After having my TW AWD with the 6G75 motor dyno'd at SKR yesterday I was just thinking...

AWD magna with 3.5L has 154KW at the fly, and typcially 95-100KW at all four wheels
2WD magna with 3.5L has 155KW at the fly and say 125-130KW at the fronts
(please correct me if wrong, but they are just rough figures for arguements sake)

Those 30KW or so difference between a 2WD and AWD would have more factors than simply drive train loss. Although the AWD has 3 times as many diffs, power loss through a diff is a percentage of the torque. To break it down: Static friction loss, viscous loss (oil) and gear torque loss as a % (speed dependant). So putting power through 3 diffs should not give you 3 times the loss in power.

The other factor that comes to mind is for a 2WD to do a meaningful dyno run, to work out the total pushing force, the power consumed to rotate the other 2 wheels should be subtracted. The un-driven wheels also represent a power loss as several KW per tyre (we know that because tyres get hot). Whilst you might be measuring power at only the front wheels (or driving wheels) you must subtract the loss of the un-driven wheels to get a realistic or comparative understand of motive force - which is the acceleration of the vehicle.... and to compare it to AWD dyno results.

I think there is an error in the measurement of power because an AWD is having to overcome tyre loss of ALL FOUR wheels (which has nothing to do with drive train) on a dyno, whereas 2WD only has half the power loss through tyres. On the road a 2WD is having to deal with those extra forces and they cannot be measured on a dyno.

It is likely that a AWD dyno run is going to produce a more realistic result and it is unfair to say a 2WD is so much less drive loss when it is actually not turning all the wheels. BTW the tyre friction I'm referring to is the rubber on the road.

Dynos measure the tractive force of the driven wheels and calculates this into equations for shootout mode or similar. This is why tyre pressures can screw with dyno readings.

The power loss of an AWD is totally justified, simply because of the power-sapping diffs and prop shaft.

I'm not in any way a dyno expert... but...

I don't think its not just 'losses' in the traditional sense that have all the effect, you also need to realise that some of the energy from the engine is also going into increasing the rotational energy stored in the wheels (and any other spinning parts, diffs etc - not necessarily dissipating it).
In that sense, I would like to see the effects of a 2wd car on a dyno where some of the energy given to the dyno, is given back to spin the undriven wheels, as in real conditions, they would spin too.

Also, I don't think its completely correct to look at 'friction' as the cause of heat in tyres (unless doing fully sick burnouts) as a wheel travelling on the road should not be slipping. I would look more at the heat generated by the deformation of the rubber as the tyre rolls (similar to the way if u repeatedly bend a spoon back and forth, it will heat up)

The 2WD dyno run is not taking into account all the loses, those being the extra 2 wheels turning on the surface. Unless you're constantly driving with 2 of the wheels in the air! Based on fuel consumption difference between AWD and FWD magnas, on the highway, an AWD magna must be only losing 10% more power than a FWD, not 20-30% more like dyno results show.

Settle it on the strip, compare timeslips.

You can only base things on averages dude
depending on the drivetrain components you are looking at:
15-25% loss in a FWD
20-30% loss in a RWD
25-35% loss in an AWD

These all being manual ofcourse.

If you want to really show how good a car is you take it to a track or the strip and compare timeslips, dyno's like many things are all a 'best guess' The values plugged in try to even it up for all vehicles but they will never be 100% on the money.

This is why people think 3.8L magnas are about 190 at the fly using a fwd drivetrain loss but if you actually go in one it is clearly more.

The 2WD dyno run is not taking into account all the loses, those being the extra 2 wheels turning on the surface. Unless you're constantly driving with 2 of the wheels in the air! Based on fuel consumption difference between AWD and FWD magnas, on the highway, an AWD magna must be only losing 10% more power than a FWD, not 20-30% more like dyno results show.

So you are going against everyone in the whole world? AWD losses are typically the same for ALL cars. Hub dynos between FWD/RWD/AWD prove this as it removes losses out of the equation from tyre-dyno friction.

