How To - Transmission Re-Design 1

[jbond]

Hi Gents,

I am building a custom vehicle (small lightweight 2 seater sports car) and got my hands on a EJ255. This is the 2.5 litre turbo boxer engine used in the 2006+ Subaru WRX.

Most of you will know that the WRX is AWD.

In my application, I want to have the engine either in the mid or rear of the vehicle.
If I mount it mid, then the car will be rear wheel drive.
If I mount it rear, then the car will be AWD.

I am investigating my AWD options, and have come to you with a question:

If I use a WRX (or STi) transmission, to obtain the AWD, and mount the engine in the rear, the car will be prone to driving backwards(!). What modifications (and is it realistically possible) can be made to the AWD transmission to convert it to a rear engine AWD application? (and to be driving forwards!)

Also, I believe that in a standard WRX, the torque will be prioritised to the rear wheels? This would mean that in my application the priority would be going to the front wheels, whereas I'd like it to be in the rear - do you have any suggestions?

Essentially, I want to be driving the rear wheels and to use a viscous centre diff to drive the front. I will also consider the Porsche C4 transmission, however I believe the Subaru solution may be more cost effective(?).

Thankyou for your time and consideration

 

[MikeHalloran]

At least some VW boxer transaxles are/were reversible by flipping the ring gear. I'm not aware of any others that have the necessary symmetry.

Corvair transaxles were not reversible, but could be driven from the front... but with the gearbox forward of the axle and the engine forward of that, you're looking at a long wheelbase or a really short engine.

Make it mid- engine and RWD.

The extra weight of the forward diff and whatever adapters you need will offset any performance margin for 4WD in a light car.




Mike Halloran
Pembroke Pines, FL, USA

 

[jbond]

I noticed that on the quaife.co.uk site, you can get replacement front and rear torsen diffs for the Subaru WRX / STi.

It seems to me, that if I replace those diffs with ones that rotate in the opposite direction, then it's problem solved (in terms of getting the car to drive in the correct direction). Albeit, torque preference to the front wheels versus the rear wheels may still be in effect.

Is the concept of what I have suggested - replacing the diffs, a viable one?

 

[jbond]

I agree that RWD is the easier solution, but the ability to lay so much power to the ground whilst cornering (as I've experienced in my friends STi) is very attractive.

I have a similarly high powered RWD car and it can't do what his does in the corners!

I may end up going with the RWD, but want to explore the AWD options firstly.

 

[DesignByDesign]

I am also building a sports car with Subaru engine & transmission.
I have an EJ207 & 6spd from an STI spec C. (2006 model) I very quickly abandoned any idea of 4WD and opted for the mid-engine configuration. One major factor for me is minimizing polar mass on the vehicle. The CG of the engine/transmission is virtually over the engine mounts, the total engine + gearbox is 220kg (dry - without the centre diff & tailshaft) & the load on the rear engine mount is only 20kg. The mounts are 200mm forward of the rear axle so to turn the engine round will shift the mass 400mm back.
With all that extra weight trying to ovetake the front of the car I don't tink you will be able to ue the extra traction.
As Mike said, the weight of the front diff, driveshafts, axles, centre diff will easily add 15% to the weight of the car.
Remember too that the front suspension is set up completely differently on front wheel drive cars to compensate for torque-steer. That may prevent you from tuning the suspension for optimum handling.
If you opt for RWD you will need a locking spool to replace the centre diff. I have found this impossible to track down for a 6 speed box (5 spd 2WD adaptors ar available) & would like to know if you have found / can find a source?

 

[IceStationZebra]

"I noticed that on the quaife.co.uk site, you can get replacement front and rear torsen diffs for the Subaru WRX / STi. It seems to me, that if I replace those diffs with ones that rotate in the opposite direction, then it's problem solved."

You can't just get a differential that "rotates in the opposite direction." As Mike was trying to say, you need to flip the ring gear from one side to the other. The pinion still turns the same direction, but the ring gear would turn the opposite way. This would require a complete one-off gear set (big $$$). And looking at the front differential of the WRX the pinion is not on the same axis as the ring gear, so even if you flipped the ring gear to the other side the geometry would be terrible.

http://forums.nasioc.com/forums/showthread.php?t=665334

ISZ

 

[MikeHalloran]

I don't know about Subaru diffs specifically, but I know a little about diffs in general.

The diff part per se, contained within the ring gear, generally doesn't care about the direction of rotation.

The ring gear generally does care, at least if it's a hypoid. Because of the non-intersecting centers, the teeth on both ring and pinion are asymmetrical in several ways.

In a plane that's transverse to the pinion axis, the ring gear teeth look a little like buttress threads. The flattish face takes the high normal load. Reverse direction, and the sloping face gets the high load, which translates to high separating forces and extra scuffing.

Not quite similarly, the pinion has a preferred direction, too. In forward, much of the reaction force is directed along the pinion axis, where the larger tapered roller bearing deals with it. In reverse, the higher (for a given torque level) separating force also induces more bending in the pinion, which is not really designed for long duty that way. Or something like that. The geometry really makes my head hurt.

