Quote:
Originally Posted by markoz28
So for giggles I am going to chime in and see if maybe I can make some sense of this myself.
If a car weighs 1000lbs and has a front diff and rear diff. The engine loses 10% of power through each diff and then 10% of power through total weight. So 30% loss in total.
Now put the front diff in neutral and transfer all power to the rear diff. It will still lose 10% of power from the rear diff and 10% from the weight but will not lose 10% from the front diff as it will just be along for the ride. So 20% loss in total.
In theory, xdrive would be better from a launch due to traction obviously. The RWD will do better from a roll due to less drivetrain loss.
Take a truck with all wheel drive and a truck with 2 wheel drive. Both weighing the same. The 2 wheel drive will have the advantage. This has been proven at the drag strip 1000 times after the 1/8th mile where traction is not a factor.
Of course I have been wrong before and willing to learn something new.
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And in this case you are wrong as well
To start with, you don't "lose" power through weight. I understand what you mean but it sounds wrong. Physics: power is needed to get work done. Accelerating weight is work. The more weight you have, the more power you need to accelerate for equal time. Since the power is the same, more weight = slower acceleration. Which is why, assuming perfect traction in both cases (!), xDrive is slower (more weight). In real life however xDrive is faster up to about 100-120 km/h as it provides significantly better traction down low.
Mainly however your understanding fails exactly where others do as well
Your front diff and axle are still set in motion! Don't forget that, that's the key here! Just because you now apply power through the rear wheels only, doesn't mean the front stop rotating! All of your front-wheel components (dif, axle, wheels...) still rotate, they still have a mass which needs to be set in motion! Where do you think the power to rotate that mass comes from?
And before you answer "the road" - think twice - does the road have power?
The only difference is that the "transmitter" of the power in the case of xDelete is the road (imagine a belt, like a timing belt, which connects the front wheels to the rear) instead of the TCU. So, same weight to be set in motion, just a different power transmission route.
Hence - no reduction in power losses.
Only time you would see power gains is on a dyno. Guess why? Because the front axle and dif stand completely still
do they stand still on the road?