904 vs 727?

volaredon

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as title says. this is in my 85 D150 w/a slant. original to the truck was (surprisingly for the mid-80s) is a lockup 727. Shifts awesome, except lockup doesn't.... (I know its a lockup version because I ran the "PK" number which says it is)
I was surprised to see the 727 in there. Yes I can tell the difference, not one of those guys that think "every" Mopar trans is a 727, haha)
I have been working on a "beefed up" slant, to replace the weak kneed original as funds have allowed. I'm not yet driving this truck though "I could," as is. Just tired/doggy. Starts easy, runs like a sewing machine.
I have never had very good "luck" with 904s. I have swapped 727s in place of 904s in many of my past vehicles. even in otherwise "stone stock" vehicles. after the 904 grenades.
Yet, I have had several tell me to downgrade to a 904, even though it is in a truck.
I have so far resisted. May need to swap converters anyway because I'm expecting to see ring gear tooth damage once I get the engine out (you should hear the starter working, you'd say the same thing)
Thing is I have had a /6,904 fall into my lap recently without looking for one. I've never before downgraded to a 904 from a 727. This 904 was said to be a "good core", I know its converter should be changed too, snout on it is all rusty from sitting under a lean-to, for IDK how long.
I know how to overhaul them, I've done more than a few. And would, as a matter of maintenance, (especially with its unknown history) before I use it. (though when I got fluid all over me when I put it onto my trailer, fluid didn't seem burnt) This 904 is non lockup, splines go all the way to end of input shaft. I haven't run the PK number yet to see what its original application was.

Might this 904 have the lower 1st, even though its non L/U?
what converter options are out there, other than exact same stock replacement for a 727 L/U? If Im gonna have to replace the converter, Id like a little "stall" (not crazy) and to see if I could maybe get a 10" version, I believe the one in it now is 11".
Was there a factory "lo stall" and "hi stall" for the L/U 727 or were they all the same in those years?
If there were a "lo" and a "hi" from the factory for a L/U 727, how low was the "lo stall" and how high was the "hi stall" version?
which trans would you run if this was yours? truck is 2wd, long bed, 3.21 geared.
truck destined for 3 season daily driver, occasionally (2-3x a year) will be used to pull a light (~2000 lb total with my "gear") pop up on mostly flat, Midwest roads.
I know I might gain some "get up and go" off the line with the 904, but what would I lose in durability? This is NOT a "race" truck!
How much more available engine "power" is eaten up by a 727 vs a 904?
 
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QuickDart360

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Just leave the 727. If ain't broke don't fix it!! I run a 727 in my dart sport 360. It has served me very well. Used to have a slant 6 and 904. That set up also did fine. In this case you already have the 727. Save your coin and leave it be.
 

318willrun

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If you are never going to race it, and still use it for a truck to pull or haul things, no need for anything but the 727. IF the truck is going to be somewhat of a "street rod" and not be used for a truck at all, then you have something to think about. This is what I'll say on 727 vs 904:
  • Factory stock, 727 is stronger hands down.
  • 904's are way stronger than you give them credit for.... mine is stock and has gone 13's and now 12.9's with no gears to help out (2.45's - 3.23's) and still shifts great after 5 years
  • 904's can be built to take big HP, and it's proven every weekend on the track!
  • 904's are smaller, lighter, and faster than 727's
  • converter availability is widely at your finger tips for both tranny's
  • 904 is considered an upgrade due to the advantages I listed above.
  • 727 is a great transmission as well, and can be built for Big HP as well.
 

volaredon

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So just how much more power does a 727 zap up to run vs a 904 with all other aspects of the vehicle unchanged???
 

volaredon

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Well I dunno in terms of tenths, never taken a vehicle down a 1/4 mile track.
I know that things like stall speed, converter diameter and probably other things factor into # of HP/ amount of torque list to just running an auto trans. But given a common, stock setup of each,
(Cuz I know that modification throws everything out the window) forget about rear gear, aerodynamics, altitude, etc for a minute,
With all else being equal (same vehicle, same gear, same tires, same driving conditions etc) just a swap between a 904 and a 727, how much difference is there? Is it a flat number of HP and torque or does it vary?
Just putzing around town, vs steady cruise on the road, vs stomping it as if to pass, etc.... There is different amounts of power available from the engine under varying conditions, but I'm thinking it still takes the same amount to pump fluid, fill the converter, keep passages within full "as necessary", under most conditions. How ever much that may be.
 

oldkimmer

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Well I can say that u never know what’s been done to a tranny after all these years. I should have a wire running to the back on the drivers side of the tranny for l u. Kim
 

volaredon

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Well I've seen that but this one under the truck don't have that. The 904 in my 88 d 100 did.
According to the PK number on the one in the truck says it was used 81-85, part of the number is also a date code and I remember at the time I wrote it down and looked it up it was well in range of the truck build date, leading me to believe it is original one to the truck.
 

