2.20 vs 2.54 ratio?
01-08-2005, 11:39 PM
#106
Drifting
I am keeping it good natured as I like reading his views. He does have some good points to make but can't believe that he may actually be wrong about some things.
I certainly won't fault his contribution to the 928 world, that is not even questioned.
I am also quite sure that this whole conversation would be a lot more enjoyable over a pitcher of Margarita's. I'll buy!
The complete point should be..........there is no perfect setup. There are perfect setup's for specific applications but change any variable even slightly and it skews the whole deal. Change the course, car weight, driver's style, weather, car balance.........you get the point.
Think I'll get off this computer and go cook me up a nasty steak. I am going to cook it for my current application, not to a recipie as the wind is from the north tonight and snow is expected and I am out of Bacardi and, and, and.........
Dave
Motorsport
I certainly won't fault his contribution to the 928 world, that is not even questioned.
I am also quite sure that this whole conversation would be a lot more enjoyable over a pitcher of Margarita's. I'll buy!
The complete point should be..........there is no perfect setup. There are perfect setup's for specific applications but change any variable even slightly and it skews the whole deal. Change the course, car weight, driver's style, weather, car balance.........you get the point.
Think I'll get off this computer and go cook me up a nasty steak. I am going to cook it for my current application, not to a recipie as the wind is from the north tonight and snow is expected and I am out of Bacardi and, and, and.........
Dave
Motorsport
01-09-2005, 02:21 AM
#108
928 Collector
Rennlist Member
Rennlist Member
Mark, I think Dave Ketchmi said it best. Same engine, same car, you start in 5th gear and I'll start in 1st gear. Let's see who gets to 40 first.
Now, you get to have 4th and 5th and I get 1st and 2nd, and we'll see who gets to 60 first.
Those two players (you and me above) are analogous to you in a tall-geared car and me in a short-geared car. Since physics doen't seem to manage to get to you, how about that .... a very simple, real-world example.
Now, you get to have 4th and 5th and I get 1st and 2nd, and we'll see who gets to 60 first.
Those two players (you and me above) are analogous to you in a tall-geared car and me in a short-geared car. Since physics doen't seem to manage to get to you, how about that .... a very simple, real-world example.
01-09-2005, 03:04 AM
#109
Rennlist Member
Glen, you missed the mark, as you admit to "ignoring" speed of the vehicle which is a critical comparison element. if you just take a single car, blindly look at torque curves, multiply them by the lowe ratio, you will miss the comparison. we have to use rates of acceleratoin over the operational range or the test is seriously flawed. Ive already said that if you look at one single gear, the advantage to a proportional lower speed would be greater. we are talkin about a speed range through the gears to determine if there is any advantage accleration wise, with a lower numerical ratio. The answer to this is "it depends" It depends on the target speeds.
the only way to conduct a clear test or simulation or example is to keep as much of the variables the same. In testing for acceleration for the taller gear box, you have to calculate the ave torque to the wheels over the range. Then, change the ratio, keeping the target speeds the same, and run through the numbers again. by using only RPMS of the engine, you are paying no attention to one of the most important factors Acceleration (again) is the second derivative of distance and the 1st derirvative of speed . my example of 0-105mph gives both gear boxes matched to the same engine a fair comparison, showing no favorites in target speed. However, if i wanted to skew the test, the goal would be 95mph (redline for shorter gear box) or 114mph (redline for taller gear box ) Point is, if you dont factor in speed or distance over time, you cannot find acceleration.
Again, Acceleration is d^2/dt or dv/dt just a force over no distance or speed, is just a force with no acceleration.
also, see my inserts!>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Mk
the only way to conduct a clear test or simulation or example is to keep as much of the variables the same. In testing for acceleration for the taller gear box, you have to calculate the ave torque to the wheels over the range. Then, change the ratio, keeping the target speeds the same, and run through the numbers again. by using only RPMS of the engine, you are paying no attention to one of the most important factors Acceleration (again) is the second derivative of distance and the 1st derirvative of speed . my example of 0-105mph gives both gear boxes matched to the same engine a fair comparison, showing no favorites in target speed. However, if i wanted to skew the test, the goal would be 95mph (redline for shorter gear box) or 114mph (redline for taller gear box ) Point is, if you dont factor in speed or distance over time, you cannot find acceleration.
Again, Acceleration is d^2/dt or dv/dt just a force over no distance or speed, is just a force with no acceleration.
also, see my inserts!>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Mk
Originally Posted by GlenL
Wow.
By "input" I mean the power being supplied by the engine to the transmission. And yes, for any input the output torque of the tranny will be 15.5% higher. But will be turning 13.4% slower. (And god, don't make me have explain why those percentages are different.) This is regardless of vehicle speed. That just doesn't matter.
>>>>>>>>>>> we all know the inverse of 115% of the rated torque due to lower gearing (or .86% of the speed for the reduction) however, it is CRITICAL to understand the speed component. tell me, what happens with torque at 60 to 75mph ?? or 95mph to 114mph?? these are areas where the taller gear box will have even more advantages, but over shorter periods of time. (dont make ME explain this again). run the numbers, its all about trade offs.
By focusing on a few select speeds you're making an artificial case that obscures the larger question.
>>>>>>> the larger question is what is the total torque (area under the curve for all gears required) required to accelerate a mass like our 928, from 0-100mph . you can do this by integrating the torques in each gear over the speed range. Ive done a manual integration that i know you have seen and listed the torques x the time element to give this information clearly. You have chosen to ignor it.
For the fun of it, i'vs cracked open my tech spec booklet and dyno graph. The ratios are:
1) 3.6010
2) 2.4646
3) 1.8194
4) 1.3433
5) 1.0000
>>>>>>>>> total ratio would be thse ratios x the rear end as all the 2.72 , 2.2 and the 2.54 (auto) have different rear ends. you do have the net differnces down, which is the 1.15 % diff. thats fine.
Now the ratios between these are:
2-1) 0.6844
3-2) 0.7382
4-3) 0.7383
5-4) 0.7444
>>>>>>>>>>key for determinig how much area under the torque curve is put to the ground.
