I ask because I have a D150 with a slant 6 and a 727. The /6 guys are all saying that the 727 saps too much power to run vs a 904.
My problem is that I have had many of both a 904seems quite weak, I don't have much luck with them.
Being that I have the 727 already it seems like a bonus to me, many are saying it is really a curse.
I have heard 15% to 25% I have heard "about 100 hp" which can't be because that is about All my stock slant makes. If that were true the truck would not be able to pull itself at any speed above a crawl.

I am going a different direction with this one, previously on past-owned vehicles, I did the typical "yank the slant 6 and put a V8 in its place" but I am experimenting with building up another slant 6 for this one, to see what I can get from it.

Via a google search I came upon a discussion on trans power consumption on a Dodge charger forum of which I am not yet a member.
I did recognize some names in that thread over there that I see here also. John Kunkel among others. Hopefully he sees this question posted here.
While I definitely realize that I will most likely have to overhaul this trans just because of it being 35 years old if nothing else. I know they used ALOT of 904s in half ton trucks as well but I'm not looking at going that direction. If I swap to a different trans it will be something with an OD so being as how I have never seen a 500 or 518 cast with a slant bell housing, it will probably be converted to a manual.
But for now the 727 stays. I may look into some other torque converter to slide in when I put my fresh engine in, and possibly some sort of a shift kit to hopefully improve on (cut down on) the amount of power that the trans robs, but definitely not looking to do anything as crazy as putting 904 guts inside as many racers do, this is a truck, and 90% of the time it will just be a daily driver, but I do occasionally want to pull my pop-up with it or maybe my utility trailer with a garden tractor on it.
Any ideas?

Playing devil's advocate here......with all else being equal how much more zip would I gain and/or mileage (which is not my #1 goal here btw, but had to ask) what would I gain by going to the weaker trans?

How much does converter choice have to with the amount of power lost/consumed by the tranny?

Another thought; (and was brought up in the thread in the Dodge charger forum)
Can I assume that a given trans will absorb the same amount of engine power if the engine ahead of it makes 100 hp or if it makes 300 hp? (For that last part let's assume for a minute that I could directly bolt up a bigger engine to this same trans)
I have heard that it takes 40-50hp to maintain a steady speed going down the road. Most HP above that that isn't eaten up by the drivetrain, is most useful in getting up to that speed at a reasonable rate.
But if my trans takes "100hp" to run and 40-50hp to maintain speed, I don't see how it is possible to even get the truck moving,and up to speed as there's 150hp right there, and supposedly stock (current) slant 6 is only good for 100hp at best "it don't compute"
150 is a common hp rating I see on even a 318 after they went to"net" ratings. So that being said, even a 318 wouldn't have enough to get the truck moving and keep it there. So those power consumption numbers can't be true.

Isn't the excepted loss hp number 20% from flywheel to rear wheel?

Many, many years ago I read the 727's were pulling ~ 45 hp, and the 904's ~ 25 hp. In my '82 Ramcharger 318 I had A998 w/o troubles. Later I went to a '91 360 with the A998, later I went with an A500 due to swapping in a 9.1/4 diff w/ 4.10 gears. Never broke any of them.

Short of a dyno there is no steadfast answer here. It varies with HP in and RPM run. Steady state versus accelerating.

Lots of variables for a one size fits all answer.

727 is much heavier than a 904 so, there is that

Chrysler did some testing in the '70's between a 727 and 904 transmissions in drag racing. With everything else being equal, including converter specs, a 904 was about .15 seconds faster in the 1/4 mile than a 727.

Nice thing about a 904 is that you can get a 2.74 low gear from a later 904/988/999 even if it's a lock-up trans.

And that is constant whether behind a weak, stock 318, or a Race Hemi? still 20%? an engine (using stock published specs as I can remember from over the years, but the variation given youll get my drift) a 150hp 318 takes 20%..... a 425hp 426 Hemi also takes 20% of its power to run the 727... so a 318 would use 30 hp and the Hemi would take around 85 hp to run the same transmission? somehow that doesn't seem right.
and mr "300".... I never really "broke" anything with a 904, they just seemed to "burn up the frictions" at what seems premature usage, to me. and definitely more often than any 727 I have ever had.....

A flat % as noted is misleading, as noted: the same trans shouldn't require more power to run with different engines.
I think it's a set HP/TQ level, but I can't recall a trans-dyno test to establish precise data.
There's no doubt some power required to drive a manual trans as well.
Seems pretty well established that a 904 consumes less power than a 727, and that the 727's beefier nature makes it more bulletproof.
As my drag car went from under 600HP to about 640 or so, its basically-stock 904 went through clutches & bands more quickly.
But to be fair the idiot driver (me) left in 2nd gear a few times.
Upgrading to a CRT 904 and keeping my hand off the shifter at launch has fixed those issues.

