Quote:
Originally Posted by SUB-FT86
So what you're trying to say is torque doesn't lose torque to the wheels, only horsepower?
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That is not what I am saying.
Still not sure if you're pulling my leg. If so, bravo sir, bravo. If not, here's a more detailed explanation:
Drivetrain losses affect both engine torque and power. Look at this RSX-S dyno chart (slightly less peak torque and a slightly higher redline than what's expected from the FT):
It shows wheel power (which is the same as engine power minus drivetrain losses) and engine torque minus drivetrain losses (which is
not the same thing as wheel torque). Note that this dyno's torque values don't exist anywhere: engine torque is amplified by gearing as it's diminished by friction, so by the time the RSX loses ~12% of its engine torque due to the drivetrain, it also gains ~220-1330% due to gearing (exchanged for a decrease in angular speed since the wheels are spinning slower than the engine).
So engine torque is slightly higher than the ~115-125 lbf*ft shown on that dyno, and wheel torque is *much* higher.
Why use these nonexistent pseudo engine torque values? (that account for drivetrain losses, but are plotted at the engine's angular speed rather than wheel speed)
A couple reasons:
- That's how this type of dyno works. By measuring force and speed at the wheels, it's very easy to figure out wheel power. Since engine speed is also measured, it's easy to then calculate engine torque (but because wheel power was used, drivetrain losses are represented).
- Including drivetrain losses with the engine torque makes it easy to calculate what wheel torque will be for various gear ratios.
In mathematical terms:
P = Power
T = Torque
w = angular speed
N = system efficiency (0.8-0.9 in most MT cars)
r = overall gear ratio
P = T * w
P_wheels = P_engine * N
T_wheels = T_engine * N * r (side note: w_wheels = w_engine / r)
T_wheels = P_wheels / w_wheels
T_wheels = P_engine * N / w_wheels [eq 1]
This basic relationship can also be described in terms of linear force at the wheels (rather than angular force, aka torque):
F = Force
v = velocity
P = F * v
F_wheels = P_wheels / velocity
F_wheels = P_engine * N / velocity [eq 2]
As eq 1 and eq 2 show, at any given vehicle speed, you increase force at the wheels if you increase engine power.
BTW, this page is handy when working out this sort of thing:
http://www.fatboyraceworks.com/gears/ (it currently only has Honda data, but you can plug in anything).
I don't know if that clarified or confused things, but either way this is off topic. If anyone wishes to discuss it further, I suggest moving to PM or at least to a different thread.