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Old 11-04-2018, 11:52 PM   #29
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Originally Posted by CHRISCAM View Post
I'm having a difficult time understanding how a given transmission and rear end assembly doesn't use the same amount of torque/ hp to turn, regardless of what engine is mounted to the front of the bellhousing.

The shop installs an electric test motor to determine the amount of torque/ hp required to turn the existing driveline at a given rpm. It's determined by this test procedure that 50 hp/ lb. ft of torque are required to turn the assembly at 3000 rpm.

Someone explain in the flaw in my logic. I'm not being sarcastic here, by the way.
Yes you are correct, having a 1000hp engine on the front of the assembly or a 50hp electric motor it would still require the same power to turn the drivetrain at a given rpm with "no" or the same load at the wheels (at the same temperatures-friction).
But it isn't turning only that we are talking about. It is transmitting differing loads that also change direction and speed. More load, more friction. Did you not read my post?
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Old 11-04-2018, 11:52 PM   #30
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Of course I'm probably only making around 2 hp on Jupiter. (if that)

Most important, how much are you pushing on Uranus?




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Old 11-05-2018, 06:44 AM   #31
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Originally Posted by CHRISCAM View Post
I'm having a difficult time understanding how a given transmission and rear end assembly doesn't use the same amount of torque/ hp to turn, regardless of what engine is mounted to the front of the bellhousing.
The powertrain does not "soak up" any fixed amount of power because friction doesn't work that way. If it did, you would be hard-pressed to turn either the transmission over by hand or turn the wheels by turning the driveshaft with the transmission in neutral and the car lifted at the chassis (drive wheels drooping free).



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Imagine a dyno shop that has a given transmission (let's say it's a T6060) mounted in their test stand, along with a 48" drive shaft, backed up by a full Gen. 5 Camaro IRS rear end and rear wheels (essentially, a full Gen 5 Camaro, minus the body and interior, just the frame with a drivetrain).

The shop installs an electric test motor to determine the amount of torque/ hp required to turn the existing driveline at a given rpm. It's determined by this test procedure that 50 hp/ lb. ft of torque are required to turn the assembly at 3000 rpm.
Against how much load? How much car acceleration would result if the same 50 HP motor was powering a real car on a real road?


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Now, if the shop then installs a 430 hp LS3 ahead of that T6060, wouldn't that driveline still consume 50 hp/ tq at 3000 rpm? That would leave 380 net hp at the rear wheels (which figure I have intentionally used in this example because 380 hp is pretty close to average rwhp on these Gen 5 LS3s, it seems).
If the LS3 was running against an identical load to your 50 HP electric motor test (or running the same amount of acceleration on the same road), it'd only be putting out the same 50 HP. You'd just be throttled way back from WOT to get that much.



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Or, per the OP's question-- 800 crank hp should have around 750 at the rear wheels with this same driveline?

Someone explain in the flaw in my logic. I'm not being sarcastic here, by the way.
You and OP are trying to say that the total of all the friction forces involved remains constant, when it's the proportion of friction to power/torque input that's the mostly constant relationship.

That line of thinking is like saying it would take the same amount of force to drag a pallet across the shop floor with a couple of engines strapped to it as it would to push the same pallet across the same floor with only something light like a trunk lid sitting on top of it instead.


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Old 11-05-2018, 12:07 PM   #32
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That’s a neat way to explain it. Cool. Thanks.
Except what he is talking about is water pushing resistance on the hand or in comparison is the resistance the wind puts on the car. This has nothing to do with horsepower that the drive train is using up. There is no increased wind resistance in a Dyno room. This factor has an effect on terminal velocity though.
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Old 11-05-2018, 06:32 PM   #33
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Which weighs more a pound of feathers or a pound of lead
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Old 11-06-2018, 12:26 AM   #34
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Originally Posted by Norm Peterson View Post
The powertrain does not "soak up" any fixed amount of power because friction doesn't work that way. If it did, you would be hard-pressed to turn either the transmission over by hand or turn the wheels by turning the driveshaft with the transmission in neutral and the car lifted at the chassis (drive wheels drooping free).




Against how much load? How much car acceleration would result if the same 50 HP motor was powering a real car on a real road?



If the LS3 was running against an identical load to your 50 HP electric motor test (or running the same amount of acceleration on the same road), it'd only be putting out the same 50 HP. You'd just be throttled way back from WOT to get that much.




You and OP are trying to say that the total of all the friction forces involved remains constant, when it's the proportion of friction to power/torque input that's the mostly constant relationship.

That line of thinking is like saying it would take the same amount of force to drag a pallet across the shop floor with a couple of engines strapped to it as it would to push the same pallet across the same floor with only something light like a trunk lid sitting on top of it instead.


