I understand the "few hundered RPM higher" deal...the torque in the current
gear would still be higher than if shifted to the next gear (see my Excel sheet
to confirm my understanding).

So...the red line you have drawn represents the RPM at which the next gear
falls (also indicating the torque values)!

Gat daym, I was looking at it correctly before, I just didn't understand
the cascading concept. I figured the next gear was shifting before HP peak
because it occured before the yellow circle on the following line.

Nice Jim.

 

I think we found the problem.... The quote in my sig by the old man should correct you on that.

Well have fun with this little topic, I'm realizing that its not as intuitive to most as it is to me. I don't know how to explain this to you boss, which leaves me dumbfounded.

Bret

 

EDIT.

 

Here's a good place to start for understanding traction...

http://www.hotrod.com/techarticles/34238/

 

its actually all really simple in my opinion. Hp is just a measurement of torque, the speed of the motor combined with its torque (or how well it breaths) will give you raw power. Which moves you down the track and gives you the win or loss. when a car launches with 400 ft lbs at lets say 2000 rpm (probably a big block or something) its actually launching with 152 hp. tonz of torque, but its only moving 2000 rpms...and speed matters just as much as torque. This is why dragsters never run under 8000 rpms, they run as high as they can get the motor without breaking it. whats funny is a motor making 160 ftlbs of torque at 5000 rpms also makes 152 hp. If in theory both these cars came off and stayed at these different rpms. Assuming everything else on them was identical. They would move at the exact same speed, because they produce the same amout of power. most people even like myself had a hard time grasping how a motor that made all that torque 400 ftlbs would not walk away from one making only a mer 160 ftlbs. But rpm makes a huge difference. I used the common torqueX RPM divided by 5250 to get these numbers, check for yourself ;]

 

No they would not.

Even though horsepower is the same, the RPM is different which means the axle speed is higher.

YOu're missing the point of the initial post, but it's already been clarified.

Thanks anyway.

 

I think we need a reprise of something JonA wrote several pages back:
You guys are all trying to solve the wrong problem, or asking the wrong question. You're all stuck on this one-fixed-gear thing, which is totally meaningless to car performance. I know of no racing class where you are only allowed one final drive ratio and only one speed transmissions.

Here are some indisputable facts:

1. Power is defined as how much work can be done within a certain time, or how fast a certain amount of work can be done; racing is defined as how fast a certain amount of work can be done, whether the work be Arny and Hulk throwing rocks or Garlitz and Prudhome completing the 1/4-mile.

2. There is a vast difference between the torque at the drive wheels and the torque at the crankshaft; the former is directly related to the acceleration of the car, the latter is not.

3. If you don't believe #2, then tell me how fast a 3000lb car that averages 300lb-ft of crankshaft torque completes the 1/4-mile, without adding info I didn't already give you (assume no drag/inertial losses, perfect traction, and no flywheel effect at launch). You can't. But you can tell me how fast the car finishes if it averages 300hp during the run.

4. Although torque at the crank and torque at the tires are only related via the gearing (at which point you know rpm and therefore are really talking about power), power at the crank and power at the tires is always the same (minus friction and inertial losses) regardless of gearing.

5a. At any given speed, and all else being equal, the car that puts the most power to the road will accelerate the hardest.

5b. Over any given distance, and all else being equal, the car that averages the most power to the road will cover that distance faster.

5c. For any given size and drag coefficient of a car, the car that can output the most power at its top speed will have the higher top speed.

5d. Therefore, in all forms of racing the goal is to have an engine put out the most power over the course as possible, and crankshaft torque is irrelevent.

All of this is not only by physics, but by definition. Power is hardly some cryptic esoterica with no real importance. Power is the one and only unit that predicts vehicle acceleration, period. And note that for 5a-d, the engine in question could have peaks 100lb-ft of torque and make 500hp at 19000rpm or 500lb-ft and make 500hp at 5252rpm. It simply doesn't matter what kind of torque it makes, only the power. Yes, F still equals MA, but that has to do with torque at the drive wheels, not the crankshaft. The reason 5a-d are true is that making the most power at the crankshaft results in the most drive-wheel torque, which provides the most accelerative force (tractive effort) at the contact patch.

And btw, the one and only purpose of a transmission is to allow an engine to stay as close to its peak power as possible. That's why we keep getting transmissions with more and more ratios, to the point where CVTs are becoming common. And as long as you have to shift through discrete ratios, then your ideal shift points are determined by the average power in each gear, not torque. You want to shift so as to maximize the average power output over your racing distance. Period. Ditto for selecting your final-drive ratio. You obviously want to select it for the most power during a lap or run.

