Hi folks, been a bit of a lurker here for a while. I don't own an Evo - shhh - but rather a Legnum VR4. This car has been fully worked now - 500hp at the crank on E85 (280kw at all four wheels), new old-stock S-AYC rear diff, built gearbox, new old-stock xfer case. Here's a pic of the car in her current state.


I like electrics, so I've been really keen on installing the ACD setup from an Evo 8/9 into my car. It would certainly be one of a kind, and the benefits of ACD are really tempting.

So....why do Evo 8/9 owners keep switching from ACD back to the viscous coupling? From what I can gather, ACD has:

• Much quicker and stronger torque lock-up front to rear
• Integration with the AYC diff (ACD and AYC will lock up in a manner that complement each other for better yaw control)
• Release of the ACD clutch when you pull the handbrake, allowing for better yaw control when doing stuff like motorkhana.

The viscous coupling in the older Evo 1-6 transfer cases is a sealed silicone fluid unit that locks up only when the fluid heats up under friction, which delays the lockup and makes AYC less effective because it needs the torque going to the rear wheels to do its job.

Yeah so I guess I'd love to know what the beef with ACD is.

 

Hey, welcome. Really like what youve done w/that VR4 wagon. Well done

Im certainly not the drivetrain expert but i do have an EvO9 w/ACD.

Side note: Many members have opted to move the ACD unit to the trunk area for a couple of reasons.

1. Better weight distribution
2. Keeping the ACD unit better protected against the elements

So, to take a crack at your question, the high cost of replacing the ACD unit if / when it fails have prompted some to consider eliminating the unit all together & using a transfer case from a non ACD EvO

Ive even considered it if my ACD ever fails. Id rather not have the pump unit & just have a pure mechanical front LSD w/out all the gizmos

Cheers,

 

Most guys do it for weight and drag racing. But the current fastest drag Evo's in the USA (English Racing's 8 and X) still run ACD. ACD is certainly superior. Removing it is going backwards from a performance aspect.

 

first off... that car is bad ***... way cool!!

I have a non acd car with the wavetrac diff.... and i love the way it trac's.
hard in corners gives just a little understeer, then as soon as you get back on the gas, it will slow drift both front and back., and gets the car to turn.
its how a car should handle.

i dont remember my acd car handling with such confidence....

i love my non acd with wavetrac...

 

Interesting. I was originally going to install an ATS plated diff in the front (with transfer bolt upgrade from 8mm to 10mm) but then I was asking myself why I didn't just go super nerdy and install ACD. The VR4 uses an open front differential from factory.

Would love some more responses folks - keep 'em coming. Especially if you have some deep technical knowledge of the difference in handling between the Viscous Coupling Unit and the ACD transfer case. I've looked at a few tech diagrams and videos, but I can't tell if the ACD transfer case is still a Viscous Coupling plus clutches, or if it's purely just clutch based (and the car would go into FWD mode if ACD ever failed).

 

If the ACD fails, you still have the mechanical link between the front and rear as the car still has a center diff, it just turns into a fully open center diff.

The front diff has nothing to do with the difference in an ACD vs VCU car.

 

ACD came from rally racing and was tuned for each course and driver. To give a private owner a taste of its potential they throw in locking programs for three surfaces. The general idea is that when the car is going straight the center differential is locked but when cornering, unlocked. This is only a taste of what the ACD is because there’s all sorts of possible variations in how much it locks or when the locking and unlocking takes place. This was some of the race tuning that gave Mitsubishi so many championships. For a private owner who doesn’t track or rally its mostly a light on the dash.

 

the main driving difference is that the acd is active and will react and start locking or de-locking right after the driver steering input... it will not wait for cornering G or difference in axle speed. It will open and close multiple times in the same corner.. Also it will open when you pull the handbrake.. etc... You can really notice it working on slippery surface. One more thing it will let you do is alter your yaw angle in the middle of the corner while you are sliding... Try that in a viscous centre diffcar..
I

 

Yeah I know that, sorry. I was only saying that I was either going to:

1. Just install an ATS plated *front* diff and leave the current centre diff/VCU; OR
2. Install an ACD transfer case with the accompanying *front* helical LSD included in it, and go nuts doing all the computer and wiring work to make ACD function in a VR4 (including finding a place to put the pump).

 

I cant speak too much for the acd alone but i know autocrossing my X i was overwhelming the acd/ayc system. The diffs would screech and howl on aggressive corner exits or just unload all together after a few seconds and make the car push hard. Its nice to have but i feel that it loses a bit of its effect once big tq, corners, stiff spring rates and big hoosiers come into effect.

 

Does anyone know if the ACD system is tied to the ABS as well? I see occasional mentions of individual wheel braking as part of ACD. Does the Evo 9 also have ASC? That might be what they are referring to.

It's a bit hilarious which cars got what, to be honest. The 1996+ Legnum VR4 had ASC (basically - electronic stability control) but only on the automatic models. Some VR4s were sold with AYC as well, some with an open rear diff.

So it was possible to have a mid '90s Legnum/Galant VR4 with auto, AYC and ASC, or an auto with just ASC and not AYC, or an auto without AYC and without ASC. But it wasn't possible to buy a manual without AYC, nor was it possible to buy a manual with ASC. Sooo confusing.

 

no ASC on the 9... X got it.
ACD and ABS share some sensors (wheel speed, G sensors..) but you can run one without the other.. No Individual wheel braking because of AYC/ACD...

 

My 2003 Evo 8 has both a steering position sensor and G sensor but no ACD or AYC. So, the only thing those sensors can control are the brakes.

 

According to the evo 9 wiring diagrams the AWD Controller takes direct input (spliced into the same outputs) from the TPS sensor, Steering angular velocity sensor, G-Sensor (Longitudinal), G-Sensor (Lateral), parking brake switch, and the stoplight switch. All the wheel speed sensors are wired directly into the ABS Controller. There are then what appear to me to be 5 data wires connecting the ABS Controller to the AWD Controller. There is one wire going from the AWD Controller to the ECU.

I'll post the diagrams if anyone is interested.

 

First time I've ever heard of this. It's certainly not an issue for the ACD in the 8/9. And my friends X doesn't have this problem. Also on big sticky tires.