Billy

iTrader: (8)

Is the logic behind the ACD units smart enough to know that when the following conditions are met

1) full acceleration as determined by g sensors
2) with zero tire slippage
3) above a certain speed

to fully open the center diff making it essentially a FWD thus minimizing drivetrain losses and making an EVO much faster when accelerating traction is abundant?

I understand that's not the primary objective of this piece of equipment but with the electronically controlled ACD, would it not simply programming smarts into it to do this? Seems like a good idea to me as I can't of any reason not to do this.

Billy

iTrader: (8)

Maybe I should clarify.

Take CAR #1 with a fictitious AWD mechanism whose center diff mechanism delivers equal amounts of torque to the front and rear wheels for a 50/50 torque split. I've heard that the parasitic drivetrain losses from the crank to the wheels can range as high as 35%. So, in this scenario, if we can get 300HP at the crank, we will be putting down 195HP at the wheels with assuming a 35% AWD drivetrain loss.

Now take CAR #2 with a fictitious FWD car with 300HP at the crank but with only a 20% (this is purported to be about right by a few people) drivetrain loss. This is much less than the AWD mechanism because, in comparison, it doesn't have to turn as many gears, shafts, etc. This car with the same engine will put down 240HP to the front wheels.

Now without dwelving into the significant differences in AWD dyno methodologies, we can say that IF traction is plentiful CAR #2 would be more ideal in an acceleration situation over CAR #1 because it is delivering more power to the ground.

The EVO7 has the capability of instantly disengaging the center diff as in the case where you pull on the e-brake. It also has sensors that recognize your forward rate of acceleration through g-force sensors.

Based on a fews viewings of a Best Motoring race where a "280" HP JDM EVO7 pulls away from a ~340HP E46 M3 on a straightaway where traction and aerodynamics probably aren't factors, I'm wondering if Mitsu worked some magic? And yes, I realize the E46 is quite a bit heavier than the E7

I'm wondering if they designed some logic into the ECU to open the center diff when a car is under full acceleration and traction is not an issue. For example, the ECU determines that driver needs straight line acceleration and in a split second, the center diff opens to minimize parasitic losses. And I whacked?

This opening of the center diff shouldn't be an on-off switch thing but rather it should be constantly monitoring slip in the front tires. As soon a slip is detected, then quickly transfer torque to the rear.

Mind you it would not explain why the E7 is still slower in a straight line over an E6 even though the E7 is slightly heavier with a slightly more torquey motor.

Incidentally do any of the Skylines do this?

cvl

Back in the day, Audi claimed that their Quattro system actually had less frictional losses and resulted in higher HP at the wheels and higher MPG than thier 2WD cars. They claimed that it actually took less energy to turn the wheels under power than to pull them along.

snelson

2 interesting issues here. First, does a 4wd car absorb more power than a 2wd car. I was aware of the Audi claim but the standard way of measuing this loss, on the rolling road, does not support this. The car is accelerated on the RR as usual and on raching max power, the clutch is disengaged. The rolling risistance of the drivechain can then be measured through the deceleration of the RR. If one considers the extra mechanicals needed for 4wd, it is no suprising that this increases the power losses.

This info is needed when considering the situation of the ACD. The ACD unit does, in fact, spot when it would be most benefitial to move the power to the front wheels under acceleration. The settings, tarmac, gravel and snow determine how much in total is distributed, with tarmac giving more power to the rear than gravel and snow.

However, the ACD does its trick to improve the handling, so it moves the drive to the front wheels coming out of a corner under acceleration, to help pull the car through without oversteer. In straight line acceleration, I believe that it keeps the "base" power distribution for the particular setting because that will reduce wheel spin the most.

The final point to note is that if one considers how the loss is usually meassured, the friction from the components that distribute the power to the rear wheels is still largely there even when power is not distributed to them. There will be a readuction, but not a great deal. Think about it - the AYC or diff, prop shaft etc still turn.

However, for fastest acceleration, it does seem that having the ACD on the snow setting appears to help, particularly on standing starts.

oli goon

snelson,

good response. I almost understood it



billy,

R32 skylines were basically RWD, only transfering grip to the front when slip was detected. This resulted in bad over steer. It is possible to change this by rearrangoing the electronics a bit. the book 21century perofoirmnace by Julian Edgar has it all in.

In this respect they are fundamentally different from Evo's. This is probably b'coz the Evo is from a rally heritage whereas Skyline isn't.