Strength of the WRX Transmission/Differential

[SkiRideDrive]

Hi everyone,

I was curious what people thought about the strength of the wrx transmission/differential in the 818 application. I spent some time looking through previous posts where people were arguing that since this car was lighter, the stock wrx transmission should have no trouble in the 818.

The part that concerns me is that all of the drive power will be going through the front differential, which was only designed to power the front wheels in the wrx. I claim no expertise in subaru's, but I would assume that the wrx is only designed to transfer 50% or less of its power/torque to the front wheels. If this is the case, subaru would only have designed the front differential and any internals related to driving the front wheels to handle half the power of the stock wrx, plus whatever safety factor they use for longevity.

In the case of the 818, all of the the engine power/torque will be going through this assembly, to the rear wheels of the 818. Granted the car is lighter, but I am not confident, considering weight transfer and the sticky tires people will be putting on this car, that the rear wheels will break free before transmitting half the power/torque of a stock wrx engine through the transmission/differential.

Any thoughts? Thank you for your input and I apologize if this has been covered before.

[Justen]

This has been covered before, and I don't think its going to be an issue. since the car is so much lighter, the for stress on the tranny won't be much more than stock, and unless your dumping the clutch at every stop sign, the stock internals shoule hold up fine. If you are worried about it, you can always put in some hardened ppg gears

[BrandonDrums]

Here's a thread where the strength of the front diff/ring gear is covered http://thefactoryfiveforum.com/showt...ull=1#post3822

I posted this in response to a similar question. I think the front diff will be in fine shape honestly.

No problemo man, I see what your saying.

The Front diff will need to be replaced anyway if you want limited-slip abilities. You can pick your poison, there are quite a few front LSD's for sale for the 5MT ranging from about $350 to well over 2k.

Regardless, the front diff almost never fails first, it's usually the center diff or one of the driving gears. Putting all of the power through a single diff probably equates to only a little bit more stress on the ring gear and worm cog given the geometry of the situation. The ring gear and worm cog have MUCH larger teeth than any of the driving gears.

See below, the large ring gear in the bottom right of the transmission below is what is in question. Compare the width and thickness of the teeth on the front diff to the remainder of the gears on the left.

No worries there.

In 818 form, I just trust that the size of the teeth and the overall diameter of the ring gear gives some mechanical advantage on a gear tooth to gear tooth basis at least that the other, smaller gears don't have.

Further to that point, in automatic form, subaru transmissions usually drive around in FWD only. The electronically controlled center diff only engages when there is detected slip so on dry pavement, it's possible that 100% of the torque is going through the front diff of an AT subie anyway.

That's why I think that the transmission will see less stress overall than in AWD configuration and the front diff is beefy enough to handle all of the torque all of the time.

[SkiRideDrive]

Thanks for the detailed response Brandon. That makes me more confident in the strength of the assembly. Also, realized you are in the triangle area, I am in Durham. I'm heading up to VIR tomorrow for the holiday laps event. For $25 you can get on the track for like 20 mins (behind a pace car, but they say it will be moving along swiftly). I am taking the motorcycle up, just thought I'd let ya know.

[PhyrraM]

Also, the exact same ring and pinion assemblies were used in FWD only models prior to 1996 (longer in non-North American countries)