legacycentral bbs

now im more confused than ever. Just wjhen I think I understand the 4eat something comes along and flis the script! Guess it's time for more research

THAWA wrote:now im more confused than ever. Just wjhen I think I understand the 4eat something comes along and flis the script! Guess it's time for more research

I can take pics if that helps? I can also take pics of what causes torque bind?

The 4EAT's AWD system is fundamentally very simple. It's just how I described. The transmission's output is always connected to the front wheels, and it's connected to the rear wheels through the multiplate transfer clutch. When the MPT clutch controller has full ATF pressure, it's fully engaged. When it has no ATF pressure, it's fully disengaged. The TCU controls how much pressure is bled away from it through Duty Solenoid C.

The 5MT's is how you describe it. The transmission's output shaft drives the center differential, and the center differential drives the front and rear driveshafts. The center differential is indeed at the rear of the transmission, but the transmission's input shaft is hollow, with the front output shaft going through it to reach the front differential.

I might have some of the terminology slightly wrong there, but that's the basic idea.

This thread's getting crazy with random info....some is correct...some is not.

As we all agree the WRX has a 3.900 final drive ratio. The rear diff has a ratio of 3.545. There is then a transfer reduction gear of 1.100 in the transmission that is mated to the rear output shaft. Again, as mentioned, that gives a final rear drive of 3.8995.

Technically since the numbers we inputted only had significant digits out to the thousandth place, that is the point at which you really need to round at. You can't have a more precise number then your original numbers. Therefore you would end up with a final rear drive of 3.900.

You however would still end up with a slight difference in shaft speeds. They won't be exactly the same.

Now in terms of will it make the car more FWD or RWD biased. I actually got this wrong in my earlier post. You have to remember with gearing is that they are all gear reductions, and to find out the actual rpm you need to take your input rpm over the reduction, because the number is actual a ratio, 1 to 3.900....etc.

The larger the number, the more reduction that will occur. If you have an engine speed of 2000 rpm, and go through 1st gear reduction of 3.454, you end up with 579.039 rpm. Then when you go through the front diff you end up with 148.471 rpm. When you go through the rear transfer reduction gear and rear differential, you end up with 148.491 rpm.

I'm pretty sure everything above is correct, and hopefully it helps explain things. If something's wrong, let me know.

Also, here are a few scans from the FSM on the AWD systems MT & AT.

The MT system wants to have the same speed difference between the front and rear driveshafts. When it doesn't have that, the fluid in the center diff heats up and causes the plates to lock. The more the speed difference the quicker the fluid heats up and probably more plates lock up, which I guess you could interpret as the amount of torque which is split/sent to front & rear.

http://main.experiencetherave.com:8080/ ... rdiff1.jpg
http://main.experiencetherave.com:8080/ ... rdiff2.jpg
http://main.experiencetherave.com:8080/ ... rdiff3.jpg
http://main.experiencetherave.com:8080/ ... iption.jpg

http://main.experiencetherave.com:8080/ ... ystem1.jpg
http://main.experiencetherave.com:8080/ ... ystem2.jpg
http://main.experiencetherave.com:8080/ ... ystem3.jpg

Legacy777 wrote:As we all agree the WRX has a 3.900 final drive ratio. The rear diff has a ratio of 3.545. There is then a transfer reduction gear of 1.100 in the transmission that is mated to the rear output shaft. Again, as mentioned, that gives a final rear drive of 3.8995.

Technically since the numbers we inputted only had significant digits out to the thousandth place, that is the point at which you really need to round at. You can't have a more precise number then your original numbers. Therefore you would end up with a final rear drive of 3.900.

You however would still end up with a slight difference in shaft speeds. They won't be exactly the same.

This I don't agree with at all. You can't use the ratio 3.545 in an actual equation expecting to get presice mesurements, because 3.545 is not a precise measurement. The actual ratio is 3.54bar or 3.9/1.1, That's the only way you can get a precise ratio and the only way it would work. Wouldn't having a front final drive of 3.9 and a rear final drive of 3.8995 make the visc fluid constantly heat up trying to limit the difference in speeds? Which would cause premature wear of it, and it needing to be replaced after x miles? It would be like saying if subaru labeled the ratios to 5 sig figs, the rear diff ratio would be 3.5454 and the rear final drive ratio would be 3.89994. How can the final drive ratio change like that? Using 3.545 as an actual ratio in precise calculations is incorrect, the front final drive ratio is 3.9 and the rear final drive ratio is 3.9

THAWA wrote:This I don't agree with at all. You can't use the ratio 3.545 in an actual equation expecting to get presice mesurements, because 3.545 is not a precise measurement. The actual ratio is 3.54bar or 3.9/1.1, That's the only way you can get a precise ratio and the only way it would work. Wouldn't having a front final drive of 3.9 and a rear final drive of 3.8995 make the visc fluid constantly heat up trying to limit the difference in speeds? Which would cause premature wear of it, and it needing to be replaced after x miles? It would be like saying if subaru labeled the ratios to 5 sig figs, the rear diff ratio would be 3.5454 and the rear final drive ratio would be 3.89994. How can the final drive ratio change like that? Using 3.545 as an actual ratio in precise calculations is incorrect, the front final drive ratio is 3.9 and the rear final drive ratio is 3.9

Yes....you are correct.....you really can't use just that number and expect to get the exact final drive ratio. However that's the only number I have access to, so that's what I used. I know the combined final drive ratio is essentually 3.9 in the rear.

But like I said, I have to use the info I was provided with. The FSM states the final gear ratio as 3.545 with a transfer reduction of 1.100. It doesn't state that it's 3.900, and you really can't use 3.900 and divide by 1.100 to get the exact rear gear ratio.

Is there a speed difference between the front and rear due to the stepper. I would say yes. How much of a difference, how does it affect handling, and will it cause premature failure......I couldn't tell you....

here's the wrx fsm page i was using for my info

http://www.main.experiencetherave.com:8 ... s_info.jpg