Optimum Shift Point
05-13-2010, 05:28 PM
#16
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05-13-2010, 05:54 PM
#17
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If you try a 1st to 2nd shift (stock GT3 transmission), I guarantee that your RPM will drop below 5,500rpm.
05-13-2010, 06:03 PM
#18
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As a side note: Going for peak torque does not take into account the torque multiplication factor of the transmission. The lower gears multiply torque most and so being at redline in a lower gear (well passed the torque peak) is likely to give more torque at the wheel than being in a taller gear at exactly the torque peak.
05-13-2010, 06:28 PM
#19
The idea is to maximize power to the wheels at all speeds. This is data from the 2010 GT3 RS, using a 335/30R18 tire. Note that only 4th, 5th and 6th are close ratio, everything else is spaced out drastically (and it is worse in the standard GT3 due to the 3.44 R&P).
2010 GT3 RS (Stock) 25.6" Hoosier A6 335/30R18 A6
Gear, Ratio, Max Speed, RPM drop on upshift at Redline
1st 3.82 44
2nd 2.26 74 5000
3rd 1.64 102 6200
4th 1.29 129 6700
5th 1.06 157 7000
6th 0.88 189 7100
Final Ratio 3.89
Redline 8500
Using this gearing information with a 100rpm split (speed in gear) and an Excel pivot table with the power to the wheels every 100 rpm (from dyno graph), you can identify the ideal shift points for the 2010 GT3 RS, which by the way is 8,500 rpm in every single gear.
At 45mph (typical low speed track turn) it is impossible to keep the RS on the powerband, the car will be bogging at 5,200rpm in 2nd gear putting 250rwhp from the 415rwhp available, but nothing can be done here due to the long spacing for the first 3 gears.
Using the same cross reference of gearing/speed and power/rpm you can determine the ideal gearing for a 6-speed transmission. The GT3 has a very narrow powerband, so it would benefit from a 8-speed transmission with very close ratios (one of those DCT close-ratio like the M3 and F458). But given the restriction to a 6-speed manual, gearing has to be compromised.
In the 997 GT3 Cup, gearing is optimized for track use, and with only 6 gears available, Porsche decided to use a very long 1st gear (3.16) and a very short 6th gear (.96) with an even shorter R&P (4.00:1), the 997 GT3 Cup cannot even reach 180mph despite of the very tall 26.85" rear tire. However, having a long 1st and a short 6th allows for better spacing in all the gears, and the Cup car (well driven) rated at 450Hp same as the street car, puts more power to wheels at a given speed than the street car. Even if you get a 2010 GT3 RS down to Cup car weight, the Cup will still accelerate better because its graph of power at speed has a larger area under the curve. At 45mph, the 997 GT3 Cup can use 1st gear, and it puts down almost 400rwhp while the 2010 GT3 RS is just running at 250rwhp, and both engines are rated at 450Hp.
2010 GT3 RS (Stock) 25.6" Hoosier A6 335/30R18 A6
Gear, Ratio, Max Speed, RPM drop on upshift at Redline
1st 3.82 44
2nd 2.26 74 5000
3rd 1.64 102 6200
4th 1.29 129 6700
5th 1.06 157 7000
6th 0.88 189 7100
Final Ratio 3.89
Redline 8500
Using this gearing information with a 100rpm split (speed in gear) and an Excel pivot table with the power to the wheels every 100 rpm (from dyno graph), you can identify the ideal shift points for the 2010 GT3 RS, which by the way is 8,500 rpm in every single gear.
At 45mph (typical low speed track turn) it is impossible to keep the RS on the powerband, the car will be bogging at 5,200rpm in 2nd gear putting 250rwhp from the 415rwhp available, but nothing can be done here due to the long spacing for the first 3 gears.
Using the same cross reference of gearing/speed and power/rpm you can determine the ideal gearing for a 6-speed transmission. The GT3 has a very narrow powerband, so it would benefit from a 8-speed transmission with very close ratios (one of those DCT close-ratio like the M3 and F458). But given the restriction to a 6-speed manual, gearing has to be compromised.
In the 997 GT3 Cup, gearing is optimized for track use, and with only 6 gears available, Porsche decided to use a very long 1st gear (3.16) and a very short 6th gear (.96) with an even shorter R&P (4.00:1), the 997 GT3 Cup cannot even reach 180mph despite of the very tall 26.85" rear tire. However, having a long 1st and a short 6th allows for better spacing in all the gears, and the Cup car (well driven) rated at 450Hp same as the street car, puts more power to wheels at a given speed than the street car. Even if you get a 2010 GT3 RS down to Cup car weight, the Cup will still accelerate better because its graph of power at speed has a larger area under the curve. At 45mph, the 997 GT3 Cup can use 1st gear, and it puts down almost 400rwhp while the 2010 GT3 RS is just running at 250rwhp, and both engines are rated at 450Hp.
