Edd W

A little excert from the Technical guide to the A8 PDF published by Audi on the launch in the US - 'Engine Mounting
Torque Support Valve N382
The purpose of Torque Support Valve
N382 is to cushion the drive axle and and
driveshaft torque. Its position at the right
front corner of the engine is ideal. This is
where the engine motion resulting from
driveshaft and drive axle torque
accumulates.'

It goes on to detail how the electro-mechanical mounts work and the way they are used to enhance cabin refinement.

There does not seem to be any mention that the engine would just sit happily if they were not bolted in place though, funnily enough!

They even make mention of the positioning to the right, to counter the torque reaction for the propshaft.

Mister Bally

S4master can provide answers to all of your torque calculations. There may be a rounding error or two though ;o)

s4master1

calculations are not/were not the point I try to make. however, on what way calculations help determine if the mounts are bad or not? can somebody answer that?
you can go to the dealer and tell that to the service writer and see his/her answer, the guy was just asking what to tell the writer about the mounts, which by experience I know they collapse and causes the front mount to break, on the A8 the reason is mounted towards the passenger side because the cooling system is larger and there's not enough room to install it in the middle like the other models, plus the fact that is aluminum.
Audi in order to avoid replacing the main mounts provides a new front/torque mount with an adjustable ring so it can be adjusted to match the height of the main mounts avoiding the expense to replace them as they are quite expensive.
the equation posted here if you look carefully does not apply to this car as it was stated, that applies to an old 4x4, center transaxle system on which the entire force reacts right at the back of the transmission with no axles attached to the engine or transmission body after that.
the mount in the front is used to stop the engine from moving up or down in the front not to prevent it from "rotating".
Am posting pictures of broken mounts/brackets from 2.7 S4s during (launching = accelerate to 5k RPM put in gear and let the clutch go), the mounts break equally on both sides not just the driver's side.
I get the point but OLD pushrod cast iron engineering is just that OLD, times have and continue changing, plus engineers usually carefully consider all factors carefully on the specific task before they make a conclusion decision or comment, no argument there as is not my field, my son is graduating as a mechanical engineer this year from one of the best schools in the country, so that will be his department.
I doubt many here have the experience on enough model/years Audi cars to even make comments or judgements specially on the newer ones.
Just an example, that discussed mount is no longer used even on the V10 or diesel engines.
does anybody knows why?
does anybody knows why the front differential is mounted on the passenger side (US) and not on the driver's side?.
Now on other cars like the PORSCHE Cayenne and Panamera with V8 engine they are not call engine mounts, is call engine suspension and also as the use a transaxle they use a torque reaction mount attached to the passenger side cylinder head. on that car I would not argue the equation even though there are factors not mentioned involved.
now look at PORSCHE 911 let's say 2002 the mounting points on the engine are right at the middle just below the crank pulley and on the very front of the transmission, so how is it that they don't use a torque mount?
on newer DIESEL Audis the mounts have vacuum filled chambers to soften and stiffen as needed, how can it be possible to have this room for vacuum if the mount has to handle all the torque of a 8 or 10 cylinder DIESEL.
engineering continues innovating, improving, evolving (if that is the right word) some of us don't.

mishar

On way of designing them properly.

Fact that it is aluminum has nothing to do with its placement. It is moved to the right in order to counter torque at the propeller shaft.

Not true. Equations are for Quattro transmission.

Wrong again. It prevents rotating in longitudinal plane, that’s what you call moving up and down, and in transversal plane, that’s why it is moved to the side.

Very good. Then ask him about forces and torques on Quattro transmission.

In order to allow more space for the gas pedal.

Edd W

I love your example of the 911 - is that not one engine and transaxle combined into one unit - the driveshafts come straight out from the sides of the transaxle so the torque reaction will not be a rotational effect in line with the car body, it will be lifting force at the front of the gearbox and a downward force on the rear of the engine - hence the requirement for mounts at the front and rear with hardly any requirement for stabilisation at the sides.

