Gear ratios...
#256
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the only predictive of vehicle performance is engine torque wrt rpm, vehicle mass and aero, transmission and cwp specs and tire rolling diameter.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
#257
Nordschleife Master
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Sorry, my tablet is terrible with quoting and editing, so I often just skip it. My comment was directed at Mech33's last comment and you just happened to post right as I was posting. I wasn't posting in response to you Bill.
#258
Nordschleife Master
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BTW, I'm bowing out of this thread. It's way far away from Pete's intent and nothing new is really going to come from further conversations. Turning notifications off...
#259
Rennlist Member
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Talking about gear ratios as force or torque multipliers generally results in intuitive conclusions that are incorrect. Sure, a transmission can gain up the torque, but it does so while gaining down the speed at the same time. The result is just different combinations of torque and angular velocity at constant *power* (ignoring losses).
I recommend keeping all analysis in the power vs. speed domain, because it is simple and intuitive.
But if you want to look at things in terms of wheel force, then now you have to take into account gear ratios (and final drive ratios, and wheel radii). But comparing two scenarios isn't too difficult:
(A) 6200 RPM, 1.13 gear ratio
- Per attached dyno chart, the GT4 engine puts at 270 ft*lbs at 6200 RPM.
(B) 6800 RPM, 1.24 gear ratio
- Per attached dyno chart, the GT4 engine puts at 250 ft*lbs at 6800 RPM.
The actual FORCE at the rear tire is easy to calculate:
Force = (engine torque) * (final drive ratio) * (gear ratio) / (tire radius)
So calculate the force in each scenario:
Force_A = (270 ft*lbs) * (final drive ratio) * (1.13) / (tire radius)
Force_B = (250 ft*lbs) * (final drive ratio) * (1.24) / (tire radius)
or
Force_A = (305 ft*lbs) * (final drive ratio) / (tire radius)
Force_B = (310 ft*lbs) * (final drive ratio) / (tire radius)
Comparing Force_B to Force_A, we see that Force_B/Force_A = 310/305 = 1.016.
So scenario A puts down 1.6% more force to the ground at *that exact speed instant* than scenario B.
But we didn't need all that analysis to show that... just compare the power curves at 6200 RPM and 6800 RPM:
6200 RPM: ~317 hp
6800 RPM: ~322 hp
Comparing that power: 322 / 317 = 1.016... or 1.6% higher. Much easier than worrying about all that gearing nonsense! :P
![](https://rennlist.com/forums/attachments/gt4/1117221d1481747335-gear-ratios-gtsgt4.jpg)
I recommend keeping all analysis in the power vs. speed domain, because it is simple and intuitive.
But if you want to look at things in terms of wheel force, then now you have to take into account gear ratios (and final drive ratios, and wheel radii). But comparing two scenarios isn't too difficult:
(A) 6200 RPM, 1.13 gear ratio
- Per attached dyno chart, the GT4 engine puts at 270 ft*lbs at 6200 RPM.
(B) 6800 RPM, 1.24 gear ratio
- Per attached dyno chart, the GT4 engine puts at 250 ft*lbs at 6800 RPM.
The actual FORCE at the rear tire is easy to calculate:
Force = (engine torque) * (final drive ratio) * (gear ratio) / (tire radius)
So calculate the force in each scenario:
Force_A = (270 ft*lbs) * (final drive ratio) * (1.13) / (tire radius)
Force_B = (250 ft*lbs) * (final drive ratio) * (1.24) / (tire radius)
or
Force_A = (305 ft*lbs) * (final drive ratio) / (tire radius)
Force_B = (310 ft*lbs) * (final drive ratio) / (tire radius)
Comparing Force_B to Force_A, we see that Force_B/Force_A = 310/305 = 1.016.
So scenario A puts down 1.6% more force to the ground at *that exact speed instant* than scenario B.
