When to shift? How to produce maximum area under the curve? 3.2 Carrera.
11-22-2006, 06:45 PM
#46
Rennlist Member
while averaging max hp seems to be a good way to figure out shift points, keep in mind, its not just area under the curve, its time spend at each point on the curve. so, if you are near 150mph and you are now going to shift to slightly early to 6th, you loose more than just the rpms short and its Hp vs the post shift HP level. you loose the time spent x the HP (total energy). thats why i like to explain it in terms of HP-seconds. he who uses the most HP-seconds wins! (At least in a straight line 
)
MK
)MK
Originally Posted by Jean
I think this is a very valid statement. If I were to save my engine, I would certainly not be shifting at peak RPMs except when I am on a straightline and trying to close the door upon turn in. The loss in thrust while shifting 500RPMs sooner vs. peak RPMs is probably low single digit %, while the engine stress is exponential with RPMs.
It has to do more with what RPMs you will be at, after shifting (per the thick redline on the second picture posted below), rather than your peak torque RPM.. The more you use the peak thrust available on your car the faster you will go.
In my particular example, the optimum shifting points are per the little attachment...At redline (7070RPM) in the first three gears, 6,770 in 4th and 6,530RPM in 5th. The engine dyno curve (the upper colour box on the left) does not really tell the story about when is best to shift based on thrust to the wheels (lower colour box on the left)
To be back on topic, if I can have the datapoints for engine (for accuracy) dyno chart of the 3.2Ltr, I can figure what are the best shifting points as far as going fastest in straightline is concerned and/or have the biggest area under the curve per the thrust chart.
Bill, 6500RPM is part of the reason I agree. Torque curves and gear staging might be the other..
It has to do more with what RPMs you will be at, after shifting (per the thick redline on the second picture posted below), rather than your peak torque RPM.. The more you use the peak thrust available on your car the faster you will go.
In my particular example, the optimum shifting points are per the little attachment...At redline (7070RPM) in the first three gears, 6,770 in 4th and 6,530RPM in 5th. The engine dyno curve (the upper colour box on the left) does not really tell the story about when is best to shift based on thrust to the wheels (lower colour box on the left)
To be back on topic, if I can have the datapoints for engine (for accuracy) dyno chart of the 3.2Ltr, I can figure what are the best shifting points as far as going fastest in straightline is concerned and/or have the biggest area under the curve per the thrust chart.
Bill, 6500RPM is part of the reason I agree. Torque curves and gear staging might be the other..
11-22-2006, 09:34 PM
#47
Instructor
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At the risk of sticking my neck out ..... What we want is to maximize acceleration... Ignoring the aero effect, the equation is:
Acceleration = (Torque x Gear Ratio) / (Mass x Tire Radius)
If I set up the attachment correctly, you can see the calculations for my SC below. Note that the acceleration at the redline is always greater than the acceleration at the equivalent speed in the next gear. (Note that from 4th to 5th gear the acceleration is almost equivalent.)
Acceleration = (Torque x Gear Ratio) / (Mass x Tire Radius)
If I set up the attachment correctly, you can see the calculations for my SC below. Note that the acceleration at the redline is always greater than the acceleration at the equivalent speed in the next gear. (Note that from 4th to 5th gear the acceleration is almost equivalent.)
11-23-2006, 12:06 AM
#48
Burning Brakes
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OK. Let me try and see if I got this right by turning the question inside out. Let's say you could pick any gears you wanted. What would be the optimum gear ratios (or relative gear ratios)? Assuming the HP and TQ in the attached dyno graph, let's say we try to keep the engine running in the power band of about 5400 RPM to 7200 RPM. Then, the gears would be spaced like in the 2nd picture.
Is this correct for maximum acceleration over all the gears? I guess this is opposite of the way manufacturers typically gear transmissions with the shorter gears farther apart and the taller gears closer together. My assumption would then be that they try to keep a subset of taller gears even closer to focus on an even higher but narrower band of HP.
To maximize HP-seconds makes intuitive sense to me. In this case, it would then not be important at what speed (gear ratio or MPH) that the shift occurred. The point would be merely to minimize the number of shifts during the MPH range of highest importance.
Is this correct for maximum acceleration over all the gears? I guess this is opposite of the way manufacturers typically gear transmissions with the shorter gears farther apart and the taller gears closer together. My assumption would then be that they try to keep a subset of taller gears even closer to focus on an even higher but narrower band of HP.
To maximize HP-seconds makes intuitive sense to me. In this case, it would then not be important at what speed (gear ratio or MPH) that the shift occurred. The point would be merely to minimize the number of shifts during the MPH range of highest importance.
11-23-2006, 12:13 AM
#49
Burning Brakes
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For reference, here's the stock gear ratios. It looks like the stock gear ratios keep the RPMs in a slightly better power band of 5800 RPM to 7200 RPM, but only once the car is traveling above 85 MPH.
P.S. My data is for a Boxster S, but the gear ratios, final drive, and tire diameter is extremely close to Jean's.
