How much power will a stock 904 handle

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carmart1975

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Hey guys it the dumb teen again i have recently came across a stock rebuilt 904 trans with about a thousand miles on it he is going to sell it to me for a smoking deal. just wondering how much power they can take i have done some research and haven’t found a good answer.
 
Meh....350, 360 maybe? Maybe more. I guess it depends on what tires and gear you have. If it has a highway gear and you put some sticky tires on it, you can break it. As long as the tires break loose relatively easily, it'll probably be ok. They can take some abuse. The two biggest things are keeping it cool and using a good fluid.
 
It depends on if they replaced that reverse wound spring in the clutch pack. It has a tendency to unwind and just gum up the whole works. If they replaced it I'd say you could go to 400 horsepower. If they didn't replace it I'd say you could go to 200 horsepower.
Just my humble opinion which I highly value.
 
“IF” it is truly rebuilt then 350 hp or more even stock, but nothing lasts if beat on.
 
Weak link in the 904 is the stamped drum. A billit drum will greatly extend the power capability
 
I ran JW Trans. built 904s in my Dart for years without any trans parts breaking due to outright fatigue. The few trans failures I've had were due to driveline failures first resulting in broken sprag or broken trans cases. Best numbers 11.14 @ 117 with 1.47 60' @ 3,540 lbs footbraking. Usually detuned a tenth or two. * 2.74 low gear set, reverse pattern manual valve body, 4.2 lever, the good red(?) clutches, probably had some oiling modifications & Amsoil SuperShift. Raced often, serviced 2-3 times a year and freshened every 3-4 years.
 
They will handle plenty of horsepower, if built correctly. I have been 9.90 et with a 3200 lb AMX, and my buddy has been 9.60 with his 68 Dart. I put over 300 runs on the AMX with zero issues.
 
It depends on if they replaced that reverse wound spring in the clutch pack. It has a tendency to unwind and just gum up the whole works. If they replaced it I'd say you could go to 400 horsepower. If they didn't replace it I'd say you could go to 200 horsepower.
Just my humble opinion which I highly value.
Where can this reverse wound spring be found?
I have an a500 here to build, and heavy duty parts are proving to be fewer and farther between on a 904/500 vs a 727/518.
I've never built a race car but I've seen plenty of 904s die behind stone stock 140-150ish hp engines over the years without any form of abuse...id be interested in hop up goodies and tips/ advice myself.
 
Don, look over on Moparts in the transmission for sale section. There's a guy that sells billet servo's, band struts and stuff. He has some kind of reverse wound accumulator spring that may be what was mentioned above. MrRandyJ is his name and you can't miss all the shiny billet parts in his ads. It seems to be reasonably priced too, for what it is.
 
OEM cars are usually built with compromises leaning towards "street manners", not meant to be "beat on" and with "comfortable" shifts. But there are different "duty" levels of transmissions. (i.e.: 904, 998, 999) I've always heard that heat is the most common cause of transmission failures, which I believe applies mainly to street & dual purpose applications. I was fortunate enough to get a '72 340 Challenger as my first car. Not sure if it was totally stock or just slightly modified at the time, but I went through three stock HD 727's in a year because I abused it on the street and just didn't know any better. It wasn't parts breakage, it was mainly band & clutch slippage. The local trans shop finally put in a shift kit and it made an obvious difference. Why? Because quicker, firmer shifts reduced overlap and slippage which generate heat. But there are other considerations also. Firmer shifts from a shift kit or aftermarket valve body are harder on bands & clutches which aren't designed to be hit much harder. Those should be upgraded also. Trans fluid is also a factor. Many have used Type F trans fluid because it's supposed to help the bands & clutches grab & hold better than other fluids. Performance synthetic fluids can also helpful because synthetics tend to handle more heat before starting to break down. Then there are the usual things regardless of application like a good trans cooler, higher capacity pans and regular servicing. There are little miscellaneous mods that trans builders can do to increase the durability as well, just like tweaks to an engine - clearances, fluid pressure, opening or restricting oil flow, etc. A proper torque converter matched to how the vehicle is driven is also important because it generates the most heat. You want an efficient converter because the more flash/stall it has, the more heat is generated. "Regular", non-lock-up converters don't form a solid path of power to the rear wheels, so there is always slippage there. But the converter has to compliment the cam and rest of the combo too, so they all work together, in the same range. It's difficult to say how much HP something can handle. I think it mostly boils down to how long a transmission built to a certain degree with certain parts can live behind x-amount of torque or HP. No simple answer. I'm no trans expert, just trying to provide a better perspective. Advice from this forum and from reputable trans builders will help in finding the right build for your needs. I was a "dumb teen" too, but eventually learned a lot, often the hard way. But there is always something more to learn. I wish the internet and these forums were around then. You're on the right track.
 
My first car was a 76 charger SE (a Cordoba with different grill and taillights) with a stone stock 318 aside from an intake and carb from the car's original 360. We swapped that 318 in at dads insistence before the car was handed to me.
When the engine got swapped dad took the 727 out of my brother's wrecked 74 charger to a trans shop for a rebuild so it was "fresh" along with the 40k original mile junkyard 318 dad bought to go into it. Dad drive that car as his work car for a few years like that (after the 318 swap) before it was handed down to me.
That charger got hit in the parking lot at work so I bought a 75 cordoba to swap everything I could into it I could from the charger. Then a 79 ramcharger. I couldn't keep a trans in that Cordoba to save my life for more than a couple of months at a time until I quit with junk 904s and put a 727 in that car too. Both the charger and the cordoba were originally 360/904 equipped.
 