But we didn't need all that analysis to show that... just compare the power curves at 6200 RPM and 6800 RPM:
6200 RPM: ~317 hp
6800 RPM: ~322 hp
Comparing that power: 322 / 317 = 1.016... or 1.6% higher. Much easier than worrying about all that gearing nonsense! :P
![](https://rennlist.com/forums/attachments/gt4/1117221d1481747335-gear-ratios-gtsgt4.jpg)
1. shift points and gear ratio changes for comfort on the street
2. shift points and gearing change effects on the race track or straight line
Mech33 nails it...
the Net net of all this.. on the street, pick the gears that make the shifiing enjoyable. on the track pick the gears that allow for the least amount of shifting , while keeping the engine at 6000 to 7000rpm at all times possible when WOT. and straight line, wont matter much due to the trade offs as long as the engine stays in the max HP range, which GT4 owners are lucky, due to the wide HP sweet spot.
![Smilie](https://rennlist.com/forums/images/smilies/smile.gif)
On a percentage basis, that GT4 torque dip from 2500 to 4500 rpm looks fairly pronounced, and seems tied to the issue under discussion. Over most of the rev range, the GT4 maintains about 20 ft-lb+ more torque than the GTS, but the difference dips to about zero at about 3000 rpm.
40 to 60mph on one car, might feel better vs 50 to 70mph, but its only because the speeds are so different, and so are the forces, partially due to the gearing changes.
#260
Rennlist Member
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the only predictive of vehicle performance is engine torque wrt rpm, vehicle mass and aero, transmission and cwp specs and tire rolling diameter.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
This is all true at any speed and any power level as well. it factors out things like gearing, wheel diameters, etc.. HP contains more information than engine torque...... it can be synonymous for most practical discussions with rear wheel force. which is what you are very familiar with , due to your thrust force curves that you provided here. they are directly proportional with each other, in every way!
![Smilie](https://rennlist.com/forums/images/smilies/smile.gif)
Im sure you make great gear boxes.. in fact, i know that you do. BUT, your understanding of the acceleration forces vs gear ratio vs power available is what i was trying to help you with. in your business , you might want have this as a knowledge base as it wlll help you and your customers with all trade offs and answers on the topic. insulting me for providing solid dynamic physics knowledge of the topic, doesnt do anyone any good
#261
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the only predictive of vehicle performance is engine torque wrt rpm, vehicle mass and aero, transmission and cwp specs and tire rolling diameter.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
power is just torque at rpm, so yes it's desirable but only because its an indication that the torque curves doesn't fall off as quickly as in a lower power situation.
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
#262
Nordschleife Master
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Bill, you can use either the power curve or torque curve as the engine output in predicting performance and calculating forces and accelerations, the math is just slightly different in each case. That is the only point here. Ignoring drag and other parasricis as that is not relevant to the discussion:
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
Last night I mentioned the history. Mark has a long history of trying to only use the latter formula in gear conversation. He does exactly what he did here saying that HP is X at point C and therefore changing the ratio will not benefit you much or at all. That's flawed.
Go to my example yesterday on Joe's lap. It was deliberately over simplified. We generally wouldn't just put in the shorter 4th without changing 3rd, even though the math has shown that there is an advantage there. It may be a small delta, but we are talking about maximizing it.
So you step back and treat it as a system, and not a single point and things change. We move it all to make the improvements. If we fix the 1-2 shift so that we are optimizing acceleration in 2nd gear, it all needs to move accordingly. Stating that the 3-4 shift doesn't need fixing because you are already had peak hp misses the point completely. This is the ongoing issue with how Mark tends to approach these conversations and why when one is talking gear ratios and gear changes I use the formula that I do, as does most everyone else I know.
#263
Rennlist Member
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The example I made is just such an instance. You have a long history of acting like HP and torque are interchangeable, and they are not. And I picked my example on purpose specifically because the better geared example falls into the range where the engine torque is falling because it exemplifies the point. I hope you don't mind that I work in Nm. This continues the 110mph, 6800 and 6200rpm assumptions made above.
FDxgear ratiox engine torque = axle torque (ignoring tire size)
3.89x1.24 x 365Nm = 1760.6Nm
3.89x 1.125x390Nm = 1706.74Nm
The car with the shorter 4th gear is going to have more power to the ground at the speed in question.
Your examples are excessively confusing to the layman reader. Your statements that you cannot make more power, you just move around how you use are accurate to a point. That works in a theoretical space. We aren't in a theoretical space, we are in a real specific space, first with Pete's example and later with Joe and LS. The assertion that the gearing on this car is good is inaccurate in both instances and falling back to the same HP argument you always make doesn't make that untrue. Or maybe it would be more correct to say that the gearing is good, but it is very easy to make it better. On paper, the car may have the HP to do the top speed it is geared for, but in practice that leaves a lot of acceleration on the table, especially in the 75-150mph range.
