Another torque versus horsepower question for a track car
01-03-2008, 02:43 PM
#151
Rennlist Member
Bill,
Im still trying to understand your great graphs.
am i missing something, when i think im seeing that it pays to stay in each gear passed redline, due to the maximized thrust force? it doesnt look like a shift in any gear earlier creates any gain in thrust force at any point.
From the graphs, shift at 7000rpm!!!! (to maximize HPseconds and acceleration overall for any vehicle speed range)
The easy way around this, is to just look at the HP curve and mark on it, the gear spacings. if at any point a redline shift produces less HP than the next gear HP, then it pays to shift earlier. how much earlier has to be determined by closer examination. since HP curves are all different shapes and sizes, it depends.
mk
Im still trying to understand your great graphs.
am i missing something, when i think im seeing that it pays to stay in each gear passed redline, due to the maximized thrust force? it doesnt look like a shift in any gear earlier creates any gain in thrust force at any point.
From the graphs, shift at 7000rpm!!!! (to maximize HPseconds and acceleration overall for any vehicle speed range)
The easy way around this, is to just look at the HP curve and mark on it, the gear spacings. if at any point a redline shift produces less HP than the next gear HP, then it pays to shift earlier. how much earlier has to be determined by closer examination. since HP curves are all different shapes and sizes, it depends.
mk
01-03-2008, 02:53 PM
#152
Rennlist Member
Great point. You wrote this before i asked the question as well in the following post. I just seemed to obvious. But, now that i think about it, Bill was always underlining the point that he was trying to maximize the torque in the next gear shift. (and i dont know why) You have pointed out, what i have been trying to say all along, it has to do with maximizing HP-seconds. From the look at his graph, its seems obvious that this would be done by shifting at 7000rpm, or am i missing something?
With each gear, the greatest thrust force is provided all the way to 7krpm, and like you said, it looks like if his engine could withstand 7500, there would be more gains as well!
Mark
With each gear, the greatest thrust force is provided all the way to 7krpm, and like you said, it looks like if his engine could withstand 7500, there would be more gains as well!
Mark
Edit: Mike and Mark replied above while I was writing this, so it covers the same issues.
Actually, Bill, I still don't think you have quite got Mark's point yet. This graph is a good example of a car that would be fastest when shifting at the 7,000 rpm red line in every gear. Look at 4th-5th, for example. At 7,000 rpm in 4th there is over 750 lbs of thrust. There is no point on the 5th gear thrust curve that hits 750 lbs of thrust, so the car is always going to accelerate faster at 7,000 rpm in 4th than at any rpm in 5th. Yes, the torque is falling, and has been since the torque peak at , what, 4,000 rpm? Yes, there is less horsepower at 7,000 rpm than at 6,500 rpm. However, you are in a lower gear, so the mechanical advantage of the lower gear means that you are accelerating faster.
Torque moves the car, but the gears act as torque multipliers. Horsepower is just the rate of producing torque. Engine torque times rpm divided by 5,252 equals horsepower in the SAE system of units. That means that 300 ft-lbs of torque at 3,000 rpm produces exactly the same amount of power as 150 ft-lbs at 6,000 rpm. 160 ft-lbs at 6,000 rpm produces more power, and, due to the mechanical advantage available through the gearing, will accelerate the car faster than 300 ft-lbs at 3,000 rpm.
In terms of Mark's "HP-seconds", just look at the amount of horsepower under the curve between 5,000 rpm and 7,000 rpm. It is more than under the curve between 4,500 rpm and 6,500 rpm. This car will always be faster if you can keep the revs between 5,000 and 7,000 rpm than if you keep it between 4,500 rpm and 6,500 rpm. In fact, it would (probably) be fastest if you raised the rev limit to 7,500 rpm. You could then operate between 4,500 and 6,500 rpm during DEs and endurance racing, 5,000 and 7,000 rpm when racing and 5,500 and 7,500 rpm when qualifying or when there is enough prize/sponsorship money to pay for the resulting engine rebuild you would soon require.
