Sounds like you've maybe got bad motor and/or transmission mounts. The FMX's are tough as nails, far stronger than a C-4.
i'll second that. i have no intentions of ever changing the FMX in my car for anything but maybe an AOD. the AOD is actually based on the FMX too, so basically it'd be like replacing my 3 speed FMX with a 4 speed FMX for the most part.
Your opinion of TCI stuff is just that---your opinion. The fact still remains the FMX was a better unit than the C-4. Just because the aftermarket bypassed them, doesn't mean they weren't. The aftermarket also bypassed the Cleveland too. And everyone knows it was the better hi-po motor platform.
What I said was that TCI does not make the best torque converters. By disagreeing with me, you imply that TCI DOES make the best torque converters, and this is patently not true. They do make a decent TC for the money but not the best. This cannot be disputed.
Was the FMX a better unit compared to a C4 or C6 back in the day? Yes.
Was the Cleveland a better block compared to a Windsor back in the day? Yes
Given today's after market parts, is somebody better served with an FMX behind their engine or a C6? There is simply a much greater choice and availability of after market premium parts for the C4 and C6 and an enthusiast can build a far better tranny out of a C4 or C6 than he can with an FMX. This cannot be disputed.
My point is about choices TODAY. I originally had an FMX in my Mach 1 but the PO swapped it out for a C6 and I am glad he did.
I added low drag needle bearings in place of all the thrust washers. Not an option with the FMX
I got a sweet R servo that is not available on the FMX.
I got a low friction, low geared (2.71 first gear as opposed to a 2.46 first gear stock) 6 pinion, steel planetary for the C6 that can take the rigors of hard launches behind my 475 hp hot 351W. This planetary does not exist for the FMX. Is the FMX strong? Yes. But so is the C6, and there is a lot more after market offerings for the C6 and C4 than for the FMX, making these the better transmissions TODAY.
Great information, thanks much. So, a bad motor mount would cause the thumping even if the car is just idling? Is there a way to isolate the tranny from the engine, say, plug the vac line and see if the thumping continues or goes away? How do I check the motor mounts? What would I look for to see if the tranny mount is bad?
All this put down of the FMX is based on what you've heard (apparently) In this post, it's apparent you've never run an FMX, so all this is simply your opinion, not fact. Until you've run an FMX behind your 351W, don't be so quick to put it down.
I am not saying the FMX is an inferior tranny compared to the C4 or C6. I am only saying that the aftermarket offers less choices for it and as such, you can build a better tranny out of a C4 or C6. As for Broader performance, that is where I got my low gear, 6 pinion steel planetary, wide bands, needle bearings, valve body, R Servo, etc. Jay at Broader Performace was very clear that given my build, a stock anything tranny wasn't going to last very long. These super hipo parts are simply not available for the FMX. Until they are, your choices with an FMX are limited.