If you read this blog regularly, you know just how frustrated I have been with the handling of my car. Coming out of a very well set-up, rear wheel drive BMW M3 (running on Hoosiers and a race suspension, of course), I have been completely frustrated with the nasty understeer and inconsistency of the Evo. The answer to fixing this, of course, could include a lot more stiffening in the suspension, a lot more lowering, and using much stickier tires, but while all of that would make the car considerably faster on a road course, it would be torn apart on the frost-heaved, pot-holed, and generally broken up roads at Targa Newfoundland.

I have been convinced for some time that one of the keys to making my particular Evo IX perform would be found in the differentials. In tarmac mode, the differentials appear to “fight” you in some corners — shifting the locking of the center diff mid-corner and keeping you from being able to consistently run hard through the corner. In the tight and twisty corners, the car is well balanced and changes direction at will, but come into a long, high-speed sweeping corner, and you get terminal understeer. Feeding in more throttle makes all 4 wheels slide, but it’s still not always that fast and is a little hard to predict (kind of a problem when you’re running on a stage and don’t know where the road goes). Left-foot braking also helps this, but you still end up scrubbing speed off and you add in the joy of beating the piss out of your brakes and tires.

Speaking with some of the racers I know and trust, each pointed to some elements of the differential(s) that they changed to make improvements here. Some swear by yanking out the factory diffs (including the active center) and installing various aftermarket parts, while others have tried to maximize the factory parts that are in the car. A call to Andrew Comrie-Picard — a fellow WORKS-backed driver, a Canadian rallyist who knows the Targa Newfoundland, the current Rally America points leader in his own Evo IX, and an all around great driver and car builder — unvealed some interesting insights. First, Andrew confirmed that the factory ACD controller is not tuned for competition and was probably a severe limit in the car. Second, he pointed out there were some great options to make this better.

One option is a programmable diff controller from Motec, Pectel, or others. With a controller like this, you can modify the pump pressure in the Active Center Differential, thereby establishing what percentage of lock you want between the front and rear. To boot, these controllers allow you to modify this lock-up at various engine speeds, throttle positions, and when the brakes are on (for us Left-Foot Brakers). This is basically what the factory ACD ECU already does, and these controllers give you the ability to fine tune those maps. This is great, but is also amazingly time consuming and the only starting point for maps is the stock settings from Mitsubishi. This is a very high level of commitment to tuning if you want to get this right.

The second option are upgraded controllers available from Ralliart! I have heard rumors of these parts on the various message boards, and rumors that they would not work due to the lack of Active Yaw Control in the U.S. cars. Well, like most rumors on the internet, there is a little bit of fact and a little bit of fiction here. The fact; these ECUs exist, and are available in both sport (basically high performance street) and race applications. Andrew confirmed that they do indeed work on the Evo IX RS, as he has run it in his own car.

I ordered one immediately. In discussing my new-found-potential-answer-to-terminal-understeer, Pete Kang at WORKS replies, “Oh. We have one of those sitting in a box here. It didn’t work on one of our cars so we never thought to use it again.” Turns out that there are either some limitations as to which cars it will work with (as evidenced by all 3 lights coming up and the car faulting out on the WORKS Evo VIII) or there is something wonky with that specific car. Either way, borrowing the WORKS Ralliart ACD ECU resulted in the unit plugging right in and working immediately in my 2006 Evo IX. Let’s test this puppy out!

NASA Time Trials @ Thunderhill Raceway Park in Willows, CA

I’m a huge fan of the NASA Time Trial program. The NorCal NASA Region is a great club — reasonably priced track time, well run events, and a huge diversity of classes and cars — but some of the HPDE (driving school) groups are populated by drivers of, shall we say, variable levels of talent. The Time Trials group offers a much better alternative; experienced drivers in a tightly controlled group with real lap timing. I signed up for the weekend in the TT-S class, which is a class for race cars, controlled by a power-to-weight ratio formula, that lets me basically run however I have the car set up at that point.

I drove the car up on Friday night after work, with the minimum of spares, tools, and supplies that I’ll need. This both let me scrub in my new set of Toyo R888’s, as well as not have to deal with the headache of loading a trailer, strapping the car down, etc. It’s really nice to have a street legal racecar.

My plan was to test the new Ralliart ACD controller for the bulk of the weekend, using my Racepak G2X data acquisition system to track and compare the performance over the weekend. Lap times would surely tell me which set up was faster, and I will be able to decide which set up I prefer, but data acquisition will confirm what the stopwatch shows.