Yes, there are some engines that actually benefit from earlier shifting. A big 2-valve V8, for example. Or a 944 turbo with the small, stock K26-6 turbo that runs out of breath at high rpm. The fastest way to drive a chipped but otherwise stock 951 boosting 15 psi or more is to shift at the 6,500 rpm redline in first, then to progressively shift sooner in higher gears, with 4th-5th coming at about 5,500 rpm. But as soon as you put on a slightly bigger turbo, even a K26-8, the torque curve falls above 6,000 rpm, but no longer plummets like it is diving off a cliff. In that case, the fastest way around the track is to shift at redline in every gear (until it breaks down, which it will.)
Actually, Bill, I still don't think you have quite got Mark's point yet. This graph is a good example of a car that would be fastest when shifting at the 7,000 rpm red line in every gear. Look at 4th-5th, for example. At 7,000 rpm in 4th there is over 750 lbs of thrust. There is no point on the 5th gear thrust curve that hits 750 lbs of thrust, so the car is always going to accelerate faster at 7,000 rpm in 4th than at any rpm in 5th. Yes, the torque is falling, and has been since the torque peak at , what, 4,000 rpm? Yes, there is less horsepower at 7,000 rpm than at 6,500 rpm. However, you are in a lower gear, so the mechanical advantage of the lower gear means that you are accelerating faster.
Torque moves the car, but the gears act as torque multipliers. Horsepower is just the rate of producing torque. Engine torque times rpm divided by 5,252 equals horsepower in the SAE system of units. That means that 300 ft-lbs of torque at 3,000 rpm produces exactly the same amount of power as 150 ft-lbs at 6,000 rpm. 160 ft-lbs at 6,000 rpm produces more power, and, due to the mechanical advantage available through the gearing, will accelerate the car faster than 300 ft-lbs at 3,000 rpm.
In terms of Mark's "HP-seconds", just look at the amount of horsepower under the curve between 5,000 rpm and 7,000 rpm. It is more than under the curve between 4,500 rpm and 6,500 rpm. This car will always be faster if you can keep the revs between 5,000 and 7,000 rpm than if you keep it between 4,500 rpm and 6,500 rpm. In fact, it would (probably) be fastest if you raised the rev limit to 7,500 rpm. You could then operate between 4,500 and 6,500 rpm during DEs and endurance racing, 5,000 and 7,000 rpm when racing and 5,500 and 7,500 rpm when qualifying or when there is enough prize/sponsorship money to pay for the resulting engine rebuild you would soon require.
Yes, there are some engines that actually benefit from earlier shifting. A big 2-valve V8, for example. Or a 944 turbo with the small, stock K26-6 turbo that runs out of breath at high rpm. The fastest way to drive a chipped but otherwise stock 951 boosting 15 psi or more is to shift at the 6,500 rpm redline in first, then to progressively shift sooner in higher gears, with 4th-5th coming at about 5,500 rpm. But as soon as you put on a slightly bigger turbo, even a K26-8, the torque curve falls above 6,000 rpm, but no longer plummets like it is diving off a cliff. In that case, the fastest way around the track is to shift at redline in every gear (until it breaks down, which it will.)
01-03-2008, 03:56 PM
#153
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Thanks everyone for hanging in there. It was a heated discussion and one I thought was going to end like most of them, where it just dies off. But , we all got on the same page in the end, in part by the website and Mike's post. Thank you Mike for the comments as well as whoever first posted the link to Craigs article. I was touting how this discussion would end in a few minutes on a white board, well, the article was the whiteboard! Great article.
You're the one in the trenches, no need to credit me whatsoever. If you weren't so patient and DaveM993 wasn't sincerely interested, we wouldn't have gotten to this point. Obviously you've been "working this issue" for ages and it looks like the recent website article pulls together what you've been presenting for eons. Now your have another tool at your disposal to use!
Putting pending questions to Bill V. regarding shift points aside (i.e Waterguy's post), presenting a car's performance potential and shift point selection either as thrust (at the tire patch) or the functional equivalent, torque at the wheels, for each gear, is great way to understand shift points. And for those who want to take it a bit further, you can overlay all gear choices to get and understanding of the importance and effect of this variable.
