When to shift? How to produce maximum area under the curve? 3.2 Carrera.
11-21-2006, 01:41 PM
#16
Rennlist Member
Originally Posted by NJ-GT
HP is what matters for acceleration.
The new 997TT has a peak torque of 460 lbs/ft between 1,900 rpm - 5,000 rpm. Beyond 5,000 rpm up to 7,000 rpm the torque falls. If torque is what determines acceleration, a driver keeping the 997TT at full gas between 2,000 rpm and 4,000 rpm will accelerate quicker than a driver keeping the same car between 5,000 rpm and 7,000 rpm, because the driver running at lower RPMs is getting more torque. In reality, the driver running at higher RPM will accelerate much quicker.
The new 997TT has a peak torque of 460 lbs/ft between 1,900 rpm - 5,000 rpm. Beyond 5,000 rpm up to 7,000 rpm the torque falls. If torque is what determines acceleration, a driver keeping the 997TT at full gas between 2,000 rpm and 4,000 rpm will accelerate quicker than a driver keeping the same car between 5,000 rpm and 7,000 rpm, because the driver running at lower RPMs is getting more torque. In reality, the driver running at higher RPM will accelerate much quicker.
What folks are perhaps confusing is the "over time" part of horsepower. Assuming away things like wheel slip, in the same gear, your example car will accelerate faster from 2-4k rpm than it will from 5-7k rpm assuming it takes the same amount of time to go from 2-4k rpm as it does from 5-7k rpm (assuming away air resistance). You ever notice a car seemingly running out of steam and "pull" at high rpms in a gear? That's the car's torque falling away (again, ignoring air resistance).
If you want to figure out max acceleration across gears (the original premise of this thread) then the calculation needs to consider the relationship of force (or power, do the conversion if you prefer to work in hp - it is merely derived from torque, anyway) across the gear ratios (no longer engine torque, per se). Because of the limited number of gears we have in our cars, it is basically always likely that applying slightly less force (torque) but doing it alot (work, high revolutions) across the unit of time (minute) is better than doing more force, but less work (lower revs) over the same amount of time (minute). Thus we generally should always shift our Porsches at redline.
RD, you want peak rpm to take advantage of maximum at the wheel torque (e.g. you want to apply as much force at the wheels as possible at any given moment in time over as much time as possible). Figure this out by plotting rear wheel torque vs. rpm using your gear calcs.
Or, if you want to know the optimal place to shift based on speed (velocity), plot power vs. velocity, then shift at max power. (Easier to derive based on P=FV).
Guess what? The two approaches will give you the same answer...
11-21-2006, 01:56 PM
#17
Rennlist Member
Thread Starter
Originally Posted by M758
IMHO there are only a few cases to shift below Redline.
here are some of them
1) Track conditions. Depening on the track and where you are you may want to "short" shift. This typically occurs when you already have all the power your car can put down or waiting to shift at redline forces shift in bad spot on the track.
2) Power falls off dramaticly in upper RPMs. This is not usally the case or Porsche motors flat sixes especially. However if the power peak is 5500 RPM at lets say 200 hp 4500 RPM is 180 hp and 6200 RPM you are down to 150 hp. In that case shift well before 6200 rpm as the engine actually makes more hp at 4500 rpm. This happens on motors with great low end torque, but limited breathing and they simply run out of steam. In a case like this what is happening is the torque curve peaks down low and really drops off at 6200 RPM. In most engines the torque drops off, but hp goes up since hp = torque times engine speed. Now if peak hp is 200 hp at 5500 Rpm and 6200 rpm is 185 I'd still probaby run to redline since this more than the hp at 4500 rpm eventhough the hp is dropping.
3) Engine life. Some engines just don't like being reved to far. The 944 engines run well to 6000 to 6200 RPM. They can run 7000, but doing so over and over again puts alot more stress on the oiling to the bearings. So I like to shift at 6000 to 6200 and on rare occasion run to 6500 if I need just a tiny bit more. Peak power is 5800 to 6000 anyway so I don't gain more hp beyond 6200. I did do some looking to see what would be optimal. In my case this was looking at hp at shift point vs the rpm in the next gear. My goal was to shift so that one in the next gear I was making at least as much hp in other gear. I might work, but got complex since this changed my shift point in each gear and often put me past my 6200 RPM redline.
So I still shift at 6000 to 6200.
here are some of them
1) Track conditions. Depening on the track and where you are you may want to "short" shift. This typically occurs when you already have all the power your car can put down or waiting to shift at redline forces shift in bad spot on the track.
