Revisiting Torque, RPM, Horsepower, and Acceleration Basics?
03-30-2003, 09:03 PM
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Revisiting Torque, RPM, Horsepower, and Acceleration Basics?
What is it that makes a car accelerate?
This is a very basic question to which almost everyone probably thinks they know the answer. Maybe they do, and maybe they don't.
Let's examine the very basics again:
In Physics we're introduced to the following basic ideas: Force, Work (Force times the distance the force moves the object under force), and Power (Rate of Work per unit Time).
A motor produces two important measurable data: Torque and RPM. Torque as you should know is Force X Distance (for some of us still expressed in Foot-Pounds). The units of Torque are the same as those for Work, which are denoted differently as Pounds-Foot. While Torque needn't imply motion, Work does. Since we're interested in a crankshaft that is actually rotating, we can treat them as the same thing, where the distance is one revolution on a one foot radius.
From Torque and RPM, we calculate Horsepower - a peculiarly defined amount of Power (33,000 ft-lb per minute).
Great - so we know what the words mean, and we know how they relate in the most basic sense. So what's the problem?
The problem is that these ideas, words, and numbers don't tell the story about acceleration simply. And as you probably know, where the communication of ideas are concerned Simple is Good. Too bad that Simple isn't good enough for serious work.
Number one simple question and associated datum: "How much power does it make?" Answer: the Peak HP number. Problem is that the car with the highest peak horsepower doesn't always accelerate the fastest.
The other simple questions we're all familiar with are "How fast does it go?", and "How quick is it?" This last one is the one we all generally care about most, and it's not susceptible to a simple answer captured in a single number - excepting of course a time to distance number like the quarter mile.
The desire to find a datum to tell the story better leads to things like:
Power to Weight ratio (unfortunately based on Peak HP).
Horsepower per unit displacement - again the same limitation.
Tapley Data (lbs force at the drive wheel (at peak torque) per ton noted in each gear. This was found in Road & Track road tests until graphical time to distance replaced it or John R. Bond let go.
BMEP (Brake Mean Effective Pressure) - an average of the Cylinder Pressure useful in comparing relative efficiency. As a single number it occurs at the Peak Torque though it can be represented as an average also.
This last point (about averaging) is important.
Unless we are running a CVT (Continuously Variable Transmission), we must run the motor thru an operating rev range, and we will only spend an instant at the Peak Torque and at the Peak HP. Clearly what we are interested in then are the average values over that range for Torque, Horsepower, and BMEP. We often refer to this as the Area under the Curve, a value that can be represented adequately by a finite Riemann sum or step average.
So, now that we have a better idea of what we're talking about, let's return to the question we started with: What makes a car accelerate?
That answer is Force at the drive wheel contact patch.
Force at the drive wheel contact patch is equal to Wheel Torque divided by the rolling radius of the driven wheel/tire.
You see where this is going right? Yep, the Engine Torque Curve. But, that's not the end of the story, really just the beginning. Don't think that we're done with Power - we're not.
So let's suppose that we've turned our original question into: "Do I care about the Average Torque or the Average Horsepower?" The answer might depend on whether or not you are stuck with fixed gear ratios.
Here are four tables. The first two each have equal area under the torque curves over a rev range of the same width, the third shows how much additional engine torque is required from the original rev range and gearing to equal the wheel torque of the second:
Forgive the flat torque curves - they are a convenience, and don't compromise the general point being made.
As you can see from the fourth table, there is a reason that F1 motors rev to 18,000 and higher. An original Cosworth DFV produced about 133 bhp per litre at 8500 rpm, with a BMEP of about 203 psi . A modern era F1 V10 makes about 270 bhp per litre at 17,000 rpm, with about the same BMEP.
By comparison a box stock Integra-R motor makes around 110 bhp per litre at around 8000 rpm, with a BMEP of about 180 psi. At somewhere between 220-250 bhp and 8500-9000 rpm you could hit a BMEP of 200.
BMEP's of more than 200 are hard to get, and harder to get as the revs climb skyward. Cylinder pressure isn't everything then you see.
It should be clear to you that while yes, Engine Torque is primarily what is pushing you down the track, the RPM at which it's developed and for which you're geared is very important. Power - Horsepower - then is not meaningless or unimportant.
Remember - these are just descriptive data meant to convey a reasonable characterization of a motors performance. It only gets more complex from here.