Mike
01-03-2008, 04:24 PM
#154
Bill,
I really like the presentation of thrust available for each gear. If you don't mind me building on what you said in an earlier post, it's torque at the wheels that accelerates the car. I assume that "Thrust" on the y-axis of your graph is simply torque at the drive wheels divided by tire radius. So the goal is to operate at the highest thrust level possible.
Question...your graph...assume you're out of a corner in 3rd gear...Muslanne-like straight ahead. You select (light blue curve) to upshift at 6250 rpm where you trade a thrust of approx 1100 lbf for about 900 lbf in 4th at 5250 rpm. Why not run 3rd to redline (6500 rpm) and get this benefit (what others would refer to as maximizing the area under the curve)?
Mike
I really like the presentation of thrust available for each gear. If you don't mind me building on what you said in an earlier post, it's torque at the wheels that accelerates the car. I assume that "Thrust" on the y-axis of your graph is simply torque at the drive wheels divided by tire radius. So the goal is to operate at the highest thrust level possible.
Question...your graph...assume you're out of a corner in 3rd gear...Muslanne-like straight ahead. You select (light blue curve) to upshift at 6250 rpm where you trade a thrust of approx 1100 lbf for about 900 lbf in 4th at 5250 rpm. Why not run 3rd to redline (6500 rpm) and get this benefit (what others would refer to as maximizing the area under the curve)?
Mike
The thrust calculation is easy.
you need;
dyno torque in ft-lb and rpm
drive wheel loaded radius in ft or loaded rev/ mile, I use revs/mi
loaded radius in ft = 5280/(2*pi*drive wheel revs/mi)
overall gear ratio = individual gear * cwp ratio
thrust = (torque*overall gear ratio)/(loaded radius)
hp =(torque*HP)/5250
***************************
Guys you are right, take it to the redline in each gear, I got too zoned on what's happening in 1 and lost the bigger picture.
01-03-2008, 04:26 PM
#155
01-03-2008, 06:03 PM
#156
Rennlist Member
Is it?
HP-seconds, Kw-hours are unit measures of work. Is torque?
Ive always understood that torque is a force in a rotational plane. I know you went through great lengths to explain the dimensional equivilance, but if you toss around torque again, and not qualify it to at least be tied to the torque at the rear wheels, or better said, tied to a force acting on the pavement, by the rear wheels, where at any speed, the rear wheel torque can vary, while the power is the same. (meaning the rear wheel forces are the same as well)
wheeew.
If we keep folks on the track of lookng at powercurves, we do everyone a big service. Just look how easy it is to see where your shift points are, without even multiplying out the thrust forces??? keep the HP maximized and thrust forcs are maximized by default. That way, anyone with a HP curve can just take their gear spacing, figure out what the rpm drop is at redline, and see if there is more torque after a shift. If there is, short shift, if there is not, keep it running to redline!
Mk
HP-seconds, Kw-hours are unit measures of work. Is torque?
Ive always understood that torque is a force in a rotational plane. I know you went through great lengths to explain the dimensional equivilance, but if you toss around torque again, and not qualify it to at least be tied to the torque at the rear wheels, or better said, tied to a force acting on the pavement, by the rear wheels, where at any speed, the rear wheel torque can vary, while the power is the same. (meaning the rear wheel forces are the same as well)
wheeew.
If we keep folks on the track of lookng at powercurves, we do everyone a big service. Just look how easy it is to see where your shift points are, without even multiplying out the thrust forces??? keep the HP maximized and thrust forcs are maximized by default. That way, anyone with a HP curve can just take their gear spacing, figure out what the rpm drop is at redline, and see if there is more torque after a shift. If there is, short shift, if there is not, keep it running to redline!
Mk
01-03-2008, 06:13 PM
#157
Is it?
HP-seconds, Kw-hours are unit measures of work. Is torque?