2) Power falls off dramaticly in upper RPMs. This is not usally the case or Porsche motors flat sixes especially. However if the power peak is 5500 RPM at lets say 200 hp 4500 RPM is 180 hp and 6200 RPM you are down to 150 hp. In that case shift well before 6200 rpm as the engine actually makes more hp at 4500 rpm. This happens on motors with great low end torque, but limited breathing and they simply run out of steam. In a case like this what is happening is the torque curve peaks down low and really drops off at 6200 RPM. In most engines the torque drops off, but hp goes up since hp = torque times engine speed. Now if peak hp is 200 hp at 5500 Rpm and 6200 rpm is 185 I'd still probaby run to redline since this more than the hp at 4500 rpm eventhough the hp is dropping.
3) Engine life. Some engines just don't like being reved to far. The 944 engines run well to 6000 to 6200 RPM. They can run 7000, but doing so over and over again puts alot more stress on the oiling to the bearings. So I like to shift at 6000 to 6200 and on rare occasion run to 6500 if I need just a tiny bit more. Peak power is 5800 to 6000 anyway so I don't gain more hp beyond 6200. I did do some looking to see what would be optimal. In my case this was looking at hp at shift point vs the rpm in the next gear. My goal was to shift so that one in the next gear I was making at least as much hp in other gear. I might work, but got complex since this changed my shift point in each gear and often put me past my 6200 RPM redline.
So I still shift at 6000 to 6200.
It's interesting that you can be faster even when power falls off, but slower if power falls off too quickly.
When I drove a diesel on the Autobahn in Germany, I shifted well before the 5,000RPM redline because the torque/HP falls off too quickly at 4,500. I ended up shifting at about 4,000 to gain speed quicker.
But in the end, I still don't see how the area under the curve is not all that you need to determine when to shift.
I used to have a formula that calculated the RPMs at a given speed with a given tire diameter and gear ratio. I will look for that and post it, because you can use it with any car. It doesn't include tire expansion due to speed nor tire slippage, but it's close.
11-21-2006, 02:28 PM
#18
Also, I'm not sure "thrust" is an applicable term here. Can you explain what you mean? Do you mean the force being applied to the car?
thrust is the push that you feel in the small of your back.
if you know the mass of the car, and neglect aero effects, thrust can be used in the old standby Newtonian formula to give acceleration
F=Ma or in this case a = F/M
11-21-2006, 02:38 PM
#19
Rennlist Member
I think you missed the point. HP determines the torque at the rear wheels , as multiplied through the gear box at any vehicle speed. as i said, torque is irrelavent, unless you know rpms. (just as rpms are a non factor, unless you know the engine torque). Since accleration is proportional to power, at any vehicle speed, (based on the netonian identity of acceleration = power/(mass x acceleration), one could assume that maximizing HP at any speed will yeild max acceleration . To do this, look at[ the HP curves and eyeball the curve that is spaced out for the gear ratio spacing. In the mike911 graph, because we know that the 911 spacing is near .7, the rpm drop will be down from 6400rpm to 4500rpm. because you can see that the left side is lower than the right side on the curve, it pays to shift as high of an rpm as possible. Using averages is a good approximation with HP curve. (rough integration)
NOT in all cases is Hp is derived. HP is also the rate of change of kinetic energy. on a dyno, you actually measure this, or rear wheel torque as made through the gears. the dyno computer then takes a spark input or RPM signal and then calculates engine HP. without it, the dyno has NO way of knowing engine torque, but it can know HP
based on your second statement, torque doesnt NOT determine the power found at the rear wheels. Here is why. engine torque without knowing speed or rpm is meaningless. knowing gear box ratios, you then can calculate rear wheel torque and then thrust forces. however, knowing HP and a vehicle speed, thrust forces can be calculated. we still may never know engine torque, but we will know the torque and relative forces at the rear wheels, resultant from gear ratio multiplication. Thats the F=ma. so again, HP will determine the forces at the rear wheels at any vehicle speed. (not to be confused with rwhp or rwt which is flywheel values as measured at the rear wheels)
Now to address your "facts" yes, acceleration follows the torque curve, BUT ONLY IN THAT GEAR! If you look at hp values at max torque rpm, you will see that it is low. better to downshift and use a lower gear that takes adantage of the higher hp of the engine. (and then produces MORE rear wheel torque because HP determines rear wheel torque as multiplied through the gear box at any vehicle speed) . so that "fact" while true, is misleading and is the most single misused fact in trying to understand the Hp/torque relationship.