RustyRatRod

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How many times do we have to have this discussion? You're obviously not going to be happy until you swap to the 904, so do it.
 

dirty white boy

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lotta folks like the 904 cause thay dont explode in the burn out box! but you ant worried bout that, a truck used as a truck can benefit from the stoughter 727,.. but that /6 alt not hurt ether!! 904 will spin up a lil quicker,.. and would be better from a mpg point of view! money wise, you got both, so buying a converter or a drive shaft is what it comes down too....its a choice only you can decide correctly on!
 
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Jadaharabi

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If it takes 12% of the power to drive the 904 and 18% of your power to drive the 727 then you have a 6% loss between the 904 and the 727.
If I remember correctly in my old age the 85 and 86 lock up Transmissions used to fail pretty much straight from the factory. What the dealership in our town used to do was plug that lock up on the valve body and do away with the lock up function. Then you just had first second and third like a normal 727 or 904.
There was no wire for the lockup it was hydraulically done through the valve body.

And no there was no low gear set for the 904 back in 85. However you can take a set and put in that transmission from a more modern transmission.
 

volaredon

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Really? I thought the low gear set in the 904 started around 79-80ish.
And I thought that it actually was used in some of the non L/U versions too. Maybe not?

How'd they plug the L/U? As I remember it was a small added on valve module on top of the VB with a ~5/16 ish diameter "S" shaped tube that plugged into the main VB but it's been a while since I have been inside of one.
 

volaredon

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lotta folks like the 904 cause thay dont explode in the burn out box! but you ant worried bout that, a truck used as a truck can benefit from the stoughter 727,.. but that /6 alt not hurt ether!! 904 will spin up a lil quicker,.. and would be better from a mpg point of view! money wise, you got both, so buying a converter or a drive shaft is what it comes down too....its a choice only you can decide correctly on!
Really? What's different in a 904 vs a 727 where the lighter duty trans wouldn't explode? From what i remember they're pretty similar inside except everything is just bigger within a 727.
And money wise I'm gonna need a converter either way, and I'd definitely have to go thru the 904 since it was acquired as a "good core" with history and whether it works or not is unknown.
Only thing within the 727 that's got me wondering is knowing that it is (was) a L/U version and that part isn't working though it works great otherwise.
And why that is the case, what it would take to get it working, whether it would be worth messing with to get that part of it working.
I did change the fluid in the 727 when I 1st got the truck running, had to because the pan gasket was leaking like a sieve. The old fluid was clean and red, no crap laying in the pan.
 

dirty white boy

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Really? What's different in a 904 vs a 727 where the lighter duty trans wouldn't explode? From what i remember they're pretty similar inside except everything is just bigger within a 727.
And money wise I'm gonna need a converter either way, and I'd definitely have to go thru the 904 since it was acquired as a "good core" with history and whether it works or not is unknown.
Only thing within the 727 that's got me wondering is knowing that it is (was) a L/U version and that part isn't working though it works great otherwise.
And why that is the case, what it would take to get it working, whether it would be worth messing with to get that part of it working.
I did change the fluid in the 727 when I 1st got the truck running, had to because the pan gasket was leaking like a sieve. The old fluid was clean and red, no crap laying in the pan.
something bout the sprag spinning astronomical rpms and coming apart,..do a search in the transmission section,..happens when drag racing not in regular duty use....like i said,..not sumptin you need to worry about....only mentioned it as why folks might prefer the 904....
 
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Locomotion

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It's the drum that explodes. They are bigger in a 727 and stress is more further out. Sprag can go bad in either one when you break a R&P or u-joint on launch or screw up your burnout. In 40+ years I never seen a 904 explode like a 727, including in my race cars. Chrysler tested 904 vs 727 in the '70 in Stock & SS cars. 904 was faster by .15, all else being equal. Low gear in later 904's is nice in certain applications.
 

Demonracer

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I swapped out my 727 RMVB with trans brake to a 904 because of the weight difference & less reciprocating mass, but my application is race only. I have no experience with the differences on the street. If it runs. drives & shifts well as it is, leave it alone & spend your money elsewhere on the truck.
 

volaredon

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How many times do we have to have this discussion? You're obviously not going to be happy until you swap to the 904, so do it.

not quite. I've dealt with both of these trannys over the years// both personally, (I am 54 and have yet to own any other brand of vehicle than Mopar) and in my living turning wrenches. Personally, I have heard many brag about how great a 904 is// yet I cant seem to get the life out of them behind a stock 318... driving how ~80% would call "normal" driving. I know either can be "built up".
and the decision at the assembly line level, as to which vehicles got which tranny is confusing. 904s in something like a 3/4 ton van with a 360, that gets USED as a van, or in a late 70s B body wagon, never made sense to me. and here i have a "lightest of the light duty bare bones pickup" that a few years earlier would have been called a "MISER" package, with a /6, with a 727.

