How to determine optimal shift points?
02-29-2008, 05:54 PM
#31
Three Wheelin'
All good to know, and were I racing this car or someone else owned it, I would be shifting close to redline. I understand that now.
For those of us who own the car and drive it just in DEs, though, where is the sweet spot where the power is very good but the cost of making that power, in engine wear, is not high? I find myself shifting around 7500 RPM, I think, in gears 2-4, but I sure shift lower (not sure where) in gear 1.
For those of us who own the car and drive it just in DEs, though, where is the sweet spot where the power is very good but the cost of making that power, in engine wear, is not high? I find myself shifting around 7500 RPM, I think, in gears 2-4, but I sure shift lower (not sure where) in gear 1.
02-29-2008, 05:58 PM
#32
From a purely physics pov, it is the torque available at the drive wheels that accelerates the car. Since the transmission multiplies this torque (by the gear ratio), the ideal shift strategy is to shift when the current torque at the wheels drops below that which is available at the same speed in the next higher gear which is a function of the engine RPM and Torque, aka HP.
02-29-2008, 05:59 PM
#33
Three Wheelin'
From a purely physics pov, it is the torque available at the drive wheels that accelerates the car. Since the transmission multiplies this torque (by the gear ratio), the ideal shift strategy is to shift when the current torque drops below that which is available at the same speed in the next higher gear. Since the GT3 makes such high torque at the top end of the RPM range, you'll see that (again, theoretically) the best shift point is redline (since the available torque there is still higher than that which is available immediately after shifting). The attached chart shows the torque v. rpm for the engine, and after the transmission.
edit: ah I see now, you have to factor in the rev drop as well. What is it generally per gear? 2k rpm?
02-29-2008, 06:04 PM
#34
I can see a benefit if you're entering a sweeper and shift up before you go into the corner instead of shifting in it. But I guess that's not what you meant right?
05-03-2011, 03:36 PM
#36
Rennlist Member
There has been a lot of talk on the subject, since this thread. Since it was referenced in a newer discussion, I read a few of the posts here.
NJ-GT, you are absolutely right. Its all about maximizing HP over the operational range of speeds. with the HP curve shown, its easy to see the arc is about 15% drop in RPM, which is the closest any close ratio gear box will be, so, even still, you need to shift at redline to maximize straightline acceleration as you say.
NJ-GT, you are absolutely right. Its all about maximizing HP over the operational range of speeds. with the HP curve shown, its easy to see the arc is about 15% drop in RPM, which is the closest any close ratio gear box will be, so, even still, you need to shift at redline to maximize straightline acceleration as you say.
redline in every gear with the exception of 4th to 5th where there is no need to run up to 8,200 rpm. 4th and 5th are so close that shifting at 8,000 rpm still gets you on the power band at 6,600 rpm. 5th is limited to 8,000 rpm and the car does not reach readline in 6th gear.
You want to have the biggest area under the horsepower curve at any gear, torque has no business on ideal shift points.
This car has more torque from 4,000 rpm to 6,000 rpm than from 6,200 rpm to 8,200 rpm (redline). If the torque was the answer, I could stay between 4,000 and 6,000 rpm and accelerate faster than another driver running between 6,200 rpm and 8,200 rpm, but this is not truth.
You want to maximize acceleration at any speed on racing conditions.
The 997 GT3 Cup is properly geared for the power characteristics of this engine. Every upshift after 2nd gear (done at the 8,000 red line) lands on next gear at 6,650-6850 rpm.
Lots of explanations to this on Racing groups. Also plenty of videos on the web with Pro drivers shifting gears on these cars where they should.
MM-Racing,
I use this internet calculator. Much better than a graph, because the graph does not consider rear tire diameter differences across multiple brands.
http://www.f-body.org/gears/
You want to have the biggest area under the horsepower curve at any gear, torque has no business on ideal shift points.
This car has more torque from 4,000 rpm to 6,000 rpm than from 6,200 rpm to 8,200 rpm (redline). If the torque was the answer, I could stay between 4,000 and 6,000 rpm and accelerate faster than another driver running between 6,200 rpm and 8,200 rpm, but this is not truth.
You want to maximize acceleration at any speed on racing conditions.
The 997 GT3 Cup is properly geared for the power characteristics of this engine. Every upshift after 2nd gear (done at the 8,000 red line) lands on next gear at 6,650-6850 rpm.
Lots of explanations to this on Racing groups. Also plenty of videos on the web with Pro drivers shifting gears on these cars where they should.
MM-Racing,
I use this internet calculator. Much better than a graph, because the graph does not consider rear tire diameter differences across multiple brands.
http://www.f-body.org/gears/
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