Yes, HP is the rate of doing work. its not the length of time, its the rate. the faster you do (force x distance) the faster you accelerate. the actual force goes down inversely proportional to speed (for constant power). If you have less hp, and you do due to the fact that we have to deal with a HP curve that goes up and down over the use of gears, acceleration will go down even faster (as do the forces) as speed goes up. if we had constant acceleration over a speed range, we would have to have HP that goes up proportionally to speed, and rear wheel torque that was constant, like a rocket!
this leads in to the other confused term or concept, that of flat torque curve. what folks really mean is "flat" hp curve. we (in racing) all use the hp curve in the peak area, which means the torque is falling dramatically. we will rarely use engines in the peak torque or flat area of the torque curve range.
I hope this helps.
Mk
QUOTE=DrJupeman]Your statement that "its HP that determines the torque found at the rear wheels" is confusing. Horsepower in all cases is derived (HP=(Torque * RPM)/5252). Torque (force) is what is measured. Therefore, more accurately, "torque determines the power found at the rear wheels". In either case you have to calculate through the gear box to determine power at the wheels (when measuring force on an engine dyno) or power at the crank (when measuring force on a chassis dyno).
Here are the facts:
Any given car, in any given gear, will accelerate at a rate that exactly matches its torque curve. If you're below peak torque you will accelerate slower than if you are at peak torque.
Why we care about HP for acceleration is that it is an indication of how much force we are applying over time. HP is simply work over time and gearing allows one to manipulate the length of time one is applying a force. Applying a force over more time than less and you will go faster...[/QUOTE]
NOT in all cases is Hp is derived. HP is also the rate of change of kinetic energy. on a dyno, you actually measure this, or rear wheel torque as made through the gears. the dyno computer then takes a spark input or RPM signal and then calculates engine HP. without it, the dyno has NO way of knowing engine torque, but it can know HP
based on your second statement, torque doesnt NOT determine the power found at the rear wheels. Here is why. engine torque without knowing speed or rpm is meaningless. knowing gear box ratios, you then can calculate rear wheel torque and then thrust forces. however, knowing HP and a vehicle speed, thrust forces can be calculated. we still may never know engine torque, but we will know the torque and relative forces at the rear wheels, resultant from gear ratio multiplication. Thats the F=ma. so again, HP will determine the forces at the rear wheels at any vehicle speed. (not to be confused with rwhp or rwt which is flywheel values as measured at the rear wheels)
Now to address your "facts" yes, acceleration follows the torque curve, BUT ONLY IN THAT GEAR! If you look at hp values at max torque rpm, you will see that it is low. better to downshift and use a lower gear that takes adantage of the higher hp of the engine. (and then produces MORE rear wheel torque because HP determines rear wheel torque as multiplied through the gear box at any vehicle speed) . so that "fact" while true, is misleading and is the most single misused fact in trying to understand the Hp/torque relationship.
Yes, HP is the rate of doing work. its not the length of time, its the rate. the faster you do (force x distance) the faster you accelerate. the actual force goes down inversely proportional to speed (for constant power). If you have less hp, and you do due to the fact that we have to deal with a HP curve that goes up and down over the use of gears, acceleration will go down even faster (as do the forces) as speed goes up. if we had constant acceleration over a speed range, we would have to have HP that goes up proportionally to speed, and rear wheel torque that was constant, like a rocket!
this leads in to the other confused term or concept, that of flat torque curve. what folks really mean is "flat" hp curve. we (in racing) all use the hp curve in the peak area, which means the torque is falling dramatically. we will rarely use engines in the peak torque or flat area of the torque curve range.
I hope this helps.
Mk
QUOTE=DrJupeman]Your statement that "its HP that determines the torque found at the rear wheels" is confusing. Horsepower in all cases is derived (HP=(Torque * RPM)/5252). Torque (force) is what is measured. Therefore, more accurately, "torque determines the power found at the rear wheels". In either case you have to calculate through the gear box to determine power at the wheels (when measuring force on an engine dyno) or power at the crank (when measuring force on a chassis dyno).
Here are the facts:
Any given car, in any given gear, will accelerate at a rate that exactly matches its torque curve. If you're below peak torque you will accelerate slower than if you are at peak torque.
Why we care about HP for acceleration is that it is an indication of how much force we are applying over time. HP is simply work over time and gearing allows one to manipulate the length of time one is applying a force. Applying a force over more time than less and you will go faster...[/QUOTE]
11-21-2006, 02:59 PM
#20
Originally Posted by coryf
Adam@Autometrics posting on Cory's computer:
If you can produce a thrust chart like the one above, the ideal shift point will become VERY clear. If 2 gears cross, the intersection is the ideal shift point. The one above shows a 5 speed that does not cross, but you can imagine that a tight 6th gear might.
