Dyno day at AWE. Photos and figures!
02-19-2007, 10:29 AM
#2
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That's a lot of Audi's!
Looks like a good turnout.
Just curious, why did that 422WHP S4 dyno in 3rd gear and not 4th? 4th is closer to 1:1 than 3rd is. 3rd still has an element of torque multiplication. Just curious.
Just curious, why did that 422WHP S4 dyno in 3rd gear and not 4th? 4th is closer to 1:1 than 3rd is. 3rd still has an element of torque multiplication. Just curious.
02-19-2007, 11:33 AM
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iirc, power is power, even after its gone thru gear ratios
IIRC from back in college lab...
So the dyno takes its roller's RPM and torque as measured by the roller, calculates the power by multiplying the two:
roller_spd [rpm] * roller_torq [ft-lb] / 5252 = roller_power [hp]
*NOTE: "stuff" in [] are the engineering units.
Since power @ the roller == engine power @ the wheels,
wheel_power = roller_power
And if you take the wheel_power and divided by the ENGINE speed, then you get the torque as a function of ENGINE speed, as measured at the wheel (ie: drive train power loss accounted for).
engine_torq@wheel [ft-lb] = wheel_power [hp] * 5252 / engine_spd [rpm]
Hence irregardless which gear the car is in... a lower gear would give higher roller torq, but lower roller rpm; while a higher gear would give lower roller torq and higher roller rpm. Thus power at the roller is the same in either gear, assuming car's drive train power (not torque) loss is the same between the 2 gears in the tranny.
So the dyno takes its roller's RPM and torque as measured by the roller, calculates the power by multiplying the two:
roller_spd [rpm] * roller_torq [ft-lb] / 5252 = roller_power [hp]
*NOTE: "stuff" in [] are the engineering units.
Since power @ the roller == engine power @ the wheels,
wheel_power = roller_power
And if you take the wheel_power and divided by the ENGINE speed, then you get the torque as a function of ENGINE speed, as measured at the wheel (ie: drive train power loss accounted for).
engine_torq@wheel [ft-lb] = wheel_power [hp] * 5252 / engine_spd [rpm]
Hence irregardless which gear the car is in... a lower gear would give higher roller torq, but lower roller rpm; while a higher gear would give lower roller torq and higher roller rpm. Thus power at the roller is the same in either gear, assuming car's drive train power (not torque) loss is the same between the 2 gears in the tranny.
02-19-2007, 12:23 PM
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In a purely English form, I could interpret that to say
that your car makes the same power in 1st gear as it does in 6th gear, right? According to you, it shouldn't matter what gear the car is in.
If I think about that statement, it doesn't make sense. For instance, on my car, the first gear ratio is 3.5:1. That means, first gear multiplies the torque output from the crank (my car is 221 lb/ft of torque) by 3.5 times. 221 * 3.5 = 773.5. Leaving my final drive ratio out of this equation (because I don't know how that factors into the actual torque that hits the wheels), the torque at the wheels of my car should be 773.5 minus whatever is lost due to the drivetrain between the tranny to the wheels (fluids, centre diff, driveshaft, rear diff, half shafts, yada yada). I'm sure there's also loss upstream of 1st gear, namely the union of the flywheel to the clutch and input shaft to the tranny.
Let's take drivetrain loss and final drive ratios out of this equation. If I dyno my car in 1st gear, my torque output would be 773.5. That's not truly representative of the engine's output. It is stepped up 3.5 times.
If I dyno in 4th gear (my 4th gear ratio is 1.034), the lb/ft of torque actually reaching the wheels should be 228 (221 * 1.034). Strangely enough, the 3.0 tiptronic's 4th gear is exactly 1.0:1. So, in the 4th gear scenario, you are getting a truer representation of the engine's power because all the components are spinning at almost the exact same speed.
I'm a simple guy, so that's how logic works in my simple mind.