Besides, the drag caused by the two rear tyres being pulled and not contributing to drive is absolutely negligable. The forces at play to turn extra diffs and shafts would absolutely be about 98-99%

I'm with Dave - I think rolling resistance from the back end on a FWD would be 1% at the most.
Interesting idea though, I do like the way your mind is working, Munkey.


As for the mention of heat & friction from tyres flexing where contacting the road surface, just run higher tyre pressure. Better fuel economy and less tyre wear from overheating.


Dyno power figures @ wheels versus flywheel and IMO:
FWD drivetrain loss is ~20% - has 1 diff and 1 gearbox
RWD drivetrain loss is ~30% - has 1 diff , 1 gearbox and 1 90 degree change in direction (driveshaft to rear diff)
AWD drivetrain loss is ~40% - has 3 diffs (Front/centre/rear), 1 gearbox, 1 transfer case and 1 90 degree change in direction.

Also, from what I remember reading around here, if I've remembered the dyno figures correctly, then my 20/30/40 estimates are pretty close.
165kW FWD VRX's dyno around 130kW = 21.2% loss.
165kW AWD VRXs dyno around 100-110kW = 39.4 to 33.3% loss. (maybe these 110kW were modified, so gained a little power?)
155kW AWD Exec dyno around 90kW = 42% loss.

So as a rough guide, I like 20/30/40 % loss for FWD/RWD/AWD.

Oggy you just gave figures from a manual FWD and auto AWD, you can't compare these...

If you put your hand out the window at 110km/h, there is a huge amount of airflow... this 110kmh air is blowing over the tyres as they roll, yet when you stop the tyres are still warm. To me this suggests a measurable amount of KW has gone into rotating the tyres on the road.

Yeah a hub dyno would be a better comparison of drive train loss between 2WD and AWD.

But the real test is how fast can you accelerate up to the speed limit, in the wet ;)

If you put your hand out the window at 110km/h, there is a huge amount of airflow... this 110kmh air is blowing over the tyres as they roll, yet when you stop the tyres are still warm. To me this suggests a measurable amount of KW has gone into rotating the tyres on the road.

Yeah a hub dyno would be a better comparison of drive train loss between 2WD and AWD.

But the real test is how fast can you accelerate up to the speed limit, in the wet ;)

I think that is more a test of traction and weight. In the dry its obvious that an average FWD will flog an AWD even if they are both auto purely due to the slightly higher loss in power and the heavier car that the awd is, awd becomes advantagous when you are talking BIG power, as soon as fwd hits about 200fwkw and equated torque figures it is no longer superior and an AWD with the same engine would not only give more reliable times but the times would be quicker.

if you put your hand out the window at 110km/h, there is a huge amount of airflow... This 110kmh air is blowing over the tyres as they roll, yet when you stop the tyres are still warm. To me this suggests a measurable amount of kw has gone into rotating the tyres on the road.

Yeah a hub dyno would be a better comparison of drive train loss between 2wd and awd.

But the real test is how fast can you accelerate up to the speed limit, in the wet ;) lol lol lol lol omg i like this one lol :)

I think that is more a test of traction and weight. In the dry its obvious that an average FWD will flog an AWD even if they are both auto purely due to the slightly higher loss in power and the heavier car that the awd is, awd becomes advantagous when you are talking BIG power, as soon as fwd hits about 200fwkw and equated torque figures it is no longer superior and an AWD with the same engine would not only give more reliable times but the times would be quicker.

Focus RS500 has around 220kw atw and 460Nm and still behaves with FWD. the limit is certainly higher in modern FWD cars that employ new tech. The barrier is now about 250kw for FWD. food for thought, thats the same wheel power as a VE HSV clubby

Focus RS500 has around 220kw atw and 460Nm and still behaves with FWD. the limit is certainly higher in modern FWD cars that employ new tech. The barrier is now about 250kw for FWD. food for thought, thats the same wheel power as a VE HSV clubby

That's because it has the majority of the weight OVER the driving wheels ;) Unlike a HSV VE...


