Offroad gearbox build

Hi all

I am just about to start building a new offroad gearbox for my sf forester so I thought I would start a thread for general discussion on the topic.

My current plan is start with a sf box, swap to L series low range, hopefully do a 4.44 final drive conversion, some form of LSD in the front and either design my own locking centre diff or use a dccd setup.

This combination would give me a 1st low of 72% lower than standard sf 1st which is a heap better than the stock 44.7% reduction.

I have considered using an SG box as a starting point as they are meant to have a stronger 1st and reverse gear but I believe that they use a different front diff (axles that go in to box) and the 1st gear is 2.6% higher meaning the overall reduction would be 67.5% lower than standard SF 1st

I have given my current box a heap of abuse and my 1st gear is still fine. I have managed to chip my reverse gear so it now sounds like a cement mixer but this was a result of trying to back up a steep rough hill in an overloaded car with no hand brake which meant massive clutch dumps. I only have myself to blame for that one. I think the 4.44 final drive will take enough stress off the drive line to mean this is unlikely to happen again.

Another issue with the 4.44 conversion is that it will be very difficult to fit it in with the low range gear set but I have heard that it has been done before.

To compensate for the lower final drive I am also hoping to install a taller 5th gear from another box. The 0.78 5th from the L series box would be a good option but I am not sure if it is compatible. The ratios from 3rd to 5th from the L series box would actually be a good conversion to compensate for the taller final drive.


Currently I only have obtained a L series box so if anyone has suggestions as to where get the other components I require on the cheap that that would be great.

Anyway this is my plan currently, please comment and suggest better ideas.

I like the idea of the SG box as well. If the reduction is 67% instead of 72% with the SG 1st gear, the difference isnt much. Plus you may be able to use the SF 1st gear so you will still get the better synchros.

Have you spoken to Rick about this? There's not much he doesnt know about Subaru gearboxes :iconwink:

Will be interested to see how much of a machining is required for the 4.44 + L low. Particularly an assessment on strength and whether it could be suitable for turbo power.

My advice is go with a decent front plated LSD like a Cusco. That and if you go down the DCCD path then buy a complete STI RA DCCD 5 speed, swap out the DCCD centre for your viscous and then flog it as a standard STI gearbox. That way you get all the parts you need for the DCCD. All the above is what I didn't do (at least first time around) and wish I had. If that's too expensive then just go with a good front LSD and do the DCCD centre later. Apparently Brian at Subabits can get Cusco's for around $1100 which is what I paid for mine imported from Japan.

6 speed sti fron lsd fits.
With the SG box, its uses the same from diff as the SF just doesn't have the stubs poking out. These stubs come out with a circlip and can be put on the SG box, which is what I'm doing.
You can buy individual gears from All Drive Subaroo for a decent price.
Another bonus of the SG box is that you get a better slider on 1st gear, while still only a single mesh synchro the slider makes a huge difference. Gets rid of the crunchy 1st gear thats common on most subaru boxes.
Im keen to see how much of the 4.44 crown wheel needs to be shaved. I'll be going with 4.44's but go 1.447 low range. The gearing works out slightly lower in 1st gear than with 4.11 and L-Series low range.

Dccd is the best option but as venom said it's expensive. I am going for an alternative route and chose a 20kg centre which I picked up at a good price. Apprentice wages don't pay much so I'll make do with what I have.

taza said:

6 speed sti fron lsd fits.
With the SG box, its uses the same from diff as the SF just doesn't have the stubs poking out. These stubs come out with a circlip and can be put on the SG box, which is what I'm doing.

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I though that might be the case but good to know.

taza said:

I'll be going with 4.44's but go 1.447 low range. The gearing works out slightly lower in 1st gear than with 4.11 and L-Series low range.

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By my calculations SF 1st with 1.447 and 4.44 is 1.6% higher than 4.11 and 1.59 and SG 1st with 1.447 and 4.44 is 4.3% higher than sf 1st with 1.59 and 4.11. Not huge amounts but every little bit counts.

Venom said:

My advice is go with a decent front plated LSD like a Cusco. That and if you go down the DCCD path then buy a complete STI RA DCCD 5 speed, swap out the DCCD centre for your viscous and then flog it as a standard STI gearbox.

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I was thinking about doing this but I would want to take the 4.44 gearing as well so it would be harder to flog as an sti with 4.11.

What gearbox is your box based on Venom and are you running 4.11s?

Venom said:

Apparently Brian at Subabits can get Cusco's for around $1100 which is what I paid for mine imported from Japan.

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Thanks for the tip. I am currently in the process of building a rear manual locker which if (big if) it works I might consider doing the same for the front. Otherwise I will just have to convince myself that I have the funds for a plated LSD

I tried that. Came up with a half decent design. Thr cost of manufacturing one wasn't worth it in the end. I scrapped the idea and went with LSD's.
Subaru's arent made to be rock crawlers. Lockers would work well but would get them into more trouble just bresking other weak components on the vehicle.

Taza, are you sure the STi 6spd front diff fits a dual range? I thought thats what Rally has & he needed some clever (& secret, lol) work to get it to fit. It would be great if it did :biggrin:

Dedman, will be great to see if you can design a locker. There'll be many people very interested in that!

Yes some minor modification needs to be done but it fits in a dual range box. I have two at home. One thats been modified to fit a 5 speed box. The other standard. I can replicate the modifications to the new diff.

taza said:

Yes some minor modification needs to be done but it fits in a dual range box. I have two at home. One thats been modified to fit a 5 speed box. The other standard. I can replicate the modifications to the new diff.

