At 400+hp, What kills A904s?

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MRGTX

MRGTX

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My new engine puts out 450+hp/520+lb-ft at the crank. I’m planning on proceeding with the A904 but I understand this is close to the generally accepted safe range for this unit.

What is the typical failure mode for these transmissions? When do they typically suffer their failures? I always assumed that the moment the tires grab after a burnout was the killer but I don’t know that for sure.

Back before I installed my last engine swap (300hp small block) I had the local shop rebuild it with upgraded bands and clutches, higher RPM governor weight, shift kit. Maybe some other stuff? I can’t remember. It was a while back. I also installed a deeper pan. I’ve been running the cooling lines through the (upgraded) radiator but I may add an external cooler for some extra insurance. I’m pretty sure I also bought an upgraded sprag but I can’t find my receipt for that part so I’m not positive.

Not sure if this makes as difference but im running a mild 2800rpm converter, 4.10 axle. I don’t plan on running drag slicks but I will be on 200 TW summer compound tires this year.

I guess I’d like to know how to be kind to this thing and minimize the conditions that kill them until I’ve saved up enough cash for the manual swap. :)

Thanks!
 
Traction with low gears will break it. Eventually.
 
MRGTX said:
My new engine puts out 450+hp/520+lb-ft at the crank. I’m planning on proceeding with the A904 but I understand this is close to the generally accepted safe range for this unit.

What is the typical failure mode for these transmissions? When do they typically suffer their failures? I always assumed that the moment the tires grab after a burnout was the killer but I don’t know that for sure.

Back before I installed my last engine swap (300hp small block) I had the local shop rebuild it with upgraded bands and clutches, higher RPM governor weight, shift kit. Maybe some other stuff? I can’t remember. It was a while back. I also installed a deeper pan. I’ve been running the cooling lines through the (upgraded) radiator but I may add an external cooler for some extra insurance. I’m pretty sure I also bought an upgraded sprag but I can’t find my receipt for that part so I’m not positive.

Not sure if this makes as difference but im running a mild 2800rpm converter, 4.10 axle. I don’t plan on running drag slicks but I will be on 200 TW summer compound tires this year.

I guess I’d like to know how to be kind to this thing and minimize the conditions that kill them until I’ve saved up enough cash for the manual swap. :)

Thanks!
Click to expand...
nothing
 
This is what I am using..
Unfortunately John is not building any longer, but he could refer you to someone who does.. There is always Andrews Racing Transmissions in Ft. Collins Colorado, also a great builder too.

 
Just had this conversation with a friend. He races stock class and between him and his sons they have 6 cars.
All are 904's. All have hundreds of runs. Cars range is between 12 seconds to mid 9's. In 20+ seasons of racing only 1 broken tail shaft housing. He also told me a racing buddy of his has a 66 Coronet Hemi in A stock. He runs a 904.
Your chances of having failure are slim to none.Now the 904 had to be well built, nothing fancy but correctly done.
When I raced the only failure I had transmission wise was a converter stator that went bad.My 904 lasted 20 seasons for me and the new car's owner has had it for 9 years now still racing it. The car was low HP at around 280 at the rear wheels.Fear not.
 
OP, do you know what year your 904 is? They're all basically the same but there are slight internal differences between earlier and later years, especially after 1978 when they became lockup style. The lockup are generally less desirable for performance applications but like the video posted above, they can be made to work.

If you can find a way to put a wider 5 clutch front drum in your unit you will be ahead of the game, more holding power. The low gear planetary sets from later units are a popular addition as well, helps torque multiplication off the line. I have held the belief that the low gear sets were installed in the later units ('78-up) because the cars of the time were heavy and under powered and needed all the help they could get to get them rolling with any alacrity.

904s respond to buildups just like any other transmission. Use quality parts and pay attention to detail and you'll be fine.

If you believe internet hyperbole, a 904 is a tenth quicker in the 1/4 mile because it has less rotating mass than a comparably built 727. They take less power to turn.

Look up "Proflite" transmissions. Basically they're 727 cases with 904 guts. They're very expensive, semi-pro level units that use mostly aftermarket parts but the idea is still the same, less rotating mass.
 
Thanks to all. So far, this is very encouraging. Are these comments all assuming the A904 in question is built to a spec? How would the conversation change if it was a stock unit?

This trans is the one that came in my ‘73 318 car.

