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Originally Posted by CHRISCAM
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.
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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.
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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).
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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.
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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