So with the rev limiter set at 6350, I need to shift at 6300. What this means, based on the previous ratios, is that these are the RPM ranges used for each gear:
1) 0 to 6300
2) 4311 to 6300
3) 4650 to 6300
4) 4651 to 6300
5) 4689 to 6300
>>>>>>>>>>>>>>>>>already gone through this, and i used 6400 as a redline as that is actual for the S4 . i also used 4500rpm as the shft rpm torque value as this gave us a 300ftlb and a 275ftlbs range I averaged to 290ftlbs and used that for the comparisons. You can use your own HP curves, what counts is the percentages, and you will be close with any 928 engine
Now it's off to the dyno graph to find the powers at these points. The peak is just past 6000rpm (woohoo!) and if you're really interested, I posted it in the last week in the intake design thread.
1) "150" to 250 (Don't know the launch power and it doesn't change the analysis)
2) 210 to 250
3) 222 to 250
4) 222 to 250
5) 225 to 250
>>>>>>>>>>>really, i have not seen too many 928s with 250ftlbs at 4500rpm that hold their torque to 225 at 6400rpm. but thats ok, close enough
I've got a 2.75 rear end, but I mention this only for reference. If it was switched to a 2.54 or 2.2 these shift points would still be optimal for accelerating the vehicle. The point that Mark won't give up is that there is some operating point somewhere that the 2.2 will be better than the 2.54. This can only occur where the 2.54 has required an up-shift while the 2.2 is still in the higher power end of the engine range.
>>>>>>>>>>>DING DING DING. a winner. ive listed these points and the time spent at these points. its almost 40% off the time on an acceleration curve from 0-105mph
In this analysis I've ignored vehicle speed. Why? Because it's a linear relationship to output RPM of the transmission. It just doesn't matter!
>>>>>>>>>>>>BZZZT wong answer. yes, it is linear, but comparing two cars, you have to use speed or you cant compare acceleration . again, speed or distance has to be used to compare acceleration. basic law of physics.
Now to complete the exercise, divide the power points before and after each shift:
1-2) 84%
2-3) 88.8%
3-4) 88.8%
4-5) 90%
>>>>>>>> I dont understand the goal here?
And then multiply by the 15.5% mechanical advantage of the taller final drive:
1-2) 97%
2-3) 102.6%
3-4) 102.6%
4-5) 104%
>>>>>>>>> what ever the goal of above, if you are just multipying all gear ranges by the new lower ratio, then what are you really getting as an answer here??
What this shows is that the higher mechanical advantage compensates for the need for an earlier shift in all except for the first shift, and then it's almost covered.
>>>>>>>>>>>>i think this is where your problem of understanding lies. your confusing need for earlier shift with torque available to accelerate after every shift, compared to a previous speed and car with higher shift points.
What is proved by this is that only when the 2.54 car shift to second is the 2.2 car producing more force on the wheels. At all other points the 2.54 car is accelerating faster.
>>>>>>>>>>>>>>>>>>>>>>>so, there is a point where the taller ratio is producing more resultant torque, but why wouldnt it happen for a period of time at all shift points of the new gear ratio??? plus, are we talking 2.54 rear end trannie, vs 2.2 tranie or the automatic. remember, ive already listed the transmissions ratios and their trade offs, plus all their resultant torques
F = ma It's The Law!
>>>>>>>>>>>>>>> yes, and here is another couple of laws. force x displacement is work. work x time is power/energy there is NO acceleration if there is no distance or speed over a time period covered.
Now if I had Excel at home I'd make graphs and tie it to vehicle speed as well.
>>>>>>>>>ill see if i can put even more information down on a graph.
By "input" I mean the power being supplied by the engine to the transmission. And yes, for any input the output torque of the tranny will be 15.5% higher. But will be turning 13.4% slower. (And god, don't make me have explain why those percentages are different.) This is regardless of vehicle speed. That just doesn't matter.
>>>>>>>>>>> we all know the inverse of 115% of the rated torque due to lower gearing (or .86% of the speed for the reduction) however, it is CRITICAL to understand the speed component. tell me, what happens with torque at 60 to 75mph ?? or 95mph to 114mph?? these are areas where the taller gear box will have even more advantages, but over shorter periods of time. (dont make ME explain this again). run the numbers, its all about trade offs.
By focusing on a few select speeds you're making an artificial case that obscures the larger question.
>>>>>>> the larger question is what is the total torque (area under the curve for all gears required) required to accelerate a mass like our 928, from 0-100mph . you can do this by integrating the torques in each gear over the speed range. Ive done a manual integration that i know you have seen and listed the torques x the time element to give this information clearly. You have chosen to ignor it.
For the fun of it, i'vs cracked open my tech spec booklet and dyno graph. The ratios are:
1) 3.6010
2) 2.4646
3) 1.8194
4) 1.3433
5) 1.0000
>>>>>>>>> total ratio would be thse ratios x the rear end as all the 2.72 , 2.2 and the 2.54 (auto) have different rear ends. you do have the net differnces down, which is the 1.15 % diff. thats fine.
Now the ratios between these are:
2-1) 0.6844
3-2) 0.7382
4-3) 0.7383
5-4) 0.7444
>>>>>>>>>>key for determinig how much area under the torque curve is put to the ground.
So with the rev limiter set at 6350, I need to shift at 6300. What this means, based on the previous ratios, is that these are the RPM ranges used for each gear:
1) 0 to 6300
2) 4311 to 6300
3) 4650 to 6300
4) 4651 to 6300
5) 4689 to 6300
>>>>>>>>>>>>>>>>>already gone through this, and i used 6400 as a redline as that is actual for the S4 . i also used 4500rpm as the shft rpm torque value as this gave us a 300ftlb and a 275ftlbs range I averaged to 290ftlbs and used that for the comparisons. You can use your own HP curves, what counts is the percentages, and you will be close with any 928 engine
Now it's off to the dyno graph to find the powers at these points. The peak is just past 6000rpm (woohoo!) and if you're really interested, I posted it in the last week in the intake design thread.