The point is, ring/pinion sets are optimized to transmit lots of torque in one direction for long periods of time, and not nearly as much torque in the reverse direction, for not nearly so long. So you can't just drive 'em backwards; they'll howl and break.

It gets worse; they're hard to make; your corner machine shop can't make an opposite hand set from a sample. Even if they could, you'd need opposite hand housings, etc.

This exercise will cost more than buying your friend's STi, and even if you manage to get the drivetrain like you want it, the odds are against you getting the rest of the car right. If you like the car, just buy it.



Mike Halloran
Pembroke Pines, FL, USA

 

[jbond]

Design by Design - I have only found 1 place, and it was for 5 not 6 speed

http://www.bremarauto.com/subaru_transaxle_conversion.htm

To Mike and the other people that have offered valuable insights, thanks for your comments, I think I'll be going RWD which will make it easier in many respects (and cheaper too!).

Now I'm thinking porsche G50 transaxle, does that sound like a good prospect or do you guys have other recommendations?

I'm expecting the engine to be making mountains of torque and power, as I'm intending to run a lot of boost pressure. So the transaxle has to be a strong one!

 

[DesignByDesign]

Thanks for the link - I contacted them last year & they said they'd never done any development work on the 6 speed box, but 'would if there was enough demand'.
I have a gearcutting company in NZ who will make a spool to my details. (I'm in Thailand, no professional machine shops here) On my last visit I took them the centre diff, but they require the 'male' splines to ensure the fit. I'm not looking forward to pulling the gearbox apart when I'm supposed to be putting it in the chassis!

I also contacted Modena Engineering

http://www.modenaengineering.com.au

& they say 'yes, we do them all the time' but I'm 99% sure they are talking about a rear-wheel conversion so I asked them to clarify - that was back in September & I'm still waiting.
Braemar did send some photos of the gear-shift rod on the Saker car they build. That's another story you'll need to solve at some point when the engine's between you & your gearbox!

 

[JohnLear]

Probably a stupid suggestion, otherwise I suspect it would have been suggested already, but is there any chance of running the gearbox upside down?

 

[jbond]

Good question, I have no idea. Can anyone else answer?

There is oil in a gearbox, correct? Is it gravity controlled?

 

[patprimmer]

I have seen it done.

The oil fill and drain holes end up in the wrong place but its not an insurmountable probem.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[jbond]

Gold. I will definately investigate my options into that.

Any advice, pointers, or links to examples or guides, that can prove useful to me?

 

[jbond]

Mounting the transmission upside down may result in a very low bell housing that gets too close to the road?

See this thread, it seems I'm not the first to explore this option:

http://forums.nasioc.com/forums/archive/index.php/t-90804.html

 

[JohnLear]

Depending on output shaft height relative to input shaft height, it might result in the inner CV joints being relatively high, thus possibly downward angled driveshafts, and probably a somewhat higher gearbox mass (free lunch anyone?).

The reason I suggested it is that I vaguely recall that some early racing transaxles were built into VW transaxle cases, and the whole box mounted 180° around and upside down for mid engine use. I think one of the early Hewland boxes might have been like this?

 

[JohnLear]

Umm, gearbox oil, I'm no gearbox wizz, but I would expect that so long as one set of gears (on a shaft) is sitting in the oil bath, then plenty of oil will get flung around. I'd be checking for baffles (internal reinforcing webs) that might block oil from running into the 'bottom' of where the differential sits and that the crown wheel is sitting in a decent depth of oil.

My advice is free, it's worth at least what you pay!

 

[shanba]

Look at how all the bearings are fed oil, some may rely on gravity feed in a hole above the bearing. Especially in the ring and pinion area. The transmission and differential breather vent will need to be relocated. Also, seal weep holes will be below the seals, need to investigate allowing drips to occur upside down. Just a few of the issues this would create.

 

[waross]

The old VW's had symmetrical diff's. The ring gear ran on one side for "Bugs" and on the other side for vans and buses. The VW vans and buses had a simple reduction gearbox behind each wheel that reversed the direction of rotation, hence the reversed diff assembly.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thruthefence]

I think the " Crown Conversion" Corvairs turned the transmission end-for-end, and then 'flipped' it to allow for the rear seat mounted V8 engine to have 4 forward gears. My Lotus 65 uses a transmission from a Renault 16,(a front wheel drive) swapped end-for-end with the ring gear swapped over for the same reason.

 

[RossABQ]

The transaxle used in Porsche 944/951 et al is pretty much the same as used in many Audi's, and there are actually some benefits to using the Audi versions, not the least of which is the Audi ones are $200 at boneyards, while the Porsche units are $800 - 1000. 5-sp with torque-tube input. The axles sit ahead of the gearbox. Good for <600 HP (upgrade for 944's with the Chev LS1 conversion):

http://www.porschehybrids.com/php/viewtopic.php?f=2&t=2957&p=26931&hilit=Audi+transaxle#p26931