P.S. My data is for a Boxster S, but the gear ratios, final drive, and tire diameter is extremely close to Jean's.
Last edited by arenared; 11-23-2006 at 01:56 AM.
11-23-2006, 03:48 AM
#50
Rennlist Member
I think you guys basically got it now.
Funny, the BMW racer buddy of mine just chewed on my ear for about an hour over this same subject! i have his gear ratios, his hp curves, and he still wants to early shift due to the appearance of the torque curve rapidly falling way short of redline and his "feel" of the acceleration at the higher rpms in each gear on the race track. finally, after plugging in the values, he saw the light. we were pretty even before in our races. I may have just screwed myself with that set of tips!!
Mk
Funny, the BMW racer buddy of mine just chewed on my ear for about an hour over this same subject! i have his gear ratios, his hp curves, and he still wants to early shift due to the appearance of the torque curve rapidly falling way short of redline and his "feel" of the acceleration at the higher rpms in each gear on the race track. finally, after plugging in the values, he saw the light. we were pretty even before in our races. I may have just screwed myself with that set of tips!!
Mk
11-25-2006, 01:19 PM
#51
Nordschleife Master
Interesting stuff.
Mark! Why didn't you PM me on this?
One thing people don't look into is what the best shift point is for each gear. On most of our cars the power peak is right near the redline so using one point doesn't substantially matter. If the torque is falling off strongly, that is the power peak is far from the redline, then where to shift will vary with the gears' ratios.
I see some nice graphs, tools and Excel work in this thread. Thought to add a link to my own home-brewed tool. Answers the main topic directly: what shift point works best? Takes dyno data, gears ratios, wheel size, weight, Cd and so on and give accelerations, velocities and times. Also some stuff on front-sump oil sloshing but that's safely ignored.
Enjoy:
http://home.earthlink.net/~morecowbell98/BenchRace.xls
Mark! Why didn't you PM me on this?
One thing people don't look into is what the best shift point is for each gear. On most of our cars the power peak is right near the redline so using one point doesn't substantially matter. If the torque is falling off strongly, that is the power peak is far from the redline, then where to shift will vary with the gears' ratios.
I see some nice graphs, tools and Excel work in this thread. Thought to add a link to my own home-brewed tool. Answers the main topic directly: what shift point works best? Takes dyno data, gears ratios, wheel size, weight, Cd and so on and give accelerations, velocities and times. Also some stuff on front-sump oil sloshing but that's safely ignored.
Enjoy:
http://home.earthlink.net/~morecowbell98/BenchRace.xls
12-03-2006, 09:51 PM
#52
Rennlist Member
Sorry, i got too wrapped up, i forgot. Plus, it was more of the same stuff we always talk about!
If you look at the hp curve, it will take all those factors into account as to not need gear ratios values, torque levels, etc. Just find the gear spacings, plot those on a HP curve and you are done. its that easy! all you then need to do is eyeball the maximized HP utilized, or again, "HP-seconds" which will take into account, the time spent at the top rpm regions of HP curve. but even taking a rough integration of the curve, will give you the shift points. rarely is it less than redline for most cars.
MK
If you look at the hp curve, it will take all those factors into account as to not need gear ratios values, torque levels, etc. Just find the gear spacings, plot those on a HP curve and you are done. its that easy! all you then need to do is eyeball the maximized HP utilized, or again, "HP-seconds" which will take into account, the time spent at the top rpm regions of HP curve. but even taking a rough integration of the curve, will give you the shift points. rarely is it less than redline for most cars.
MK
Originally Posted by GlenL
Interesting stuff.
Mark! Why didn't you PM me on this?
One thing people don't look into is what the best shift point is for each gear. On most of our cars the power peak is right near the redline so using one point doesn't substantially matter. If the torque is falling off strongly, that is the power peak is far from the redline, then where to shift will vary with the gears' ratios.
I see some nice graphs, tools and Excel work in this thread. Thought to add a link to my own home-brewed tool. Answers the main topic directly: what shift point works best? Takes dyno data, gears ratios, wheel size, weight, Cd and so on and give accelerations, velocities and times. Also some stuff on front-sump oil sloshing but that's safely ignored.
Enjoy:
http://home.earthlink.net/~morecowbell98/BenchRace.xls
Mark! Why didn't you PM me on this?
One thing people don't look into is what the best shift point is for each gear. On most of our cars the power peak is right near the redline so using one point doesn't substantially matter. If the torque is falling off strongly, that is the power peak is far from the redline, then where to shift will vary with the gears' ratios.
I see some nice graphs, tools and Excel work in this thread. Thought to add a link to my own home-brewed tool. Answers the main topic directly: what shift point works best? Takes dyno data, gears ratios, wheel size, weight, Cd and so on and give accelerations, velocities and times. Also some stuff on front-sump oil sloshing but that's safely ignored.
Enjoy:
http://home.earthlink.net/~morecowbell98/BenchRace.xls