FDxgear ratiox engine torque = axle torque (ignoring tire size)
3.89x1.24 x 365Nm = 1760.6Nm
3.89x 1.125x390Nm = 1706.74Nm
The car with the shorter 4th gear is going to have more power to the ground at the speed in question.
Your examples are excessively confusing to the layman reader. Your statements that you cannot make more power, you just move around how you use are accurate to a point. That works in a theoretical space. We aren't in a theoretical space, we are in a real specific space, first with Pete's example and later with Joe and LS. The assertion that the gearing on this car is good is inaccurate in both instances and falling back to the same HP argument you always make doesn't make that untrue. Or maybe it would be more correct to say that the gearing is good, but it is very easy to make it better. On paper, the car may have the HP to do the top speed it is geared for, but in practice that leaves a lot of acceleration on the table, especially in the 75-150mph range.
there is NO sacrifice of the car's performance from 75 to 150 performance IF you spend time in the max HP range. gearing becomes less of a component if it allows you to be in the max HP range over all shifts, which the GT4 does VERY well. ironically, i seemed to remember that the car redlines at 145mph in 4th, so if you wanted to spend time at 145 to 150mph, you would be in 5th, but as long as you shift and are in the max HP zone 6000rpm to 7500rpm, the power would be the same, so forces would be near identical, so there is no need to change gearing to optimize.. you are already optimized. Porsche knew this. Knows this! im trying to help you here Matt.
Matt, let me be very clear.. ive never said torque doesnt matter, I've never said "HP and torque are interchangeable" i said "ENGINE TORQUE" doesnt matter for comparisons. sure you need it in the equation to calculate power, or rear wheel forces, but its a variable.
AND, HP determines rear wheel forces at any speed, period! so does engine torque, but you need all sorts of other factors to determine rear wheel forces.. make sense? that's a big difference... you have taken what i have said out of context. HP and rear wheel forces can be used interchangeably for comparisons.
what am i missing here...... im mean, "the double dipping" part.. was there another point?
Last edited by mark kibort; 12-15-2016 at 01:53 PM. Reason: removed opening emotion and spelling
#264
Rennlist Member
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Bill, you can use either the power curve or torque curve as the engine output in predicting performance and calculating forces and accelerations, the math is just slightly different in each case. That is the only point here. Ignoring drag and other parasricis as that is not relevant to the discussion:
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
Nobody here is denying that both formulas work. The problem, when the topic is gearing, is that only one of the two formulas allows you do treat gear ratio as a variable.
Last night I mentioned the history. Mark has a long history of trying to only use the latter formula in gear conversation. He does exactly what he did here saying that HP is X at point C and therefore changing the ratio will not benefit you much or at all. That's flawed.
Go to my example yesterday on Joe's lap. It was deliberately over simplified. We generally wouldn't just put in the shorter 4th without changing 3rd, even though the math has shown that there is an advantage there. It may be a small delta, but we are talking about maximizing it.
So you step back and treat it as a system, and not a single point and things change. We move it all to make the improvements. If we fix the 1-2 shift so that we are optimizing acceleration in 2nd gear, it all needs to move accordingly. Stating that the 3-4 shift doesn't need fixing because you are already had peak hp misses the point completely. This is the ongoing issue with how Mark tends to approach these conversations and why when one is talking gear ratios and gear changes I use the formula that I do, as does most everyone else I know.
Last night I mentioned the history. Mark has a long history of trying to only use the latter formula in gear conversation. He does exactly what he did here saying that HP is X at point C and therefore changing the ratio will not benefit you much or at all. That's flawed.
Go to my example yesterday on Joe's lap. It was deliberately over simplified. We generally wouldn't just put in the shorter 4th without changing 3rd, even though the math has shown that there is an advantage there. It may be a small delta, but we are talking about maximizing it.
