Just bought 69 Cougar 351W - 4V XR-7. The FMX is shot, should I rebuild the FMX or swap to C-6? I don’t know what the swap would entail. I plan on building the 351w to 350-450hp. Need a reliable transmission to handle that power. Would like to hear from people who have done the swap or swapped in an AOD transmission.
I can’t believe that nobody has responded yet.
I’ll start by saying that I’m biased; and that bias is the reason for my response. You’ll hear all kinds of stories about how the cast-iron-cased FMX is too heavy, how it’s a dog or a slug or a “bad design”; how NOBODY has decent performance parts for it. BULL! to all four statements. You’ll hear that you should toss it into the nearest dumpster and find a small bell C6 or a C4. That’s the “conventional wisdom”, rehashed 100’s of times over the years.
Okay, there’s nothing wrong with a C6… I put a lot of miles hauling and pulling heavy loads with a C6 behind a “homebrew” junkyard-dog 406 in my '72 F100 years ago. Really good in a pickup, behind a gas-guzzling FE. It also has more parasitic drag than an FMX - that means more TQ/HP is spent spinning a heavier gearset and less is spent spinning the tires. And C4’s are great little automatics and yes you can build the snot out of them; once you have a “core” transmission with good gearsets and not-too-much iron filings in the pan… but you have an FMX, and you can get “performance” parts for it. I’d start running off a list of tech articles and suppliers, but my wife just announced that supper is ready, and I still have a teenaged son that LOVES TO EAT at home; so I’m gonna just say “Google is your friend” and suggest the keywords “FMX transmission performance”.
Besides which, Milo (Cougrrcj) will eventually see this thread and effectively squash a lot of the “conventional wisdom” with one of his (and my) favorite pictures. That of his friend’s 70-something Mustang quarter-miler with A LOT of air between the front tires and the pavement as it leaves the Christmas Tree; powered by a lot-more-than-460CID Lima engine, power delivered to the driveshaft by an… FMX transmission.
My personal bias? My first car had a bone-stock FMX behind a somewhat “snorty” (for a Cleveland with 2-barrell heads) but streetable 351. I had to replace u-joints a couple times, but NEVER did the FMX give me trouble. That says something - for if I, in my teenage years through early 20’s, couldn’t break my FMX; d@mn few others could either! All I ever did was change the fluid and filter now-and-then; it never even burped.
OBTW: You mentioned that you might be interested in going to and AOD. Look up that trans in Wikipedia, see upon WHAT transmission design that AOD was based.
Just Sayin’ Copyright 2002 Desert Dawg/StangDreamin’ T3 MUST hit the Triple C’s “Donate” button AND water the shrubbery if he wants to use this term 
And so it begins… 
My personal preference would be a C6. But I have nothing bad to say about an FMX. One item on my wish list is the transmission that I read about a couple years ago. A guy was running it on his Mustang pushing about 600HP through it. It was a C4. 
Shrubbery
Just verbalizing…
Dang it Dawg :shakefist: that keyboard was almost new
DeputyDawg pretty-pleased me to post this pic…
That car at the time had a 484-inch 460 stroker backed by an FMX trans (using a tall-block 351M/FMX bellhousing). At the time, that car was running 11.teens in the quarter-mile with 1.4x-second 60-foot times. BTW, that was a stock rebuild on the FMX, using the old Fairbanks shift kit and a 4000rpm stall converter. Does that say anything to how strong FMXs are? Now that car is getting a new 545-inch motor, still backed by the same FMX trans it had when it rolled out of the factory as a 351C-2V back in August of 1970!
That’s a cool pic… 
Now, if you’re asking about AOD swaps, I have one of those behind a 480hp 393W and it is holding up just fine. Of course, it is not a stock AOD by any means. I had it built with the best parts - the drums with the most clutches, the later stamped-steel direct drum so I could use the later 2" o/d band, the later ‘mechanical diode’ one-way clutch, the A++ o/d servo, a Baumann shift kit (but now has SilverFox SPT-R valvebody) and a hardened inner input shaft so I still have lock-up in D and o/d…
Oh, and the wide-ratio gearset from a 4R70W trans that gives ratios of 2.84/1.55/1.00/.70 versus the stock ratios of 2.40/1.45/1.00/.67. The lower gearset doesn’t sound like much, but it is 18% lower First gear, making my 3.55 rear gears feel like 4.19s off the line, yet 1.00 in Third and a .70 o/d ratio so at highway speeds the engine is only turning 1700rpm! Best of both worlds!!!
Of course, there is the other faction that likes electronic controls so they used a 4R70W from a 3.8 or 4.2 V-6 to get the low gearing and no hardened inner shaft is necessary… But then you have to buy a controller… To MAY-toe, to-MAH-toe…
I see you couldn’t resist the temptation, Milo. And GOOD ON YA for it! 
That has to be one of my favorite pics; “Old Gray Mare” rearing up to strike at her challenger.
I don’t see anyone getting ‘feisty’ over it, either. Only response was from Todd; and he liked it. Not even a little dig about it being a Mustang.
Surprising because, well… he’s Todd 
Actually, that pic is with me too
i have seen it plenty of times, and every time it’s posted again, i enjoy another nice close look
I think I’m going to stick with my FMX and have it rebuilt.
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