Modifying Full Throttle Upshifts BW M12

Trader, I wonder if you can do the blip throttle trick for the GM TH400, for proper manual down shift. The hitting the accelerator into a down shift, keeps rotating inertia from attacking low band and sprag. Some where I found that info, but it is not talked about much.

Being not familiar with operation of the M series tranny mechanicals, its just a curiosity of sorts.

Oddly enough with the Torque Command, you can down shift hard in any forward gear. Which I was a bit of a mad man with my 73 when I owned it.

Trader, they are talking about the older M-8 in the Jags, but might be an M-11. When the M-12 came out I'm pretty sure the old "D2 D1" selections (on Rambler) were gone, the more common "D 2 1" taking over. I do recall that if left in 2 the trans would upshift anyway at a certain speed for safety considerations.... to prevent over revving the motor. 5400 rpm/70 mph sounds about right...

ramblinrev wrote: Isn't there a note in the Owner's Manual, that a BW, in the "2" position will automatically upshift to 3rd at 70-something MPH?

Edited by Steve_P - Nov/26/2021 at 6:06pm

If your transmission is upshifting at 4000 rpm at full throttle then you can certainly adjust the vacuum modulator to move the shift points (both partial throttle and full throttle) upward. At 4000 rpm the vacuum modulator is controlling / limiting the upshift points, not the governor. The factory recommended pressure settings were set for smoothness and fuel economy not performance. AMC did not set up the automatic pressure specs any differently for the AMX and Javelin performance engines though they should have.

By raising the operating pressure you'll get firmer, faster shifts and less slippage which will increase the life of the bands and clutches.
I believe that you'll want to move the modulator adjustment up for a full throttle upshift to begin at about 4800 rpm. The trans. governor is set up to force an upshift at about 5100 rpm irrespective of the position of the shift lever.

You will find however that initial engagement of reverse or drive ranges when shifting out of park or neutral will be a lot firmer and the closed throttle 3-1 or 2-1 downshift will be much more noticeable.
Tweak your modulator adjustment clockwise to raise the line pressure a couple of turns at a time and then do a road test to find your optimum point.

Just as a follow up, prior to the 1968 model year when all the AMC / BW automatics were referred to as "Shift Command", there was the "Flash-O-Matic designation which dated way back. All Flash-O-Matics used the D-1, D-2 and L forward drive designations. D-1 was the normally used position that provided all 3 speeds with a first gear start and D-2 provided a second gear start with an automatic upshift to 3rd. The intention was to use D-2 for slippery surfaces. L range held the car in first gear with no automatic upshifts. Placing the shifter in L when driving at road speeds would automatically downshift the transmission to second gear and if/when the car slowed down below about 20 mph on closed throttle the transmission would downshift again to first gear.
Shift Command was introduced in 1967 in the full sized cars and was the transmission version used with the floor shift/console option. The shift quadrant changed to the "more modern" D, 2, 1 designations. D gives only a first gear start with 3 forward gears, 2 gives you second gear only and 1 replaced the L (Low) range of the Flash-O-Matic by only making first gear available with no upshifts.

All the functional differences between the Flash-O-Matic and the Shift Command versions were due to changes in the valve bodies.

Dono, as for adjusting the modulator you'll most likely find that you'll want to set the pressure so that the 2 to 3 full throttle upshift point begins to occur somewhat below 5000 rpm. Otherwise your part throttle upshift points may be too high for normal driving in traffic situations. If your cam allows rpm much past 5000 then you can always shift manually to hold first and second longer though much may not be gained by shifting past the horsepower peak.
Note, that as mentioned in earlier posts, that the valve body is designed to force upshifts when engine rpm exceeds much above about 5200 rpm irrespective if you're holding the gear shifter in a lower gear position.