Ive always understood that torque is force x distance, or a force in a rotational plane. I know you went through great lengths to explain the dimensional equivilance, but if you toss around torque again, and not qualify it to at least be tied to the torque at the rear wheels, or better said, tied to a force acting on the pavement, by the rear wheels, where at any speed, the rear wheel torque can vary, while the power is the same. (meaning the rear wheel forces are the same as well)
HP-seconds, Kw-hours are unit measures of work. Is torque?
Ive always understood that torque is force x distance, or a force in a rotational plane. I know you went through great lengths to explain the dimensional equivilance, but if you toss around torque again, and not qualify it to at least be tied to the torque at the rear wheels, or better said, tied to a force acting on the pavement, by the rear wheels, where at any speed, the rear wheel torque can vary, while the power is the same. (meaning the rear wheel forces are the same as well)
work = force x linear distance
unit ft-lb
torque = force x a circular distance
unit ft- lb
hp =work/time
unit ft-lb/s
hp-sec = hp x sec
unit ft-lb because the s in the numerator and denominator cancel
they all measure the same thing
01-03-2008, 06:21 PM
#158
Rennlist Member
Got it. Not as intuitive for me, but the algebra doesnt lie.
thanks
Mark
thanks
Mark
the dimensional analysis verifires it but i'll try a differnt explanation.
work = force x linear distance
unit ft-lb
torque = force x a circular distance
unit ft- lb
hp =work/time
unit ft-lb/s
hp-sec = hp x sec
unit ft-lb because the s in the numerator and denominator cancel
they all measure the same thing
work = force x linear distance
unit ft-lb
torque = force x a circular distance
unit ft- lb
hp =work/time
unit ft-lb/s
hp-sec = hp x sec
unit ft-lb because the s in the numerator and denominator cancel
they all measure the same thing
01-03-2008, 06:42 PM
#159
Three Wheelin'
the dimensional analysis verifires it but i'll try a differnt explanation.
work = force x linear distance
unit ft-lb
torque = force x a circular distance
unit ft- lb
hp =work/time
unit ft-lb/s
hp-sec = hp x sec
unit ft-lb because the s in the numerator and denominator cancel
they all measure the same thing
work = force x linear distance
unit ft-lb
torque = force x a circular distance
unit ft- lb
hp =work/time
unit ft-lb/s
hp-sec = hp x sec
unit ft-lb because the s in the numerator and denominator cancel
they all measure the same thing
01-03-2008, 07:06 PM
#160
No, technically they don't. They do have the same units, but one is a measure of energy (scalar quantity), and the other is the measure of an angular force (vector quantity.) If your eyes have glazed over after reading this, carry on with your life. For those interested in a (relatively) simple explanation: http://en.wikipedia.org/wiki/Torque
01-03-2008, 07:33 PM
#161
Three Wheelin'
Well, as the article says, they have the same units but are not the same quantities. For the purposes of this discussion, you can take a dyno torque curve, then factor in all the variables that translate it from a rotational force to a linear force (gear ratio, final drive ratio, tire diameter, rpm) and calculate out a thrust (or acceleration if you know other variables such as drag coefficient and weight.) The results may surprise you, because the shape of the torque curve tells you nothing directly about what rpm band is going to get you the fastest result.
On the other hand, Mark does not need to do those calculations. Since horsepower is a measure of the rate of doing linear work, he just says "I need to be in whatever gear that will keep me in the highest part of the horspower curve for the longest period of time." And he will be right.
On the other hand, Mark does not need to do those calculations. Since horsepower is a measure of the rate of doing linear work, he just says "I need to be in whatever gear that will keep me in the highest part of the horspower curve for the longest period of time." And he will be right.
01-03-2008, 08:48 PM
#162
Rennlist Member
In the article with the V8 vs the turbo I4, (both 500hp, but much different engine torques) They used the same gear boxes as they went from 3-4 6 speeds. Hardly a fair comparison.
I took the power curves and found something interesting, that maybe the author didn't think of. If we are comparing different engines with the same HP, why not match the gears to the proportional speeds of the respective engines, with the same exact ratios?? He makes a point, but the point could be better made with gear boxes that fit the engines, and keeps things honest.