Before moving on let's consider the case, which applies to most of us, where the gearbox ratios are fixed. In that case you are left to increase the average torque over the useful powerband, where the useful powerband is defined by your critical gearing steps and the peculiarities of the track(s) you are running on.
You are now required to consider the varying ways of looking at the torque distribution - since we rarely have a perfectly flat torque curve, and indeed might not want one.
Are we traction limited? Then let's not have a lump of torque at the bottom. If not, then do we want it all to come in as soon as possible? Maybe.
What's the distribution of time over the rev range over a lap? If you're spending most of your time in the bottom (or top) of the powerband, don't you want as much as you can get there?
So it kinda looks like you've got just a few choices:
Are you trying to build a useful powerband around fixed gearing?
Or are you trying to select gearing to make the most use of a given powerband?
Unless you're giving something big away to get it, it's intuitive that a nice smooth (even flat) torque curve is both versatile and easy to drive.
But what if somebody is offering to sell you more than that? Maybe they call it Transient Response. (Yeah, this again).
I was reminded of all that again while flipping thru this months RaceCar Engineering, in which there's a picture representing Cosworth's new Transient Dyno - not that I wasn't going to return to that subject in this post anyway.
Allow me to start over on the subject from the beginning. In the common vernacular exists a phrase called throttle response. It generally refers to one of two things.
First thing: Imagine a motor with a really heavy flywheel. When you're sitting in neutral and you blip the throttle, it gains revs with a certain speed. Then change out the flywheel for a super light one. Now when you blip the throttle it rev's quicker. It also accelerates a bit faster when you're running thru the gears. That's nice isn't it?
Second thing: Imagine a motor that has a throttle tip in mixture control deficiency. The motor doesn't run strong when subjected to a sudden change in manifold pressure. Imagine then that you've dialed in your A/F and Enrichment scheme and now it responds to the throttle much more crisply. That's what I'm talking about.
So what else is there?
Other than the two things I've mentioned above, and the aforementioned distribution of torque within a useful powerband, there are a few other relevant topics that fit within the realm of Transient Power Characteristics.
If you run a motor thru it's powerband against a load controlled to maintain a constant rate of acceleration, and log the output at several different rates, there will be differences in the measured output. This is due to complex unsteady gas flow phenomenon. Tuning about such phenomena is something that MAY take place at the highest levels of motorsport - I wouldn't know.
If you've read Mark Donohue's book, you may remember the part where he tells the Porsche engineers that until he can spin the tires in all gears at all speeds he won't admit to having enough power. In current F1 drivers run off the bottom of the powerband to manage traction during corner exit transition. Management of the slope of the torque curve below the normal WOT working range is a prime example of real world tuning of Transient Power Characteristics.
For most of us, this kind of thing, while interesting, is irrelevant. We aren't generally traction limited, and we don't tend toward explosive power delivery - we generally want (and can handle) all we can get wherever we can get it - but I'll say it again: I would generally opt for a nice smooth wide flat torque curve so it doesn't complicate my job in the corners.
Some people make a big deal about biasing their torque curves toward the bottom of the powerband. Perhaps this is on the theory that to get to the top half of the powerband you will normally have to transit the bottom half. This wouldn't hold true in a corner in which you used up the bottom half of the powerband at part throttle though for example. On the other hand if that's what you were using to pull off the corner onto a short shute it might be very useful. See how it's all conditional?
Another thing some people make a big deal about is "Recovery", as in recovery to the WOT curve power output following an upshift. SAE Paper 2002-01-0196 is concerned with Direct Torque Measurement on F1 cars. Putting it as plainly as I can, the transient phenomena associated with an upshift are largely restricted to decreasing amplitude oscillations about the WOT curve reflecting torsional drivetrain oscillations and the impulse from the angular momentum of the rotating assembly running faster than the rest of the drivetrain before the clutch is released.
SAE Paper 890877 is interesting as well. It's a brief on the Honda RA165E thru RA168E - 1.5 Litre V6 F1 Motors from 85-88 by several members of the Honda R&D team. One of the many interesting graphics is a bar chart of Senna's throttle position at the 88 San Marino GP. About 64% was spent at WOT, about 20% closed, and intermediate throttle positions were about equally distributed with a slight linear bias toward small increment.