So you step back and treat it as a system, and not a single point and things change. We move it all to make the improvements. If we fix the 1-2 shift so that we are optimizing acceleration in 2nd gear, it all needs to move accordingly. Stating that the 3-4 shift doesn't need fixing because you are already had peak hp misses the point completely. This is the ongoing issue with how Mark tends to approach these conversations and why when one is talking gear ratios and gear changes I use the formula that I do, as does most everyone else I know.
I think you might be well served to step back and think about it again. yes, you did point out your case last night , but as i understand it, if you have the same HP at any speed, the rear wheel forces will be the same. I dont know if you realize it, but you made my point exactly by showing the example you laid out.....
1. changing the gear ratio at the same speed produced the same torque (Nm)
2. you then said that because the ratio was shorter, you had more power available . which is false. same speed... same force... same power.. its the way it works.
3. I never said "because you already hit max HP" i clearly said, "Because you are in the max HP "range", you have the same rear wheel forces regardless of gearing or engine RPM in that range" (range of top HP level, and this conversation is easier due to the GT4 having a flat HP curve in that range of 6000rpm to 7500rpm)
4. this will help your business in a huge way if you are one of the few that really gets this.
#265
Rennlist Member
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That's not true, power is a derived quantity, you measure the fundamental quantities of torque and time then calculate power.
there are 2 types of dyno
absorption types measure force(torque) and speed(rpm), these are the type used for tuning because the PAU(Power Absorption Unit) can be set to absorb and hold specific load. Outside the manufacturing community one of the more common absorption types is Dynapack. These eliminate the tire by bolting directly to the hub, the overall drive ratio and engine rpm needs to be correctly input for data to be meaningful.
inertial types measure the acceleration rate of a known mass and this gives the torque which is then used to calculate power. Yes these will need to have the tire rolling diameter as well as the gear ratios and rpms to derive meaningful results
there are 2 types of dyno
absorption types measure force(torque) and speed(rpm), these are the type used for tuning because the PAU(Power Absorption Unit) can be set to absorb and hold specific load. Outside the manufacturing community one of the more common absorption types is Dynapack. These eliminate the tire by bolting directly to the hub, the overall drive ratio and engine rpm needs to be correctly input for data to be meaningful.
inertial types measure the acceleration rate of a known mass and this gives the torque which is then used to calculate power. Yes these will need to have the tire rolling diameter as well as the gear ratios and rpms to derive meaningful results
As for torque not being relevant... for a motor, you only need the torque vs RPM curve, OR the power vs RPM curve, since either can be directly calculated from the other. They are simply related and not independent.
So when you talk about torque, you're just taking about a specific mathematical transform of the power (and RPM). Similarly, when you talk about owner, you're just talking about a transform of the torque (and RPM). Remember:
Power (in HP) = Torque (in ft*lbs) * RPM / 5252
This is why the power in HP and torque in ft*lbs dynos always cross at 5252 RPM! (an artifact of the units)
So neither is irrelevant, but you only need to talk about one of them to have all of the relevant information.
In my opinion, it's easier to use power when trying to predict performance or make tuning decisions because it is invariant with gearing (per my earlier example). You could also choose to talk torque, but then you leave yourself open to incorrect conclusions if you don't correctly factor in gearing.
Just trying to tone it down a notch. ![Smilie](https://rennlist.com/forums/images/smilies/smile.gif)
Plenty of respect for all the gear manufacturing discussions!
My point was only that I don't even need the engine torque curve or wheel force calculations to determine which gear is going to make the car accelerate at a higher rate at a given speed. I just need the horsepower vs RPM curve, and what RPM each gear would put me at. Higher power, more acceleration. Easy, intuitive.
Obviously acceleration at one specific speed is not a be all end all... the overall combination of lots of acceleration over wide ranges of speed is more indicative of overall acceleration performance. But it's hard to have that latter discussion without being on the same page on the 1st principles of what power is and how it relates to vehicle acceleration.
![Smilie](https://rennlist.com/forums/images/smilies/smile.gif)
Plenty of respect for all the gear manufacturing discussions!
My point was only that I don't even need the engine torque curve or wheel force calculations to determine which gear is going to make the car accelerate at a higher rate at a given speed. I just need the horsepower vs RPM curve, and what RPM each gear would put me at. Higher power, more acceleration. Easy, intuitive.
