Love driving these old street/strip Hydramatics, but I'm often asked about the "kickdown linkage." If you're unfamiliar with early Hydros, there's a pressure lever that shares the shifter shaft and bracket. Since it ties to the throttle linkage, most ***ume it's a kickdown lever — it is not. This lever controls the internal line pressure and can pull at 40 lbs of force, so a simple cable system won’t hold up on the street. Too little adjustment and you go nowhere; too much and you’ll kill the trans. Both the original B&M and Hydro-Motive instructions share different but similar ways to build the linkage, but they don’t work well in early cars with close and narrow firewalls. I think of a local '32 that had his going through the firewall and then back out the floorboard. To solve my clearance issue in an early car, I use a two-bellcrank system. Since I’ve been swapping intakes and need to modify the upper mount and crank, I thought I’d share this info for the next guy. Lower bellcrank: Try to keep a parallel line off the pressure lever coming out of the transmission. Make the bellcrank with unequal arms in case you need to change or alter the ratio of travel (the distance the pressure lever travels is not the same as the distance the throttle moves). Adding an extra adjustment hole can help later too. Mount this to the pivot ball location for the clutch Z-bar — or on the side of the bellhousing if you have a boss. I used a 1/4" rod and heavy ends for the lower linkage. Moving up the firewall toward the intake: I used a smaller rod (3/16") from piano wire and made two 45° bends to go around the bellhousing and move my line closer to the center of the engine. My reason for the smaller rod: by adding the bends, it's more likely to flex in that area under a very heavy foot and take some pressure off, reducing the chance of bending or breaking stops. Upper bellcrank: Same as above, but a couple of things: Hydro-Motive instructions reference “spongy travel,” and this is part of the over-center of the radius. I made my upper crank with mounts less than 90° in relation to the pivot point. Now you need to find the common axis between idle and full throttle for your carb linkage to mount. Make and mount the linkage rod to the carb. With the carb closed (idle), mark the arc line on the bellcrank. Get some help and open the carbs to full throttle, move the bellcrank to full pressure, and mark your second arc. That intersection is where you want your rod end for the carb linkage. When done, it should be tight and smooth, with just a small amount of flex if needed. For final adjustment, I tightened the carb-to-linkage rod 1/4" and followed the Hydro-Motive instructions.
I built a very similar linkage for my Hydro on my '52 Hemi with Holley 3bbl. Not shown is the 1/4" rod going to the carb. I set it to have the TV totally off at zero throttle, totally buried at full throttle. Has worked well for 30 or so years.
Looks like that should work great! I used a modified Lokar Torqueflite kick-down cable on mine, so far has worked well.
This is a clean and well thought out set up. I am also using a cable attached to a bellcrank without any issues (yet). I tried a rod ***embly but it would bind up, your use of a second lower lever would have fixed that.
Good idea with a cable system - technology has improved since these were original built, bet they didnt have anything smooth as Lokar and should work good too.
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