If you took the high revving engine and gave it a .78 rpm drop gear box. (less than even a cup car set up) and left the V8 torquer alone at near .7 rpm drop per shift, you could make the speeds match the HPs almost identically for both cars. Kind of contrary to the authors point, you could make both cars accelerate almost exactly same at ANY speed. I think this would further drive home the point that its the power at any speed that determines acceleration.
(a point he makes throughout his article, as well as quoting the: Acceleration=power/(mass x velocity))
One last interesting about using a closer ratio gear box on the high revving 500hp I4 turbo motor, is that suddenly, the HP curve is such, that it doesn't need to be shifted at redline. in fact, with a .78 drop in rpms, you would want to short shift it from 9000rpm to down to 8700rpm. This could actually get lower if the gears got closer, like a cup car .85rpm drop, down to short shifting at 8500rpm. The curves were hypothetical, but the point is true. This would be difficult to determine by looking at the torque curves and would be a lot of tedious calculations. The HP curves and knowing that its all about maximizing hp, makes this exercise, very easy.
Mk
I took the power curves and found something interesting, that maybe the author didn't think of. If we are comparing different engines with the same HP, why not match the gears to the proportional speeds of the respective engines, with the same exact ratios?? He makes a point, but the point could be better made with gear boxes that fit the engines, and keeps things honest.
If you took the high revving engine and gave it a .78 rpm drop gear box. (less than even a cup car set up) and left the V8 torquer alone at near .7 rpm drop per shift, you could make the speeds match the HPs almost identically for both cars. Kind of contrary to the authors point, you could make both cars accelerate almost exactly same at ANY speed. I think this would further drive home the point that its the power at any speed that determines acceleration.
(a point he makes throughout his article, as well as quoting the: Acceleration=power/(mass x velocity))
One last interesting about using a closer ratio gear box on the high revving 500hp I4 turbo motor, is that suddenly, the HP curve is such, that it doesn't need to be shifted at redline. in fact, with a .78 drop in rpms, you would want to short shift it from 9000rpm to down to 8700rpm. This could actually get lower if the gears got closer, like a cup car .85rpm drop, down to short shifting at 8500rpm. The curves were hypothetical, but the point is true. This would be difficult to determine by looking at the torque curves and would be a lot of tedious calculations. The HP curves and knowing that its all about maximizing hp, makes this exercise, very easy.
Mk
01-04-2008, 07:57 AM
#164
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Bill,
Im still trying to understand your great graphs.
am i missing something, when i think im seeing that it pays to stay in each gear passed redline, due to the maximized thrust force? it doesnt look like a shift in any gear earlier creates any gain in thrust force at any point.
From the graphs, shift at 7000rpm!!!! (to maximize HPseconds and acceleration overall for any vehicle speed range)
Im still trying to understand your great graphs.
am i missing something, when i think im seeing that it pays to stay in each gear passed redline, due to the maximized thrust force? it doesnt look like a shift in any gear earlier creates any gain in thrust force at any point.
From the graphs, shift at 7000rpm!!!! (to maximize HPseconds and acceleration overall for any vehicle speed range)
The graph below is based on my stock 993TT, the numbers are not from the factory dyno, but from my datalogger, therefore real life data of acceleration in 3rd, 4th and 5th gears, from which I drew the thrust curves and the resulting HP and torque curves. These are more accurate than the factory obviously for my particular car in my particular conditions as they take into consideration real life numbers instead of a generic factory dyno.
My optimum shift points to maximize acceleration are redline in 1st and 2nd., 6740RPM in 3rd. 6,760 in 4th. and 6,520 in 5th to 6th. Pretty close to redline.
I thought this thread was slowing down too much.
01-04-2008, 01:30 PM
#165
Rennlist Member
Thread Starter
Mark in B.,
I am the poster child for this now. I like to think I have an open mind...and to be honest I have learned something...or more accurately...expanded my understanding of the application of power from an engine.