Obviously acceleration at one specific speed is not a be all end all... the overall combination of lots of acceleration over wide ranges of speed is more indicative of overall acceleration performance. But it's hard to have that latter discussion without being on the same page on the 1st principles of what power is and how it relates to vehicle acceleration.
Garbage in, garbage out. That's my point, not just being argumentative as you suggest. When I calculate forces relative to gear ratios I look at an engine dyno plot. You may not consider it materially relevant but if you are going to attempt to teach me something please be accurate. It's not too much to ask. And I definitely don't need a lesson on what a dyno is or how it works. Give respect, get respect.
P:I saw your post and read it before the ninja edit.
P:I saw your post and read it before the ninja edit.
I think you missed the point. Think about it again... Your math should have been using ENGINE power numbers but you pulled them from a WHEEL dyno. Drive-line loss has already been subtracted. That loss can be as much as 15% depending on the particular dyno used. When we calculate how much power is going to be put down through the gearbox (ignoring friction) we want to base it on actual engine output.
Ps. Gear ratios do have something to do with wheel dyno plots, which is why they usually run them in 4th gear and try to get close to 1:1.![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Ps. Gear ratios do have something to do with wheel dyno plots, which is why they usually run them in 4th gear and try to get close to 1:1.
![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
you are correct, there is a differnce between the losses in 3rd and 4th for example. mainly due to 4th (some cars 5th) running directly to the output shaft in some cases. but other gears there is not level of difference.
"Wheel torque" on a dyno is a misnomer. Know how they typically measure / create that dyno curve? They measure *power* at the rear wheels (because it is invariant of gearing, other than frictional losses), and call that "wheel horsepower". Then they simply back-calculate the *engine* torque via torque = power / (angular velocity) by using the angular velocity of the engine in that calculation (engine RPM, converted to the appropriate units of course).
So "wheel horsepower" *is* the actual power measured at the wheels, but "wheel torque" is *not* the torque output about the wheel itself... to calculate the wheel torque, they would need an accurate measure of the wheel radius, and the magnitude of the torque values would be nothing like what you see on the dyno charts.
Gear ratios have nothing to do with that dyno plot...
So "wheel horsepower" *is* the actual power measured at the wheels, but "wheel torque" is *not* the torque output about the wheel itself... to calculate the wheel torque, they would need an accurate measure of the wheel radius, and the magnitude of the torque values would be nothing like what you see on the dyno charts.
Gear ratios have nothing to do with that dyno plot...
a tremendous example of this is say, you shift at 110mph in 3rd into 4th. if you shift at 7500rpm and you end up at 6200rpm post shift, and both HP levels are at the same level, the net force to the wheels will be the same (sans the small loss difference between gears)
#266
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Bill, you can use either the power curve or torque curve as the engine output in predicting performance and calculating forces and accelerations, the math is just slightly different in each case. That is the only point here. Ignoring drag and other parasricis as that is not relevant to the discussion:
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
F = (torque) * (final drive ratio) * (gear ratio) / (wheel radius)
or
F = (power) / (velocity)
This is just semantics at this point.
My response was directed at your comments as to what can and actually is actually be measured by a dyno, and that is only force(torque) and rpm in an absorption dyno and the acceleration rate of a known mass and rpm in an inertial dyno
to predict acceleration you use the
F = MA changed into A = F/M
F is the applied force in lbs in the British Engineering system or Newtons in the MKS system
M is in slugs in BES and Kg in MKS
to get the above F(aka Thrust) which is a linear vector quantity you need to convert engine torque in lb-ft by the sum total of the mechanical ratios(cwp+individual gear ratio +tire rolling radius) between the flywheel and tire-road interface which will be measured in ft( the ft comes solely from the tire radius as the other 2 are dimensionless quantities)
yes, the equations you present are correct but the statement about what a dyno measures is not. There is really no need to know power(other than for casual comparison purposes) only torque & rpm modified by gearing tell the story, what you want is torque @ high rpm because that allows the use of gearing to change that into linear speed.
#267
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yes but if you use the power curve you first convert it to torque.