The reason the front mount is important on almost any quattro Audi is to ensure the torque effect on the engine and gearbox from the front driveshafts is controlled and that it does not cause too much deflection of the propshaft.

Remember, the engine and gearbox assembly is being subjected to rotational forces in two planes, unlike most front or rear wheel drive cars.

Edd W

Another good example - motor bikes.

On shaft drive bikes the additional twisting torque the rear suspension has to handle is identical to that car engine mounts are subjected to.

A bike with a chain drive only has the torque effect which lifts the front wheel - imagine the rear suspension on a chain drive bike as like the front drive shafts in relation to the engine in a quattro Audi and the shaft drive bike as like the prop shaft going to the rear wheels.

In the 1980's when Peugeot were developing the 205 T16 rally car they had a real issue with drivers lifting off the throttle over a jump - the negative torque reaction was so strong it was causing the car to nose-dive. It was a transversly mounted mid engined car and the car was pivotting around the crankshaft. I cannot remember how they cured it but I think it was as crude as simply telling their drivers to resist the urge to lift-off... or die!

s4master1

great examples on the rally cars and the bikes.
as far as I know that was the only instructions given to the drivers, but remember how light those cars are and the position of the motors.
never argue the torque forces and reactions.
as a perfect example of how technology/engineering continues to advance is the bikes, specially chain driven with large displacement engines.
on my Aprilia RSV1000R if you not careful releasing the clutch the front wheel lifts very easily, no argument there.
next week am picking up a new Bmw S1000RR which is equipped with DTC= (dynamic traction control), that program prevents the front wheel from lifting up on take off, as well limiting the throttle if the bike leans more than 38 degrees on street and rain modes, (allows up to 53 degrees on racing mode).
all this accomplished by software and gyroscopic sensor/s.
great explanation on the 911, it almost makes a perfect cross between the mounts and axles counteracting each other, glad to see somebody can explain this in plain simple language without any technical stuff needed.
Now back to the W12 A8, the engine was designed before the newer engine management software and was easier installing the "torque mount" than leaving the bracket empty or redesigning the alternator/filter housing. This is the kind of part called "WISDOM TOOTH" if is there sure it can do something but if is removed you will not miss it.
there's a reason this car has 2 engine management control units.
hope many of you have experience with newer model cars specifically Audi, On newer cars you can not accelerate to RED LINE like older cars while sitting at idle, the computer will limit acceleration to 2900 RPM only.
while in gear and pressing the brake you will only be able to accelerate at 1200 RPM (still have to check on the w12 the limit), and while in basic settings mode you can push the throttle to the floor and will only let you accelerate up to whatever channel you working requires to.
Now I too know cars like camaro, mustang and many other high torque equipped cars if you accelerate the engine it tries to rotate, specially if in gear it almost wants to jump out of the car when the mounts or left mount are broken.
look at new/newer Audi models they don't have anything to counter or dampen this force is all accomplished thru the management software.
never wanted to open an argument here just plain simple way of explaining why the main mounts need replacement at the same time, As I wrote Audi now offers new mount with an adjustment ring to avoid replacing them all, but mostly is only used if the car is under warranty, if not they will sell you all the mounts together with a humongous labor bill.
I explain this situation on a regular basis to customers. Does Mr Smith gives a flying kite how the electrons flow when his window motor is not working?, he just wants it done at the best possible price period.
Now if Mr Smith comes to the service writer giving him/her a class/lecture of how things ARE OR SHOULD be designed, the writer will tell the technician to do the very minimum possible and give Mr Smith's car what is call the SUNSHINE SERVICE".

s4master1

and also let's not forget the new position of the front axles and transmission mount.(no longer 2 mounts)

notice on the picture the position of the front differencial

Gzila850

42,000 miles... dealer service department in Houston area reports "excessive fluid leaking" from motor mount. $1285 to replace. Exploring options....

Brozee

That sounds about the right price. Charged me $1100 to partially remove the engine to replace a coolant sensor. While you have the mount replaced have them thoroughly go over everything else as you don't want to have it dropped again for any other miscellaneous part; coolant sensor, O2 sensor (one of many, many), starter, etc...