The stock ACD controller is mounted on the chassis slightly behind and to the right of the glovebox. With the right side kick panel removed, you can reach up and unplug the wiring harness to the stock controller. For the purpose of testing, I zip tied the new controller to an Accusump line running through the cabin (pictured above), where it would be within reach of the factory plugs.

On Saturday morning, I started out running the new controller in tarmac mode. The time trial group starts each Saturday morning with open qualifying, with the fastest time in that session setting a running order for the next session. Each subsequent sessio’s times than re-seed that order, with the design of this being that the cars always go out in order of fastest to slowest, theoretically giving everyone clear track for at least a few laps. Unfortunately, my first session had me stuck behind a bunch of traffic, and it wasn’t until my last few laps that I got any clear running. My last two laps were a 2:06.7 and a 2:06.6. Not a bad time when I compared it to my last time I ran at Thunderhill a year ago, where my fastest lap was a 2:11. That said, I didn’t really have a great feel for the controller and couldn’t tell what was the controller and what was the other changes I’ve made to the car since last year.

The remainder of the day was spent getting comfortable and trying to make the most of the new set up. With my brain back up to speed and being comfortable back in the car, I managed to click off dead consistent laps with the new diff controller. I banged off three laps in the 2:05’s in my second session — a 2:05.8, a 2:05.3, and a 2:05.1 — and never saw a lap more than a second off of that time unless I was buried behind a slow car.

The new Ralliart diff controller is a revelation!

One complaint that I have always had about the Evo has been my personal inability to click off consistent lap times. I have never been the absolute fastest driver in the groups I’ve run with, but I have always been able to be very, very consistent. Like turning laps within tenths of each other kind of consistent. Since I bought the Evo, I have been completely confounded by my inability to run a string of fast laps. I could always bang off one or two fast laps, but this usually resulted in having to push the car so hard that it would overheat the tires and brakes to the point where I couldn’t keep up that pace for a full session. The new diff controller makes the car easier to drive, with a lot less fighting the chassis and more consistency in the handling throughout a long run. This resulted in faster laps across the board.

The difference in feel between the stock diff controller and the Ralliart part (both running in Tarmac mode) is dramatic. With the stock ACD controller, corner entry would be fine (especially if you trail brake in), but the mid corner resulted in horrific understeer. Corner exit would be manageable as you got back on the throttle, as all 4 wheels would slide on the exit, but the exit speed would always be compromised by the mid-corner behavior of the car. To deal with the mid-corner behavior of the car, you could lift — which would actually usually increase understeer — or you could left foot brake, which would tighten up the angle of the car but would scrub speed. So, you inevitably enter the corner slower and build speed out (classic slow-in, fast-out), but even then the mid-corner behavior would be inconsistent. Gravel settings improved things slightly (especially for left foot braking), but it still isn’t right.

With the new diff controller in place, the behavior is completely different. In general, the car went from feeling like a front wheel drive car to feeling like a rear wheel drive car. Corner entry remained unchanged, with a slightly improved entry coming from more stability while trail braking. Mid-corner and corner exit are completely different. Rather than having to left foot brake to manage car position mid-corner, you can throttle steer the car! Lifting off throttle tightens the line, while adding in more throttle sets up a four wheel drift towards the outside of the corner. Very easy to manage without having to alternate throttle and brakes while sliding the car. On corner exit, the car tends more to traditional throttle-on understeer, but it feels like it has more overall grip as you add in more gas.

On Sunday, after re-establishing my baseline in the morning, I then switched to gravel and snow settings on the Ralliart diff controller, and experimented with more left foot braking. Neither mode was faster than tarmac, and less left foot braking and more careful use of the throttle appeared to be the best solution. This is counter to the stock computer, where gravel mode and left foot braking has proven fastest for me.

The final test came later in the day when I reconnected the stock ACD controller. After spending the weekend getting comfortable with the new set up, I reset back to what I was driving for the past couple of years. With a rabbit in front of me on the road (another Evo, well set up and being driven very well), I managed to set my fastest lap of the session — a 2:04.6. Could the stock ECU be better? Nope. That lap then followed with a succession of slower and slower laps as the tires and brakes overheated. But the data confirmed my suspicions more than the stopwatch; my theoretical fastest lap (calculated by looking at your fastest segments — think of this as your personal goal in an ideal world) with the Ralliart ACD controller was calculated at a 2:03.6, while the factory ECU fastest theoretical was a 2:04.2!

The Ralliart ECU appears to be the best set-up on the car, and now it is on me to learn to make the most of the new handling of my Evo.