If you can produce a thrust chart like the one above, the ideal shift point will become VERY clear. If 2 gears cross, the intersection is the ideal shift point. The one above shows a 5 speed that does not cross, but you can imagine that a tight 6th gear might.
here are 993 thrust curves w/ different grearing
Red is stock US g50/20 blue is w/ g50/30 Cup box
11-21-2006, 03:03 PM
#21
Rennlist Member
see my inserts:>>>>>>>
Originally Posted by DrJupeman
Here's what I said, written a different way:
A car with, say, 200 ft/lbs of torque will accelerate at 2000 rpm at the same rate as it would in that gear with 200 ft/lbs at 4000 rpm (ignoring wind resistance). This despite HP being double at 4000 rpm. Acceleration will always be greatest at the torque peak in any given gear. Don't miss the given gear statement in my example.
>>>>>>a given gear is the problem, you are comparing two acceleration rates at two entirely different speeds. common problem in analysing these kind of problems. you are comparing acceleration at 2000rpm say 50mph, and then you are comparing acceleration at 100mph at 4000rpm. even though the torque for both rpm points is 200ft-lbs, acceleration will not be the same becasue the power to accelerate at the higher speed goes up with the cube of speed. but, if we are ingoring friction and aero forces, and we are talking in space, yes, force would be constant and so would acceleration. Interesting, look at what happened to hp it went up by a factor of two to keep acceleration constant ( or thrust force, rear wheel torque, whatever)
Also, I'm not sure "thrust" is an applicable term here. Can you explain what you mean? Do you mean the force being applied to the car? Do you mean acceleration? Short of attaching a jet engine or similar thrust producing device, I'm not sure how to treat "thrust" in any car acceleration equations.
>>>>>>>yes, he means force or thrust. it can be extrapolated to the rotating wheel where torque would be the term. if we have a 24"diameter tire, that would be 1" foot radius and the torque would be 1lb of thrust or force, for every 1ft-lb of torque on the CV joint.
Let's be clear on terms:
Torque is a measure of force.
Work is the measure of force over distance.
Power (hp) is the measure of work over time.
Newton taught us that F=MA. Thus if the force is constant (my 200 ft/lbs at 2000 or 4000 rpm) and the car is constant (mass the same), the acceleration will be constant. There is no RPM in Newton's equation. Indisputable, best I can figure.
>>>>>>But newton also taught us, acceleration is proportional to power
acceleration =power/(mass x velocity) this means acceleration will go down with speed, if power is constant. IT also means that over a speed range, torque (or F) will goe down proportionally witih speed. even though you may have 200ft-lbs at 1000rpm all the way to 6000rpm, because hp would be rising, the potential for acceleration would be greatest if you could use gears to multiply the 200ftlbs for any given VEHICLE SPEED. an infinitely variable gear box would operate at max hp, not at max engine torque. close ratio gear boxes allow you to keep the rpms in the max hp range longer over an speed range. When talking about engines what makes all this so complicated is that we are talking about A=F/M, but its at the rear wheels, not at the flywheel. HP determines this at any vehicle speed. Key point!
Now, when you talk about the question posed here, where to shift, and you consider that cars rarely have such nice flat peak torque curves, then what you care about is torque at the wheels. This takes into consideration gearing.
>>>>>no, think of flat HP curve as being nicer!. you want top hp and a flat of a HP curve possible. Ill take a plumetting torque curve any day of the week at the track, as long as i get high hp.
So what you want to do to accelerate the fastest is apply the maximum amount of torque at the wheels for the longest amount of time. To find optimal acceleration across gear shifts, plot your torque at the wheels vs. rpm in each gear.
Note, also, that you can determine optimal shifting by plotting power vs. velocity given the relationship P=FV.
>>>>> yes, exactly, just finid the power curve, plot out gear spacing and you are done. no need to plot anything. the long way will be to look at torque curves and multply torque vs gear ratios at any speed. the easy way is to look at a HP curve, take the gear spacind and find the points that give the max HP to the wheels. averaging is a crude, but effective way to integrate this, but if you really want to do this right, you need time spent at the rpms and HP levels too.
A car with, say, 200 ft/lbs of torque will accelerate at 2000 rpm at the same rate as it would in that gear with 200 ft/lbs at 4000 rpm (ignoring wind resistance). This despite HP being double at 4000 rpm. Acceleration will always be greatest at the torque peak in any given gear. Don't miss the given gear statement in my example.
