Bill Verburg

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.

GTgears

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.

GTgears

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...

mark kibort

Fantasic post.. everyone take note.. someone took the time to explain this perfectly, with the sceanaro that Matt had laid out.. not to pick on Matt, but his conclusion was really about part of the discussion here.
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.

Not everyone does this, due to the stability issue on the exit. 2nd can get you into trouble, but based on your car control , i see no issue for you, BUT, its a part of the track that can bite. i do it every time, for rear wheel stability on the entrance too.. exit, i respect the forces and never have issues. just be careful and respectful of 2nd down the corkscrew. turn 5.... have at it!!

Yes, this is true!! funny!

interesting perspective, and certainly one that will effect perception of torque (i.e. acceleration forces). however, if you focus on the HP differences at each RPM range, this is easier to compare, as it takes everything into account . But, since the RPM ranges are the same, using torque is fine too. so, to the point of this thread, why doesnt the GT4 feel a lot different, rather than a little different? basically, its due to the fact that the engine output is not really all that different... 20hp is hard to feel in the lower RPM ranges, and that mixed with the perception and changes of relative vehicle speeds. its there can be some factors that sway your senses.
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.

mark kibort

Bill, this is plain not true. power IS by definition, the cars capacity to accelerate. by definition power is the rate of change of KE... so, its why dynos can measure power, without ever knowing torque of any kind! all you need to know is the change of speed of the dyno drum (or car on the road), and you get HP! very simple. you need NONE of the mass, aereo, transmission CWP, specs, as longs as you are comparing two of the same cars. now, two different cars, yes..... in this discussion we are talking about the effects on the same car.. certis paribus... all things being equal.
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!

Matt, you ask questions, you make statements and then before we can respond , you go away. this IS exactly on point and on topic to the discussion. Gear changes can change perceived performance. not so much on actual, and thats what ive been trying to prove here. gearing can be changed to optimize shift points at the track. not much will change in a straight line to max speed , and on the street............that's personal preference with shifting frequency vs speed range preferences, AND the all important RPM at hyway speed in top gear. so, knowing what is truth and what is feeling is a values to the GT4 owners that are looking to improve for their uses.
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

Mech33

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.

GTgears

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.

mark kibort

Matt... ok again.. try and put down your preconceived notions of torque and gearing. your example is meaningless. I dont understand your point... you show us two different gear ratios at the same speed (110mph) and you show the same resultant torque in (Nm). that makes my point perfectly. if you have the same rear wheel forces at the same speed then the power will have to be the same... how is it not... im confused . what am i missing? you say that the shorter 4th gear will put down more power at the ground, yet you quote the near exact same force... (wheel torque). that would mean , it would be the same hp.

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 is your point here? wheel torque/HP, engine torque vs engine HP, there is no difference but a tiny efficiency factor that you mention of about 2-3hp between gears. sure, doing the test in 4th gear if its 1:1 vs 3rd is a slight difference, but this added factor is just noise for those trying to understand the concepts and math here.... in the power transmission world, when we add another gearing stage for greater reduction of gear ratios, there is an efficiency factor that goes down. usually its small, and depends on many factors . so put it in there.............you think that's a motivation of getting into 4th earlier if possible, to save 2-3 HP..... sure, you are right, but for the fraction of a second of time spend longer in 3rd, is it worth the change for that reason alone?????? its easy to calculate, but generally, no.
what am i missing here...... im mean, "the double dipping" part.. was there another point?

mark kibort

yes, and thats the problem.. the semantics. taking it step further, you dont even need the forces to determine the power. just rate of change of KE works too. x Joules, at start, y Joules at end of 1 second will give power. (divide by 746)

Matt,

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.

mark kibort

yes, true about the two types, but a rolling dyno, more applicable for the discussion because of the topic, will show accurate HP numbers with out ever knowing engine torque. doesnt even calculate rear axle torque either. just rate of change of KE ,by knowing the time between measurements and the speed change of those two, or successive points. on a dyno with no tach input, no engine speed values... only MPH of the drum and a stopwatch (and known rolling mass values) you get accurate HP values

mech33, he is referring to my statement of "engine torque" being irrelevant in comparisons, which it is. not that it isnt needed for HP /torque curve calculations, but its irrelevant when comparing two cars, unless you are taking all the other factors into account. tire diameter, gearing , etc... HP just contains more info. the trade offs of RPM and torque happen naturally, and it doesnt matter for comparisons what they are. again, for comparisons.

Dont shoot the messenger here.. those that understand the physics or want to, can find great benefits of the basics i present. i don't say torque is irrelevant, i say HP contains more information and is easier to compare with due to it containing more info. engine torque can be irrelevant in comparisons of two cars, where RPM and torque can be exchanged freely with no net difference. for our comparisons, you brought up the engine torque and gear ratios. for this discussion it is relevant to bring up or check my work with engine torque vs gear ratios, but as you have seen, its just easier to use engine HP

Great point, that's why the GT4 discussion is very easy, because its almost cheating, because it has a near flat HP curve from 6000 to 7500rpm. this implies that if you are ever in this range, at any vehicle speed, you will put the same rear wheel forces to the ground! (sans the small differences in gear ratio efficiency that i mentioned in an earlier point)

Lets be accurate. when you gave me the scenario, you found the same rear wheel torque.. you mentioned that the power available would be different and more optimal for the lower ratio.... why do you say that? you need to be accurate and it doesnt seem to be in this case. Im giving you the respect because i think you do great work... We are discussing the application here which might be different for you .

this is irrelevant... Because that 15% is made up of many losses, (and is variable depending on speed) and will be near the same for any car with different ratios. We are talking about changes in gearing losses, not the losses in aggregate.
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.
exactly.... as I mentioned, a dyno can accurately measure hp with no RPM signal. the output is HP vs MPH.... all other factors are not needed. its the basis for my argument for same HP same acceleration on the track............ if you ever are at the same hp level at the track, that will be the maximum you can accelerate... independent of gear ratio values and changes to those values. the Laguna Seca example is a great example of this... if you are operating from 6200rpm to 7500rpm you are in the max HP range and that is the greatest rate of acceleration regardless of the gear ratio or gear you are in.

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)

Bill Verburg

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.

Mech33

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...

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.

mark kibort

Exactly! nice!

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.

As Mech33 said, this is not exactly true. power=Force x velocity... all you need is a speed and power at that speed, and we can find the acceleration force. (minus all the losses and if there are some, and they stay the same for comparisons, then they cancel out ) and because of close to a 24" diameter tire, (because the radius is a foot), lbs of force becomes lb-Ft of torque at the axles.

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.

Bill Verburg

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

mark kibort

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.