Currently running 727, but considering switching to 904, for the lock up converter feature. I assume the 904 lock up is electronic controlled?
What is "everything" I need to make the conversion. My car (hot rod) is driven regurlarly, and I'm thinking the lock up feature will improve gas mileage?

If all you want is lockup you can accomplish that with he 727 you have.

You need the converter, valve body and reaction shaft support (or pump asssembly).

I would use the later electronically controlled lockup valve body.

You would have to do the same on the 904 if it was. It a lockup unit.

Nice thing about the 904 you can find a low gear set that will make it scoot a little better off the line and not screw up mileage.

& it would be lighter (search to find out how much). Weigh all your options to decide on the best plan of action (time/money/effort) for the amt of gain to be had. Just for mileage there might be other avenues.

If you're making any decent power stick with the 727. 904 is a weak point especially driven reguarely.

what kind of car?

or simply upgrade to the A518... a modern 727 with OD and lockup.

On the other side a decenty built 904 could get you some HP, being lighter rotation mass and doesn't have to blow up! many racers use the 904 for that reason!

904s are not a weak point in most drivetrains if built with even minor upgrades. Also most cars that are swapped from a 727 to a low geared 904 are very noticeably quicker, I would not hesitate one bit for most cars to swap in a 904.

Lock up was totally hydraulically controlled till about 89 then they added a solenoid to de-activate it but not activate it, they still used hydraulics to turn it on but the electronics could overide it usually for cold or heavy throttle driving.

I thought so too but was proven wrong, it locks AND unlocks electrically.

Then my trans in my 68 cuda was malfunctioning, I never hooked up electricity to the solenoid ever but it positively locked up.

It is a 48 DeSoto, 1st Gen 392, 727, 83/4 open rear.

Very early lockup units were purely hydraulic control. You had no control over lockup. Many people complained about lugging, drone, growling once in lockup and the only way to change it slightly was by changing the lockup spring rate. The valve body had a lockup module on it with this spring inside.

Then the electronic lockup was developed to reduce if not eliminate the drone and lugging. It was just a simple lockup solenoid that either opened or closed a vent orifice which moved the lockup valve in the valve body.

Same design was used on the later model 4 speed overdrives, had two solenoids, one for lockup and one for overdrive. Both run by the PCM.

In all cases (3 and 4 speed) the electronic lockup will unlock and lock depending on inputs such as vehicle speed, throttle pressure ( on injected engines from TPS ), etc.

You're limited to what you can put behind the 392 without some sort of adapter.

Only trucks had the electronics. If you stick with a car one you'll have hydraulic lockup.



I've killed many 904s over the years all with 360 daily drivers. There used to be a picture here of a wrecked 904 with the parts comparison between a 727 for the weak points. I definitely wouldn't want to run one in what he has myself, I like reliability

Wish you weren't so far away. The best tranny guy near me sucks, plus doesn't want to do anything.

Speaking of electronics controlling lockup, very simple to manufacture your own controls.

You don’t need a computer to do it. Simple pressure switch, open/close switch on the throttle and you have complete control of lockup.

Don’t discount your abilities. Buy a service manual for a 1978 or later 727 equipped vehicle.
The pictures and procedures should represent a lockup trans. You can easily swap the pump and valve body, the input shaft (I forgot to mention that earlier in the post) and set up the end play.

What may be easier is find a van or pickup truck with the trans you need. Find a crashed one - at least you know the trans was good enough to get the vehicle to the scene of the accident.