>>>>>>a given gear is the problem, you are comparing two acceleration rates at two entirely different speeds. common problem in analysing these kind of problems. you are comparing acceleration at 2000rpm say 50mph, and then you are comparing acceleration at 100mph at 4000rpm. even though the torque for both rpm points is 200ft-lbs, acceleration will not be the same becasue the power to accelerate at the higher speed goes up with the cube of speed. but, if we are ingoring friction and aero forces, and we are talking in space, yes, force would be constant and so would acceleration. Interesting, look at what happened to hp it went up by a factor of two to keep acceleration constant ( or thrust force, rear wheel torque, whatever)
Also, I'm not sure "thrust" is an applicable term here. Can you explain what you mean? Do you mean the force being applied to the car? Do you mean acceleration? Short of attaching a jet engine or similar thrust producing device, I'm not sure how to treat "thrust" in any car acceleration equations.
>>>>>>>yes, he means force or thrust. it can be extrapolated to the rotating wheel where torque would be the term. if we have a 24"diameter tire, that would be 1" foot radius and the torque would be 1lb of thrust or force, for every 1ft-lb of torque on the CV joint.
Let's be clear on terms:
Torque is a measure of force.
Work is the measure of force over distance.
Power (hp) is the measure of work over time.
Newton taught us that F=MA. Thus if the force is constant (my 200 ft/lbs at 2000 or 4000 rpm) and the car is constant (mass the same), the acceleration will be constant. There is no RPM in Newton's equation. Indisputable, best I can figure.
>>>>>>But newton also taught us, acceleration is proportional to power
acceleration =power/(mass x velocity) this means acceleration will go down with speed, if power is constant. IT also means that over a speed range, torque (or F) will goe down proportionally witih speed. even though you may have 200ft-lbs at 1000rpm all the way to 6000rpm, because hp would be rising, the potential for acceleration would be greatest if you could use gears to multiply the 200ftlbs for any given VEHICLE SPEED. an infinitely variable gear box would operate at max hp, not at max engine torque. close ratio gear boxes allow you to keep the rpms in the max hp range longer over an speed range. When talking about engines what makes all this so complicated is that we are talking about A=F/M, but its at the rear wheels, not at the flywheel. HP determines this at any vehicle speed. Key point!
Now, when you talk about the question posed here, where to shift, and you consider that cars rarely have such nice flat peak torque curves, then what you care about is torque at the wheels. This takes into consideration gearing.
>>>>>no, think of flat HP curve as being nicer!. you want top hp and a flat of a HP curve possible. Ill take a plumetting torque curve any day of the week at the track, as long as i get high hp.
So what you want to do to accelerate the fastest is apply the maximum amount of torque at the wheels for the longest amount of time. To find optimal acceleration across gear shifts, plot your torque at the wheels vs. rpm in each gear.
Note, also, that you can determine optimal shifting by plotting power vs. velocity given the relationship P=FV.
>>>>> yes, exactly, just finid the power curve, plot out gear spacing and you are done. no need to plot anything. the long way will be to look at torque curves and multply torque vs gear ratios at any speed. the easy way is to look at a HP curve, take the gear spacind and find the points that give the max HP to the wheels. averaging is a crude, but effective way to integrate this, but if you really want to do this right, you need time spent at the rpms and HP levels too.
11-21-2006, 03:05 PM
#22
Rennlist Member
Thread Starter
I've located a couple formulas that will help in these calculations. This can be used in any car, since you can plug in MPH, RPM, Tire size, gear ratio. I should be able to compare the RPM drop between shifts with the torque/HP chart to determine when to shift. I will work on that a little bit later, but for now, here goes. Please indicate if there are any errors in my math/formulas....
11-21-2006, 03:09 PM
#23
Rennlist Member
I helped a few of my BMW racer buddies that were misled with this kind of thinking. when they finally got the fact that acceleration is proportional to power at any speed, they started shifting at redline all the time and their lap times went down. short shifting, leaves HP on the table! you have to know your hp curve. just because your torque falls off fast, is no reason to shift. you need to know the HP!! take a look at the comparison between a stock porsche 928 v8 and a stock e46M3. one has 100ftlbs more engine torque, yet at ANY speed, both cars will accelerate exactly the same!
MK
MK
Originally Posted by murphyslaw1978
These are very good points, M758. What's interesting is that most engines (even F1 engines) have power falling off at the upper RPM ranges. In fact, I would argue that the engine is not designed properly if you're peak HP is at redline, because it seems that you can gain from running beyond that RPM.