Just a couple comments, first about cruising horsepower. If you have a vehicle that is getting 20 mpg at 60 mph, you are burning 3 gallons/hour. A gallon of gasoline weighs about 6.3 pounds so you are burning about 18.9 lbs/hr. A reasonable estimate for BSFC is 0.5 lb/hp-hr so you end up with about 38 hp. As to a TF "consuming" power, where would that due to? Hydraulic losses due to the pump and thermal losses due to the converter. What are those? I haven't a clue but I can't believe figures in the 50-100 hp range. Even the old Fluid Drives weren't that bad!

You may get different results based on which Wallace calculator you use. But on one of them, I was getting 20-25 HP difference will make about a .15 second ET difference in the 1/4 mile in an 11.- second car.

I don't think different converters use different amounts of power. They determine how efficiently the power is being used based on their stall/flash/slippage configuration in relationship to the engine's power curve and the application it's being used in.

Check his response here:

JK Response linky

These numbers are from an article in Car Craft Magazine.

Powerglide_____18 hp
TH-350________36 hp
TH-400________44 hp
Ford_C-6______55-60 hp
Ford_C-4______28 hp
Ford_FMX______25 hp
Chrysler_A904__25 hp
Chrysler_727___45 hp

Sniper above mentioned steady state vs accelerating. Changing the rate of acceleration on an engine dyno will effect the measured power output. The quicker the RPMs come up, the more power is lost. On the strip, the greater rotating mass of the 727 may cost 30 hp, but at a steady 2500 rpm the difference may be minimal.

Car Craft isn't know to be overly scientific in it's procedures.

I can pretty much guarantee a 727 isn't sucking 45 HP out of a 100HP slant six.

Don, put the 727 in it and tell people it's a 904. That way everybody happy!

If your concerned about how much power a 727 may take away from your slant 6, after you install the 727, add more power to the slant 6. Problem solved!

If you had both transmissions in the same vehicle, I suspect the 904 would feel faster, the lower 1st gear and the smaller rotating assembly would both be something you could feel, if both were in the same vehicle. To compare the two while in different vehicles, it would be pretty hard to pin any difference on the trans alone. While riding in a vehicle, I doubt there is anyone that could tell you which transmission was in the vehicle by the ride alone, without looking at it.

As far as breakage, I've broke them both. from my perspective, the weakest point on a 904 seems to be the front pump/torque converter hub. Every failed 904 I've had, had a cracked converter hub and a damaged pump. Gene

I don't have to put the 727 in.... the factory did that for me on the assembly line already!

More curious I guess, than "concerned". just because of what I'm seeing on a couple of other forums I am on.....

Back in the 80s/90s, I replaced more dead 904s with 727s than I care to try to think back and count. Most of the dead 904s were just "burnt up" clutches, steels and seals..... , not "hard part" carnage//// though I do remember a few cracked converter hubs.
Back then, one of them that I 727 swapped, was in a 318 powered '81 B-150 short wheelbase vans. It belonged to a friend's dad and he pulled a pretty big camper with. He burnt up the 904, had it rebuilt. (by who I don't remember.) I did the R&R and he took it for a "bench rebuild" somewhere. We added a pretty big aux fluid cooler along with the rebuild. A couple years later, it blew again. This time I talked him into a 727. That trans never blew again. My buddy's dad had bought this van brand new, and it was his daily driver for work in between camp trips. As I remember, I believe he said at one point he "lost" a MPG or maybe 2, since the 727 went in on daily driver duty.

among others that I had done 904-to-727 in, were a 76 Charger, a 75 Cordoba, an 85 Diplomat, and then some. I did do a few 904-904 swaps back then too.... I did notice that stock converters in 904s seemed "whinier" than 727s did, especially when driving slow as you would going up+down rows at a store looking for a parking spot where you would hold 1st for a while..... I did notice the difference in take off power between 904s between ones with and ones without the lower 1st.....

and Gene..... I have another slant 6 in the works, that is getting that "Tim Taylor" "more power" treatment for that truck, as we speak.

I've killed at least 2 904s more then just clutch wear and also end up going to 727. These were always behind 360s. I never had much problems with 318s.
For a slant 6 if you already have the parts to switch to a 904 that's the route I would go because you probably would notice a bit of a difference with it.

I run 904's in my junk. The only one I've wounded was a pump gear, and it cracked the snout on my PTC converter. 4000 on the chip on the brake could have something to do with it.