:::::::READ CAREFULLY::::::: Now that there's been decent exposure about the benefits of modifying the rear differential (especially considering its not assembled properly from the factory), I'd like to turn some attention BACK to ACD Tuning - I'd like to reiterate a few points - Motivation: - Owners with evos overseas that come WITH AYC choose to disable the function in favor of a competent mechanical set up - think TRE or Cusco - This is particularly helpful for USDM evos, that came with neither. If you consider that Evos with AYC swap to upgraded 1.5 way mechanicals to improve peformance (Cusco's LSD wasn't just built for the USDM market), this should imply that a USDM swapped with a Cusco should perform on par with a JDM with a Cusco - basically they start at different places in terms of performance, but end up pretty much the same (if everything else is the same). - Anyway, moving forward from that, what's next? In terms of our European and JDM counterparts, the main focus is not AYC, but ACD TUNING - Tuning the ACD will actually complement the function of a competent mechanical rear differential, less and more lock up can dramatically improve both turn in and corner exit. - I'm sure many people will doubt the benefit of this kind of mod, as they did with the rear differential mod in the beginning - however, if you consider how the factory would tune the ACD (for safety) and considering the dynamics of an AWD car, it should be pretty clear that there can be a lot of benefit from getting the ACD tuned. - Some people, especially those with rear differentials, will wonder - do I need more? Won't I oversteer too much? The answer is no. If you think about it, that is dependent on setup. This will allow the car to exit out of corners faster, period. - New options on the market allow for quick and easy turnaround time, and makes the ACD reflash a very simple modification. - You can bet no serious time attack evo is running without the ACD modded. While most race teams went with the very expensive MoTeC or another equivalent custom map tuning system, there are now much more cost effective and efficient methods for anyone with an ACD to get it tuned. - This may slip through as a "costless" mod in terms of points in certain motorsports classifications, if not many, as it is such a niche modification With all that said, here are the options that I've been compiling for ACD tuning - it's come a long way since when I first made this post, and there are now even custom maps available for your particular driving. ACD Tuning Options: ACD-Tuning Aptly named, ACD-Tuning offers a wide varieties of maps, from Circuit to Auto-X and other custom maps - http://www.acd-tuning.com/. You can expect that these maps have had a LOT of time put into the programming to take advantage of all the different inputs the car receives. Price is competitive with Gruppe-S and they are in my eyes the first real option to Gruppe-S. However, you can expect that the reflashes here won't have the nasty low-speed binding of the center diff that the Gruppe-S reflash has received complaints about. ACD-Tuning claims at least a 1-2 second improvement on any given 60 second stretch of track - no small beans. Anyone who would like to challenge this claim - I'd love to hear about it. Honestly, at this point, this tune renders the Gruppe-S reflash obsolete. You can also contact lan_evo_mr9 for more information on this flash. *For those of us who already have the Gruppe-S ACD reflash, ACD Tuning will be offering discounted prices to make retuning the ACD a little easier to swallow. Gruppe-S This is the option that opened my eyes to ACD Reflash found here: ACD Reflashes Now Available!!!. Turn around time is quick and the difference is real. I at least suggest anyone who has seen it to give it a try. There was a thread about it, but it got thrown into service reviews, and ultimately not a large population of different people viewed it. Beyond this, at least for USDM evos, the options get a little less reasonable except for the more hardcore. Ralliart Next, there's the Ralliart ACD controller found here http://www.rhdjapan.com/ralliart-act...ion-viii-54389 . It is confirmed to work with the USDM cars, as discussed here Ralliart Active Center Differential Controller and here Ralliart ACD ECU . Unless you're doing rally, the second thread suggests that the Asphalt version, RMR-0728A, is the way to go for street driven USDM cars. Ralliart has had years more experience than anyone in the US when it comes to ACDs, so I think this is a great option for CT9A cars. This mod also disables AYC, had we had it anyway. Motec Finally, there is the MoTeC unit sold here http://www.worksevo.com/store/produc...roducts_id=664 . This is the priciest option by far, and while it offers many maps to choose from on the fly, this is most likely overkill for people who want something more like "high-speed map," "low-speed map" to set it and forget it. However, for the most hardcore and pro/semi-pro racers, this is probably the preferable option. It allows you to customize the maps how you'd like according to your personal driving, and it requires a lot of hours behind the wheel on the track to do so. It was pointed out later in the thread that the ability to have many different custom maps that you can switch between all on the fly can be a huge benefit in terms of rally driving, where you have a large variation in the types of surfaces you can come across, all within the same race. Kaps http://eng.kaps-transmissions.com/ne...ancer-934.html - seems to be rally oriented, plus a nice little video. However I'd like to iterate here that ACD tuning is not just beneficial for rally driving, as some may think. Please check out the video Concluding: There have been a lot of reviews generated already and it really makes a difference in how the Evo handles. It eliminates a much of that midcorner-corner exit understeer you've no doubt experienced. Now that the rear diff mod has become so popular, I'm sure there's a question of either/or here - people asking "should I do the the ACD flash OR the Rear Diff?" and "I don't want TOO much oversteer" - this is a misunderstanding - both of these mods will compliment each other. The ACD reflash allows the center diff to lockup more efficiently, and the rear diff will manage that power better. Oversteer is a suspension issue. These differentials working together will allow the evo to rocket out of corners the way it was designed to. Leaving suspension tuning to be more about increasing overall grip, not just setting up the car to handle how you like it. Anyone saying well I don't need x, I already have y - that's like saying you don't need another second off of your time. Hopefully information about the ACD will become more widespread as a result of this thread. Anyone who doubts the effectiveness, I challenge you to try it.