Yes, in a given gear the acceleration of that vehicle will be at its maximum at the RPM point of max torque. This is where I thought it ended. But then I realized we are not talking about a given gear. We are talking about a given speed....which Mark kept pounding on...and frankly I wasn't listening. From that perspective, if you had the ability to choose from an infinite number of gears at a given speed, the gear that will give you the highest amount of acceleration at the that speed will be the one that puts the rpm at the max HP. In the concept of exiting a corner you are at a given speed...you choose the gear...not the other way around, this is why it is important. I got stuck on>>>when an engine is revving up through the rpm when does it develop max. acceleration, at the point of max torque....however, the next part of the story is...OK at that speed, in that gear, if you could suddenly shift to a lower gear to bring your revs up to the point of max HP you would have a higher acceleration!!!
A third way - if you had an ideal CVT you would want to run the engine rpm (which would stay constant) at the point of highest HP not Torque. I made this observation very early on in the thread...but I didn't make the connection to what Mark K. was talking about. Thanks Mark K. for your patience!
So now that the physics is settled...the real debate is when do you shift and where? I now realize there is no hard a fast answer to that question, it depends, every engine has a different torque curve, it depends on the dynamics of the corner...blah, blah. I am still not convinced that shifting at redline will lead to the greatest average acceleration, ALWAYS...as it is entirely dependent on what happens to engine torque AFTER max HP.
I have always shifted around (bracketed) the point of max HP...in my car this allows me to go to redline...I still have good torque there. But I think it is wrong to say one would do that in every case for every car.
I am going to go back to this link https://rennlist.com/forums/off-topic/400308-greatest-ownage-ever-on-a-car-forum.html(it is the OT link ...but the link to NSXprime is in the first post) to see some of the greatest pwnage of all time. The poor admins on this site are dealing with a worldwide onslaught of hits...you may see nothing but a donkey. But if you have 4 hours it is worth the read...if you can get through.
cheers
I am the poster child for this now. I like to think I have an open mind...and to be honest I have learned something...or more accurately...expanded my understanding of the application of power from an engine.
Yes, in a given gear the acceleration of that vehicle will be at its maximum at the RPM point of max torque. This is where I thought it ended. But then I realized we are not talking about a given gear. We are talking about a given speed....which Mark kept pounding on...and frankly I wasn't listening. From that perspective, if you had the ability to choose from an infinite number of gears at a given speed, the gear that will give you the highest amount of acceleration at the that speed will be the one that puts the rpm at the max HP. In the concept of exiting a corner you are at a given speed...you choose the gear...not the other way around, this is why it is important. I got stuck on>>>when an engine is revving up through the rpm when does it develop max. acceleration, at the point of max torque....however, the next part of the story is...OK at that speed, in that gear, if you could suddenly shift to a lower gear to bring your revs up to the point of max HP you would have a higher acceleration!!!
A third way - if you had an ideal CVT you would want to run the engine rpm (which would stay constant) at the point of highest HP not Torque. I made this observation very early on in the thread...but I didn't make the connection to what Mark K. was talking about. Thanks Mark K. for your patience!
So now that the physics is settled...the real debate is when do you shift and where? I now realize there is no hard a fast answer to that question, it depends, every engine has a different torque curve, it depends on the dynamics of the corner...blah, blah. I am still not convinced that shifting at redline will lead to the greatest average acceleration, ALWAYS...as it is entirely dependent on what happens to engine torque AFTER max HP.
I have always shifted around (bracketed) the point of max HP...in my car this allows me to go to redline...I still have good torque there. But I think it is wrong to say one would do that in every case for every car.
I am going to go back to this link https://rennlist.com/forums/off-topic/400308-greatest-ownage-ever-on-a-car-forum.html(it is the OT link ...but the link to NSXprime is in the first post) to see some of the greatest pwnage of all time. The poor admins on this site are dealing with a worldwide onslaught of hits...you may see nothing but a donkey. But if you have 4 hours it is worth the read...if you can get through.
cheers
Thanks for taking the time to post all of this information!