Over Powering an Automatic

TomC · November 15, 2010, 08:03:30 AM

Many have been talking about using the MT644 behind the 8V-71 then bumping up the horsepower. In case some of you don't know, an automatic works by having to over power the engine to bring the rpm down to the next shift point at which point the clutches lock in with the next gear shift. All the planetary gears are rotating at all times, but engage one at a time through the use of multiple disc packs powered by the hydraulics of the transmission. Hence the most wearing and heat producing time during shifting to the next gear.
On the new 4000 series (B500) Allison, they have a split rating of 1650lb/ft in first and second, then up to 1850lb/ft third through sixth. If you're running a hot engine like I am against a modest transmission (my engine is rated at 375hp and 1125lb/ft torque while the V730 is rated at 275hp and 750lb/ft torque [if I remember right]), the trick is to number one not be flooring it on acceleration, or if you do, manually shift it and let up on the gas pedal during the shift. Once the Allison automatic is in gear, it can take alot of additional power over its' rating.
On my truck HT740, it has the RV smooth shift option. The transmission has the normal 1C-2C-2L-3L-4L shifting, but the on the shifts between 2-3 and 3-4 where the converter is locked up, during the shift, the converter instead releases out of lockup during the shift and relocks up immedeiately after going into the next gear. This makes for a very smooth shift and alot less strain on the individual gear clutches. The actual time it is out of lockup is about 2 seconds. Hope this helps. Good Luck, TomC

kyle4501 · November 15, 2010, 07:57:07 PM

According to the book, the shift sequence for my HT754cr is 1C-2C-2L-3L-4L-5L.

Is it possible to change it to 1C-2C-2L-3C-3L-4C-4L-5C-5L ?
Would that make for softer shifts? Why wasn't it programmed that way?

Inquiring minds . . . .

TomC · November 15, 2010, 08:59:41 PM

Kyle-in the 80's, they did have that kind of programming (getting another shift valve body) since the engines back then had a much narrow power band (like the 8V-71) then do the engines now. It was an effort by Allison to be more appealing to truck drivers with the additional "shift" points. But-what they found out that if the truck happened to be at the exact speed where the transmission was in converter mode and climbing a hill, it cooked the transmission. Hence that programming was eliminated. If you want smoother shifts- either shift it manually and release the pedal, or look into the momentary release of the torque converter as I previously stated for your trans (I'm going to look into it also). Good Luck, TomC

luvrbus · November 16, 2010, 05:38:40 AM

Kyle, when you get close give me a call I'll walk you through getting smooth shifts on a Allison 754 CR,start shopping for a Stone/Bennett shifter (hydraulic type) not the electric over air.The close ratio can be made to shift really smooth me I liked the FS setup on the Allisons

good luck

lostagain · November 16, 2010, 06:59:25 AM

Luvrbus, TomC, my HT740 shifts rough unless I do it manually, and is still quite firm from 3rd to 2nd. Like I have to turn the Jakes off before I down shift into 2nd. Can it be made smoother, or is it normal? I don't know the name of the shifter, but it is a cable from handle to trans. There is also a cable from trans to the governor.

Thanks,

JC

Ed Hackenbruch · November 16, 2010, 07:24:34 AM

JC, when i switched to transynd my 644 started shifting a little smoother.

TomC · November 16, 2010, 07:26:00 AM

I would first make sure the cable running from the engine to the transmission is adjusted correctly and getting full travel. Should be completely closed at idle and full open at full throttle.
Then changing to the Castrol Synthetic trans fluid can make a difference. Good Luck, TomC

kyle4501 · November 16, 2010, 10:27:10 AM

Quote from: luvrbus on November 16, 2010, 05:38:40 AM
Kyle, when you get close give me a call I'll walk you through getting smooth shifts on a Allison 754 CR,start shopping for a Stone/Bennett shifter (hydraulic type) not the electric over air.The close ratio can be made to shift really smooth me I liked the FS setup on the Allisons

good luck

I've got a cable shifter - no cable yet, I wanted to know the exact length requires before I buy a new one.

I've got the mechanical modulator on the transmission that is connected to the engine throttle. . . .

Do I need something different?