So if we look at the whole picture, we can see that our cornering speeds are going to be reasonably predictable, and that once they're maxed out we're left with braking and accelerating. We spend a lot more time on the gas than we do on the brakes, and I don't need to remind you how important that is. So most high dollar engine development programs necessarily are of the sort defined above as developing a powerband around the gearing. This development is the type for which a sophisticated Transient Dyno is intended. The purpose of the dyno is to simulate the actual operating conditions of the motor, with one goal being the tailoring of output to the available number of gear ratios and the demands of the course layout. Some of the other purposes are development of component reliability and precise fuel consumption. It should be obvious that this will almost always entail a compromise that is most accurately arrived at by precisely such a device as a Transient Dyno.
Transient Dyno's are nothing new in the simple sense of variable load for one pass continuous data logging. There are thousands of small engine room dyno's with variable load control on their pumps. One of the primary benefits of such control is the elimination of large amounts of run time under steady state that has been referred to often as "wearing out a motor on the dyno." High end Transient Dyno's are more correctly referred to in distinction as simulation devices.
Some day when we are all using Electronically controlled Continuously Variable Transmissions much of this will be irrelevant. Here are a couple of nice tables I found at http://www.v8914.com:
Note that the CVT gives max wheel torque at Peak Power, whereas with the fixed gear gives max wheel torque at the Peak Torque. Note that at equal wheel speed of 280 the CVT wheel torque is 6964, whereas the fixed gear wheel torque is 5235. Exciting huh? Forget Sequential - give me CVT!
I hope this helps you understand somewhat better the surprisingly confusing relationship between Torque, Power, and Acceleration.
Scott, who used writing this as an excuse to watch less television...
[Modified by RR98ITR, 10:45 PM 3/30/2003]
This is a very basic question to which almost everyone probably thinks they know the answer. Maybe they do, and maybe they don't.
Let's examine the very basics again:
In Physics we're introduced to the following basic ideas: Force, Work (Force times the distance the force moves the object under force), and Power (Rate of Work per unit Time).
A motor produces two important measurable data: Torque and RPM. Torque as you should know is Force X Distance (for some of us still expressed in Foot-Pounds). The units of Torque are the same as those for Work, which are denoted differently as Pounds-Foot. While Torque needn't imply motion, Work does. Since we're interested in a crankshaft that is actually rotating, we can treat them as the same thing, where the distance is one revolution on a one foot radius.
From Torque and RPM, we calculate Horsepower - a peculiarly defined amount of Power (33,000 ft-lb per minute).
Great - so we know what the words mean, and we know how they relate in the most basic sense. So what's the problem?
The problem is that these ideas, words, and numbers don't tell the story about acceleration simply. And as you probably know, where the communication of ideas are concerned Simple is Good. Too bad that Simple isn't good enough for serious work.
Number one simple question and associated datum: "How much power does it make?" Answer: the Peak HP number. Problem is that the car with the highest peak horsepower doesn't always accelerate the fastest.
The other simple questions we're all familiar with are "How fast does it go?", and "How quick is it?" This last one is the one we all generally care about most, and it's not susceptible to a simple answer captured in a single number - excepting of course a time to distance number like the quarter mile.
The desire to find a datum to tell the story better leads to things like:
Power to Weight ratio (unfortunately based on Peak HP).
Horsepower per unit displacement - again the same limitation.
Tapley Data (lbs force at the drive wheel (at peak torque) per ton noted in each gear. This was found in Road & Track road tests until graphical time to distance replaced it or John R. Bond let go.
BMEP (Brake Mean Effective Pressure) - an average of the Cylinder Pressure useful in comparing relative efficiency. As a single number it occurs at the Peak Torque though it can be represented as an average also.
This last point (about averaging) is important.
Unless we are running a CVT (Continuously Variable Transmission), we must run the motor thru an operating rev range, and we will only spend an instant at the Peak Torque and at the Peak HP. Clearly what we are interested in then are the average values over that range for Torque, Horsepower, and BMEP. We often refer to this as the Area under the Curve, a value that can be represented adequately by a finite Riemann sum or step average.
So, now that we have a better idea of what we're talking about, let's return to the question we started with: What makes a car accelerate?
That answer is Force at the drive wheel contact patch.
Force at the drive wheel contact patch is equal to Wheel Torque divided by the rolling radius of the driven wheel/tire.
You see where this is going right? Yep, the Engine Torque Curve. But, that's not the end of the story, really just the beginning. Don't think that we're done with Power - we're not.
So let's suppose that we've turned our original question into: "Do I care about the Average Torque or the Average Horsepower?" The answer might depend on whether or not you are stuck with fixed gear ratios.
