converting an Impreza 2.5i to RWD

[benz88]

I actually know a guy who Has a bugeye 2.5 N/A thats quick, best time was high 13's i believe. It was fun as hell to drive in even though it wasnt a Turbo Car,

[kramerbuccs24]

Well there's something to that, but it seems like there's an 8-10% WHP increase. So, assuming it's possible to get the 2.5 to 200 flywheel HP, I'm looking at 170whp vs. 150WHP. Plus -what I imagine would be- greater fuel efficiency that comes from not having to operate all the extra drivetrain.

I have a feeling those numbers are very inflated. If I had to guess you wouldn't gain more than 8whp by removing the front driveshafts.

Search on NASIOC for this idea, I guarantee it has been discussed and completed multiple times.

[Hillbilly]

I have a feeling those numbers are very inflated. If I had to guess you wouldn't gain more than 8whp by removing the front driveshafts.

They may be. I'm just going by the old standard: 18-25% driveline loss on awd vs. 10-15% DLL for RWD.

Search on NASIOC for this idea, I guarantee it has been discussed and completed multiple times.

like I said in 1st post, I've been searching extensively for the better part of 2days. The few places I've found anything other than "take out the front axles/weld the diff" has mentioned using an RX7 tranny and then in a discussion on a Datsun forum they were talking a/b using some Nissan units (from Silvias, i think). I didn't know any of those cats, so I wasn't sure if they knew what they were talking a/b +those threads were several yrs old. Figured I'd post here b/c I know you guys, and with the broad spectrum of rides here I figured someone might -at least!- know someone that knows something a/b this.

[twastheglow]

I fully understand what you're trying to achieve here and I'm all about inovation, but...

1) If it were as easy as you're so desperately searching it to be, there would be a wealth of information on the subject. There are probably reasons why you're not finding more information about the Mazda/Nissan working out, ie it either simply wouldn't work or those select people stopped trying because it was cost prohibitive.

2) What you're specifically looking to do is without a doubt going to need fabrication. That will consist of labor and materials. The budget you're shooting for will simply not suffice.

The last thing you want to do to a daily driver with a small budget is to reinvent what all these well trained engineers have designed, tested and proven to work. Not only work but work well.

I would look to a different platform. I know you've already stated all the reasons you don't want to and I can respect that, but I've stated all the reasons you should. Nonetheless, good luck with whatever you decide.

[twastheglow]

And as a side note, the only place you're going to find the answers you're looking for (if there are such answers) is on a Subaru only forum with very technical minded people.

[Hillbilly]

So on a couple other forums I'd posted this question in, I had 2 guys suggest a T5.

So here's my thoughts on this... I've been researching this off and on for a long time, thinking about doing a RWD WRX, so I've collected some good info already. I'm probably going to buy another WRX to convert, because mine is fairly low mileage and I like the AWD.

First of all, forget about that rear diff. Putting all the available power through only the rear diff (instead of 4 wheels) is not going to be a good idea with any amount of torque. It may work fine with a 2.5i if you're going to use it as a gentle daily driver, but that would probably defeat the entire purpose of doing the conversion in the first place.

Second, the transfer shafts aren't very strong either, so if you get a welded diff (or a spool to replace the center diff) you're likely to break a shaft for the same reasons as the rear diff.

So there aren't any good easy options without finding a weak link somewhere in the drivetrain.


So, this can definitely be done, and it's been done before. You can have someone make you a transmission adapter plate and mate a different transmission to it, and there are also lots of options out there for rear diffs. Most everything that you do will require some custom work, but I'm sure you know that. You'd need a custom driveshaft and axles. custom diff cradle, adapter plate or custom bellhousing, and some misc other things to mount the trans to the chassis, etc. You'd need a shift linkage also, which would obviously depend on the type of transmission.


Here's what I think would be an achievable setup, and one that would be probably lower priced than a lot of options:

Most of this depends on what's easily available and there is some aftermarket support for, so I wouldn't choose a rear diff housing that no one makes a limited slip differential for, and I wouldn't choose something that had a totally oddball spline pattern that no one would be able to hobb or broach splines for.

