FMX transmission shifting RPMs

[vintageman]

Good evening everyone, for those with an FMX transmission and a 351C, can anyone tell me what your shifting RPMs are when going from 1st to 2nd, and 3rd? I have a 3:55 rear end gears. Mine is shifting from 1st to 2nd at around 3000 rpms and 2nd to 3rd a little later. Acceleration is great, I am just wondering if my shift RPMs may be a little late and maybe I need to change the modulator as I have heard that will change the shift point. At this moment from memory, and I may be mistaken, I believe I have the modulator with the green strip. BTW, I do have to change my speedo gear because I still have the original from the 2:73 (I think) gears. Thanks and I appreciate the future advice.

Tom

 

[Bentworker]

Good evening everyone, for those with an FMX transmission and a 351C, can anyone tell me what your shifting RPMs are when going from 1st to 2nd, and 3rd? I have a 3:55 rear end gears. Mine is shifting from 1st to 2nd at around 3000 rpms and 2nd to 3rd a little later. Acceleration is great, I am just wondering if my shift RPMs may be a little late and maybe I need to change the modulator as I have heard that will change the shift point. At this moment from memory, and I may be mistaken, I believe I have the modulator with the green strip. BTW, I do have to change my speedo gear because I still have the original from the 2:73 (I think) gears. Thanks and I appreciate the future advice.

Tom

Is that at WOT?

What is it like when you coast to a stop? Do you feel it downshift?

Also see https://7173mustangs.com/threads/fmx-valve-body-info.39371/

 

[vintageman]

Thanks Bentworker, with just normal acceleration and stomping on it, it feels like it shifts at those RPMs. I will be honest, I don't do a lot of "normal" acceleration when driving it, I do step on it a lot when starting from a complete stop. When cruising at a specific speed, the RPMs are not high and are constant and feel smooth with no flat spots when accelerating or slowing down. I don't think that I can feel it downshift but probably need to pay a little more attention to see if I can feel it. I will check out the link and after a few more drives, (both slow and spirited) report back.

Thanks
Tom

 

[giantpune]

Are you sure your modulator is working and that its getting vacuum signal? It could be a matter of the modulator not doing anything at all and the transmission is falling back to the mechanical governor.

If there is vacuum signal making it all the way back there, it may be worth it to try the adjustable modulator and just dial it in.

 

[steves73]

My stock 1973 Fmx would shift at 1500 rpm in normal driving. When I had the engine modified to way more horsepower and changed to 3.25 ratio rear, it stayed the same, so first gear changed in like 2 seconds.
At full throttle, it was always 4500 rpm.

 

[BUCKWHEAT]

With my C4, I got an adjustable modulator from my transmission guy. First, I confirmed vacuum at the modulator consistent with the Vac diagram. Then I adjusted the new modulator to get firm up-shift that I wanted. No more "iffy" shift, and firm shift at WOT. FMX may vary...

 

[vintageman]

Thanks everyone, I am going to try to do some testing tonight, that is if the rain doesn't hit me again. I appreciate all the comments and advice.

Tom

 

[Don C]

Here are the shift speeds for an FMX and 351C



As you can see, your axle ratio is not shown, so you'll have to extrapolate the shown speeds to estimate the shift speeds of your setup.
Yes, you need to change the driven speedometer gear. There is more information in this thread:
https://7173mustangs.com/threads/speedometer-gears.15595/

And, your kickdown linkage also has to be properly adjusted.

 

[vintageman]

Thanks Don.