Here are four tables. The first two each have equal area under the torque curves over a rev range of the same width, the third shows how much additional engine torque is required from the original rev range and gearing to equal the wheel torque of the second:
Forgive the flat torque curves - they are a convenience, and don't compromise the general point being made.
As you can see from the fourth table, there is a reason that F1 motors rev to 18,000 and higher. An original Cosworth DFV produced about 133 bhp per litre at 8500 rpm, with a BMEP of about 203 psi . A modern era F1 V10 makes about 270 bhp per litre at 17,000 rpm, with about the same BMEP.
By comparison a box stock Integra-R motor makes around 110 bhp per litre at around 8000 rpm, with a BMEP of about 180 psi. At somewhere between 220-250 bhp and 8500-9000 rpm you could hit a BMEP of 200.
BMEP's of more than 200 are hard to get, and harder to get as the revs climb skyward. Cylinder pressure isn't everything then you see.
It should be clear to you that while yes, Engine Torque is primarily what is pushing you down the track, the RPM at which it's developed and for which you're geared is very important. Power - Horsepower - then is not meaningless or unimportant.
Remember - these are just descriptive data meant to convey a reasonable characterization of a motors performance. It only gets more complex from here.
Before moving on let's consider the case, which applies to most of us, where the gearbox ratios are fixed. In that case you are left to increase the average torque over the useful powerband, where the useful powerband is defined by your critical gearing steps and the peculiarities of the track(s) you are running on.
You are now required to consider the varying ways of looking at the torque distribution - since we rarely have a perfectly flat torque curve, and indeed might not want one.
Are we traction limited? Then let's not have a lump of torque at the bottom. If not, then do we want it all to come in as soon as possible? Maybe.
What's the distribution of time over the rev range over a lap? If you're spending most of your time in the bottom (or top) of the powerband, don't you want as much as you can get there?
So it kinda looks like you've got just a few choices:
Are you trying to build a useful powerband around fixed gearing?
Or are you trying to select gearing to make the most use of a given powerband?
Unless you're giving something big away to get it, it's intuitive that a nice smooth (even flat) torque curve is both versatile and easy to drive.
But what if somebody is offering to sell you more than that? Maybe they call it Transient Response. (Yeah, this again).
I was reminded of all that again while flipping thru this months RaceCar Engineering, in which there's a picture representing Cosworth's new Transient Dyno - not that I wasn't going to return to that subject in this post anyway.
Allow me to start over on the subject from the beginning. In the common vernacular exists a phrase called throttle response. It generally refers to one of two things.
First thing: Imagine a motor with a really heavy flywheel. When you're sitting in neutral and you blip the throttle, it gains revs with a certain speed. Then change out the flywheel for a super light one. Now when you blip the throttle it rev's quicker. It also accelerates a bit faster when you're running thru the gears. That's nice isn't it?
Second thing: Imagine a motor that has a throttle tip in mixture control deficiency. The motor doesn't run strong when subjected to a sudden change in manifold pressure. Imagine then that you've dialed in your A/F and Enrichment scheme and now it responds to the throttle much more crisply. That's what I'm talking about.
So what else is there?
Other than the two things I've mentioned above, and the aforementioned distribution of torque within a useful powerband, there are a few other relevant topics that fit within the realm of Transient Power Characteristics.
If you run a motor thru it's powerband against a load controlled to maintain a constant rate of acceleration, and log the output at several different rates, there will be differences in the measured output. This is due to complex unsteady gas flow phenomenon. Tuning about such phenomena is something that MAY take place at the highest levels of motorsport - I wouldn't know.
If you've read Mark Donohue's book, you may remember the part where he tells the Porsche engineers that until he can spin the tires in all gears at all speeds he won't admit to having enough power. In current F1 drivers run off the bottom of the powerband to manage traction during corner exit transition. Management of the slope of the torque curve below the normal WOT working range is a prime example of real world tuning of Transient Power Characteristics.
For most of us, this kind of thing, while interesting, is irrelevant. We aren't generally traction limited, and we don't tend toward explosive power delivery - we generally want (and can handle) all we can get wherever we can get it - but I'll say it again: I would generally opt for a nice smooth wide flat torque curve so it doesn't complicate my job in the corners.
Some people make a big deal about biasing their torque curves toward the bottom of the powerband. Perhaps this is on the theory that to get to the top half of the powerband you will normally have to transit the bottom half. This wouldn't hold true in a corner in which you used up the bottom half of the powerband at part throttle though for example. On the other hand if that's what you were using to pull off the corner onto a short shute it might be very useful. See how it's all conditional?