Power is very real, very predictive of performance, and very important. You'll never see boat engines rated for torque, nor aviation engines, nor locomotives, nor industrial powerplants. That's because if you want to know how fast a pumping station will drain New Orleans, you need to know the power it makes, not the torque (because again, power is work/time). What is automotive esoterica is any discussion of what determines the acceleration of a car in one fixed gear!

 

Well said. I thought Jim's chart showing axle torque vs speed with the Horespower peaks circled was quite a good visual for those who ask the wrong question.

 

Except for starts, restarts and pit outs, most NASCAR circle track races do use one gear and often a maximum specified final drive ratio. They sometimes use 50% of the engines rpm range twice a lap. To them it may have some meaning. Of course sometimes they only use about 300 rpm change per lap, or even less, but still in one gear.

Be careful with "indisputable" around this bunch. Some folks might dispute your "facts". To others, who have their minds made up, "facts", indisputable or otherwise are irrelavent.

Thanks for the input.

 

Either way pops you gotta like this guys fire under his *** here! I can definately FEEL his intensitiy in his writing, and I can understand the angst.

Bret

 

I wasn't aware there was a maximum final drive ratio at any NASCAR event. But even so, teams still have a choice of gear ratios up to that limit, as well as having some control over tire diameter. But more importantly, regardless of any limits on gearing, the car that applies the most power to the ground will still be faster (all else equal).

And I agree that unless a car has a CVT, its engine will be required to output power across a range of rpm, be that a wide or narrow range. So that brings in another issue that is often overlooked: peak power is not usually relevent, but average power is what actually predicts a car's acceleration. I had tried to not overtly bring this up before, because it's one more issue that confuses people. But since power must take place over some time interval (power can't occur during zero time), we're really always talking about average power.

Heh! Well, it's a free country and one can dispute anything he chooses. But he won't be correct.

 

Yep, power to the ground is the thing. As you pointed out power at the wheels is torque at the wheels x rpm. My take is that some folks get confused because applying torque at a rate (aka power) isn't always intuitive when you have the "torque wrench" concept of torque.

FWIW, with radial Cup tires there is very little tire diameter control. Air pressure changes spring rate, not diameter/circumference.

Of course it's average power, or average torque, for that matter, that the vehicle feels. That has come up a lot around here, even in this thread. Don't be hesitant to bring up relevant points. Some folks are able to juggle more than one ball at a time; some aren't.

I'm sure you are familiar with the Engine Masters Challenge. The scoring is based on average power and average torque thru an rpm range. For "dyno racing", that may be the best way to compare engines.

My take is that NASCAR Cup engines are built to give max average power over the rpm range they will see on the track. There's a good chance that the torque curve is "managed" especially in short track engines where traction out of the corners (forward bite) is lacking and the engine might overpower the tires. Traction control is illegal , so one should be able to manage the rearwheel torque at the source, given enough development time and money.

A few head/cam packages which are frequently used, especially by the LT1 guys, specifically target average power/average torque over the operating range. It's reflected in the vehicle performance.

Of course we fragile humans can't change how Ma Nature works. No matter how we try to explain it, She has the utimate word. An old friend said that well: "You can't rape Mother Nature."

We are fortunate to have you as our final source for what is correct, B4C.


Jon

 

Probably so, at least for the big$ teams. Because if the tires are spinning then much of the power is not being applied to the ground, but rather to making heat in the tire carcass.

All hail me, the final arbiter of truth! Of course, if there are any physicists who want to discuss Quantum theory or String theory and incorporate Planck's laws, then they actually could dispute me and I couldn't prove them wrong (even if I knew much about that stuff). But I'm actually gambling that everyone here will be sticking to Newtonian physics.

 

I'm old enough that Issac was my Physics 101 instructor. I sat between Albert and Max. Albert was always taking us off on "thought experiments" and Max was always either too hot or too cold. Interesting semester.

Gambling? "God does not throw dice." -Albert referring to quantum theory.

Jon

 

I read back on this thread and shake my head.

Just think, if my torque converter was a lock-up type, this would have never
started.

Jon, as you suggested I've been reading more on "Stall Torque Ratio (STR)".

Here is a small, but representitive write-up (of many I found):

http://www.converter.com/whatsnew_dynamometer.htm

EDIT (Another link):
http://www.converter.cc/tech.htm

In other words, the torque converter is going to multiply the output torque
according to the input torque...STR is not a fixed number - it will vary according
to engine output, transmission gear, vehicle weight, etc.

So...if my dyno was taken in 1st gear, the crank output would have had a
higher load on the transmission input which would have allowed more multiplication
throughout the sample.

Instead, the dyno samples are taken in 3rd gear to reduce the mulitplication
effect. This would send the torque converter fluid into a vortex flow sooner.

This might explain the differences in my dyno torque curve vs. the 1st gear
and second gear g force data acquired from the accelerometer.

Can I get some love NOW?!