1) "150" to 250 (Don't know the launch power and it doesn't change the analysis)
2) 210 to 250
3) 222 to 250
4) 222 to 250
5) 225 to 250
>>>>>>>>>>>really, i have not seen too many 928s with 250ftlbs at 4500rpm that hold their torque to 225 at 6400rpm. but thats ok, close enough
I've got a 2.75 rear end, but I mention this only for reference. If it was switched to a 2.54 or 2.2 these shift points would still be optimal for accelerating the vehicle. The point that Mark won't give up is that there is some operating point somewhere that the 2.2 will be better than the 2.54. This can only occur where the 2.54 has required an up-shift while the 2.2 is still in the higher power end of the engine range.
>>>>>>>>>>>DING DING DING. a winner. ive listed these points and the time spent at these points. its almost 40% off the time on an acceleration curve from 0-105mph
In this analysis I've ignored vehicle speed. Why? Because it's a linear relationship to output RPM of the transmission. It just doesn't matter!
>>>>>>>>>>>>BZZZT wong answer. yes, it is linear, but comparing two cars, you have to use speed or you cant compare acceleration . again, speed or distance has to be used to compare acceleration. basic law of physics.
Now to complete the exercise, divide the power points before and after each shift:
1-2) 84%
2-3) 88.8%
3-4) 88.8%
4-5) 90%
>>>>>>>> I dont understand the goal here?
And then multiply by the 15.5% mechanical advantage of the taller final drive:
1-2) 97%
2-3) 102.6%
3-4) 102.6%
4-5) 104%
>>>>>>>>> what ever the goal of above, if you are just multipying all gear ranges by the new lower ratio, then what are you really getting as an answer here??
What this shows is that the higher mechanical advantage compensates for the need for an earlier shift in all except for the first shift, and then it's almost covered.
>>>>>>>>>>>>i think this is where your problem of understanding lies. your confusing need for earlier shift with torque available to accelerate after every shift, compared to a previous speed and car with higher shift points.
What is proved by this is that only when the 2.54 car shift to second is the 2.2 car producing more force on the wheels. At all other points the 2.54 car is accelerating faster.
>>>>>>>>>>>>>>>>>>>>>>>so, there is a point where the taller ratio is producing more resultant torque, but why wouldnt it happen for a period of time at all shift points of the new gear ratio??? plus, are we talking 2.54 rear end trannie, vs 2.2 tranie or the automatic. remember, ive already listed the transmissions ratios and their trade offs, plus all their resultant torques
F = ma It's The Law!
>>>>>>>>>>>>>>> yes, and here is another couple of laws. force x displacement is work. work x time is power/energy there is NO acceleration if there is no distance or speed over a time period covered.
Now if I had Excel at home I'd make graphs and tie it to vehicle speed as well.
>>>>>>>>>ill see if i can put even more information down on a graph.
01-09-2005, 03:17 AM
#110
Rennlist Member
Look, im not making my old physics instructor roll over in his grave here. Ive passed this thread on to several others over drinks tonight and there was not ONE wrong point that i have made. could some of the 30 folks be wrong here and a little misled some pretty brilliant minds ive spoken to seem to think so.
Your comment below sums up (and so for dave) that you just arent getting it. there is no argument that a lower gear will have a greater acceleration to a proportional lesser speed. Ive already posted in detail the resultant wheel torques through the appropriate gear ratios over the entire speed range!!
You reallly are missing the boat Heinrich. seriously, speak to a mechanical enineer or physics guy that you may know and pass my information to him.
what im saying is that over a speed range, gearing higher or lower will have a series of trade offs of greater and lower torque to accelerate with, with each and every gear change. if we had the race from 1st gear to redline, you would be correct, but that would be me going to 40mph and you going to 35mph. (this makes no sence!!) we need to get an operational range and look at the ratios that are used to get to the SAME top speed, whatever it is. I selected a top speed that didnt give much of an advantage either way.
again, gearing only effects the efficiency of application of torque to the rear wheels over a specific operational speed range. (ie 0-60, 0-100, 60 - 100mph, etc)
In racing, this is why racers just dont use a transmission that has the highest ratio in the rear end. they will change it to match the projected speeds of the car so that every straightaway, the car/engine and transmission package can reach redline before breaking. of course, there is no way to match every curve, so they pick ratios that help areas where the car spends time accelerating the longest. the more you can apply the max engine hp to the wheels, the faster and quicker you will be. (and the most torque you will apply to the wheels over those speed ranges as well)
Ive hear more lame interpretations of physics than ive heard in my entire life. more terms used out of context that i can keep track of. Ill have to make a list of that and post it.
I shoud hear about 30 appologies for doubting me very soon, or as soon as someone either cracks a book or ask someone they know to help them understand what the heck im talking about!!!!
MK
Your comment below sums up (and so for dave) that you just arent getting it. there is no argument that a lower gear will have a greater acceleration to a proportional lesser speed. Ive already posted in detail the resultant wheel torques through the appropriate gear ratios over the entire speed range!!
You reallly are missing the boat Heinrich. seriously, speak to a mechanical enineer or physics guy that you may know and pass my information to him.
what im saying is that over a speed range, gearing higher or lower will have a series of trade offs of greater and lower torque to accelerate with, with each and every gear change. if we had the race from 1st gear to redline, you would be correct, but that would be me going to 40mph and you going to 35mph. (this makes no sence!!) we need to get an operational range and look at the ratios that are used to get to the SAME top speed, whatever it is. I selected a top speed that didnt give much of an advantage either way.
again, gearing only effects the efficiency of application of torque to the rear wheels over a specific operational speed range. (ie 0-60, 0-100, 60 - 100mph, etc)
In racing, this is why racers just dont use a transmission that has the highest ratio in the rear end. they will change it to match the projected speeds of the car so that every straightaway, the car/engine and transmission package can reach redline before breaking. of course, there is no way to match every curve, so they pick ratios that help areas where the car spends time accelerating the longest. the more you can apply the max engine hp to the wheels, the faster and quicker you will be. (and the most torque you will apply to the wheels over those speed ranges as well)
Ive hear more lame interpretations of physics than ive heard in my entire life. more terms used out of context that i can keep track of. Ill have to make a list of that and post it.