Another thing some people make a big deal about is "Recovery", as in recovery to the WOT curve power output following an upshift. SAE Paper 2002-01-0196 is concerned with Direct Torque Measurement on F1 cars. Putting it as plainly as I can, the transient phenomena associated with an upshift are largely restricted to decreasing amplitude oscillations about the WOT curve reflecting torsional drivetrain oscillations and the impulse from the angular momentum of the rotating assembly running faster than the rest of the drivetrain before the clutch is released.
SAE Paper 890877 is interesting as well. It's a brief on the Honda RA165E thru RA168E - 1.5 Litre V6 F1 Motors from 85-88 by several members of the Honda R&D team. One of the many interesting graphics is a bar chart of Senna's throttle position at the 88 San Marino GP. About 64% was spent at WOT, about 20% closed, and intermediate throttle positions were about equally distributed with a slight linear bias toward small increment.
So if we look at the whole picture, we can see that our cornering speeds are going to be reasonably predictable, and that once they're maxed out we're left with braking and accelerating. We spend a lot more time on the gas than we do on the brakes, and I don't need to remind you how important that is. So most high dollar engine development programs necessarily are of the sort defined above as developing a powerband around the gearing. This development is the type for which a sophisticated Transient Dyno is intended. The purpose of the dyno is to simulate the actual operating conditions of the motor, with one goal being the tailoring of output to the available number of gear ratios and the demands of the course layout. Some of the other purposes are development of component reliability and precise fuel consumption. It should be obvious that this will almost always entail a compromise that is most accurately arrived at by precisely such a device as a Transient Dyno.
Transient Dyno's are nothing new in the simple sense of variable load for one pass continuous data logging. There are thousands of small engine room dyno's with variable load control on their pumps. One of the primary benefits of such control is the elimination of large amounts of run time under steady state that has been referred to often as "wearing out a motor on the dyno." High end Transient Dyno's are more correctly referred to in distinction as simulation devices.
Some day when we are all using Electronically controlled Continuously Variable Transmissions much of this will be irrelevant. Here are a couple of nice tables I found at http://www.v8914.com:
Note that the CVT gives max wheel torque at Peak Power, whereas with the fixed gear gives max wheel torque at the Peak Torque. Note that at equal wheel speed of 280 the CVT wheel torque is 6964, whereas the fixed gear wheel torque is 5235. Exciting huh? Forget Sequential - give me CVT!
I hope this helps you understand somewhat better the surprisingly confusing relationship between Torque, Power, and Acceleration.
Scott, who used writing this as an excuse to watch less television...
[Modified by RR98ITR, 10:45 PM 3/30/2003]
03-30-2003, 09:10 PM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (RR98ITR)
ok that was interesting but all those ? are confusing.. some should be ' and others ?. might want to fix it all.
edit: good deal, everything looks nice now
[Modified by baonest, 1:45 AM 3/31/2003]
edit: good deal, everything looks nice now
[Modified by baonest, 1:45 AM 3/31/2003]
03-30-2003, 10:07 PM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (RR98ITR)
If you've read Mark Donohue's book, you may remember the part where he tells the Porsche engineers that until he can spin the tires in all gears at all speeds he won't admit to having enough power.
Great thread.
I related very closely to this portion of your post. Why?
Well, very early on I became unhappy with my itr's powerband... stock that is. I decided that a 13.0 1/4 mile time would be acceptable acceleration to me, and once I met that I'd be happy with playing with the rest of the car... lack of usable tq at lower rpm's was just not pleasing...
Well, I came up with the idea that I could use a higher final drive (4.9) - way back when very few people on this board had even heard of or tried it... I figured this would be in effect a tq multiplier and put more power to the wheels... it certainly "felt" like it did... I also upped the horsepower of my car... by as much as 28whp - much of that across the board and in the midrange as well as peak. It definitely made the car very much fun to drive - much of an improvement over stock. But I still wasn't close to my 13.0 goal... 14.5 was the best I could do in the 1/4 mile, and part of that was the stock clutch as well as stock weight of the vehicle... I also was using the stock wheel and tire combo... My theory was that if I wasn't spinning tires in 4th gear, then I could use a higher final drive... shorter gears up top might help too of course...
I'm now of the opinion that some very short gears might be better than the final drive... but I wasn't serious enough to run slicks or do rev-limit clutch drops...
I think at this point, I'm going to go the other route and see if I can't make some obscenely high hp in order to meet that goal... but I suspect that drivability will be even worse then...
