Hey guys. So I currently have the stock FMX and think its starting to slip and shift a bit sloppy, so before I’m left stranded somewhere I want to look at my options. Im just checking to see if Im missing anything about switching to either a c6 or c4 from the fmx. I know i’ll probably need a new driveshaft, is there anything else im not thinking of?
I would just rebuild the FMX.
You might be better off rebuilding the FMX. You could build it more stout for when you are at the track. If you put in a C4 or C6 that you don’t know where it has been before you got it, then you could be in a similar situation as you are with your current FMX.
The other thing about a C6 is that they are great for a big block, but in a small block they actually rob power from your engine.
I realize the FMX is actually a very stout factory tranny and has less parasitic loss than the c6. The power loss is a concern with the c6, and its not my first choice. The c4 has the least parasotic loss and is in the ballpark of 50lbs lighter. I know its not as strong in stock form but if we are talking about having to do a rebuild anyways, I would guess that rebuilding one of the most common transmissions with huge aftermarket support might be more cost effective than a fmx rebuild. I still need to talk to my tranny guy about the options and prices though.
I’m less concerned with picking a tranny, I’m more curious about swap interchangeability with the trannies. is a c4 and fmx flexplate and starter the same? I’m used to gm stuff where anything bolts together
Yes you would keep the same flex plate and starter. You would need to buy a different dipstick / tube, cross member, transmission mount, driveshaft yoke, drive shaft, shift connector rod, neutral safety switch and torque converter along with the new replacement transmission, either C4 or C6.
If it were me I would just have the FMX rebuilt. It is a common Borg Warner transmission used in many makes of automobile. Any competent transmission shop will have no problem doing the job.
I have a C4 behind the 351W in my Red Mach and I like it a lot. The last FMX I had rebuilt required me to haul in my spare FMX so the guy could scavenge parts out of it as there are some things you can’t get for the FMX any longer. I’d go with the C4…maybe it’s in my head, but the Red Mach just seems more “nimble” with that combo than the 351W/FMXs that I’ve had over the years.
If you rebuild the FMX there is no worries about making accommodations for swapping in something that wasn’t there. C-4 or C-6 is going to gain you nothing over the FMX. They are all non-lock-up 3 speed trans. All with the same gear ratios. If you want to go through the effort of putting something in that wasn’t there before, swap in an AOD. This will give you the benefit of overdrive in 4th gear----which is wonderful if you have a rear gear bigger than 3.00:1.
If you have any performance with your engine and decide to have the FMX rebuilt, go with Trans-Go’s FMX-3 kit. Kinda pricey, but really makes the trans perform. If you are planning to have it rebuilt stock. Go with the small shift kit. Probably 3-67 or if sometime in i’s life the FMX had a later V-B installed the kit could be 3-73. Don’t worry about parts. They are all available. No new hard parts. Some parts are expensive. But you don’t know what you need until it’s apart.
You decision should consist of FMX or AOD or manual.
I realize that i could be insane, but I dont think I’m interested in an Aod. The difference between a c4 and fmx is the weight and power loss. an aod is also heavy with more parasitic loss. I’m not saying I wouldn’t ever use it, but I dont think its what I’m after right now. Also I have had 3.90 gears for over a year and haven’t felt the need to go on the freeway much. I would be really interested in doing a tremec tko swap, but for what they are asking for those trannies it will probably be a future swap
As you already know, to get the best 1/4 miles times, a C-4 would be the way to go.
My little sister just got a 400 and a 12 bolt with 3.73’s for her 69 pontiac. If I want to continue to beat her, (that should be a obvious yes), then I need to use any advantage I can.
Best bang for the buck is gears, regardless of trans.
I have 3.90’s in it right now and feel pretty good about that.
I have this swap in place on my 70. I would highly recommend the FMX rebuild but let’s assume you want to go forward with the C6.
Here’s a summary of things that I recall about this swap:
Starter, flexplate are the same.
I used the same trans mount and cross member as the FMX.
Driveshaft yoke is identical on FMX to C6.
Driveshaft needs to be shorted about 3 inches.
New cooling lines (a complete pain in the a$$)
Trans & converter. I had a C6 without the top side fins which made it fit without hitting the tunnel.
Kickdown and linkage had to be modified. The C6 linkage bends around slightly different from the FMX and therefore the FMX linkage does not fit directly.
You also need a C6 with the correct linkage on it. If it’s a 70 or later and you want to reinstall the lockout rod there you need to make sure you have a later model C6 shift lever with the lockout rod mechanism.
I had an aftermarket exhaust so I don’t know if you’d run into trouble with the exhaust system on your car.
The FMX is really a good trans so if you’re not looking to put serious power down then I would highly consider sticking with the FMX!!