behind the 351c 4v and will my FMX transmission be okay if I put cast iron 4v heads on my 351c?? I'm only planning to get about 350hp. I'm going for a street cruiser with some xtra power, but I don't want to change my tranny. Will the FMX be okay if I make this conversion?

4reboy

Yes the FMX should be good with that power, they can take a LOT. And if you don't have the FMX the only other thing that should be behind there is a C6, on the 71-73's I believe they put C6's and FMX's on the 351 engines, but not sure.

1971mach1

My 71 with a 351C-4v origionally came with a C6.
I have heard the c6 is stronger than the FMX. Not to say the FMX is weak though.
They're both pretty heavy (weight) trannys though.
My car sure feels lighter now with the TKO 5-speed in it.
If you want to replace the FMX with another auto, you can get a beefed up C4 now, alot lighter too.
Personally, I'd wait until the FMX has problems though, unless you just have to shave off about 100lbs weight.

orange88

I have an originally 351-2v with a swapped intake for 4v carb and the fmx works just fine

THUMPIN455

Like said above, the FMX will work for you, a C4 will take a relatively stock Cleveland with a few upgrades, its an easy job for an FMX or a C6. Finding the C6 will take more effort and they soak up more power, but they are out there if you want one.