It's interesting that you can be faster even when power falls off, but slower if power falls off too quickly.
When I drove a diesel on the Autobahn in Germany, I shifted well before the 5,000RPM redline because the torque/HP falls off too quickly at 4,500. I ended up shifting at about 4,000 to gain speed quicker.
But in the end, I still don't see how the area under the curve is not all that you need to determine when to shift.
I used to have a formula that calculated the RPMs at a given speed with a given tire diameter and gear ratio. I will look for that and post it, because you can use it with any car. It doesn't include tire expansion due to speed nor tire slippage, but it's close.
It's interesting that you can be faster even when power falls off, but slower if power falls off too quickly.
When I drove a diesel on the Autobahn in Germany, I shifted well before the 5,000RPM redline because the torque/HP falls off too quickly at 4,500. I ended up shifting at about 4,000 to gain speed quicker.
But in the end, I still don't see how the area under the curve is not all that you need to determine when to shift.
I used to have a formula that calculated the RPMs at a given speed with a given tire diameter and gear ratio. I will look for that and post it, because you can use it with any car. It doesn't include tire expansion due to speed nor tire slippage, but it's close.
Last edited by mark kibort; 07-15-2009 at 06:17 PM.
11-21-2006, 03:15 PM
#24
Rennlist Member
again, shifting at a point that maximizes hp is key. not shifting at max hp, but shifting at near redline (depending on gear spacing and hp curve shape)
in a cup car where many times peak hp is redline, that is true, but most cars have an arc around max hp. so, it pays to shift above max hp , to max RPMs allowed or desired.
to your first statement, we really should focus on "at any speed" . then, the acceleration that is "hardest" will be at a point closest to max hp. you find me peak torque acceleration at "any point in time" and ill show you faster rate of acceleration at that same speed in a lower gear, closer to max HP!
MK
in a cup car where many times peak hp is redline, that is true, but most cars have an arc around max hp. so, it pays to shift above max hp , to max RPMs allowed or desired.
to your first statement, we really should focus on "at any speed" . then, the acceleration that is "hardest" will be at a point closest to max hp. you find me peak torque acceleration at "any point in time" and ill show you faster rate of acceleration at that same speed in a lower gear, closer to max HP!
MK
Originally Posted by DrJupeman
Ignoring gear changes, wrong. At a given point in time, the car will accelerate the hardest in that gear at its peak torque. Its physics. Until this thread, we (the human race) even called it a law.
What folks are perhaps confusing is the "over time" part of horsepower. Assuming away things like wheel slip, in the same gear, your example car will accelerate faster from 2-4k rpm than it will from 5-7k rpm assuming it takes the same amount of time to go from 2-4k rpm as it does from 5-7k rpm (assuming away air resistance). You ever notice a car seemingly running out of steam and "pull" at high rpms in a gear? That's the car's torque falling away (again, ignoring air resistance).
If you want to figure out max acceleration across gears (the original premise of this thread) then the calculation needs to consider the relationship of force (or power, do the conversion if you prefer to work in hp - it is merely derived from torque, anyway) across the gear ratios (no longer engine torque, per se). Because of the limited number of gears we have in our cars, it is basically always likely that applying slightly less force (torque) but doing it alot (work, high revolutions) across the unit of time (minute) is better than doing more force, but less work (lower revs) over the same amount of time (minute). Thus we generally should always shift our Porsches at redline.
RD, you want peak rpm to take advantage of maximum at the wheel torque (e.g. you want to apply as much force at the wheels as possible at any given moment in time over as much time as possible). Figure this out by plotting rear wheel torque vs. rpm using your gear calcs.
Or, if you want to know the optimal place to shift based on speed (velocity), plot power vs. velocity, then shift at max power. (Easier to derive based on P=FV).
Guess what? The two approaches will give you the same answer...
What folks are perhaps confusing is the "over time" part of horsepower. Assuming away things like wheel slip, in the same gear, your example car will accelerate faster from 2-4k rpm than it will from 5-7k rpm assuming it takes the same amount of time to go from 2-4k rpm as it does from 5-7k rpm (assuming away air resistance). You ever notice a car seemingly running out of steam and "pull" at high rpms in a gear? That's the car's torque falling away (again, ignoring air resistance).
If you want to figure out max acceleration across gears (the original premise of this thread) then the calculation needs to consider the relationship of force (or power, do the conversion if you prefer to work in hp - it is merely derived from torque, anyway) across the gear ratios (no longer engine torque, per se). Because of the limited number of gears we have in our cars, it is basically always likely that applying slightly less force (torque) but doing it alot (work, high revolutions) across the unit of time (minute) is better than doing more force, but less work (lower revs) over the same amount of time (minute). Thus we generally should always shift our Porsches at redline.