Surprisingly the best 904 I ever had was the one in my 87 Dodge B 250 van. That was the most trouble free 904 of the many I have owned.
My 78 fury has one in it too, so far so good (but hell, that car has only 56k miles) but is starting to show issues from age... Hardened seals within the clutches, etc. That one I am gonna overhaul and put back in but not without a few upgrades.
 
Where can this reverse wound spring be found?
I have an a500 here to build, and heavy duty parts are proving to be fewer and farther between on a 904/500 vs a 727/518.
I've never built a race car but I've seen plenty of 904s die behind stone stock 140-150ish hp engines over the years without any form of abuse...id be interested in hop up goodies and tips/ advice myself.
What we have here is a reverse wound direct clutch pack spring for the 904, 998, 999, 500, 42RH, 42RE, 44RH, 44RE Dodge, Plymouth, Chrysler, Jeep, or AMC transmission. The stock spring in the direct clutch drum loses its tension at high RPM and causes the direct clutches to drag in 1st gear. This spring is wound in the opposite direction which eliminates that problem and keeps the clutches from burning up and also helps eliminate 2-3 shift overlap. This spring should be used in any transmission that will see 5500 RPM or more. This is a direct replacement for the stock spring, no other parts or modifications are necessary. These springs have been through a shot peening process that drastically increases cycle life and they are 100% made in the USA. Please ask any questions before purchasing. Check my other listings for billet dual ring accumulators, billet 4.2 and 5.0 levers, HD band struts, 904 billet rear servo kits, billet speedometer hole plugs, and billet 727 front and rear servo kits. Thanks for

REVERSE WOUND DIRECT CLUTCH DRUM SPRING FOR 904 500 42RH 42RE TRANSMISSIONS
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They will handle plenty of horsepower, if built correctly. I have been 9.90 et with a 3200 lb AMX, and my buddy has been 9.60 with his 68 Dart. I put over 300 runs on the AMX with zero issues.
But I bet that was not with a stock 904, and that was what the question was.
 
Yeah, some of you yahoos are missin the EFFIN point here. He said STOCK 904. You ain't gonna run 11s LONG with a stock 904 so lets quit bein stupid.
 
a 904 will handle a warmed over small block if it's been rebuilt properly. if it's got good parts and a shift kit it'll likely be fine for a long, long time. if you service it regularly.
 
My dad's stock 904 went 12s, my brother's stock 904 went 12.60, and both could have gone 11s. So put a stock 904 behind a stock engine and not worry about it
 
i have recently came across a stock rebuilt 904 trans with about a thousand miles on it he is going to sell it to me for a smoking deal.

i guess it really depends who rebuilt it and if they rebuilt it correctly. it coud blow up behind 100hp or survive years at 500hp,
 
And stock tranny's is what I've talked about on my experiences.
Like rusty said "stock won't last long" (or very many trips down the track) under those conditions.
I have an inquiry to Randy for a package price for 1 of each of his products
Accumulator, both servos, a spring, and billet front band anchor and strut. Some of his prices struck me as "not bad"while others seemed high.
I seem to be able to find the rest besides the spring and the rear servo "around" but 904 "beef up" parts in general seem higher than what I paid for equivalent 727 ones. And the rear servo has to be had " piecemeal" instead of "all in one" as it comes for the 727.
 
A related question to the original question of this post that I've (tried) to ask and can never seem to get a good answer for is how much power does a 904 consume to run as compared to say an 833, 833od, a 727, and/or an a500 with everything else being equal?
 
A related question to the original question of this post that I've (tried) to ask and can never seem to get a good answer for is how much power does a 904 consume to run as compared to say an 833, 833od, a 727, and/or an a500 with everything else being equal?
The best info I've heard was from testing Chrysler did between a 727 and 904 in the '70's. With both transmissions and their respective converters built to the same specs and the vehicle weight adjusted for the different transmission weights, the 904 was about .15 seconds faster in the 1/4 mile. It may have been in a Super Stock car. Apparently the benefit came from less rotating weight due to smaller parts and less friction due to smaller friction areas.

On a related note, it's my understanding that 904's are less prone to blow up like a 727 because of smaller, lighter parts, like the drum. I had one or two 727's "grenade" in my early years but never had or saw a 904 do it.
 
Shift kit, deep pan and aux cooler will go a long way towards making a stock 904 last. The one in my Duster is a bit of a mystery, rebuilt with supposedly some upgraded hard parts but the guy I bought it from circa 2010 didn't know much about the details. FWIW I put about 35k miles on it behind a 300 hp 360, about 10k miles behind its current 360 at high altitude (~370 hp) then another 3k or so since moving to near sea level (~450 hp) about a year ago. I put a lightly-modified stock late-70s valve body in it when the original one cracked a few years back, once the power and torque jumped up from the altitude change I felt like the 2-3 shift couldn't keep up so I did a TransGo TF-2 kit on mostly "max" settings, works great now. Full rebuild would of course be ideal but I just need it to hold up until I have the funds for a TKX 5-speed swap.
 
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