Norm
Norm, no offense, but that line of reasoning is flawed too. A given transmission and rear end, as used in my hypothetical chassis dyno setup, will require the same amount of torque to turn it at a given rpm, regardless of what the source of that torque is.
Using your analogy of a pallet being dragged across a shop floor with two dissimilar weights strapped to them is NOT what I was describing. I specifically mentioned using the same transmission and rear end assembly in both examples precisely so that it would keep that source of frictional loss in the assembly as a constant.
I'm not saying that two different engines will have the same frictional losses in their respective rotating assemblies, I'm saying if those two engines (and my hypothetical electric test motor) were all mated to that same transmission and rear end assembly, that trans. and rear end would require the same amount of torque to turn them.
That, in my mind means that a given transmission and rear end assembly should, at least in most cases, provide a constant factor of hp/torque loss when paired to a given source of twisting force.
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Old 11-06-2018, 06:54 AM   #35
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Originally Posted by CHRISCAM View Post
Norm, no offense, but that line of reasoning is flawed too. A given transmission and rear end, as used in my hypothetical chassis dyno setup, will require the same amount of torque to turn it at a given rpm, regardless of what the source of that torque is.
Using your analogy of a pallet being dragged across a shop floor with two dissimilar weights strapped to them is NOT what I was describing. I specifically mentioned using the same transmission and rear end assembly in both examples precisely so that it would keep that source of frictional loss in the assembly as a constant.
I'm not saying that two different engines will have the same frictional losses in their respective rotating assemblies, I'm saying if those two engines (and my hypothetical electric test motor) were all mated to that same transmission and rear end assembly, that trans. and rear end would require the same amount of torque to turn them.
That, in my mind means that a given transmission and rear end assembly should, at least in most cases, provide a constant factor of hp/torque loss when paired to a given source of twisting force.

I think you have to figure acceleration in somewhere.
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Old 11-06-2018, 07:12 AM   #36
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Originally Posted by CHRISCAM View Post
Norm, no offense, but that line of reasoning is flawed too. A given transmission and rear end, as used in my hypothetical chassis dyno setup, will require the same amount of torque to turn it at a given rpm, regardless of what the source of that torque is.
As long as the load that it is working against never changes, of course. But are you willing to always use (to use the test example numbers) exactly 50 HP and 3000 rpm in your driving so that you could make 'absolute powertrain losses remain constant' a true statement for you and your driving?


Quote:
Using your analogy of a pallet being dragged across a shop floor with two dissimilar weights strapped to them is NOT what I was describing. I specifically mentioned using the same transmission and rear end assembly in both examples precisely so that it would keep that source of frictional loss in the assembly as a constant.
You're missing the point. The amount of friction depends on the amount of force between two objects (whether they're gear teeth or pallets on the shop floor is irrelevant) and the coefficient of friction that exists between them when one of them is forced to slide across the surface of the other one.

What you're still not getting is that the forces between the gear teeth when you're transmitting 400 HP through them is much higher than if you've only got 50 to work with. And power transmitted is where the majority of the friction losses are coming from. Very little power is needed to simply turn the gears, this being why you don't need to be Superman to turn the transmission or the differential over at low speed by hand (you're not working against much load, so you don't need to be very powerful).


Quote:
I'm not saying that two different engines will have the same frictional losses in their respective rotating assemblies, I'm saying if those two engines (and my hypothetical electric test motor) were all mated to that same transmission and rear end assembly, that trans. and rear end would require the same amount of torque to turn them.
That, in my mind means that a given transmission and rear end assembly should, at least in most cases, provide a constant factor of hp/torque loss when paired to a given source of twisting force.
Constant factor (e.g. 15%) isn't the same thing as constant amount (some flat amount of HP lost). Try to avoid confusing the two.


One last try . . . if the absolute value of friction losses remained constant (like OP thought had to be the case), nobody over on M6G would be complaining about differential temperature warnings when they take their PP2 out to the track, there wouldn't be legal action involving GT350's that didn't get this cooling, and Camaros wouldn't have differential and transmission cooling systems either.


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Old 11-06-2018, 07:12 AM   #37
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Originally Posted by CHRISCAM View Post
Norm, no offense, but that line of reasoning is flawed too. A given transmission and rear end, as used in my hypothetical chassis dyno setup, will require the same amount of torque to turn it at a given rpm, regardless of what the source of that torque is.
Using your analogy of a pallet being dragged across a shop floor with two dissimilar weights strapped to them is NOT what I was describing. I specifically mentioned using the same transmission and rear end assembly in both examples precisely so that it would keep that source of frictional loss in the assembly as a constant.
I'm not saying that two different engines will have the same frictional losses in their respective rotating assemblies, I'm saying if those two engines (and my hypothetical electric test motor) were all mated to that same transmission and rear end assembly, that trans. and rear end would require the same amount of torque to turn them.
That, in my mind means that a given transmission and rear end assembly should, at least in most cases, provide a constant factor of hp/torque loss when paired to a given source of twisting force.
You're correct. At a given steady constant speed.

The losses vary when you attempt to spin up that mass TO that speed at different acceleration rates.
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