RD, you want peak rpm to take advantage of maximum at the wheel torque (e.g. you want to apply as much force at the wheels as possible at any given moment in time over as much time as possible). Figure this out by plotting rear wheel torque vs. rpm using your gear calcs.
Or, if you want to know the optimal place to shift based on speed (velocity), plot power vs. velocity, then shift at max power. (Easier to derive based on P=FV).
Guess what? The two approaches will give you the same answer...
11-21-2006, 03:15 PM
#25
Rennlist Member
Thread Starter
Originally Posted by mark kibort
I helped a few of my BMW racer buddies that were misled with this kind of thinking. when they finally got the fact that acceleration is proportional to power at any speed, they started shifting at redline all the time and their lap times went down. short shifting, leaves HP on the table! you have to know your hp curve. just because your torque falls off fast, is no reason to shift. you need to know the HP!! take a look at the comparison between a stock porsche 928 v8 and a stock e46M3. one has 100ftlbs more engine torque, yet at ANY speed, both cars will accelerate exactly the same!
MK
MK
11-21-2006, 03:25 PM
#26
Rennlist Member
Adam again...
As far as the 2-4k vs 5-7k rpm statement goes. That is true but not really relevent, as the car will be going different speeds. If you really want to compare 2 rpm ranges, the car must be going the same speed in each test. Basically, it would be 5-7k in 1st vs 2-4k in 2nd. The HP curve tells me that 5-7k in 1st would be faster. This is why HP matters rather than torque... HP inherintly takes gearing into account.
Also, a power vs velocity chart is a typical dyno graph. Wheel speed and rpm are essentailly the same. It has already been established that you do not always shift at peak power.
Might be petty but there's some unit problems going in this thread.
Force. lbs or N. This is the F in F=ma. It refers to the force from the rear tire to the ground, we're calling it thrust. Not HP or torque.
Work (or Energy). Force multiplied by distance. FtLbs or Nm (J). This is where torque comes in.
Power. Work per unit time. 550 ftlbs/s=1HP or kW=1000Nm/s (J/s)
P=FV is a steady state equation relating HP, ground speed and drag (aerodynamic and mecahnical) and is really only relevant if you are NOT considering acceleration.
And on a GT3 Cup car, the thrust curves DO cross. It is apparent on paper, and I've confirmed it with data.
Originally Posted by DrJupeman
in the same gear, your example car will accelerate faster from 2-4k rpm than it will from 5-7k rpm assuming it takes the same amount of time to go from 2-4k rpm as it does from 5-7k rpm
Or, if you want to know the optimal place to shift based on speed (velocity), plot power vs. velocity, then shift at max power. (Easier to derive based on P=FV).
Or, if you want to know the optimal place to shift based on speed (velocity), plot power vs. velocity, then shift at max power. (Easier to derive based on P=FV).
Also, a power vs velocity chart is a typical dyno graph. Wheel speed and rpm are essentailly the same. It has already been established that you do not always shift at peak power.
Might be petty but there's some unit problems going in this thread.
Force. lbs or N. This is the F in F=ma. It refers to the force from the rear tire to the ground, we're calling it thrust. Not HP or torque.
Work (or Energy). Force multiplied by distance. FtLbs or Nm (J). This is where torque comes in.
Power. Work per unit time. 550 ftlbs/s=1HP or kW=1000Nm/s (J/s)
P=FV is a steady state equation relating HP, ground speed and drag (aerodynamic and mecahnical) and is really only relevant if you are NOT considering acceleration.
And on a GT3 Cup car, the thrust curves DO cross. It is apparent on paper, and I've confirmed it with data.
11-21-2006, 03:26 PM
#27
Rennlist Member
also a little misleding as it leads you to believe that a lower gear box creates more thrust over every gear. there are no intercections, nor would you expect there to be, but there are trade offs. you shift at 15% lower rpm gain 15% more thrust, and the comparitive gear box would still be in a 30% higher for that period of time. torque in the old gear after that shift is the trade off, and continues until you run out of gears. (gearing doesnt make hp, it makes more effective the hp over a desired speed range of operation) now, if you plotted the HP utilized vs speed, this would be become more clear. (or acceleration vs speed)
Originally Posted by Bill Verburg
Even on a very close ratio box like a G50/30 they don't cross.
here are 993 thrust curves w/ different grearing
Red is stock US g50/20 blue is w/ g50/30 Cup box
here are 993 thrust curves w/ different grearing
Red is stock US g50/20 blue is w/ g50/30 Cup box
Last edited by mark kibort; 07-15-2009 at 06:17 PM.
