4EAT, how does it work?

[70SS454LS6]

I've read many detailed descriptions of how the 5MT works, 90%-10% front to rear normal torque distribution, 50%-50% torque distribution with max front slip, viscous coupling, strictly mechanical, no electronics.

But I've yet to read a similar description of how the 4EAT works. What is normal 4EAT torque distribution and max torque distribution? Is it like 5MT, also 90%-10% and 50%-50%? What do the electronics do to control the 4EAT box or clutch pack? What are the mechanical elements of the 4EAT box and how do these elements interact to transfer torque front and rear?

And the key question, which box is better, 5MT or 4EAT, for street driving in all weather conditions, dry, rain, snow?

I'd appreciate your posting URLs that describe the workings of the 4EAT.

Thank you.

[Andyjo]

Well, i've heard that the 5MT has a standard 50/50 split (roughly) all the time.. where as the 4EAT has a 90/10 split, until the computer tells it not to have one... or if you just throw a switch on the duty solenoid C you can lock up the center clutch pack (read center diff mech.) and get a near 50/50 split.

How it works... well there's a signal from the computer, and a pulse width modifyer in the computer, and it sents that signal to the solenoid which opens & closes, moving hydrolic fluid all around. I might actually be able to 'borrow' an occiliscope from the school to test out the signal one of these days

other than that... what do you need to know?

[jamal]

the center diff has clutch packs that can lock up to varying degrees. The lowest is sending 10% of power back, and it can lock up to the point where it sends 50% back. So even when it's sending as much as it can to the rear, there's still some slippage. That also allows it to turn without binding up and stuff.

 

It's kind of wierd visualizing how 50-50 split works. If it were capable of sending 100% of power to the rear, it would be as if the center was locked and the fronts were on rollers or ice or something. In that case the rear wheels would obviously be the only ones doing any work to move the car.

 

There are like 3 or 4 endwrench articles on the 4eat. Check them out.

[NOMAD327]

There are two speed sensors built into the transmission housing that tell the transmission computer what the front and rear axle speeds are. The computer favors the 90 front 10 rear arrangement normally. Should the front axle speed be higher, the clutch will be engaged more and more to shift power to the rear axle. 50/50 is the maximum split, which is essentially as though there was no center differential. As there is no limited slip rear end on most of these cars, you can still have one front and one rear wheel spin and not be able to move. In practice, I believe the 90/10 split gives a very good driving feel, and 50/50 will take you through anything you will likely ever encounter in highway driving.

[idosubaru]

the center diff has clutch packs

 

the 4EAT doesn't have a center diff. the clutch packs serve a similar function. 90/10 normal and 60/40 at most is the break down i've seen.

 

There are like 3 or 4 endwrench articles on the 4eat. Check them out.

amen, check out endwrench.com or whatever it is, it's Subaru's official documentation website or whatever you want to call it. excellent information on the details and workings of the 4EAT.

 

for snow/rain and stuff it depends. in general the manual trans are better. the 4EAT doesn't add any traction until it detects slip, well who wants to wait until then to gain traction?

 

that being said the 4EAT is nearly as good if not better than the manual transmission if you install a switch to control the clutch lock up. when it's snowing, you "lock" it and have your front and rear wheels "locked" together. in the models that offer 5 speeds without any differential lock, the 4EAT with this switch installed would be better in snow and off road driving.

[OB99W]

amen, check out endwrench.com or whatever it is, it's Subaru's official documentation website or whatever you want to call it. excellent information on the details and workings of the 4EAT.

Here's some info to start with:

http://endwrench.com/pdf/feb2004pdf/4EAT.pdf

http://endwrench.com/images/pdfs/4EAatPh2Win04.pdf

http://endwrench.com/images/pdfs/4EAT.pdf

http://www.endwrench.com/pdf/drivetrain/02Winter4EATTransOp.pdf

 

The above and more on the drivetrain are at:

http://www.endwrench.com/archive/sysdrive.html

[70SS454LS6]

Many thanks for your comprehensive responses, OBW99W, grossgary, Nomad327, jamal, and Andyjo. I'm studying them all. I've been driving a 97-OBS-5MT for ten years, but I just bought a 96-sedan-4EAT and the trans was a mystery. I had no idea that the 4EAT was such an intelligent device and the Subaru Owner's Manual was no help in explaining how it works. I'm more acquainted with stick-shift and 1960s and 1970s vintage Chevy TH-400 auto trans with P/N/D/L/R and no smarts but with 400-500 ft-lbs of torque. Subarus are much more fun.