As you're probably well aware, the WRX, like all imprezas, understeers no matter what you do. I believe this is an intentional part of the design - rallyists tend to pitch the car into a turn and are quite happy using the understeer to straighten the pitched car out of the turn. This should be a pretty effective technique for very tight tarmac or for low traction surfaces. However, for race tracks, that's not very practical. Pitching scrubs way more speed than is necessary, is highly inefficient, and kills tires.
So, you need to get the car to oversteer slightly, naturally, to work well on the track. We thought we'd accomplish that in the neutral or power-on conditions by biasing more power to the rear of the car - so the 35:65 seemed like a great idea. In our search for oversteer, we've gone to some extremes. We were running 950 pound rear springs with 600 pound front springs. We
were getting ludicrous amounts of high speed oversteer with still some low speed understeer. We fixed that with a radical wing in the rear to plant the rear at high speeds and reduce oversteer, but we're still fighting the battle.
With all that stiffness, including a really stiff cage, we started getting issues of rear wheel lift-off. That's generally not a problem - see GTIs and other such cars. Rear wheels always come off in braking zones. But in trailbraking situations, what happened for us was that one wheel would lift. It would lock since the driver was on the brakes. When it locked,
the viscous center diff would try to feed the same power to the rear. That wheel with all the weight on it (the outside) was not going to double in speed (super sticky hoosiers), so what happened instead was that either the front wheels would have to speed up to compensate (since engine rpm is fixed) or the engine would have to bog down. Since the fronts can't
speed up (you're under braking and anyway, how can hitting the brakes result in acceleration?
), the engine would bog down.
We went to that cusco diff to give us the rear bias under throttle, while hopefully not worsening the engine bog issue. What we discovered was not what we wanted. The diff is not a locked diff, as some people believe it is (as we did). It is definitely a limited slip differential - there is no other way to do non-50:50 power splits that I can think of. What we expected was when we got trail-braking induced inside rear wheel lift-off, was some form of anti-lock - since power had nowhere to go but the rear wheels, it should be harder to lock up the rears. That would help out immensely. But instead, what happened was a worsening of the engine-bog issue. It was nuts. Gary would trail-brake into a turn and completely fall off boost and out of the rpm ranges.
So that's my primary gripe with that diff. As for an advantage the rest of the time? We didn't notice much, but we were running close to stock power levels (intake, exhaust, but stock engine, turbo, boost, etc). I think you'll see more of an advantage on a car that can take advantage of powering the rears...
I hope that's not too long of a story. Lemme know if you have more questions.
Joel Gat
Crew Chief
Sheehan Motor Racing
http://www.TeamSMR.com