I shoud hear about 30 appologies for doubting me very soon, or as soon as someone either cracks a book or ask someone they know to help them understand what the heck im talking about!!!!
MK
Originally Posted by heinrich
Mark, I think Dave Ketchmi said it best. Same engine, same car, you start in 5th gear and I'll start in 1st gear. Let's see who gets to 40 first.
Now, you get to have 4th and 5th and I get 1st and 2nd, and we'll see who gets to 60 first.
Those two players (you and me above) are analogous to you in a tall-geared car and me in a short-geared car. Since physics doen't seem to manage to get to you, how about that .... a very simple, real-world example.
Now, you get to have 4th and 5th and I get 1st and 2nd, and we'll see who gets to 60 first.
Those two players (you and me above) are analogous to you in a tall-geared car and me in a short-geared car. Since physics doen't seem to manage to get to you, how about that .... a very simple, real-world example.
01-09-2005, 03:24 AM
#111
Rennlist Member
YOUR ON!! i could use a margarita just about now!!!
Look forward to it. Thanks! Hope sometime soon
Im keeping it light, even though the written word can be a little harsh.
If im wrong about anything ive posted, i certainly would like to know about it. so far, 4 qualified engineers, one PHD, have reviewed this and agreed on all my points( and have shaken their heads also about the misinterpretation of the physics discussed here!) this is really simple suff guys, but your making it much harder than it has to be.
Hey, if you disagree with anything, let me know . but , show your work. I have!
but your right about one thing , there is no perfect set up. and this is why my original comment. for street driving, rear end ratio changes have very little effect on performance. heck, just look at the 0-100mph tests for all of these combinatons in road and track. there is not much difference in those tests, much of differences are driver, and or conditions anyway.
MK
Look forward to it. Thanks! Hope sometime soon
Im keeping it light, even though the written word can be a little harsh.
If im wrong about anything ive posted, i certainly would like to know about it. so far, 4 qualified engineers, one PHD, have reviewed this and agreed on all my points( and have shaken their heads also about the misinterpretation of the physics discussed here!) this is really simple suff guys, but your making it much harder than it has to be.
Hey, if you disagree with anything, let me know . but , show your work. I have!
but your right about one thing , there is no perfect set up. and this is why my original comment. for street driving, rear end ratio changes have very little effect on performance. heck, just look at the 0-100mph tests for all of these combinatons in road and track. there is not much difference in those tests, much of differences are driver, and or conditions anyway.
MK
Originally Posted by Ketchmi
I am keeping it good natured as I like reading his views. He does have some good points to make but can't believe that he may actually be wrong about some things.
I certainly won't fault his contribution to the 928 world, that is not even questioned.
I am also quite sure that this whole conversation would be a lot more enjoyable over a pitcher of Margarita's. I'll buy!
The complete point should be..........there is no perfect setup. There are perfect setup's for specific applications but change any variable even slightly and it skews the whole deal. Change the course, car weight, driver's style, weather, car balance.........you get the point.
Think I'll get off this computer and go cook me up a nasty steak. I am going to cook it for my current application, not to a recipie as the wind is from the north tonight and snow is expected and I am out of Bacardi and, and, and.........
Dave
Motorsport
I certainly won't fault his contribution to the 928 world, that is not even questioned.
I am also quite sure that this whole conversation would be a lot more enjoyable over a pitcher of Margarita's. I'll buy!
The complete point should be..........there is no perfect setup. There are perfect setup's for specific applications but change any variable even slightly and it skews the whole deal. Change the course, car weight, driver's style, weather, car balance.........you get the point.
Think I'll get off this computer and go cook me up a nasty steak. I am going to cook it for my current application, not to a recipie as the wind is from the north tonight and snow is expected and I am out of Bacardi and, and, and.........
Dave
Motorsport
01-09-2005, 04:12 AM
#112
Burning Brakes
Join Date: Jan 2002
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I've read this whole thread so far....super interesting and educational. I hope nobody minds if I try to make sense of this to myself out loud.
I think I understand Mark's point: The time it takes for two cars to go a certain distance (1/4 mile) is determined by the frequency and range of each cars multiple torque peaks relative to one another. So, a higher geared car could beat a lower geared car if the distance travelled and the timing of the power curves favored the higher gear. So the AVERAGE ACCELERATION of the car would be faster.
On the other hand: A lower geared car reaches a torque peak faster, it may not stay in its peak as long, but the ACTUAL ACCELERATION from peak to peak is faster. Therefore; ACTUAL ACCELERATION of a car is faster, but when other variables are factored in, such as number/spacing of gears and distance, then the AVERAGE ACCELERATION at the finish line actually determines who wins. If there was only 1 gear and infinite power/rpms, then the lower gear would make acceleration time faster from the start point and it would remain faster because it is constant ACTUAL ACCELERATION. Multiple gears enters into the picture to deal with the problem of a non-constant torque line and that is where AVERAGE ACCELERATION across the gears comes into play.
So, to sum up my thoughts:
One party is arguing from the point of AVERAGE acceleration.
The other party is arguing from the point of ACTUAL acceleration.
Does this sound about right??? or am I completely lost.....
PS. Is the difference between 2.20 and 2.54 a difference of the gears themselves (within the same transmission) or just the ring and pinion?
I think I understand Mark's point: The time it takes for two cars to go a certain distance (1/4 mile) is determined by the frequency and range of each cars multiple torque peaks relative to one another. So, a higher geared car could beat a lower geared car if the distance travelled and the timing of the power curves favored the higher gear. So the AVERAGE ACCELERATION of the car would be faster.
On the other hand: A lower geared car reaches a torque peak faster, it may not stay in its peak as long, but the ACTUAL ACCELERATION from peak to peak is faster. Therefore; ACTUAL ACCELERATION of a car is faster, but when other variables are factored in, such as number/spacing of gears and distance, then the AVERAGE ACCELERATION at the finish line actually determines who wins. If there was only 1 gear and infinite power/rpms, then the lower gear would make acceleration time faster from the start point and it would remain faster because it is constant ACTUAL ACCELERATION. Multiple gears enters into the picture to deal with the problem of a non-constant torque line and that is where AVERAGE ACCELERATION across the gears comes into play.
