First/Reverse only Transmissions

First/reverse only is a concept that has been around in demo derby for years and years. i remember walking around in the pits with my dad talking to people back in the 80’s as a kid. they modified their stuff for first/reverse only….or had a transmission shop do it….along with water in the tires……and running all thread instead of wire and chaining the hood. Anyways, when i started out to build a first/reverse only transmission(which started as a wager), i wanted to not only keep it from shifting, but eliminate moving parts. in essence K.I.S.S.(keep it simple…..stuipid).

the actual modification to prevent shift is actually rather simple. you unplug the modulator valve from engine vacuum and disable the governor. modulator valve is low speed shift control. governor drives just like a speedometer off the output shaft.  on both the th350 and th400, line oil pressure from the pump is supplied to the gear drive end of the governor. as road speed increases, weights fly out in a governor action working against supply oil(instead of a spring like an engine governor would work). the faster you go- the more signal oil is let by. this signal oil is then sent to the 1-2 and 2-3 shift valves. the tension of the springs in the valving controls shift point……and the size of the holes in the mid plate control how fast the gear applies.

on a th400, my method for disabling shift is to remove the governor and both supply lines entirely. you then drill and tap the oil supply port at the rear of the valve body which is meant to supply the governor. you leave the signal oil/return port open. this does two things: you line pressure is not lost anywhere in the governor circuit cause it is essentially completely removed. the other is there is no bleeding past of the fluid on any of the valving that may eventually build up and try to trigger a shift. if you are familiar with case drain on a hydraulic system, in essence that’s what it is.

Now, after298116275 the fact i figured out that by simply modifying the governor to stay closed would give you the same effect. contrary to what anyone claims, i had 15-20 transmissions running successfully in events across northern IL before i ran across this modified governor from a builder out west. it does work but it would still have the effect of wanting to shift into 2nd at high speed. here again this was caused by case drain effect. so i discovered people sticking clips into the 2nd gear shift valve. i never did this on anything i built. in fact, i use to modify my pile of old governors and sell em for a few bucks at shows years ago. this still works on th350’s and th400…….just not as effectively as my method. i would say this is about 85% effective whereas the other is 100%.

The other part unique to my 1st/reverse design is locking in the first gear holding member. on any automatic transmission to get gear reduction you have to have one holding member and one driving member to get what is called planetary action. on a th350 it is a piston style holding clutch to the rear of the case. on a th400 it is a rear 3 rib band on the front planetary gearset. a 727 also uses a band if i remember right. anyways,on both the 350/400 transmissions, reverse and 1st gear are the same gear-powerflow. the forward clutch drives first gear, while the forward clutch releases and the direct clutch is engaged to drive in reverse. so i reasoned that by permanently anchoring the holding member you not only eliminate moving parts but ensure the gearset is held. you also reduce friction and strain on your first gear holding sprag.

on th350 transmissions, i tried many different methods of doing this, as well as my friend Jim. it was hit and miss one trans would win derby after derby….the other wouldn’t pull out of the driveway. after a ton of thought i realized what was happening and found the fix, but elected to quit building th350 for demo derby cause i started seeing gearset damage and snapped output shafts. for this reason i quit building th350 for derby specific. i will rebuild one and use the 4L60 center support … can use it for whatever you want after it leaves my shop and it will work!

on a th400, i had much more luck. just find a way to lock the rear servo completely engaged so the the rear band doesn’t dissengage. the picture to the right was my first attempt at figuring this out using a modified rear servo cover and a hommemade adjustment screw. i then removed all the seals and pistons from the bore, plugged the oil ports off, and set the tension on the rear band manually. this didn’t work out at all… sucked royally. so i took a different approach to it entirely.396007827 it happened qute by coincidence i had a GM 8.5 rear end i was setting up in the shop at the same time…..looked at the servo……looked at the shim kit of that rear axle……and that worked. i eventually abandonned the idea of blocking off the oil ports to the rear servo and ended up suing the stock rear servo/accumulator with new seals but both return springs removed. you then use shim washers to shim the piston down in the bore to tension the rear band. HOWEVER, i also discovered this is an adjustable thickness of shim. there are 3 different pins used on the rear servo all different lengths….and you also have wear to the fron planetary and rear low 1 holding band. i have used a specific washer from lawson products, and on average you use 2-3 washers….sometimes one.

the most successful way to set the rear servo/band tension like this is remove the rear servo…….remove both both springs and seals ….set the accumulator on the bench…..then reassemble it back into the bore. you then stick about two washers under the servo cover and press down to the case. you want the servo cover to barely hover above the aluminum case. if there is a giant gap there it is too much. if there is no gap and you don’t feel the tension on the band……you need more washer/shim. when you get it where you want it, you then install your seals, the inner accumulator, shim washers selected washerand torque it in.

Now, it is very important that you do not overload the rear servo tension. you are suppose to prevent movement moreso that set tension. you are still getting oil flow to it in low 1/rev for holding. what we are doing here is preventing it from dissengaging fully causeing friction/heat. it will still move slightly even when shimmed. if you overtension it, it will warp the case and even screw up how the whole planetary gearset rides in the case/center support. i showed several different people how to build my style 1st/reverse only. pictured here to the right is precisely how not to do it. the 3/8″ washers will float around against the rear servo cover and your apply will jump all over the place. you would be better off just running a stock rear servo set up over this. the yellow spring works out quite well for rear servo return/accumulator springs.

There are numerous other small things i do to my derby transmission builds, and they all kinda work together. first/reverse only in my opinion is somewhat overrated in modern demo derby. with these huge geared rear axles and what not, you almost need more than one gear. and for a wire car it is a bit overkill for what your going for. a properly built stock th400 transmission driven wisely and maintained properly can win any derby, and have. the whole object of going 1st/reverse only or 1-2/reverse manual shift is to add longevity and performance to a transmission for a performance edge


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