My response was directed at your comments as to what can and actually is actually be measured by a dyno, and that is only force(torque) and rpm in an absorption dyno and the acceleration rate of a known mass and rpm in an inertial dyno
to predict acceleration you use the
F = MA changed into A = F/M
F is the applied force in lbs in the British Engineering system or Newtons in the MKS system
M is in slugs in BES and Kg in MKS
to get the above F(aka Thrust) which is a linear vector quantity you need to convert engine torque in lb-ft by the sum total of the mechanical ratios(cwp+individual gear ratio +tire rolling radius) between the flywheel and tire-road interface which will be measured in ft( the ft comes solely from the tire radius as the other 2 are dimensionless quantities)
yes, the equations you present are correct but the statement about what a dyno measures is not. There is really no need to know power(other than for casual comparison purposes) only torque & rpm modified by gearing tell the story, what you want is torque @ high rpm because that allows the use of gearing to change that into linear speed.
My response was directed at your comments as to what can and actually is actually be measured by a dyno, and that is only force(torque) and rpm in an absorption dyno and the acceleration rate of a known mass and rpm in an inertial dyno
to predict acceleration you use the
F = MA changed into A = F/M
F is the applied force in lbs in the British Engineering system or Newtons in the MKS system
M is in slugs in BES and Kg in MKS
to get the above F(aka Thrust) which is a linear vector quantity you need to convert engine torque in lb-ft by the sum total of the mechanical ratios(cwp+individual gear ratio +tire rolling radius) between the flywheel and tire-road interface which will be measured in ft( the ft comes solely from the tire radius as the other 2 are dimensionless quantities)
yes, the equations you present are correct but the statement about what a dyno measures is not. There is really no need to know power(other than for casual comparison purposes) only torque & rpm modified by gearing tell the story, what you want is torque @ high rpm because that allows the use of gearing to change that into linear speed.
You can generate your "thrust force vs. velocity" charts for any gear like you posted earlier with only the power curve vs. velocity for that gear. Just divide the two! Give it a shot if you are skeptical...
![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Mathematically it is all equivalent.
As for dyne's, I was referring to the common dynamic / inertial dynos most shops use. It is far easier to measure the acceleration of an inertial drum or other known inertial load than to measure actual torque of the spinning drum. But braking dunks that try to hold the engine at fixed operating speeds of course do this for specific tuning needs. Those may physically measure reaction torque. Depends on whatever is easiest to derive the quantity of interest.
#268
Rennlist Member
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Bill, you don't need to convert to torque to calculate thrust. Take the power being put into the body (which is just the power at the engine minus any losses getting to the wheels, which you can ignore for simplicity for now), and divide by the body velocity. This give you the thrust force. Easy.
You can generate your "thrust force vs. velocity" charts for any gear like you posted earlier with only the power curve vs. velocity for that gear. Just divide the two! Give it a shot if you are skeptical...![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Mathematically it is all equivalent.
.
You can generate your "thrust force vs. velocity" charts for any gear like you posted earlier with only the power curve vs. velocity for that gear. Just divide the two! Give it a shot if you are skeptical...
![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Mathematically it is all equivalent.
.
Matt makes fun of the Hp-seconds term , in estimation of comparative performance, but its very relevant to our discussion....rate of change of KE is power, and rate of change is acceleration by definition, so its very easy to see that the same amount of available HP will accelerate a mass exactly the same at any same speed.
So, contrary to Matt's statement, if you have the same power in any gear, regardless of the ratio, at any same speed, you will accelerate at the same rate and produce the same rear wheel force. He already proved this with his own data. this is why changing gears out to optimize HP that is already being optimized is a waste of effort. i can tell you many stories of guys that dropped 4:45s in their BMW from the 4:11 and only ended up shifting more in awkward places on the track, and yielded the same or even worse lap times. however i will say, its much more critical for cars with engines that dont have flat HP curves in the usable range.... such as the GT4.
yes but if you use the power curve you first convert it to torque.
My response was directed at your comments as to what can and actually is actually be measured by a dyno, and that is only force(torque) and rpm in an absorption dyno and the acceleration rate of a known mass and rpm in an inertial dyno
to get the above F(aka Thrust) which is a linear vector quantity you need to convert engine torque in lb-ft by the sum total of the mechanical ratios(cwp+individual gear ratio +tire rolling radius) between the flywheel and tire-road interface which will be measured in ft( the ft comes solely from the tire radius as the other 2 are dimensionless quantities)
yes, the equations you present are correct but the statement about what a dyno measures is not. There is really no need to know power(other than for casual comparison purposes) only torque & rpm modified by gearing tell the story, what you want is torque @ high rpm because that allows the use of gearing to change that into linear speed.
