I was wondering if anyone could let me know which one of these transmissions is a better performer? I have heard varying opinions and was hoping to get it cleared up so I can decide whether to rebuild my fmx or move to a C6.
Here's a general breakdown, pretty much lines up with Bosscat's remark:
C4:
Weak in stock form.
Can be built very strong.
Dollars invested = Strength
Compact
Light
Lowest parasitic power loss.
No mods to driveshaft if swapping between FMX
Best aftermarket support.
C6:
Strongest in stock form.
Very heavy
High parasitic HP loss.
Need to shorten driveshaft if swapping from FMX or C4.
Big, bulky
Good aftermarket support.
Many headers interfere with the tranny due to width.
This may or may not be the case with your car, depends on year, header, engine family, etc...
FMX:
Cast iron, so heavy tops the list.
Least aftermarket support.
Some cast iron internals are fragile.
Swaps easily with C4.
Was designed in the mid 50s.
What year car.
If it came with an FMX, it likely will fit a C6 fine.
Some cars it is a tight fit though, and some cars may require tranny crossmember mods.
Edit: Just saw that you likely have a 72 Stang
Being a little bigger car, you would definately want to build a C4 strong. The C6 will swap in with a little driveshaft surgery.
Have you considered an AOD?
They are weak also in stock form, but should be built strong anyway before use, just like the C4.
Someone will correct me if I am mistaken, but I believe it would be an easier swap than the C6 just due to the driveshaft issue. The AOD should take to the FMX/C4 driveshaft fine.
Anyway, the AOD would make the car a much more street friendly cruiser with the OD.
C4 would be a better choice IMO. Don't get me wrong I really like my C6 and I can thow anything at it without worrying if it will break but the C4 looses a lot less power to the rear wheels, Can be built just as strong as a stock C6 and its lighter. I fortunately didn't have any issues with headers fitting around my C6 but I have heard that it can be an issue. Is there something wrong with your FMX. They are pretty stout trannies too. Whats your budget too?
Kevin
A built C4 that can handle 600 h.p. can be had for a little over $1,000. Cheaper if you don't need that much...about $850. Much lighter than C6, less drag, as stated. If stock is the only option, stick with the C6 for aftermarket and stock replacement parts availability.
General concensus is that the FMX is actually a pretty good transmission. It will rival the C6 in regards to strength in stock form and is generally a pretty reliable transmission. However it has 2 major downfalls... very poor aftermarket support and very heavy (slightly heavier than the C6 which weighs alot).
Since you already have the FMX... I guess it depends on what you plan to do with it. If you are going for high performance swap to a built C4. If you want mostly stock performance then the FMX should serve you fine. Like the others have posted, a C6 swap doesn't make alot of sense.
I just finished swapping out my FMX for a T5. Even after a 10 year old rebuild with shift kit and 2500rpm stall that transmission was still working perfectly the day I pulled it out. And I can't say I've babied it... especially in my younger years.
The FMX will get you home no mater what. Can't say that about a C-4. The one weak point in the FMX was the two "ears" on the torque converter hub driving the front pump (or so I've been told) "Back in the day" it was THE automatic of choice for 351's. The AOD was based on it. Unless I was given a C-4 (which I have had done with my recent 77 Comet buy) I'd keep the FMX. When the C-4 I have craters, I have an FMX ready to replace it. Then comes the Toploader 4 speed.
the FMX is almost indestructible in stock form, as my 30 mph reverse drops in my youth will attest and not just one or two, it was a pretty regular thing. the biggest weak point with the FMX is the stock cast iron 2nd gear band and really only if you have a really stout shift kit. there isn't a lot of aftermarket performance support but there are shift kits and stall canvertors available and many internal AOD parts are interchangeable, like clutches and steels. it really doesn't have much more parasitic loss than a c4 something like 1 percent but the c6 is about double what a c4 is and almost double what the FMX is. the FMX is definitely a heavy pig but all that weight is basically in the center of the car so it's not a big deal as far as F/R weight balance.
if you are going to swap the FMX for anything swap it for an AOD, the FMX can handle a LOT more power than most people give it credit for, it was the only transmission ( well an early form of it, anyway) available for big blocks before the advent of the c6 and the only transmission ford used behind 351w's and c's in passenger cars except for a very few c6's used in hi-po 4V clevelands in mustangs and torinos and even then it had to be special ordered and wasn't standard equipment. it was also used behind 351m's and 400's for the first couple of years of their lifespan until the c6 became available for them.
that's a ton of mustangs, torinos, ltds, galaxy 500's, thunderbirds, etc. and their mercury cousins built from 69 until about 1980 or 1981!!!!! even some 302 cars got the FMX. and how many of those cars do you ever see getting the transmission rebuilt? about 1 in about 10,000 or so, i'd guess. in fact most of the time the FMX will long outlast the car was originally in.
so my vote is officially, keep the FMX or swap to an AOD if you swap it all.