- most tracks have a 2nd gear 45mph turn and if they happen to be at the beginning of a long straight the time magnification benefit of launching out with near 400hp versus 250hp adds up
- still not optimal and very track dependent - but it can be useful
05-13-2010, 11:01 PM
#20
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NJ - GT: Can you please explain? I agree with your point about the first to second shift, where the rpm's drop below 5500 due to gearing. On the subsequent upshifts, is it not a good rule of thumb to make sure the revs do not drop below 5500 rpm?
05-13-2010, 11:17 PM
#21
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Short answer, staying above 5500 rpm (max torque) won't guarantee that the car will accelerate the best.
You want maximum power at a given speed. At 130mph, the 2010 GT3 RS can be in 4th, 5th or 6th gear and in these three cases you will be above 5,500 rpm (you didn't drop below the maximum torque), but the car in 4th gear will be putting over 400hp to the wheels, the car in 5th around 350rwhp, and the car in 6th (the one closest to max torque) just around 280rwhp. I'll take the car with 400rwhp.
Even shorter explanation: F1 car, 900Hp and less torque than a 2010 GT3 RS.
05-14-2010, 12:14 AM
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05-14-2010, 10:05 AM
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The best way to understand this is to think in terms of energy, not force. Torque is force. Power is about energy usage.
Power is the rate of processing energy. In car terms just think of power as the rate at which the engine converts fuel energy into kinetic energy by accelerating the vehicle to speed. Clearly you want this to be maxed at all times. So you want the engine running at max power, or as close to max as possible, at all times. It doesn't matter where the engine's torque peak is, you always want to be near the power peak - where you're converting fuel energy to kinetic energy as fast as possible.
In a CVT car, many people incorrectly assume that if you floor the throttle the revs should hold at peak torque for best acceleration. This is not the case. It should hold at max power, not max torque.
Torque is purely a rotational force, nothing more. To get acceleration from torque, you need to multiply torque by revs. It turns out the higher revs at peak power is more important than the reduced torque up there. Peak power gives max acceleration.
Power is the rate of processing energy. In car terms just think of power as the rate at which the engine converts fuel energy into kinetic energy by accelerating the vehicle to speed. Clearly you want this to be maxed at all times. So you want the engine running at max power, or as close to max as possible, at all times. It doesn't matter where the engine's torque peak is, you always want to be near the power peak - where you're converting fuel energy to kinetic energy as fast as possible.
In a CVT car, many people incorrectly assume that if you floor the throttle the revs should hold at peak torque for best acceleration. This is not the case. It should hold at max power, not max torque.
Torque is purely a rotational force, nothing more. To get acceleration from torque, you need to multiply torque by revs. It turns out the higher revs at peak power is more important than the reduced torque up there. Peak power gives max acceleration.
05-14-2010, 11:40 AM
#24
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Some good explanations have already been given here. Here's some more explanation on the physics of acceleration: Torque is just a measurement of force at one instant in time at one place. With my own body, I can create enormous torque on a wheel if you give me enough leverage, e.g. a long enough lever arm. However, if that torque is not sustained over many wheel revs, then the object (the car in this case) is not going to continue to accelerate. The ability to sustain that force is what keeps the car accelerating and that's exactly what horsepower does for you.
Another way to look at it is to realize that maximizing horsepower at the rear wheels is totally equivalent to maximizing torque at the rear wheels. It is NOT the same as maximizing engine torque because rear wheel torque must take gear ratio multiplication into account. When you do that, you find that it's all about maximizing horsepower.
Another way to look at it is to realize that maximizing horsepower at the rear wheels is totally equivalent to maximizing torque at the rear wheels. It is NOT the same as maximizing engine torque because rear wheel torque must take gear ratio multiplication into account. When you do that, you find that it's all about maximizing horsepower.
05-14-2010, 02:00 PM
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As you stated earlier, it's easy to put big torque on a stationary wheel with a big lever. And you do it using zero power because no energy is being used, just force. But if that wheel is spinning and you want to make it spin faster still, you need to apply torque while it's spinning. This requires energy input, thus power. The faster it's spinning, the faster you need to feed in energy to keep up the torque, thus more power is required (faster energy usage).