11-21-2006, 03:30 PM
#28
Rennlist Member
Yes, and thats the point. at any speed, the hp is the same, even though torque (at the engine) is very different (up to 100ftlbs difference at some points)
MK
MK
Originally Posted by murphyslaw1978
Isn't the HP area under the curve the same for both cars in your example?
11-21-2006, 03:40 PM
#29
Rennlist Member
adam, you again? does cory know you are doing this to him? 
anyway, great points again. I hope its all sinking in.
one small point, to your last paragraph, is that yes, the F is tallking about the torque at the rear wheels as multplied at the gear box, at any speed. however, the hp we are dealing with on the dyno charts is measured there, so it's relavent and ok to use.
also, i can think of several ways where Power=FV is relavent even when not accelerating. think of my wing drag numbers. 100lbs more downforce at 100mph. thats 10lbs more drag, and at 4.5:1 3rd gear ratio, thats near 2ft-lbs of torque at the engine. (or a cost of 2hp at the engine) or you can calcululate it with power=FV too! (side note
)
anyway, great points again. I hope its all sinking in.
one small point, to your last paragraph, is that yes, the F is tallking about the torque at the rear wheels as multplied at the gear box, at any speed. however, the hp we are dealing with on the dyno charts is measured there, so it's relavent and ok to use.
also, i can think of several ways where Power=FV is relavent even when not accelerating. think of my wing drag numbers. 100lbs more downforce at 100mph. thats 10lbs more drag, and at 4.5:1 3rd gear ratio, thats near 2ft-lbs of torque at the engine. (or a cost of 2hp at the engine) or you can calcululate it with power=FV too! (side note
)
Originally Posted by coryf
Adam again...
As far as the 2-4k vs 5-7k rpm statement goes. That is true but not really relevent, as the car will be going different speeds. If you really want to compare 2 rpm ranges, the car must be going the same speed in each test. Basically, it would be 5-7k in 1st vs 2-4k in 2nd. The HP curve tells me that 5-7k in 1st would be faster. This is why HP matters rather than torque... HP inherintly takes gearing into account.
Also, a power vs velocity chart is a typical dyno graph. Wheel speed and rpm are essentailly the same. It has already been established that you do not always shift at peak power.
Might be petty but there's some unit problems going in this thread.
Force. lbs or N. This is the F in F=ma. It refers to the force from the rear tire to the ground. Not HP or torque.
Work (or Energy). Force multiplied by distance. FtLbs or Nm (J). This is where torque comes in.
Power. Work per unit time. 550 ftlbs/s=1HP or kW=1000Nm/s (J/s)
P=FV is a steady state equation relating HP, ground speed and drag (aerodynamic and mecahnical) and is really only relevant if you are NOT considering acceleration.
And on a GT3 Cup car, the thrust curves DO cross. It is apparent on paper, and I've confirmed it with data.
As far as the 2-4k vs 5-7k rpm statement goes. That is true but not really relevent, as the car will be going different speeds. If you really want to compare 2 rpm ranges, the car must be going the same speed in each test. Basically, it would be 5-7k in 1st vs 2-4k in 2nd. The HP curve tells me that 5-7k in 1st would be faster. This is why HP matters rather than torque... HP inherintly takes gearing into account.
Also, a power vs velocity chart is a typical dyno graph. Wheel speed and rpm are essentailly the same. It has already been established that you do not always shift at peak power.
Might be petty but there's some unit problems going in this thread.
Force. lbs or N. This is the F in F=ma. It refers to the force from the rear tire to the ground. Not HP or torque.
Work (or Energy). Force multiplied by distance. FtLbs or Nm (J). This is where torque comes in.
Power. Work per unit time. 550 ftlbs/s=1HP or kW=1000Nm/s (J/s)
P=FV is a steady state equation relating HP, ground speed and drag (aerodynamic and mecahnical) and is really only relevant if you are NOT considering acceleration.
And on a GT3 Cup car, the thrust curves DO cross. It is apparent on paper, and I've confirmed it with data.
11-21-2006, 03:43 PM
#30
Rennlist Member
Thread Starter
Originally Posted by mark kibort
Yes, and thats the point. at any speed, the hp is the same, even though torque (at the engine) is very different (up to 100ftlbs difference at some points)
MK
MK