trans power consumption Q

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volaredon

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I have an 85 D150 with a Slant and a 727. (YES I know the difference between 904 and 727)
How much power does a 727 consume? vs a 904 (OK, 998,999, or whatever variant of the lighter duty trans that you might come across) how bout vs a stick like a 833OD or maybe even a T5?
Does stall speed in the converter play a part there?
Is the power consumption of the trans a percentage of engine output? cuz I can't see a given trans behind a 500 hp big block taking up the same percentage of overall power as it would in my slant..... Im thinking there is a "set amount" of power needed, to spin the converter full of fluid, also to turn the pump pushing "X" number of PSI of fluid pressure, spin each of the interior drums, shafts, etc that would be the same regardless of what is in front of it....
(pulling numbers out of the air for a moment here) that if it takes say 40hp to "run" the trans in a big block, that it will take the same 40hp to run the trans behind my /6....
not talking aerodynamics, rear gear, tire size, etc here. just the power to run the trans.
 
The only solid info I have is that in the '70's, Chrysler did some back-to-back drag strip testing of 727 and 904 transmissions. It was probably in Super Stock because back then the car had to have the original type of transmission in Stock. The results were that all else being equal (gearing, converter, overall car weight, etc.), a 904 was about .15 seconds faster in the 1/4 mile than a 727.

The main reasons for the results were the weight and diameter of the internal rotating components and amount of bearing and gear surface friction. Today, racers have machined 904 and smaller, lighter components to fit inside of a 727 case in applications where a 904 won't bolt up (i.e.: 904 to a big block).

I believe that the more torque an engine has, the less difference there will be between the 2. But the overall "efficiency" boils down to the same reasons: smaller, lighter parts and amount of friction surfaces.
 
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volaredon said:
I have an 85 D150 with a Slant and a 727. (YES I know the difference between 904 and 727)
How much power does a 727 consume? vs a 904 (OK, 998,999, or whatever variant of the lighter duty trans that you might come across) how bout vs a stick like a 833OD or maybe even a T5?
Does stall speed in the converter play a part there?
Is the power consumption of the trans a percentage of engine output? cuz I can't see a given trans behind a 500 hp big block taking up the same percentage of overall power as it would in my slant..... Im thinking there is a "set amount" of power needed, to spin the converter full of fluid, also to turn the pump pushing "X" number of PSI of fluid pressure, spin each of the interior drums, shafts, etc that would be the same regardless of what is in front of it....
(pulling numbers out of the air for a moment here) that if it takes say 40hp to "run" the trans in a big block, that it will take the same 40hp to run the trans behind my /6....
not talking aerodynamics, rear gear, tire size, etc here. just the power to run the trans.
Click to expand...

The increased parasitic drag on a 727 is low compared to the drag a 500 or 518 overdrive trans adds. To be upfront, I have never dynoed or tracked cars to find the differences, but I read posts from several others that lost nearly a half second when they installed them in their cars.
 
Many years ago Car Craft printed the hp needed by trans ...GM Ford and Mopar.

727 used 45 hp
904 used 18 hp
 
What happens is some dummy puts in a 727 out of a New Yorker along with the torque converter and wonder why they lose take off power. With the same stall and the kd adjusted right, it's .15 seconds slower in the quarter mile. You'll hear the same stories about the difference between a th350 and a th400, C4 to a C6 (.25)...all that info is "available" online..
 
this truck came with the 727 behind the slant..... from the factory this isn't a "transplant"
Not why I am asking, but as an aside I know this trans was original to this truck because when it starts I am getting some gear noise like it has a bad ring gear.... I found what I thought to be a good deal on a B&M Tork master converter, but ran the PK number on the side of the trans and discovered that this is a lockup version (meaning that converter that I thought was a good deal, won't work for me)
some years ago my buddy's Dad had an 81 B150 shorty van with a 318/904, that had killed 904 a couple of times, pulling his camper. His son and I convinced him to swap to a 727.never had another issue pulling the camper with the van/ but he did say he lost a little MPG driving that van back n forth to work during the week.... but who buys a B van or a pickup with mileage as #1 priority? Not me.....

I am working on a replacement /6 for my truck (more power I hope) and while I have the engine out I will be looking into the ring gear issue (it is NOT the starter, have swapped that out just to see since I have spare starters collecting dust) and depending on what I find I may go to something else on a trans rather than getting another converter.... or seeing if there might be a different converter available that would make the 727 less of a power hog..... Id much rather keep the 727 vs the downgrade to a nasty 904.... (I don't have much luck with 904s but I have never "built one up" beyond a stock rebuild before either)
Since there isn't an easy solution to add an OD/auto to a slant motor, If I swap it out, the truck just may grow a 3rd pedal for a clutch, and a shifter sticking thru the tunnel..... "but not for a while".
 
now as i have said in other threads where I talk about this truck, it will never be a "race truck". I never intended that. It will be a daily driver in seasons that don't involve snow, ice and road salt, and as such will occasionally be used as a typical 1/2 ton truck would reasonably be expected to be used..... trips to places like Lowes and Menards, and to occasionally pull a utility trailer with a ~900lb garden tractor with either a mower deck under it or a PTO driven tiller on the 3 point hitch (of the tractor)
with the original (existing) engine it barely gets out of its own way empty.... and I am trying to see what I can do with it as a /6, by not having to spend the additional $$ for a V8 trans, a V8 radiator, V8 exhaust, V8 oil pan, V8 mounts, etc that means I basically have "more" to spend on beefing up the 6...... I can always put the v8 in later. (I have a "plan B' for the /6 if I cant make it work in the truck) but they did put a gazillion slants in trucks, over the years)
 
Your buddy's dad's van's tranny had a lower first gear and probably a higher stall converter; thus more gas needed to take off. If you rebuild the slant just deck it for premium gas, it will save money on gas mileage; way more than the extra cost of premium. If not rebuilding, then mill the head. Sometimes there's room for 198 rods.
 
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