Power is a function of torque x revs. As revs climb beyond peak engine torque, wheel torque begins to drop off too (in any one gear). But the revs continue to climb faster than the torque drops, so power continues to climb. At some point torque begins to fall faster than the revs are climbing and thus power begins to fall. That is the power peak. It's the point at which the engine is converting fuel energy to kinetic energy as fast as it can and thus providing the fastest acceleration. The optimum shift point is somewhere beyond that (but not too far) so your next gear falls close to the power max.
When thinking about acceleration, think about energy usage, not force (torque). It's counter-intuitive but that's how the physics works.
05-14-2010, 04:03 PM
#26
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That is only partly true. If it were entirely true then in any one gear, max horsepower would occur at max engine torque because that's where the wheels would be getting the most torque. In any one gear, max engine torque equates to max wheel torque because the gear ratio is constant.
05-14-2010, 06:56 PM
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Let me rephrase my statement because I know what you are saying and you are correct. At a given vehicle speed (not engine speed), you generate maximum rear wheel torque when you shift into the gear that operates the engine in an rpm range as close as possible to its horsepower peak. If you can downshift to a lower gear and generate more horsepower (without exceeding redline), then you do that. You take advantage of torque multiplication of the larger gear ratios. This just reaffirms what others have already said so we are in agreement. Bottom line: always think in terms of delivering maximum horsepower to the rear wheels at all times by gear selection if you want to maximize acceleration.
"At a given vehicle speed (not engine speed), you generate maximum rear wheel torque when you shift into the gear that operates the engine in an rpm range as close as possible to its horsepower peak."
The wheels will not have maximum torque near the horsepower peak, they will have maximum torque at the torque peak. This is regardless of the gear you're in or vehicle speed. But to understand how to achieve maximum acceleration, it is not really helpful to think in terms of torque. It's more complicated than it seems.
People have never-ending trouble trying to understand why you wouldn't want to be at max torque - where the wheels are delivering maximum force to the ground. It seems so intuitive. But it's really not that intuitive because if you think torque, then you also have to think revs, gearing, speed, etc., in order to understand how that torque translates to acceleration. It seems intuitive but it's not. It's confusing because there are too many factors involved.
But if you ignore the force equations and simply think energy, it all becomes very simple; For maximum acceleration you need to feed the car with as much kinetic energy as possible as fast as possible. That's all. And that's exactly what maximum horsepower is - the maximum possible rate of burning fuel and converting that chemical energy to kinetic energy by accelerating the vehicle.
You want maximum power at the wheels, which surprisingly is not where they're giving maximum torque.
Last edited by SpeedGeek; 05-14-2010 at 07:36 PM.
05-14-2010, 07:01 PM
#28
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It is a long explanation. Do a quick search here and the race forum.
Short answer, staying above 5500 rpm (max torque) won't guarantee that the car will accelerate the best.
You want maximum power at a given speed. At 130mph, the 2010 GT3 RS can be in 4th, 5th or 6th gear and in these three cases you will be above 5,500 rpm (you didn't drop below the maximum torque), but the car in 4th gear will be putting over 400hp to the wheels, the car in 5th around 350rwhp, and the car in 6th (the one closest to max torque) just around 280rwhp. I'll take the car with 400rwhp.
Even shorter explanation: F1 car, 900Hp and less torque than a 2010 GT3 RS.
Short answer, staying above 5500 rpm (max torque) won't guarantee that the car will accelerate the best.
You want maximum power at a given speed. At 130mph, the 2010 GT3 RS can be in 4th, 5th or 6th gear and in these three cases you will be above 5,500 rpm (you didn't drop below the maximum torque), but the car in 4th gear will be putting over 400hp to the wheels, the car in 5th around 350rwhp, and the car in 6th (the one closest to max torque) just around 280rwhp. I'll take the car with 400rwhp.
Even shorter explanation: F1 car, 900Hp and less torque than a 2010 GT3 RS.
NJ-GT: Thanks for the explanation, especially the part where you discuss power to the wheels based on the gear engaged.
05-14-2010, 07:30 PM
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05-14-2010, 09:08 PM
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The gist of what you're saying is correct, but this statement is not correct:
"At a given vehicle speed (not engine speed), you generate maximum rear wheel torque when you shift into the gear that operates the engine in an rpm range as close as possible to its horsepower peak."
"At a given vehicle speed (not engine speed), you generate maximum rear wheel torque when you shift into the gear that operates the engine in an rpm range as close as possible to its horsepower peak."