So, to sum up my thoughts:
One party is arguing from the point of AVERAGE acceleration.
The other party is arguing from the point of ACTUAL acceleration.
Does this sound about right??? or am I completely lost.....
PS. Is the difference between 2.20 and 2.54 a difference of the gears themselves (within the same transmission) or just the ring and pinion?
Last edited by JKelly; 01-09-2005 at 04:31 AM.
01-09-2005, 04:24 AM
#113
Rennlist Member
Here is the NET NET of the gear ratio change effects. the only think i didnt do was plot it out on a neat graph. So, we have below, the total gear ratios for the two cars with the same engine. one having a GT gear box, and one a S4 gear box. (ie 2.2 vs 2.72 gear boxes)
I give the final ratios for the two gear boxes for the speed we would be using in a race or street condition 1-4 gears are used for reasons of equal spacing to keep the comparison fair.
GEAR RATIOS LISTED FOR 2 928s. S4 2.2:1 and a GT 2.72:1
S4 1st 8.9:1 2nd 5.9:1 3rd 4.2 4th 3.2 5th 2.2 (spacing of .66.71.76) .71ave
GT 1st 10.2:1 2nd 6.8:1 3rd 4.8 4th 3.6 5th 2.72 (spacing of .66.71.75).71 ave
The GT compared to the S4 has these mechanical advantages per gear
115% for 1st
115% for 2nd
114% for3rd
113% for 4th
(and a rear end difference of 124%, or 5th gear)
{Now, your 4 speed automatic for reference with the 2.2 vs the 2.54:1
(and yes, the actual ratios are different between the two gear boxes, but ive listed the final results of the ratios here)
2.2 1st 8.1:1 2nd 5.3 3rd 3.0 4th 2.2 (spacing of .65.56.73 ) (.64ave)
2.54 1st 9.8:1 2nd 5.7 3rd 3.65 4th 2.54 (spacing of .58.64.69) (.64 ave)
compared to the 2.2, the 2.54 trannie has these mechanical advantages per gear:
120% 1st
107%, 2nd
121%, 3rd
(and final drive difference of 115% )}
Since, accleration is actually only due to area under all the torque curves over the operational speed range, torque in ftlbs seconds can show relative differences in the differnet rear end/transmissions of our discussion topic.
Below, see the actual torque through the appropriate gear and miltiplied by its ratio for total ft/lbs delivered to the rear wheels at any speed.
MPH---0-35 -------35-40----- 40-60--------60-75---------75-96------96-105
GT--1st 2958----2nd 1972---2nd 1972---3rd 1392----3rd 1392-----4th 1044ft lbs
S4--1st 2581----1st 2581----2nd 1711---2nd 1711----3rd 1229-----3rd 1229ft lbs
TIME---2sec------ 1 sec------- 3 sec--------- 2 sec------ 3 sec----------- 3sec
The times spent in each gear are approx based on actual tests and road and track values . roughly, 0-60 in seconds, 60 to 105mph in 8 seconds.
anyway, these are the torque values in each gear for two hypothetical cars , both 928 S4, but one with the GT gear ratios (2.72:1 rear end gear box vs 2.2:1 gear box) the difference of the total ratios are in the 15% range
Taking the totals of all the resultant torques and multiplying them by the time spent, we get a good estimation of what we would get if we actually used calculus and used the area under the torque curves. call it a "manual integration"
the resultant total torques over the speed range of 0-105mph is:
S4 3944ft lbs/ seconds
GT 3982ft lb /seconds
the totals are just the torque of the engine over the operational range (we use 290 which is an average) X the gear ratio X the time spent in that gear.
The math for torque produced in each gear x the time spent in that gear:
(GT)5916+1972+5916 +2784+4176 +3132= 23896/6 for a total of 3982ftlb/sec
(S4)5162+2581 +5133+3422+3687+3787 = 23672/6 for a total of 3945ftlb/sec
This is a 1% difference of the total acceleration torque, put to the wheels over a speed range of 0-105mph. make the test to 100mph, and the GT has a paltry 2% gain, make it to 115mph, and the S4 has a very slight advantage.
Go even faster, the trade offs go on til 155mph.
(as a note, the times listed in each gear duration in seconds would normally be different slightly for each advantage mechanically. However these second order differences as small as they are, would be cancellled out over the operational range)
These values are an integration of the torque to the wheels over the operational speed range though the appropriate gear ratios
this is the accelerating force over the speed and time shown above.
This clearly shows the result of a gear box change to a lower ratio with a 15% average mechanical advantage. you see the net advantage over the operational range is much less, infact, depending on the speed target range, it can be a disadvantage. (one of the reasons i still race with a S4 gear box for the tracks visit and power i have.
NOW for Force!!!!
As far as a definition of F=ma
Accleration is the 2nd derivative of distance and the first derivative of speed.
newtons 2nd law (again) says that acceleration is produced by an UNBALANCED force acting on a body proportional to the net force.
That is:
delta velocity/delta time
or in terms of distance:
Delta^2 distance / delta time
F=ma refers to an UNBALANCED force. if you don't have an UNBALANCED force, you dont have acceleration.
So, if you dont have a delta distance or a delta speed, you dont have acceleration PERIOD.
Force , acceleration and speed are vectors which consists of two parts, a magnatude and direction.
the rate of change of speed over a particular time inteval (ie average acceleration) will result in a specific average force in a particular direction. to expand this, work is force through a distance (ive provided these values ) and work x time is power. Or if you hung up on distance, speed times time is distance.
I hope this helps. if you have questions, pick up a survey text in physics so you dont get hung up on the calculus.
Mk
I give the final ratios for the two gear boxes for the speed we would be using in a race or street condition 1-4 gears are used for reasons of equal spacing to keep the comparison fair.