I have said this before, but sometimes I post things for the "good" of others"> Rusty and me had talked about this subject a while back, in a different thread.
I thought it might be good in a more appropriate section ("tranny and drivetrain") as a standalone question.

alot of good points have been brought up. like the "blocking off of the lockup" in those years, when they presented issues. Id like someone to expand on that. If only for possible red flags to beware of, next time I pull the pan. Like how to "undo" what was done, if I so decide.

also the rear drum explosion thing.... common sense would say that the bigger heavier built parts would be thought to "hold up" better.
But I'd like someone that could xpand on 1 thing (at least) the "IF a 904 eats 12% of engine power, a 727 will eat 18%". is this a hard and fast rule? are those accurate numbers? or pulled out of the air? Do all 727s eat half again, as much power to run vs a 904?

Has anyone (or would it be possible) to somehow dyno a 904, and then run a 727 on same dyno, and measure power consumption? I cant be the only one who'd be interested in that sort of info.

Is there a set HP number it takes to run either trans, before the rear wheels get "any" power, to push the car ahead?
Granted, I know this would be impossible due to bellhousing bolt pattern differences/ but, suppose for a minute, that the same Mopar 3 speed auto, "could" bolt up to a /6, or small block, or big block. We're using same converter. What HP does each trans take to run?
pressures are the same, inertia is the same, pump load is the same, if it takes (lets say, number pulled from air) 30HP to run a 727 behind a /6, then it should take the same 30HP, to run the same 727, behind a 500hp big block. The percentage of power loss to the trans would be less with the bigger engine, 100hp /6, divide by 30 hp to run the trans is 30% of available engine power. while 500 divide by 30, is (guessing) about 7-1/2% of available power. but still 30HP. (or whatever the real number is)
What effect does stall speed have, on power eaten by a given trans? This is before gearing, cams, manifolds, various carb differences, aerodynamics, etc comes into play. how bout billet accumulators and servos vs factory plastic ones? or various trans mods?

Inquiring minds want to know.
 

RustyRatRod

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not quite. I've dealt with both of these trannys over the years// both personally, (I am 54 and have yet to own any other brand of vehicle than Mopar) and in my living turning wrenches. Personally, I have heard many brag about how great a 904 is// yet I cant seem to get the life out of them behind a stock 318... driving how ~80% would call "normal" driving. I know either can be "built up".
and the decision at the assembly line level, as to which vehicles got which tranny is confusing. 904s in something like a 3/4 ton van with a 360, that gets USED as a van, or in a late 70s B body wagon, never made sense to me. and here i have a "lightest of the light duty bare bones pickup" that a few years earlier would have been called a "MISER" package, with a /6, with a 727.

I have said this before, but sometimes I post things for the "good" of others"> Rusty and me had talked about this subject a while back, in a different thread.
I thought it might be good in a more appropriate section ("tranny and drivetrain") as a standalone question.

alot of good points have been brought up. like the "blocking off of the lockup" in those years, when they presented issues. Id like someone to expand on that. If only for possible red flags to beware of, next time I pull the pan. Like how to "undo" what was done, if I so decide.

also the rear drum explosion thing.... common sense would say that the bigger heavier built parts would be thought to "hold up" better.
But I'd like someone that could xpand on 1 thing (at least) the "IF a 904 eats 12% of engine power, a 727 will eat 18%". is this a hard and fast rule? are those accurate numbers? or pulled out of the air? Do all 727s eat half again, as much power to run vs a 904?

Has anyone (or would it be possible) to somehow dyno a 904, and then run a 727 on same dyno, and measure power consumption? I cant be the only one who'd be interested in that sort of info.

Is there a set HP number it takes to run either trans, before the rear wheels get "any" power, to push the car ahead?
Granted, I know this would be impossible due to bellhousing bolt pattern differences/ but, suppose for a minute, that the same Mopar 3 speed auto, "could" bolt up to a /6, or small block, or big block. We're using same converter. What HP does each trans take to run?
pressures are the same, inertia is the same, pump load is the same, if it takes (lets say, number pulled from air) 30HP to run a 727 behind a /6, then it should take the same 30HP, to run the same 727, behind a 500hp big block. The percentage of power loss to the trans would be less with the bigger engine, 100hp /6, divide by 30 hp to run the trans is 30% of available engine power. while 500 divide by 30, is (guessing) about 7-1/2% of available power. but still 30HP. (or whatever the real number is)
What effect does stall speed have, on power eaten by a given trans? This is before gearing, cams, manifolds, various carb differences, aerodynamics, etc comes into play. how bout billet accumulators and servos vs factory plastic ones? or various trans mods?

Inquiring minds want to know.

Trust me. I understand your curiosity. I gotta tell you though, even in an A body, if I had a good working 727, there's no way in the world I'd swap. Much less a truck. I just wouldn't do it.
 

volaredon

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Trust me. I understand your curiosity. I gotta tell you though, even in an A body, if I had a good working 727, there's no way in the world I'd swap. Much less a truck. I just wouldn't do it.
Well I got a volare too that I hope to finish some day
 

RustyRatRod

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I put a 904 behind a 408 Six Pack motor. That should suggest something?

Well sure. All I'm talkin about is something already established with a 727. Something gettin built from scratch might be different.
 
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