A bit of Info:

I am a regular driver with NASA (South East and Mid-South Regions) where I run in the TTS group and also instruct. My car is a ‘work in progress’, slowly being turned into a decent TTS car as the driver also continues to improve himself. And I need at least as much work as the car (if not a lot more).

Like most anyone who has started tracking their Evo and gotten more and more track time (ie gone faster and faster), Evo understeer has been the primary handling issue from day one. I started on a bone stock suspension for my first HPDE and the list of modifications is very lengthy since then. The full list can be read through on my garage link:

https://www.evolutionm.net/forums/ga...Davenport.html

I made changes one at a time for the most part, hit the track, re-evaluate, learn to drive the improved version (IF it was improved), plot the next step. I have gotten some very helpful feedback from track regulars on this board, and the car has slowly but surely gotten better. But understeer is a fact of life in these cars. And getting it dialed out takes time, effort and patience.

The latest upgrade I made in trying to improve the car was a reflash of the ACD computer from ACD Tuning. Until this point, the single MOST EFFECTIVE mod to the car had been the Weir 12-plate rear diff upgrade. With the upgraded rear diff, corner exits were not longer a grinding tire exercise as you mashed the gas and the car began to plow off apex. With the upgraded rear diff, as the throttle moved to power, the rear will get involved and rotate the car coming off apex under power. None of the suspension mods and adjustments came close to the huge change the rear diff upgrade was. Most folks who have done the Weir (DIY) or had the Shep or TRE rear upgrades done will tell you the same thing.

So with the higher throttle drive off the corner working much better, I began to analyze what was going on mid-corner, especially on long flat sweepers. I did some rain driving on track and started playing with throttle, inducing understeer on purpose. What I relialized is that the OEM ACD unit does not get the rear end involved until you are at higher power (50% plus) throttle in most situations. On long fast sweepers where you are feathering throttle it still basically act like a FWD car and understeers painfully at times.

So off went my ACD unit to ACD tuning. I swapped many e-mails with Matt, discussing the car mods and tracks I frequent. I opted to get two custom maps in the Rain and Snow settings and leave the Tarmac map as OEM for a fallback setting and comparison point. He set the Rain map up as a moderate map with the Snow being and aggressive re-map.

My first outing with the new ACD unit was at Memphis with NASA Mid-South. Memphis is an odd track, but there are two long sweeping corners at each end. I drove the first session in Tarmac (OEM) to establish a baseline. The next two sessions I drove in the Rain (moderate) map. The last session of the day I drove the Snow (Aggressive) map and drove that map all day Sunday.

Reprogramming the ACD unit allowed me to use much smaller amount the throttle to control car rotation through the long sweepers. In the Snow map the ACD is much more aggressive in diverting power to the rear wheels. Driving a long sweeper close to traction limit, as you begin to mash the gas, there is a brief moment of the car starting to push, then you feel the rear go active and it starts to rotate nicely. At this point you can just lightly modulate the throttle to control rotation all the way through the corner. As you make apex and begin to track out and roll into throttle, the reprogrammed ACD unit and upgrade rear diff work in concert to deliver the kind of AWD experience that puts a grin on your face.

Another area the ACD reprogramming seems to help is in how quickly the ACD unit responds in other situations. There is a very slow series of tight turns on the back side of Memphis called the M. Driving this in an Evo is a matter of going in slow and working the line through the M to set up a high throttle exit through the end of it. Going in hot and hitting understeer puts you behind and you will fight the car all the way through. With the ACD upgrade, the partial throttle response through the M was nicely improved, cutting down on understeer. As you near the exit of these short-sharp turns you line up to pour on the coal through the last corner and here the ACD upgrade also showed dividends. The upgraded unit seems to respond much quicker. As I spied the exit line and rolled into throttle, the car immediately starts driving all 4 wheels and just claws it’s way out of the last turn. As you roll aggressively into throttle the upgraded ACD wastes no time in interpreting your intentions. It quickly sends power to both axles allowing you to just hang on and drive.

At Memphis on some very tired Hoosier A6’s, I cut 0.8 seconds off my time (77 second lap time). On some fresher tires, the improvement would have been better. But the pavement at Memphis is very rough, and I won’t go there on good fresh tires to just chew them up.
I have since done two more events, Road Atlanta and back at Memphis. I did lower my personal best at Road Atlanta slightly, but a jacked up TT schedule, people dropping copious amounts of oil on the track, and some traffic issues kept me from really pressing it at RA.

I really want to get this upgraded setup back at CMP (Carolina Motorsports Park) where there are 14 turns and 13 of them are flat sweepers. I hope to do that this fall. When I do I will update this mini-review. The last time I was there, this track brought out the worst in Evo understeer of any I visit.

Matt at ACD Tuning was very responsive and turned my unit around quickly. The Rain map was a slight improvement over stock, but just noticeable. The Snow (aggressive) setting is where I run the car now and it is a very noticeable change over OEM. This does give the car a ‘looser’ feel under power through turns. You can easily make the rear end step out if you get foolhardy with the throttle in a long sweeper. But controlled use of throttle balances the car in these situations.

A final note, I do not know how effective this mod would be without the rear diff upgrade. Sending more power to the rear wheels through the castrated US Spec OEM rear diff might be like pouring water in a cloth sack and expecting it to hold. Without the ability to channel some of that power to the outside rear wheel, I would question how much benefit one would see. With the OEM diff, once the inside rear wheel gets very light (or even lifts) it will just spin away while the outside rear just rolls along. I suspect these two mods (rear diff and ACD re-map) go together like ‘peas and carrots’ as Forrest would say.

If you seriously track your Evo with the ACD system, this is an upgrade worth looking into. it is not an all inclusive cure for understeer. But it is another very useful piece of the solution.

 

I've noticed this, as well, albeit under less controlled circumstances. With this in mind, I have a question. Perhaps my understanding is incorrect, the ACD varies the lock between the axles and our torque split is always 50-50. So, how could it turn into (essentially) FWD under given conditions? I'm obviously missing a crucial piece of the puzzle. TIA

 

This may stand to be corrected by those that understand the ACD system better than I do, but I think when the ACD pump is at zero pressure, the car is only driving the front wheels. And as the pump increases pressure, it increases the lock between front and rear.

Perhaps someone can further elaborate?

 

As far as I understand the ACD is like a variable lock up LSD. It doesn't "engage" the rear as needed, but it limits torque differential front and rear as pressure goes up. It just so happens the diff would rather send more torque to the front than rear under those cornering conditions with the ACD pressure being lower. If you ran with a broken pump the car is still awd but with a fully open center diff.

 

just saw this, but the car is always driving the front and rear wheels, and the center diff controls the lockup - much like the car is always driving the rear left and rear right wheels, and the diff manages the torque between them