I just got a 85 brat with an auto, goes good, but i was looking under it and found the mustache bar was pulled out of uni-body on one side, but the question is arent all autos susposed to have a 3.70 mine has a 3.90 rear. is that normal?

anyone know???

Yeah a EA81 with an Auto should have a 3.70 rear diff

Not only is the rear diff a 3.9 but the front diff is a 3.7!!!

Not only is the rear diff a 3.9 but the front diff is a 3.7!!!

:confused:

ah how does that work?

 

 

To much Fosters

okay you have made me break out my FSM and owners manuals

 

EA81 4wd AUTO

front diff is 3.454 :1

Transfer gear .933:1

rear diff is 3.7:1

One point that has failed to be mentioned - none of this matters in the slightest unless you actually drive in 4WD on a hard surface. But it does sound like maybe someone tried that...... replacing it with a 3.7 = good idea.

 

GD

Got no idea there - maybe I should have switched my brain on first!!!!

 

The correct ratio for the rear though is def 3.9 do not put a 3.7 on there or you will destroy the rear clutch pack.

Your cars must be different as our autos definitely have 3.9 rears

see now i am confused a little, My books said 3:70 for the front, but kinda vague on the rear, but one service man says 3:70, think that is why it has pulled out of the body, itis fixableeasy enough, but I would hate to pull it back through again. Iprobably wont be using it off road, so i may never put it into 4wd again. I have an off-road subaru for that.

There are no clutch packs in a Brat....unless its got an EJ series or EA FT4WD trans.

 

And according to some math....you'd better put a 3.70 diff back there for any 4WD action.

 

3AT Brat owners!! Whats your rear diff ratio?

Oh i forgot to tell everyone it is a 3 speed sorry,

All the ea81 autos are 3 speeds, at least 4wd.

 

fwd manuals are 5 speeds, are fwd autos 4 speeds?

There are no clutch packs in a Brat....unless its got an EJ series or EA FT4WD trans.

 

And according to some math....you'd better put a 3.70 diff back there for any 4WD action.

 

3AT Brat owners!! Whats your rear diff ratio?

What??

 

All auto transmissions have clutch packs, thats what engages the gearsets. The 3 spd autos have a clutch pack in the back that engages drive to the rear wheels. It is an identical set up to the later awd 4 speeds, only they are on/off only not ecu controlled permanent. It is possible to hook up a system to make the 3spds constant 4wd (granted the system would be completely custom and so not really worth it)

 

I have owned both an auto 3 speed ea81 touring wagon and an auto 3 speed coupe and both have had 3.9 rear diffs - but then, like I said - maybe the Australian market is different, after all our version of the Legacy RS got 3.9 rears whilst the rest of the world got 4.11.

I know there are clutch packs in the trans for the gear sets. What I dont know is there are clutch packs for the 4wd mechanism. That means that its technically an AWD trans, and you can use it in 4wd all the time for autos then. I really dont think so.

This should be the same tranny as my 3AT 4wd Turbo Coupe, and my diff is a 3.70.

okay you have made me break out my FSM and owners manuals

 

EA81 4wd AUTO

front diff is 3.454 :1

Transfer gear .933:1

rear diff is 3.7:1

 

front diff is 3.7, the first gear is 3.454,

Going off of the 87, which is EA-82, but it works for comparison,

Front diff 3.7

Front reduction .974

Rear reduction .948

rear diff 3.9

thats for the 3AT, SPFI/Carb, for the turbo the rear is 3.7 and the rear reduction is .933. so there you have it, an auto with a 3.7 front and a 3.9 rear. btw thats a 4wd GL wagon

I know there are clutch packs in the trans for the gear sets. What I dont know is there are clutch packs for the 4wd mechanism. That means that its technically an AWD trans, and you can use it in 4wd all the time for autos then. I really dont think so.

You are partly correct in saying that technically it is an AWD trans. The set up is identical to the Legacy 4 speed. The MAJOR difference is that the 3 spd has a on/off switch to control the 4wd clutch, the 4 spd has a computer to control the 4wd clutch.

 

Not sure how much you know about the autos, but basically they just use a shift solenoid on the 4wd actuator to actuate the clutch and drive the rear. The 3spd has a switch and so the clutch pack give full drive to the rear, and is a 50/50 split. the 4 spd has a computer that uses pulse width modulation to switch the solenoid. (Remember that a solenoid is either on or off and not inbetween, so a method is required to vary the amount the solenoid is actuated.) The Pulse Width Modulation uses a 12v 50hz (50 cycles per second) square wave to drive the solenoid. At 100% duty cycle the clutch is disengaged (works opposite to the 3spd where 12v engages the clutch) and at 0% duty cycle the clutch is engaged giving full 50/50 split to the rear. At any percentage of duty cycle in between a direct relationship is drawn to the rear split. At 50% duty cycle you have a 75/25 front rear drive split. The transmission ecu takes input from speed, throttle, gear selection and power/hold switch to decide on the level of split necessary.

 

It is entirely possible to design a PWM circuit that will take the inputs above and drive the 4wd solenoid on the 3spd auto which will make it just as AWD as a 4spd Legacy.

 

Sorry for the thread hijack!!!!