My response was directed at your comments as to what can and actually is actually be measured by a dyno, and that is only force(torque) and rpm in an absorption dyno and the acceleration rate of a known mass and rpm in an inertial dyno
to get the above F(aka Thrust) which is a linear vector quantity you need to convert engine torque in lb-ft by the sum total of the mechanical ratios(cwp+individual gear ratio +tire rolling radius) between the flywheel and tire-road interface which will be measured in ft( the ft comes solely from the tire radius as the other 2 are dimensionless quantities)
yes, the equations you present are correct but the statement about what a dyno measures is not. There is really no need to know power(other than for casual comparison purposes) only torque & rpm modified by gearing tell the story, what you want is torque @ high rpm because that allows the use of gearing to change that into linear speed.
I love your thrust curves.. I've copied and used for reference many times, But if you look at the thrust curves, they are identical in proportions to HP curves vs speed as well. All you need is the RPM % drop after you shift, and you got the operational range that the car can absorb the engines HP.. the short shift points, if any, are very apparent via the cross over of shift point vs post shift HP values. its just much easier and just as accurate.
#269
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Bill, you don't need to convert to torque to calculate thrust. Take the power being put into the body (which is just the power at the engine minus any losses getting to the wheels, which you can ignore for simplicity for now), and divide by the body velocity. This give you the thrust force. Easy.
You can generate your "thrust force vs. velocity" charts for any gear like you posted earlier with only the power curve vs. velocity for that gear. Just divide the two! Give it a shot if you are skeptical...![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Mathematically it is all equivalent.
As for dyne's, I was referring to the common dynamic / inertial dynos most shops use. It is far easier to measure the acceleration of an inertial drum or other known inertial load than to measure actual torque of the spinning drum. But braking dunks that try to hold the engine at fixed operating speeds of course do this for specific tuning needs. Those may physically measure reaction torque. Depends on whatever is easiest to derive the quantity of interest.
You can generate your "thrust force vs. velocity" charts for any gear like you posted earlier with only the power curve vs. velocity for that gear. Just divide the two! Give it a shot if you are skeptical...
![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
Mathematically it is all equivalent.
As for dyne's, I was referring to the common dynamic / inertial dynos most shops use. It is far easier to measure the acceleration of an inertial drum or other known inertial load than to measure actual torque of the spinning drum. But braking dunks that try to hold the engine at fixed operating speeds of course do this for specific tuning needs. Those may physically measure reaction torque. Depends on whatever is easiest to derive the quantity of interest.
yes, I understand all the mathematical gymnastics
measurement was/is the point
you do not and cannot measure power
you can only measure things that let you derive a power number
things that can be measured are called fundamental quantities mass(M), length(L) & time(T) are the basic fundamental quantities from which all others here are derived
F is MLTexp-2 This is what a dyno actually measures either directly of indirectly
Power is MLexp2Texp-2 This is what is calculated from F
#270
Rennlist Member
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yes, this is math gymnastics...
now, please bare with me:
so if move 550lbs up 1ft in 1 second I've required a 1 hp do do this rate of work. I guess you could say because i know the 550lbs and the distance, and the time, we have used the fundamental quantities. Same thing on the dyno, but in a rotational plane. (drum weight size, change in speed d/t, etc)
But, what have you given us.... we know only have the rear wheel forces.. we dont have the engine torque values do we? we need to calculate using the the factors that bring us to engine torque, where as , with finding power we only need the rate of change of KE. easy to find the force by F=P/v. coincidental enough very similar to : acceleration =power/(mass x velocity)
it just shows that acceleration is proportional to acceleration at any vehicle speed.