That and the fact that I enjoyed having my car on the road course and b00st wasn't an option... I'll be better off staying stock for a while on my next car.
If you figure out the best gear ratios for a 190-200whp itr that'll give the quickest acceleration (at the brink of breaking traction at each gearshift), then please post it up.
-the r0cker, who is apologizes in advance for the mild forage OT.....
03-30-2003, 11:17 PM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (Black R)
If you figure out the best gear ratios for a 190-200whp itr that'll give the quickest acceleration (at the brink of breaking traction at each gearshift), then please post it up.
Seriously though, I don't think being at the brink of breaking traction with every gearshift is desirable. If you are going to get custom gear ratios it's possible, but it really doesn't make any sense to me. Do you really want to be able to lay down rubber on a 4-5 shift at the expense of the car topping out at 75 mph? Or maybe you're just planning on running a 10 speed tranny? 
03-31-2003, 03:10 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (HecticRhythms)
cliffnotes anyone?
Trending Topics
03-31-2003, 05:41 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (RR98ITR)
Great information, though i dont understand it quite yet. Ill have to translate it into dummy terms for myself (im a little slow) and read it again, when i dont have to worry about supervisors seeing me on the internet! hehe.
03-31-2003, 06:04 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (RR98ITR)
that is alot of info to read. I'll finished reading it later. Thanks for posting about this.
03-31-2003, 06:52 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (urbanlegend21)
cliffnotes anyone?
if you are too lazy to read this post then don't worry about cliff notes, you wouldn't get it anyway
if you are too lazy to read this post then don't worry about cliff notes, you wouldn't get it anyway
If you can't keep your attention on the subject for that long, please don't comment.
Very interesting info indeed. I knew somehow that there was going to be something about Transient Response/dynos in here.
[Modified by Chris N, 10:17 AM 3/31/2003]
03-31-2003, 08:15 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (urbanlegend21)
Cliffnotes for the lazy
Torque is what makes the car accelerate (Torque is equal to a force per distance)
To make the car move at speeds that are both lower and higher than the engine speed, gearing must be used. The point of proper gearing is to maintain near maximum torque through each gear. This is done via a large torque band throughout an RPM range. For 4 cylinder engines large RPMS must be attained to create such power because the momentum is very small when compared to a 6 or 8 cylinder engine.
To select a gear ratio, considerations of the torque band, top speed, and operating RPMS must be considered.
Finally to be very fast, you need a large pocket, and a few hours with a pencil paper, and calulator to figure it out.
Method for gear selection
Step 1: Dyno the car in the closest gear ratio of 1:1
determine power band
Decide ranges of acceleration you want 0-60? 50-90?
determine gear ratio that translates peak RPM band to those speed ranges.
Spend 6k on the gearbox and smile
[Modified by Audipwr1, 5:16 PM 3/31/2003]
[Modified by Audipwr1, 6:37 PM 3/31/2003]
Torque is what makes the car accelerate (Torque is equal to a force per distance)
To make the car move at speeds that are both lower and higher than the engine speed, gearing must be used. The point of proper gearing is to maintain near maximum torque through each gear. This is done via a large torque band throughout an RPM range. For 4 cylinder engines large RPMS must be attained to create such power because the momentum is very small when compared to a 6 or 8 cylinder engine.
To select a gear ratio, considerations of the torque band, top speed, and operating RPMS must be considered.
Finally to be very fast, you need a large pocket, and a few hours with a pencil paper, and calulator to figure it out.
Method for gear selection
Step 1: Dyno the car in the closest gear ratio of 1:1
determine power band
Decide ranges of acceleration you want 0-60? 50-90?
determine gear ratio that translates peak RPM band to those speed ranges.
Spend 6k on the gearbox and smile
[Modified by Audipwr1, 5:16 PM 3/31/2003]
[Modified by Audipwr1, 6:37 PM 3/31/2003]
03-31-2003, 08:36 AM
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Re: Revisiting Torque, RPM, Horsepower, and Acceleration Basics? (py1091)
There's a well-written, widely-distributed write-up on this topic, written by Bruce Augenstein. The version with some notes about the ITR was on this website</A> which is down as of this writing. Another copy (with some BMW references) can be accessed <A HREF="http://www.boston-bmwcca.org/bimmer/1999/03/horsepower-and-torque.html" TARGET="_blank">here.
http://web.archive.org/web/200101220...ue-and-hp.html
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