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Is that a 5 spd dual range? Can you share how its modified? :iconwink:

taza said:

I tried that. Came up with a half decent design. Thr cost of manufacturing one wasn't worth it in the end. I scrapped the idea and went with LSD's.
Subaru's arent made to be rock crawlers. Lockers would work well but would get them into more trouble just bresking other weak components on the vehicle.

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I disagree, a locker allows you to slow down and drive more carefully on the same terrain, crawl instead of bash your way through.

I have actually made the locking diff already, just need to find time to design the actuator and test it. The is a very very very high probability that it is not strong enough but I will let you all know when I get that far.

NachaLuva said:

Plus you may be able to use the SF 1st gear so you will still get the better synchros.

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1st gear and reverse are the worst gears to try swap as the smaller gear is part of the input shaft meaning you have to swap the whole thing.
Im not all that fussed about better synchros as mine work fine for the most part

Yes you can crawl but a locker puts more stress on your diff, gearbox and whole drivetrain.
Having been offroad with numerous people with L-Series' s that have welded diffs. Yes they go amazing offroad and can go almost anywhere the next weekest link is the CV's. They snap easy. Ive even seen where a welded diff caused 9 of the teeth on 2nd gear to snap off.

Go for it though. Keen to see a locker in action. If money is no issue you can buy a navars R180 locker and fit it into an STI, r180 rear diff. It's been done but in the end all comes down to money.

taza said:

Yes you can crawl but a locker puts more stress on your diff, gearbox and whole drivetrain.
Having been offroad with numerous people with L-Series' s that have welded diffs. Yes they go amazing offroad and can go almost anywhere the next weekest link is the CV's. They snap easy. Ive even seen where a welded diff caused 9 of the teeth on 2nd gear to snap off.

Go for it though. Keen to see a locker in action. If money is no issue you can buy a navars R180 locker and fit it into an STI, r180 rear diff. It's been done but in the end all comes down to money.

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The question is if the damage was a result of a locked diff when a locked diff was required or the effects of a diff being locked when not required. Given 2nd gear was damaged I am guessing it was the latter in this case.

I would love a commercial locker but the price is a bit of an issue. I also believe it is not a remotely simple process fitting a locker in to the r180 diff.

Well, I think it was clever and I guess to some extent it is secret, or as it is known these days, intellectual property. Trial and error, R&D or whatever it is,
if the bloke who develops something goes to that sort of effort and expense, why would he tell the world. I just gave him the bits and he did what he did and the result has been outstanding. I really don't know why you'd go to all the trouble of trying to put locker diffs in (especially in the front), but then again everyone has a different purpose for their car and that's fair enough. I think you need to really appreciate the effect locked and plated front diffs will have on the steering.

I had a fault recently with my DCCD controller in my WRX, whereby the auto function failed and I had to use full manual control. Just by locking the centre diff, I could hardly turn the car and I had to manhandle the thing just to swerve through witches hats. The car had fantastic grip, but the car would just not turn. So think what a locked front diff would do to your steering.

NachaLuva said:

Taza, are you sure the STi 6spd front diff fits a dual range? I thought thats what Rally has & he needed some clever (& secret, lol) work to get it to fit. It would be great if it did :biggrin:

Dedman, will be great to see if you can design a locker. There'll be many people very interested in that!

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Rally said:

So think what a locked front diff would do to your steering.

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Yes I agree completely. Front locked diff = essentially no steering at all. It would really only be for a few seconds here and there to overcome airborne wheels when crawling. Locked rear diff on the other hand is very manageable in terms of steering.

I think a locking front would be ideal. If it's your daily driver there's no need for a front lsd on the road in an awd car. Your talking activating it on the odd climb or mud hole when needed. Maybe a fraction of the time spent driving the vehicle. In those circumstances your also rarely going to be doing full lock turning. Also think comparing a welded diff to a locker in terms of durability isn't a fair comparison. A welded diff is punishing the cvs and driveshafts fulltime, weaking the cvs and cv joints over time. Them yes at a time when they're under more extreme stress they let go. the whole idwa of a selectable locker is its easy on the drivetrain the 99.9% of the time its not needed.

That's all true. But the phantom switchable locker is, as far as I know, MIA. Or has one been released for the front of a Subaru? If so, then that would be the bee's knees. Otherwise, use the factory STI front diff and make do. Having bent suspension arms in my previous offroading, I am not sure that I'd want my Subaru taken any further offroad than where I have managed to take mine- I was not exactly thrashing the thing. And i was never wanting for grip. Still, for anyone wanting to go to that nth degree, it will be a case of diminishing returns as far as bang for your buck goes. I still chuckle when I think of the big 4WD's I have shown up with the diff set up I have. None of those occassions was on the surface Subaru's tend to beat the big 4WD's- sand. One was on a slippery, muddy uphill section (with me on road tyres towing a camper trailer!) and the other 2 were on a rocky up hill track.

My thoughts exactly Venom.

I've found a decent plated rear diff has helped, but whenever I lift a front wheel I lose all momentum. I also worry about the wildly spinning wheel coming down on the ground & suddenly getting grip, which obviously is how you snap an axle or CV. A locked front diff would stop that. Used only when needed, it would actually protect the driveline.

I also like Rallys setup, giving good grip & driveability all the time.

Dedman, can your locking diff be used with a plated diff or does it replace the centre?

NachaLuva said:

Dedman, can your locking diff be used with a plated diff or does it replace the centre?

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Replaces the centre. But don't count your chickens before they hatch! It is most likely going to eventuate to nothing.