I guess the point is, sure at the end of the day, its the force that causes the acceleration.. we all get that.. BUT, if you know the HP and we do based on the dyno sheets, we can compare performance at any given speed without knowing anything else. HP just has more info contained in it that we can utilize. the RPM and gear ratios and engine torque values can change til the cows come home, but if we have HP, we dont need any of the other factors... no calculations, ... it just makes life easier.
you put the car at 6200rpm or 6800rpm or 7500rpm and if the HP is the same, the rear wheel forces will be the same, regardless of the engine torque and gear ratios...... this is the main point. ("regardless of engine torque", doesn't mean torque is irrelevant, it means you dont need to care its actual value)
EDIT: I need to apologize to MATT! : Matt was right with his comparison. There was a difference between the 3rd gear at 110mph at 6200rpm vs a change of gear giving RPM levels of 6800rpm. We did a quick calculation by using the published wheel HP graph and he was using other data from another GT4 dyno run. He saw a 3% difference, by changing the gear ratios and we only gave credit to the 1.5% change. (i didnt see the data correctly from him.. need some new glasses!
)
He understands the concepts we are talking about and is really trying to optimize every area of the GT4's performance by making some small changes to ratios. i get it. whether it be street or a particular track, there are somethings to be optimized and no one should be criticized for trying to make things better. With the theoretical info and his practical experience you can have the best of both worlds and make the decision that makes sense for your use of the car.
![Smilie](https://rennlist.com/forums/images/smilies/smile.gif)
now, please bare with me:
so if move 550lbs up 1ft in 1 second I've required a 1 hp do do this rate of work. I guess you could say because i know the 550lbs and the distance, and the time, we have used the fundamental quantities. Same thing on the dyno, but in a rotational plane. (drum weight size, change in speed d/t, etc)
But, what have you given us.... we know only have the rear wheel forces.. we dont have the engine torque values do we? we need to calculate using the the factors that bring us to engine torque, where as , with finding power we only need the rate of change of KE. easy to find the force by F=P/v. coincidental enough very similar to : acceleration =power/(mass x velocity)
it just shows that acceleration is proportional to acceleration at any vehicle speed.
I guess the point is, sure at the end of the day, its the force that causes the acceleration.. we all get that.. BUT, if you know the HP and we do based on the dyno sheets, we can compare performance at any given speed without knowing anything else. HP just has more info contained in it that we can utilize. the RPM and gear ratios and engine torque values can change til the cows come home, but if we have HP, we dont need any of the other factors... no calculations, ... it just makes life easier.
you put the car at 6200rpm or 6800rpm or 7500rpm and if the HP is the same, the rear wheel forces will be the same, regardless of the engine torque and gear ratios...... this is the main point. ("regardless of engine torque", doesn't mean torque is irrelevant, it means you dont need to care its actual value)
EDIT: I need to apologize to MATT! : Matt was right with his comparison. There was a difference between the 3rd gear at 110mph at 6200rpm vs a change of gear giving RPM levels of 6800rpm. We did a quick calculation by using the published wheel HP graph and he was using other data from another GT4 dyno run. He saw a 3% difference, by changing the gear ratios and we only gave credit to the 1.5% change. (i didnt see the data correctly from him.. need some new glasses!
![Wink](https://rennlist.com/forums/images/smilies/wink.gif)
He understands the concepts we are talking about and is really trying to optimize every area of the GT4's performance by making some small changes to ratios. i get it. whether it be street or a particular track, there are somethings to be optimized and no one should be criticized for trying to make things better. With the theoretical info and his practical experience you can have the best of both worlds and make the decision that makes sense for your use of the car.
Again and for the last time
yes, I understand all the mathematical gymnastics
measurement was/is the point
you do not and cannot measure power
you can only measure things that let you derive a power number
things that can be measured are called fundamental quantities mass(M), length(L) & time(T) are the basic fundamental quantities from which all others here are derived
F is MLTexp-2 This is what a dyno actually measures either directly of indirectly
Power is MLexp2Texp-2 This is what is calculated from F
yes, I understand all the mathematical gymnastics
measurement was/is the point
you do not and cannot measure power
you can only measure things that let you derive a power number
things that can be measured are called fundamental quantities mass(M), length(L) & time(T) are the basic fundamental quantities from which all others here are derived
F is MLTexp-2 This is what a dyno actually measures either directly of indirectly
Power is MLexp2Texp-2 This is what is calculated from F
Last edited by mark kibort; 12-15-2016 at 05:57 PM.