If I think about that statement, it doesn't make sense. For instance, on my car, the first gear ratio is 3.5:1. That means, first gear multiplies the torque output from the crank (my car is 221 lb/ft of torque) by 3.5 times. 221 * 3.5 = 773.5. Leaving my final drive ratio out of this equation (because I don't know how that factors into the actual torque that hits the wheels), the torque at the wheels of my car should be 773.5 minus whatever is lost due to the drivetrain between the tranny to the wheels (fluids, centre diff, driveshaft, rear diff, half shafts, yada yada). I'm sure there's also loss upstream of 1st gear, namely the union of the flywheel to the clutch and input shaft to the tranny.
Let's take drivetrain loss and final drive ratios out of this equation. If I dyno my car in 1st gear, my torque output would be 773.5. That's not truly representative of the engine's output. It is stepped up 3.5 times.
If I dyno in 4th gear (my 4th gear ratio is 1.034), the lb/ft of torque actually reaching the wheels should be 228 (221 * 1.034). Strangely enough, the 3.0 tiptronic's 4th gear is exactly 1.0:1. So, in the 4th gear scenario, you are getting a truer representation of the engine's power because all the components are spinning at almost the exact same speed.
I'm a simple guy, so that's how logic works in my simple mind.
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02-19-2007, 12:34 PM
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torque will vary, but power will not (given same engine rpm)
The key is that POWER is different from TORQUE.
Remember that:
power = torque * rotational_speed
First gear will give uber(sp?) torque, but little wheel rotational speed. while 5th gear will give little torque, but moocho rotational speed. For a given drive train input POWER (ie: engine @ same rpm), the POWER at the wheel is same regardless of gear. (ignoring variation in drive train POWER loss) Different gearing is just a torque multiplier & rotational_speed divider (or vise-versa if you go up vs down a gear).
I found a very good layman's explanation relating these physics equations to car performance talk. Let me see if I could find it again... I'll post link once found.
Remember that:
power = torque * rotational_speed
First gear will give uber(sp?) torque, but little wheel rotational speed. while 5th gear will give little torque, but moocho rotational speed. For a given drive train input POWER (ie: engine @ same rpm), the POWER at the wheel is same regardless of gear. (ignoring variation in drive train POWER loss) Different gearing is just a torque multiplier & rotational_speed divider (or vise-versa if you go up vs down a gear).
I found a very good layman's explanation relating these physics equations to car performance talk. Let me see if I could find it again... I'll post link once found.
02-19-2007, 12:43 PM
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great explanation, found!
<a href="http://forum.miata.net/vb/showthread.php?t=127306">clicky</a>
It's the 4th post down by "FormerDatsun510Man" in this Miata.net forum thread. The thread title is "Definition: Motor: Torque vs Horse Power", and it's under the site's "good threads that we saved" section... I thought it was a very very good every-day-man/woman explanation on the subject.
It's the 4th post down by "FormerDatsun510Man" in this Miata.net forum thread. The thread title is "Definition: Motor: Torque vs Horse Power", and it's under the site's "good threads that we saved" section... I thought it was a very very good every-day-man/woman explanation on the subject.
02-19-2007, 01:16 PM
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That is an excellent read!
My calculations were way simple compared to his but I'm glad I was at least on the right path.
Now, after having read all that, I come away with two things. One, torque is really all that counts. Two, HP is a calculation, not a real output of the engine's crankshaft.
Finally, it still doesn't convince me that dyno-ing in any gear will yield the same torque output, which we've already determined is all that counts. I'm still convinced that dyno-ing in the gear that is closest to 1:1 makes the most sense.
That was an excellent write-up though and I learned a good deal from it. I'm definitely saving it.
Now, after having read all that, I come away with two things. One, torque is really all that counts. Two, HP is a calculation, not a real output of the engine's crankshaft.
Finally, it still doesn't convince me that dyno-ing in any gear will yield the same torque output, which we've already determined is all that counts. I'm still convinced that dyno-ing in the gear that is closest to 1:1 makes the most sense.
That was an excellent write-up though and I learned a good deal from it. I'm definitely saving it.