I have read a Big block 727, takes about 35-40HP to operate the transmission.
That being said what is the avg jump in MPG, going from A727, to M833 transmission?
Hang on that is nonsense. If a 727 took 35/40 hp, no slanty in the world could turn it, and there would never have been thousands and thousands of taxi-cabs that ran them.
The number needs a qualifier. Like this;
My 440 made 450 hp on the dyno, but by the time I installed it, she was down to 360 rear wheel hp at 5000rpm. See now you have a useful piece of information.
Forgedabowd hp at cruising speed.
But Yes the manual trans is good for better mpgs, that cannot be denied. But for this application, I don't see any other advantage, and in the big picture, more disadvantage.
Here's why;
To get the mpgs, the rpm has to be brought way way down. Like 2.76s are barely low enough.
With an A833, that makes the
starter gear 2.66x2.76= 7.26, so to get moving, you are gonna have to rev it up a lil to find whatever power you are looking for, to satisfy your need for acceleration. But what's worse is that at 30 mph cruising in
second gear; 30=1970rpm and even a BB is gonna complain about trying to accelerate from there;so you find yourself forced to downshift. And then the Rs will jump to 30mph =2740rpm. If the tires are hooked, this will not be Rapid Transit.
with an automatic,
you have TM, Torque Multiplication inside the TC working for you. With 2.76s; 65= 2230 rpm locked up. Your BB can make pretty good fuel mileage there, if you tune for it.
But your 727 does not have a lock up, so you want your TC to stall at less than that, so say a 2200.
your starter gear is
2.76 x2.45= 6.76, which at first glance looks worse than the A833 at 7.26. But hang on, there is an hydraulic multiplier that varies infinitely between about 1.1 to 1.8 ; depending on the Torque differential thru it. The multiplier is greatest at zero mph, and drops with speed.... unless the engine torque is simultaneously rising. So at zero mph, say the multiplier is 1.8, then 6.76 x1.8=12.17 and that will totally blow away the 833, when both start from the same rpm. 12.17 is like 4.56s for the A833.
So that is the first reason to use an automatic.The second is 30mph
Again with the 727, 30mph= 1490rpm. Of course that will be at a cruising throttle opening. That is compared to 1970 with the A833. But when you stomp it and before the trans downshifts, the Rs will rise to stall, or 2200ish which being ~2030 rpm higher than the A833. But it gets better; whereas the A833 will be stuck at 2.76 x1.92=5.30 ratio, your TC still has a multiplier in it. I'm gonna guess about 1.4, so for the 727 this is
2.76 x1.45 x1.4= 5.60.. nice, still better than the A833.
But when the trans kicks down, the Rs at WOT will rise to 2520ish, and the new ratio could be
2.76 x2.45 x 1.5 =10.14, whoa.... compared to the A833 at 2740rpm at 7.34 ratio in first.
Which would you rather have? 10.14 of course.
Second gear will get you to waaaaaaaaay past speeding so there is no point in working any more math.
The bottom line, is this; With 2.76s, and assuming no tirespin,and that both cars start at whatever rpm stall the 727 has; the 727 is gonna waste the A833 in any acceleration contest , that ends at the speed-limit. It's all in the TC.
The only handicap, is the hiway rpm can be in the neighborhood of 5% to 8 % higher with the 727, with 2.76s that could be up to about 180 rpm, depending on the TC.
So if yur sharp, you might have noticed that the TC is acting like a two-speed, with an automatically adjusting ratio in each gear, depending on the torque differential thru it; pretty sweet.
At it's most basic level, you can think of the 727 as a 4 speed, with the lowest gear(the TC) shifting itself.
If you are gonna run gears this low, you pretty much have to use an automatic else, with the A833, you will have a severe take-off handicap.
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