09-11-2013, 07:25 PM | #45 |
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Hmm...there seems to be a lot of confusion brewing here, I'm no expert, but I think these fundamental things need be understood.
1) The purpose of the transmission is to allow the engine flywheel to apply a torque at the wheels for a large range of wheel speeds. The internal combustion engine cannot produce torque at 5 RPM nor 20k RPM (something an electric motor excels at). One of the consequences of this is that there is a "lever arm", or mechanical advantage which depends on which gear is engaged. The mechanical advantage is HIGHER when the transmission is in LOWER gears. The wheel torque is engineTorque * mechAdvantge. So the engine actually generates MORE torque *at the wheels* when in lower gears. Keep in mind that the iDrive sports display is showing the torque *at the CRANK*, so it doesn't matter what gear you're in (else it would be very confusing - "why do I have 1200 HP?!"). 2) Wheel dyno runs are done in 3rd or 4th gear because that is where the mechanical advantage is approximately 1, so torque at the wheels is roughly torque at the crank. Edit - this is wrong, since the final drive ratio will screw this up. Maybe it makes it more accurate? Maybe someone else can chime in. 3) HP is a function of Torque and engine speed. The equation is HP = Tq * rpm / 5252 It is in general thought of as a signal, i.e. function of RPM HP(rpm) = Tq(rpm) * rpm / 5252, where the Tq(rpm) is what is recorded by the wheel or crank dyno run. The HP rating of the car is generally found as the max{HP(rpm)} over the set of all possible rpm. 4) The torque curve as a function of RPM is only *approximately* flat during Wide Open Throttle (WOT) after all transients have died out (such as turbo lag). There is a big difference in the torque response you'll feel over 2 sec if try to go WOT in 2nd gear going 5 MPH vs downshifting from 4th going 25 MPH. Edit - you can learn a lot by looking at this plot, which shows the *wheel torque* from a tuned BRZ as a function of *vehicle velocity* during a WOT run. (Note: final drive ratio is 3.7) Last edited by drob23; 09-11-2013 at 09:31 PM.. |
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09-11-2013, 08:03 PM | #46 |
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I'm aware of what's the display is showing HP/T values.
The BMW advertised specifications don't specify rear wheel HP or bHP. If they do, I've not seen it.
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09-11-2013, 08:03 PM | #47 | |
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let me see if I can shoot a video of the sport displays from a standstill to say 60 (for legal purposes lets stick to 60 just like top gears Jeremy clarkson stuck to 60 when driving a Vette on US soil in Vegas) We have come this far and I like the points you laid out so we can test what we know against the sport displays. I have the car tomorrow hopefully I can do it then otherwise I will do it on Saturday
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09-11-2013, 08:20 PM | #48 | |
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Just look at the BRZ plot I've linked to above...wheel torque varies from 2100 ft-lbs to 300 (and would go to 0 if you take off throttle), this is just too confusing. |
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09-11-2013, 09:15 PM | #49 |
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It's great info and makes sense as when spec'ing a engine most provide bHP. Learning more about one's ride is always fun. I like the techie stuff.
Looking forward to 300HP's vid and see what it shows.
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