It looks like a really good option for shift linkage options and bellhousing options is a Tremec trans from a Mustang. There's good aftermarket support, everyone and their momma makes driveshafts for them, and you can choose a bunch of different flavors from different years because they're everywhere. It looks like the STi clutch housing is probably the way to go, because it's the closest thing to a modular bellhousing like pretty much all other RWD transmissions have (the WRX has a split-case transmission with an integrated bellhousing). But, the STi clutch housing looks way too long to use without a custom input shaft for the RWD transmission. It's about 165mm/6.5" from STi clutch housing front flange to front axle centerline (which is about the length of most RWD trans input shafts) and the rear flange is a few inches behind that, so the input shaft would have to be a few inches longer than normal when used with the STi clutch housing.

For the rear diff, a Ford 8.8 from a IRS Cobra or a IRS Explorer sport would be the way to go. They can carry a LOT of power compared to the Subaru stuff. The housing is the same for sll of them (there's an iron housing Lincoln unit also), but the covers are different and the Explorer cover looks like it could mount similarly to the R160 Subaru diff. A new diff cradle would be needed, but it's similar enough that a custom cradle could mount up to the stock chassis mounting points with no drama. The IRS and solid axle Ford 8.8s are the same, so any Ford 8.8 LSD or gears will work. The other convenient part is that you could then use a custom length Mustang driveshaft if you're also using a Mustang Tremec.

Axles would be a combination of Subaru parts for the outboard sections, and Ford parts to mate with the diff. Again, probably just off-the-shelf parts with some custom length adjustments.

Now the rest of the fun part... Because the stock Impreza engine is mounted so far forward (because of the front axle locations) it would be nice to move the engine a lot farther back into the chassis for better weight distribution, which would also make the shift linkage a lot closer to stock. That would need a front mount intercooler, and the turbo would most likely need to be relocated unless the firewall is seriously modified for clearance. That means custom downpipe, etc...

As I'm not wanting turbo seems like maybe a swap to R200 might be cheaper than trying to custom fit the Ford rear.

So looks like I'd need custom d'shaft T5-R200, STI bellhousing fabbed to mate w/ t5 and long input shaft for the t5. Did I miss anything?
Anyone have any guesses what all this would cost?
It is a little more involved than I'd hoped it would be. I'm not really surprised, but I did have my fingers crossed.

[integral]

interesting

[PavelK313]

My friend has 2.5RS with SR in it and RWD. Super fun car!

[Average_Jerk]

My friend has 2.5RS with SR in it and RWD. Super fun car!

thats probably the easiest and cheapest way to do it

[gundal.chung]

rwd would reduce the amount of drivetrain loss, ive seen a couple STi's done like that but not an Impreza... Good luck on the swap...

[jungledorifuto]

a buddy of mine is going on two years with the stated "simple rwd conversion, leaving the front axle stubs and just removing a drive shaft.. a sti diff is pretty much level zero when it comes to transferring all the power rwd. a 240 pumpkin works easily as well.. he wants to go SR

[Anonymoose]

Sounds like "The worlds fastest W8" to me

[e36cruiser]

Sorry to bump an old thread but how did you make out with this? Thinking about just taking out my front driveshafts, mainly cuz I miss rwd but also cuz I want to run staggered.

[Monster]

Bill Hincher is the man to see, He has made a bellhousing to mate a toyota rwd trans to a subie motor. He started out converting 4G63 to rwd years ago and has moved onto other projects as well, myself with a few others on another forum are waiting for him to finish up some bellhousings for us.

http://www.billsautofab.com/forums/index.asp

[Broche]

On how to make it RWD, this is how you do it.

-RWD center diff, it will replace your center diff so that all the power goes to the rear OR you can weld your center diff to do the same thing
-Stronger Rear diff, the stock diff will not be able to handle all of the power coming to it
-Strong Rear axles, the stock axles with not be able to handle all the power coming to it
-LOTS of front suspension modification with Caster so that the car will act RWD and not like and AWD car with steering response.

It is very doable, but not very practical, especially considering what you will get from it and unless you plan on drifting it, it is a waste of time because the car will not really seem to be all that much faster.