transmission options for my new XT Turbo

[idosubaru]

i'm converting my FT4WD XT Turbo to NA and building this thing for the best gas mileage possible.

 

a 3.7 transmission should get better gas mileage than a 3.9 right? or do the 4 speed trans get less than the 5 speeds? i guess the determining factor is the final drive ratios? anyone know which trans has the best final drive ratio for highway gas mileage?

 

i'm thinking maybe swapping a 3.7 front diff into my FT4WD 5 speed trans may be the way to go if that's possible.

[Vanislru]

5spd 3.9 is the better for fuel economy. Run a quality synthetic in the trannie and motor.

[NorthWet]

The subject of what gives better mileage was argued to a non-conclusion a week or 2 ago in another thread. I happen to belong to the "slower-engine speed is better" camp.

 

Just for grins and giggles, are you sure that your XT has a 3.9?

[idosubaru]

87.5 XT Turbo FT4WD, i thought those were all 3.9's? but i'm just learning all this EA82 interchange stuff. quite confusing. any idea what thread that was, i didn't see it when i was searching.

 

a rear 3.7 turns fewer times at a given speed than a 3.9 so i'd think having a 3.7 rear diff would offer the best mileage for highway driving (and that's how it plays out with my auto 3.7 and manual 3.9's on the XT6's). so i'd like to have a 3.7 in my XT when i'm done. but - if the 3.9 trans are better then maybe i could get the 3.7 front diff into my FT4WD trans (if it's 3.9 now, which i thought it had to be).

[TROGDOR!]

Your FT4WD 5 speed is already 3.7.

 

Best gas mileage I ever got was in a 2WD nonturbo XT, which I believe has the same kind of gearing as a 3.7 final drive trans. It got 33mpg on the highway in the mountains.

[idosubaru]

i originally was looking for a manual FWD XT. i pushed 40 mpg in my old one driving all highway miles. it was only 3 years old when i got it in 1991. i could get 500 miles on a tank on all highway trips. that's what i'm aiming for now. this FT4WD XT Turbo was close, had a blown motor (i wanted to a put a new one in anyway) and from someone from the board so it was a good match. hopefully it does well, i'm converting it to NA, i'll be removing the turbo.

 

mine is a push buttom 4WD 5 speed right?

 

TROGDOR, i should be moving out to morgantown next year and be going through there a few times between now and then maybe we'll meet up?

 

which trans is it that is FWD until you put it in 4WD? wouldn't those get better mileage since they are FWD unless you put it in 4WD? i guess it doesn't make much difference really....

 

you guys are great putting up with all my stupid questions...i was pretty confident it was a 3.9 trans, i'm still learning all this as you can tell.

thanks for all the help folks,

[NorthWet]

The thread I was thinking about was relative to 4th gear vs. 5th, but I think that it is still relaveant:

http://www.ultimatesubaru.org/forum/showthread.php?t=4168

 

If it says "fulltime" or "AWD" then you don't have a way of switching it between FWD/4WD. A "pushbutton" 4WD tranny has just that" A pushbutton on the shifter that engages/disengages the transfer gears. Yes, the parttime 4WD or FWD-only trannies should theoretically get better fuel economy than a FT4WD/AWD one.

 

I agree with trogdor that it is very likely that you have the 3.7 ratio already.

[idosubaru]

AWESOME!

glad i was incorrect!

thanks again all.

[TROGDOR!]

I agree with trogdor that it is very likely that you have the 3.7 ratio already.

 

Could just pop your head under the back of the car and look at the diff tag. I'm 90% sure all 4cyl XT's are 3.7 regardless of equipment.

 

If you want to go part time 4WD, I should have a trans available soon, from an 89 nonturbo XT. I bet many people would love to get their hands on a FT4WD box.

[idosubaru]

i haven't looked at it yet other than the body and quick glance of the interior. i'll have a peak at the diff.

 

if it's a 3.7 then i see no need to swap trans. what's the difference between a FT4WD and the part time 4WD? why do people like the FT4WD so much? they both 5 speed and 3.7?

[TROGDOR!]

The part time 4WD runs in FWD until you push the button, which makes it lock into standard 4WD, with front and rear axles turning at the same speed.

 

The FT4WD is an early version of AWD, driving all 4 wheels all the time. The diff lock button locks it into standard 4WD like the part time trans. This transmission NEVER runs in 2WD.

[idosubaru]

i understand those differences, still wondering why people prefer the FT4WD trans over the part time 4WD? or maybe they don't.

[NorthWet]

... what's the difference between a FT4WD and the part time 4WD? why do people like the FT4WD so much? they both 5 speed and 3.7?

The big difference is the center diff that allows it to be fulltime... you can run it in 4WD on dry pavement without loading up, and possibly damaging, the drivetrain. With the selectable 4WD (either second lever or pushbutton) you are limited to slippery/loose surfaces for running 4WD.

[idosubaru]

ah that makes sense. is there any difference between the FT4WD and the AWD on the XT6 (speaking of manual trans)?

[Turbone]

I thought all XT6's were AWD? AT or MT. But you can select between 2wd and 4wd on the 4EAT, just pull the fuse

Or if you want, rig a switch up. Plus the duty solenoid c trick too.

[idosubaru]

I thought all XT6's were AWD? AT or MT. But you can select between 2wd and 4wd on the 4EAT, just pull the fuse

Or if you want, rig a switch up. Plus the duty solenoid c trick too.

 

not all were AWD, there was a FWD (auto only) option as well.

maybe you misread my post, i was asking if the XTTurbo FT4WD trans was the same as the AWD XT6 trans. sounds like the FT4WD XT works the same as the AWD XT6 trans. yep, i have the duty solenoid C switch installed. excellent little mod.

[Numbchux]

i understand those differences, still wondering why people prefer the FT4WD trans over the part time 4WD? or maybe they don't.

 

I don't!

 

Well, if there was one that was like the RX Dual-Range FT4WD box, with the same lo range ratio as the PT4WD D/R box, and locking center, I'd LOVE that!

 

I hate 2WD, but like having true lo range 4wd more for offroad. If I had a car that I was only running on-road, I'd want AWD or FT4WD.

 

a PT4WD tranny with the 3.7 ratio will probalby yeild the best mileage. though you still have to turn the rear diff and driveshaft, so it won't be as good as just FWD.

[TROGDOR!]

Yes, this trans uses exactly the same technology as the XT6 manual trans. They're identical except for the gearing- the '6 has a 3.9 final drive, and the individual gears are set up to be better suited for a 6 cylinder. The turbo transmission's gear ratios are designed to match the 4 cylinder engine better- I've heard that an XT6 trans behind a 4 cylinder engine doesn't run too well.

[idosubaru]

What is the gear ratio of the 1989 XT PT4WD?

 

I searched but couldn't answer. I am putting a new motor in my XT Turbo this weekend and have three trans to choose from:

 

PT4WD (89 XT non-turbo)

FT4WD (original XT Turbo trans in the car now)

Dual Range 88 RX trans.

 

i want my RPM's to be as low as possible at highway speeds.

RX and XT Turbo appear to have the same gear ratios (.871 in 5th).

[TROGDOR!]

All XT's had a 3.7 final drive, turbo and nonturbo. I believe the turbo and nonturbo PT4WD transmissions are actually completely the same.

[idosubaru]

thanks steve, i'm actually looking for the gear ratio of 5th gear, i know you told me before this was a 3.7 and i saw it through the searching i've been doing today. can't find the gear ratio though. i believe it might be 0.725 but the charts are kind of confusing. if it is that low i want that one in my car.