GEAR RATIOS LISTED FOR 2 928s. S4 2.2:1 and a GT 2.72:1
S4 1st 8.9:1 2nd 5.9:1 3rd 4.2 4th 3.2 5th 2.2 (spacing of .66.71.76) .71ave
GT 1st 10.2:1 2nd 6.8:1 3rd 4.8 4th 3.6 5th 2.72 (spacing of .66.71.75).71 ave
The GT compared to the S4 has these mechanical advantages per gear
115% for 1st
115% for 2nd
114% for3rd
113% for 4th
(and a rear end difference of 124%, or 5th gear)
{Now, your 4 speed automatic for reference with the 2.2 vs the 2.54:1
(and yes, the actual ratios are different between the two gear boxes, but ive listed the final results of the ratios here)
2.2 1st 8.1:1 2nd 5.3 3rd 3.0 4th 2.2 (spacing of .65.56.73 ) (.64ave)
2.54 1st 9.8:1 2nd 5.7 3rd 3.65 4th 2.54 (spacing of .58.64.69) (.64 ave)
compared to the 2.2, the 2.54 trannie has these mechanical advantages per gear:
120% 1st
107%, 2nd
121%, 3rd
(and final drive difference of 115% )}
Since, accleration is actually only due to area under all the torque curves over the operational speed range, torque in ftlbs seconds can show relative differences in the differnet rear end/transmissions of our discussion topic.
Below, see the actual torque through the appropriate gear and miltiplied by its ratio for total ft/lbs delivered to the rear wheels at any speed.
MPH---0-35 -------35-40----- 40-60--------60-75---------75-96------96-105
GT--1st 2958----2nd 1972---2nd 1972---3rd 1392----3rd 1392-----4th 1044ft lbs
S4--1st 2581----1st 2581----2nd 1711---2nd 1711----3rd 1229-----3rd 1229ft lbs
TIME---2sec------ 1 sec------- 3 sec--------- 2 sec------ 3 sec----------- 3sec
The times spent in each gear are approx based on actual tests and road and track values . roughly, 0-60 in seconds, 60 to 105mph in 8 seconds.
anyway, these are the torque values in each gear for two hypothetical cars , both 928 S4, but one with the GT gear ratios (2.72:1 rear end gear box vs 2.2:1 gear box) the difference of the total ratios are in the 15% range
Taking the totals of all the resultant torques and multiplying them by the time spent, we get a good estimation of what we would get if we actually used calculus and used the area under the torque curves. call it a "manual integration"
the resultant total torques over the speed range of 0-105mph is:
S4 3944ft lbs/ seconds
GT 3982ft lb /seconds
the totals are just the torque of the engine over the operational range (we use 290 which is an average) X the gear ratio X the time spent in that gear.
The math for torque produced in each gear x the time spent in that gear:
(GT)5916+1972+5916 +2784+4176 +3132= 23896/6 for a total of 3982ftlb/sec
(S4)5162+2581 +5133+3422+3687+3787 = 23672/6 for a total of 3945ftlb/sec
This is a 1% difference of the total acceleration torque, put to the wheels over a speed range of 0-105mph. make the test to 100mph, and the GT has a paltry 2% gain, make it to 115mph, and the S4 has a very slight advantage.
Go even faster, the trade offs go on til 155mph.
(as a note, the times listed in each gear duration in seconds would normally be different slightly for each advantage mechanically. However these second order differences as small as they are, would be cancellled out over the operational range)
These values are an integration of the torque to the wheels over the operational speed range though the appropriate gear ratios
this is the accelerating force over the speed and time shown above.
This clearly shows the result of a gear box change to a lower ratio with a 15% average mechanical advantage. you see the net advantage over the operational range is much less, infact, depending on the speed target range, it can be a disadvantage. (one of the reasons i still race with a S4 gear box for the tracks visit and power i have.
NOW for Force!!!!
As far as a definition of F=ma
Accleration is the 2nd derivative of distance and the first derivative of speed.
newtons 2nd law (again) says that acceleration is produced by an UNBALANCED force acting on a body proportional to the net force.
That is:
delta velocity/delta time
or in terms of distance:
Delta^2 distance / delta time
F=ma refers to an UNBALANCED force. if you don't have an UNBALANCED force, you dont have acceleration.
So, if you dont have a delta distance or a delta speed, you dont have acceleration PERIOD.
Force , acceleration and speed are vectors which consists of two parts, a magnatude and direction.
the rate of change of speed over a particular time inteval (ie average acceleration) will result in a specific average force in a particular direction. to expand this, work is force through a distance (ive provided these values ) and work x time is power. Or if you hung up on distance, speed times time is distance.
I hope this helps. if you have questions, pick up a survey text in physics so you dont get hung up on the calculus.
Mk
01-09-2005, 04:33 AM
#114
Rennlist Member
a breath of fresh air. you get a gold star (and a video CD of the 2004 race season, send me your address)
actually, both cars run from their peak torque to redline in proportion to the gear reduction they have. but, its in your understanding of the overlap concept and average torque, that shows you are on the right track. the acceleration over all is what we are looking for. this average acceleration or area under the torque curves used, is what determines the rates of acceleration. at any point in the speed range, either car can be faster or slower. yes, there are more points where the S4 tall ratio is slower, but there is a few points (actually 6seconds vs 8 seconds) where the S4 is much faster for a shorter period of time. Net net, the cars will be even for most of our concerns. (unless you pick a specific speed range to accelerate to, like 0-60 which would clearly favor the GT. as you say, we are not racing 1st gear to 1st gear. this would be silly. (ie one car runs to redline at 35mph and the other to 40?? makes no sense)
thanks for the post. Ok, one of the 30 folks gets it, anymore?
MK
actually, both cars run from their peak torque to redline in proportion to the gear reduction they have. but, its in your understanding of the overlap concept and average torque, that shows you are on the right track. the acceleration over all is what we are looking for. this average acceleration or area under the torque curves used, is what determines the rates of acceleration. at any point in the speed range, either car can be faster or slower. yes, there are more points where the S4 tall ratio is slower, but there is a few points (actually 6seconds vs 8 seconds) where the S4 is much faster for a shorter period of time. Net net, the cars will be even for most of our concerns. (unless you pick a specific speed range to accelerate to, like 0-60 which would clearly favor the GT. as you say, we are not racing 1st gear to 1st gear. this would be silly. (ie one car runs to redline at 35mph and the other to 40?? makes no sense)
thanks for the post. Ok, one of the 30 folks gets it, anymore?