The car is a ‘73 Sport with a handful of weight reduction mods. Fiberglass hood, inner bumper plate pulled, ditched power steering, running a mini starter, aluminum heads/intake/water pump, aluminum wheels, aluminum driveshaft, swapped the bench for some light bucket seats. Add a few back in for the dual exhaust, extra cables/box for the battery relocation, subframe connectors/torque boxes…etc. If it started at 3,300lbs (without me in it) it’s probably in the 3,100 and change range as it sits. I’m 200lbs give it or take. Let’s just say 3,400…?
 
Right, no real help. Just, "build it right"..lol...how much money did these racers pay for their trans? A LOT.
 
Check out Andrews Racing Transmissions in Ft. Collins Co. He is all mopar all the time..
 
Hope those snarky comments weren't directed at me... never said the guy needs a fully-prepped race trans, just citing it because it illustrates the 904 is an inherently strong design that can handle most anything you throw at it. Everything has a breaking point, throw enough power at it, you'll figure out it's limits pretty quick.

And as far as "building it right" - would you rather people on forums say just throw it together with offshore rebuild kit parts without checking clearances and run it behind a race motor?

The 904 I built for my Duster was well under $1,000 even with some nicer parts and an A&A "race prepped" case. 6 clutch front drum, 5 clutch rear, billet spring retainer, deep pan, T/A FMVB... borderline overkill for a 500 hp small block in an A body. Not much else to do to it unless you start rollerizing the whole thing, using lightened sun shells, billet this and that...none of which is even remotely necessary at his current level.
 
MRGTX said:
a stock unit?
Click to expand...
Mine were stock (Different cars, engine combo's over the years)
Manual shift valve bodies and a better intermediate band, all other internals stock on mine and my friend's cars.
The only Mega 904 is the one behind the Hemi car $$$$$.
 
My 904's broke when something else let go first, like a u-joint or ring & pinion. Running mid-low 11's @ 3,540+ lbs & 1.47 - 1.53 60' times with the wheels in the air!
 
Finally somebody mentioned an actual cost with a list of upgrades!! Under $1,000, race prepped case, 6 disc direct, 5 disc forward, billet servo, TA FMVB, deep pan; EXCELLENT! What would the cost be with today's prices? $1,500? OP, do you want to spend $1,500 on parts for your trans? Let's say it will save you 1/4 second in the quarter...worth it? You can slap a cheap kit into a 727 and it's STRONGER.
 
CudaFactHackJob said:
Finally somebody mentioned an actual cost with a list of upgrades!! Under $1,000, race prepped case, 6 disc direct, 5 disc forward, billet servo, TA FMVB, deep pan; EXCELLENT! What would the cost be with today's prices? $1,500? OP, do you want to spend $1,500 on parts for your trans? Let's say it will save you 1/4 second in the quarter...worth it? You can slap a cheap kit into a 727 and it's STRONGER.
Click to expand...
ProTrans 904 and be done with it.....too many cheap asses on here that don't appreciate a car that doesn't break.
 
Yup, I'm a cheap *** allright. I build EVERY 904 and 727 with FIVE .061 Red Eagles and a Kevlar band for $600; along with drilled vb plate as per TransGo. I do the 350, 400, C4 and C6 same way. Problem is actually that too many guys are too chickenshit to do all the work needed to change over to a 727. Gimme a break, oem frictions are $2.50; Reds are $7.50 ea....$25 more per trans..big deal...Kevlar bands $22 free shipping...
 
CudaFactHackJob said:
Yup, I'm a cheap *** allright. I build EVERY 904 and 727 with FIVE .061 Red Eagles and a Kevlar band for $600; along with drilled vb plate as per TransGo. I do the 350, 400, C4 and C6 same way. Problem is actually that too many guys are too chickenshit to do all the work needed to change over to a 727. Gimme a break, oem frictions are $2.50; Reds are $7.50 ea....$25 more per trans..big deal...Kevlar bands $22 free shipping...
Click to expand...
You don't have no power
 
I'm in Southern California. This is the horsepower capitol of the world. I got fools comin at me with 7-800 horse all the time; street cars.
 
CudaFactHackJob said:
I'm in Southern California. This is the horsepower capitol of the world. I got fools comin at me with 7-800 horse all the time; street cars.
Click to expand...
I will rephrase that for you......you live in a liberal schithole and any COMPETENT trans builder would be recommending at a minimum a billet input shaft at those power levels for durability but since you make no mention of them you must be the secret sauce 99.95 out the door 904 will live at all power levels builder. I will stick with Pro Trans and enjoy running more power than that with ZERO issues.
 
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