I think that is more a test of traction and weight. In the dry its obvious that an average FWD will flog an AWD even if they are both auto purely due to the slightly higher loss in power and the heavier car that the awd is, awd becomes advantagous when you are talking BIG power, as soon as fwd hits about 200fwkw and equated torque figures it is no longer superior and an AWD with the same engine would not only give more reliable times but the times would be quicker.

Or AWD for accelerating onto a highway after stopping on the off-camber ball bearing gravel road side lol
On a serious note, i only started this thread after a thought i had... not saying i'm right, but i wanted to discuss the dyno result differences.

That's because it has the majority of the weight OVER the driving wheels ;) Unlike a HSV VE...



You need to understand that under acceleration, mass shifts backwards, allowing RWD to put power down as it squashes the rear of the car. FWD is the opposite and under acceleration mass shifts OFF the driving wheels and affects traction

Focus RS500 has around 220kw atw and 460Nm and still behaves with FWD. the limit is certainly higher in modern FWD cars that employ new tech. The barrier is now about 250kw for FWD. food for thought, thats the same wheel power as a VE HSV clubby

Important talking magna chassis.

Good theory, can I slap a spin on this.... Lets say we had three magnas, all with the same engine power all the same weight ( I know they wouldn't be, but for this let's just say they are )
*1 is FWD
*2 is RWD
*3 is AWD

Now, if none of the cars are moving and up in the air... We all know that the power at the wheels from highest to lowest would be FWD TO AWD.., that's just with the wheels just spinning in the air.. Because even with the wheels not touching anything there would still be power loss from the friction of the drivetrain, "shorter drivetrain less friction"
And also power loss through diffs etc,

Okay so now we slap the car back on the ground....
Now every wheel has friction when you drive.., and friction makes heat.. so it doesn't matter if it's FWD, RWD, AWD.. The wheels are spinning at the same speed and same weight

Roll road dyno's are very clever.. They are built to be 98% correct, and has different setting for FWD, RWD, AWD making sure to give you a power figure that it tells you.. what your getting when all four wheels are spinning on a road, that's why rolling road dynos give a more correct power figure then hub dynos, as it takes in more factors

Hope that helps, ... Hehe though it doesn't matter if you do loss a little bit of power by having AWD, you'll always win every time, it corners and on the wet.. It's worth the loss ;)



After having my TW AWD with the 6G75 motor dyno'd at SKR yesterday I was just thinking...

AWD magna with 3.5L has 154KW at the fly, and typcially 95-100KW at all four wheels
2WD magna with 3.5L has 155KW at the fly and say 125-130KW at the fronts
(please correct me if wrong, but they are just rough figures for arguements sake)

Those 30KW or so difference between a 2WD and AWD would have more factors than simply drive train loss. Although the AWD has 3 times as many diffs, power loss through a diff is a percentage of the torque. To break it down: Static friction loss, viscous loss (oil) and gear torque loss as a % (speed dependant). So putting power through 3 diffs should not give you 3 times the loss in power.

The other factor that comes to mind is for a 2WD to do a meaningful dyno run, to work out the total pushing force, the power consumed to rotate the other 2 wheels should be subtracted. The un-driven wheels also represent a power loss as several KW per tyre (we know that because tyres get hot). Whilst you might be measuring power at only the front wheels (or driving wheels) you must subtract the loss of the un-driven wheels to get a realistic or comparative understand of motive force - which is the acceleration of the vehicle.... and to compare it to AWD dyno results.

I think there is an error in the measurement of power because an AWD is having to overcome tyre loss of ALL FOUR wheels (which has nothing to do with drive train) on a dyno, whereas 2WD only has half the power loss through tyres. On the road a 2WD is having to deal with those extra forces and they cannot be measured on a dyno.

It is likely that a AWD dyno run is going to produce a more realistic result and it is unfair to say a 2WD is so much less drive loss when it is actually not turning all the wheels. BTW the tyre friction I'm referring to is the rubber on the road.

Important talking magna chassis.

You just said FWD though, nothing about FWD Magna

You just said FWD though, nothing about FWD Magna

It is presumed context being written on a magna forum, not to mension it is fwd vs awd, which only applies to certain cars as most don't come as both.