MK
Originally Posted by JKelly
I've read this whole thread so far....super interesting and educational. I hope nobody minds if I try to make sense of this to myself out loud.
I think I understand Mark's point: The time it takes for two cars to go a certain distance (1/4 mile) is determined by the frequency and range of each cars multiple torque peaks relative to one another. So, a higher geared car could beat a lower geared car if the distance travelled and the timing of the power curves favored the higher gear. So the AVERAGE ACCELERATION of the car would be faster.
On the other hand: A lower geared car reaches a torque peak faster, it may not stay in its peak as long, but the ACTUAL ACCELERATION from peak to peak is faster. Therefore; ACTUAL ACCELERATION of a car is faster, but when other variables are factored in, such as number/spacing of gears and distance, then the AVERAGE ACCELERATION at the finish line actually determines who wins. If there was only 1 gear and infinite power/rpms, then the lower gear would make acceleration time faster from the start point and it would remain faster because it is constant ACTUAL ACCELERATION. Multiple gears enters into the picture to deal with the problem of a non-constant torque line and that is where AVERAGE ACCELERATION across the gears comes into play.
So, to sum up my thoughts:
One party is arguing from the point of AVERAGE acceleration.
The other party is arguing from the point of ACTUAL acceleration.
Does this sound about right??? or am I completely lost.....
I think I understand Mark's point: The time it takes for two cars to go a certain distance (1/4 mile) is determined by the frequency and range of each cars multiple torque peaks relative to one another. So, a higher geared car could beat a lower geared car if the distance travelled and the timing of the power curves favored the higher gear. So the AVERAGE ACCELERATION of the car would be faster.
On the other hand: A lower geared car reaches a torque peak faster, it may not stay in its peak as long, but the ACTUAL ACCELERATION from peak to peak is faster. Therefore; ACTUAL ACCELERATION of a car is faster, but when other variables are factored in, such as number/spacing of gears and distance, then the AVERAGE ACCELERATION at the finish line actually determines who wins. If there was only 1 gear and infinite power/rpms, then the lower gear would make acceleration time faster from the start point and it would remain faster because it is constant ACTUAL ACCELERATION. Multiple gears enters into the picture to deal with the problem of a non-constant torque line and that is where AVERAGE ACCELERATION across the gears comes into play.
So, to sum up my thoughts:
One party is arguing from the point of AVERAGE acceleration.
The other party is arguing from the point of ACTUAL acceleration.
Does this sound about right??? or am I completely lost.....
01-09-2005, 06:56 AM
#115
Drifting
mark, you ARE wrong. maybe some of what youve posted is correct, but in your first post you miss the point and in the first line make a statement which when read in context would seem blatantly incorrect.
the question being asked is how much difference the 2.54 rear end makes in an S4 auto compared to the 2.2 and your initial response is 'zero'. this is not physics, this is not mechanical engineering or doctorate level hypothesis, just simple everyday 'my car goes like stink what about yours?' kind of stuff.
and the answer is about 0.3 seconds to 60 mph which is not zero. even if we compared acceleration all the way to V max for both auto S4 cars there would be variance - dosent matter how much, but any variance at all is not zero. im sorry but you are wrong!!
the question being asked is how much difference the 2.54 rear end makes in an S4 auto compared to the 2.2 and your initial response is 'zero'. this is not physics, this is not mechanical engineering or doctorate level hypothesis, just simple everyday 'my car goes like stink what about yours?' kind of stuff.
and the answer is about 0.3 seconds to 60 mph which is not zero. even if we compared acceleration all the way to V max for both auto S4 cars there would be variance - dosent matter how much, but any variance at all is not zero. im sorry but you are wrong!!
01-09-2005, 11:00 AM
#116
Nordschleife Master
Mark,
In my example one car was used with different final drive ratios. That's good science. At the end where the evedence is clear, you can't follow the logic because it disproves your point.
What you miss is that we're talking acceleration and I'm working it through F = ma re-arranged to a = F/m. As m is constant, examining F gives acceleration directly. But this abstraction doesn't follow your model so, apparently, you can't grasp it. Your "BZZZT wrong answer" is itself wrong.
That said, I've done enough. I'll bet all the people who "agree" with you are hoping to get to another topic, too.
In my example one car was used with different final drive ratios. That's good science. At the end where the evedence is clear, you can't follow the logic because it disproves your point.
What you miss is that we're talking acceleration and I'm working it through F = ma re-arranged to a = F/m. As m is constant, examining F gives acceleration directly. But this abstraction doesn't follow your model so, apparently, you can't grasp it. Your "BZZZT wrong answer" is itself wrong.
That said, I've done enough. I'll bet all the people who "agree" with you are hoping to get to another topic, too.
01-09-2005, 11:54 AM
#117
928 Collector
Rennlist Member
Rennlist Member
Let's remember who Mark Kibort is. He is no drag racer. He is a racing driver.
What does this mean ....
It means that he understands that he can select the right gear at any given time and make enough "GO" (I like to call it torque and he likes to call it horsepower but whatever) to beat any car of equal rating, whether that car has taller or shorter gears.
I think I understand very clearly the major misunderstanding on Mark's part here. He thinks that by saying that taller gearing accelerates more slowly, we're saying he cannot select gears well enough in his 2.2 to stay in its torque band. Of course, we're not saying that. We're stating the obvious ... ad nauseam .....
What does this mean ....
It means that he understands that he can select the right gear at any given time and make enough "GO" (I like to call it torque and he likes to call it horsepower but whatever) to beat any car of equal rating, whether that car has taller or shorter gears.
I think I understand very clearly the major misunderstanding on Mark's part here. He thinks that by saying that taller gearing accelerates more slowly, we're saying he cannot select gears well enough in his 2.2 to stay in its torque band. Of course, we're not saying that. We're stating the obvious ... ad nauseam .....
01-09-2005, 12:00 PM
#118
928 Collector
Rennlist Member
Rennlist Member
Originally Posted by mark kibort
.........Ive hear more lame interpretations of physics than ive heard in my entire life. more terms used out of context that i can keep track of. Ill have to make a list of that and post it.MK
Originally Posted by mark kibort
.........
.... HP does move the car forward, BY DEFINITION!!! torque x speed is HP. without speed, you aint moving!!
.... HP does move the car forward, BY DEFINITION!!! torque x speed is HP. without speed, you aint moving!!
01-09-2005, 12:56 PM
#119
Burning Brakes
Join Date: Jan 2002
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Original question:
Mark's answer:
I think the only mistake in this post is the second sentence. "Gearing has little to do with acceleration for street applications. its an effeciency factor."
Maybe what it should say is "Gearing effects acceleration relative to a target speed or target range."
In the second paragraph, the folks that he is refering to may be folks that use 1st, 2nd, and 3rd, the majority of the time. Therefore; the 2.54 would "feel" as if it accerated faster....because it probably did from stop light to stop light.
In the third paragraph, he refers to 0-60 runs, which is most important to all of those who are "socially conditioned" armchair racers. In which case he points out that the lower GTS, GT, or 79 gear box would be better.
In sum, acceleration is relative to who is at the finish line first, whether it is 10 feet or 10 miles.
Originally Posted by IcemanG17
How much of a difference does the 89+ 2.54 axle ratio make in terms of acceleration vs the 87-88 2.20 axle ratio (automatic)? My 2.20 seems fairly strong, the 2.54 must really fly!
Originally Posted by mark kibort
0 , none, nothing. Gearing has little to do with acceleration for street applications. its an effeciency factor. however, it can effect acceleration for a target speed or target range. gearing doesnt produce hp, but it does determine how efficient the application of hp is over an operational range.
Folks have chimed in about how much faster certain gear boxes were when tested unscientifically, and some even had changed gears so much that they created the exact same ratios. (just with different numbers attached to them, like 2nd becoming 1st, or 4th becoming 3rd, etc)
the way to look at gears is trade offs when it comes to the rear end especially. with a larger numerical rear end, you will have less torque in the 1st gear you start at, but when you change in tho the next gear, it will have proportionately more torque in that gear than the lower numerical rear end for a longer period of time. In general terms, its a trade off, and why wouldnt it be. you are not creating more acceleration power, just redistributing it. the only way gearing really effects accleration , is when you make the gears closer together, and right now, most all 928s up to 4th gear have the same spacing. (or if you select a target speed that makes one of the gears hit it at redline. example, if your goal is to run 0-60 and nothing else matters, you want a 928 gear box and rear end that allows for redline at 60mph in 2nd gear. this is probably the GTS, GT or 79 gear box . However, any other speed target, all bets are off for which is the best. all about trade offs)
MK
Folks have chimed in about how much faster certain gear boxes were when tested unscientifically, and some even had changed gears so much that they created the exact same ratios. (just with different numbers attached to them, like 2nd becoming 1st, or 4th becoming 3rd, etc)
the way to look at gears is trade offs when it comes to the rear end especially. with a larger numerical rear end, you will have less torque in the 1st gear you start at, but when you change in tho the next gear, it will have proportionately more torque in that gear than the lower numerical rear end for a longer period of time. In general terms, its a trade off, and why wouldnt it be. you are not creating more acceleration power, just redistributing it. the only way gearing really effects accleration , is when you make the gears closer together, and right now, most all 928s up to 4th gear have the same spacing. (or if you select a target speed that makes one of the gears hit it at redline. example, if your goal is to run 0-60 and nothing else matters, you want a 928 gear box and rear end that allows for redline at 60mph in 2nd gear. this is probably the GTS, GT or 79 gear box . However, any other speed target, all bets are off for which is the best. all about trade offs)
MK
Maybe what it should say is "Gearing effects acceleration relative to a target speed or target range."
In the second paragraph, the folks that he is refering to may be folks that use 1st, 2nd, and 3rd, the majority of the time. Therefore; the 2.54 would "feel" as if it accerated faster....because it probably did from stop light to stop light.
In the third paragraph, he refers to 0-60 runs, which is most important to all of those who are "socially conditioned" armchair racers. In which case he points out that the lower GTS, GT, or 79 gear box would be better.
In sum, acceleration is relative to who is at the finish line first, whether it is 10 feet or 10 miles.
01-09-2005, 01:03 PM
#120
Rennlist Member
You didnt read it all. i qualify the statement by saying "for an infinite range of speeds"
as i said, 0-60mph would favor the 2.72 box., but 0-75mph would favor the 2.2 box.
this is the point. So, there will always be a variance. (the trade offs I speak of) Again and again, its the trade offs that i speak of.
so, what am i wrong about?
Mk
as i said, 0-60mph would favor the 2.72 box., but 0-75mph would favor the 2.2 box.
this is the point. So, there will always be a variance. (the trade offs I speak of) Again and again, its the trade offs that i speak of.
so, what am i wrong about?
Mk
Originally Posted by drnick
mark, you ARE wrong. maybe some of what youve posted is correct, but in your first post you miss the point and in the first line make a statement which when read in context would seem blatantly incorrect.
the question being asked is how much difference the 2.54 rear end makes in an S4 auto compared to the 2.2 and your initial response is 'zero'. this is not physics, this is not mechanical engineering or doctorate level hypothesis, just simple everyday 'my car goes like stink what about yours?' kind of stuff.
and the answer is about 0.3 seconds to 60 mph which is not zero. even if we compared acceleration all the way to V max for both auto S4 cars there would be variance - dosent matter how much, but any variance at all is not zero. im sorry but you are wrong!!
the question being asked is how much difference the 2.54 rear end makes in an S4 auto compared to the 2.2 and your initial response is 'zero'. this is not physics, this is not mechanical engineering or doctorate level hypothesis, just simple everyday 'my car goes like stink what about yours?' kind of stuff.
and the answer is about 0.3 seconds to 60 mph which is not zero. even if we compared acceleration all the way to V max for both auto S4 cars there would be variance - dosent matter how much, but any variance at all is not zero. im sorry but you are wrong!!