Yes, it does take more power to accelerate a heavier vehicle. It's simple physics. F=ma. For a given amount of torque (force), a heavier vehicle will accelerate slower. The rocket car example you give deals with totally different issues (thrust vs power to the ground). The perfect example of powerband and length of race is how an LS1 F-body (300 HP) will roast a Ferrari 360 Modena (405 HP) stoplight to stoplight, but from a roll where the Ferrari can be in it's powerband from the word go, the Ferrari will roast the F-body.

 

Cooter, did you play with the shift points to optomize ET for each "car"?

Also try changing the TC stall RPM and so launch RPM to get each car into it's power band quickly keeping the coeficient of frictions adequate to avoid wheel spin.

I agree low end HP (i.e. "torque") feels great but, as mentioned, it's the average amount of HP at the rear wheels that wins the race.

BTW, Drag2000 is a great tool with which to optimize your Vette's performance in the 1/4 mi. It shows you how much time you spend in each gear so you know where to increase HP (or torque).

 

Careful on your physics there, N=F x R is for torque, its not technically a force. Its a force crossed by a lever arm and such is perpendicular to the force applied.

Though, essentially you are correct. The HP and TQ integrals are all that matters in engine design. And if someone raises the mass of the car, more force will need to be produced to make it accelerate.

 

Yes, I know the physics, I just didn't say what I meant to say...I've spent the last several days working on a takehome test for a PhD statistics course I'm taking while fighting off a cold...I'm a little out of it :crazy:

 

way off topic here, but rockets are actually very interesting for the reason that they appear to both be constant thrust, and constant power. i'll be damn impressed if anyone knows why!

 

http://vettenet.org/torquehp.html

This link will tell you all about torque and horsepower. It even has a comparison of LT1 vs. L98.

Please, do yourself a favor guys. Read and understand this short text before you say any more about torque and horsepower.

It look's like a couple of you know what you are talking about. You other guys click on the link.

 

I moved the shift points to the H/P peak of each engine and left all the other settings between the 'cars' the same because Z51 said "all things being equal, horsepower will win. Period." My point that I was trying to make is that it is a total package- you have to have everything optimized to the engine's power to get the best performance.

 

I knew someone was going to have be smart when I said "all else being equal". What I meant was that if you have two cars with the same weight, same driver, same suspension, etc the higher HP car will win. Properly gearing the transmission and rear end to fit the power band of the motor was implicit in the discussion, or so I thought. Geez folks, do we really need to dig around for something to "make a point" about? Let's please use a little common sense here.

Now, if you look at the two cars in a race from a roll, where they're moving fast enough that the higher RPM car is in it's powerband, the point of the discussion is that the lower RPM car will be in a higher gear than the higher RPM car, and that you can go through all of the calculations to figure in the torque and the gear ratios, etc, etc, etc, or you can simply look at the power to weight ratios of the two cars and know that the higher RPM car will always win any race in which it can spend the same amount of time in it's powerband as the lower RPM car, whether the lower RPM car makes more torque or not (this is, of course, just to clarify, in regards to the earlier established example of the 400 lb-ft at 3000 vs 3000 lb-ft at 6000 cars). In the end, assuming two cars both spend the same amount of time in their respective powerbands, the car with the higher power-to-weight ratio will win (again, just to cover my bases here) unless significant aerodynamic effects come into play.


[Modified by CorvetteZ51Racer, 3:14 AM 4/8/2003]

 

Well let me throw this around. IF all things being equal, both cars have the same peak HP,same weigh, same gearing, but one makes more torque than the other, isn't the one with MORE TORQUE going to win? Example, small displacement high HP low torque vs. big displacement high torque unable to breath lower HP than should be. 300hp 250ft/lb vs. 300hp 350ft/lbs. I'm betting on the second.

 

http://vettenet.org/torquehp.html
This link will tell you all about torque and horsepower. It even has a comparison of LT1 vs. L98.

Please, do yourself a favor guys. Read and understand this short text before you say any more about torque and horsepower.

READ THIS LINK IT WILL ANSWER ALL YOUR QUESTIONS AND YOU WILL LEARN ALL ABOUT THIS TOPIC.




[Modified by antenna, 10:55 AM 4/8/2003]

 

That brings up a interesting question. But the car with less torque will make it's HP higher in the RPM range. Thus allowing it to stay in first gear longer making it faster as per the one link above. More torque will get you off the line faster up to your torque peak.


When does more HP over come the jump start torque will give you? And it the HP higher up in the RPM range enugh to over come that hole shot? I dunno but my head is starting to hurt. :boxing

 

Rockets have a constant thrust when fired. I don't know ALL the details, but given that Power = Force * velocity, the velocity in question is the speed of the gases coming out of the rocket engine itself. Given this velocity doesn't change (well, much, depending on the design) with regards to vehicle speed, the output power is constant. However, you can't use a constant hp number at 0 vehicle speed, otherwise you would calculate infinite thrust. :crazy:

Anyway, I'm probably off on something there.

One thing I haven't seen mentioned in this thread is about torque and hp at the rear wheels. It has been talked about with specific calculations to determine how much thrust the vehicle in question is under, but the following point hasn't been made yet. Output Torque at the rear wheels is dependent on overall gear ratio, engine speed, and the torque at that engine speed. However, output Power is dependent only on engine speed and the power at that engine speed. When shifting properly, one focuses on keeping the engine as close to peak power as possible. You shift past the peak of the power curve (if your engine rev limiter lets you get that high) so that the next gear is near the power at the point you shifted. However, some cars are rev limited just past the peak power point, so you hold your shift to your max RPM. It's not the most efficient setup, but it does prevent you from blowing your engine. :)

One point I'd like to make as well is that for any given gear, the most acceleration you will get out of that gear is at the peak of the Torque curve. However, if one could perfectly gear the car for that particular speed, one would get even more acceleration by gearing the car for the peak of the Power curve. This is why for some of the new CVT cars, the car will accelerate off the line by revving the engine up with a constant gear ratio. Once the engine hits the peak of the power curve, the CVT begins to change the gear ratio on the fly while holding the engine at the peak of the Power curve, not the peak of the torque curve.

However, those of us with traditional transmissions do not have the luxury of just holding the car at the peak power point for the whole 1/4 mile. We might be able to change our rear end a bit, but for the most part, we stick with the stock transmission gear ratios, again, for the most part; I realize some people change transmissions out completely for new gear ratios, but I digress. Here is where Lingenfelter's string of thought comes in. His theory is to increase engine torque IN THE MOST USED RPM RANGE. What does he mean by this? Let's say you have an older car with a 3-speed. Upon shifting, you go from 5500 RPM to 2500 RPM (just an example). Where do you want to increase your torque the most? Near the middle of this range, around 4000 RPM. However, lets say someone else has a very similar engine and a 6-speed transmission. This car will shift from 5500 RPM down to only 4000 RPM (again, just examples off the top of my head). The second car and engine will want to have more torque concentrated at around 4750 RPM. Let's say each car is increased by the same amount of torque at these RPM levels. Given previous calculations and examples, the second car will have more hp than the first now, and given the different gearing, it will likely be faster in the 1/4 than the first. However, the first car will probably get a bit of a jump off the line over the second but will likely lose at the end of the race. Does this mean something is wrong with the first car? Not at all. Given its gearing, the car has been improved overall for what it is, but still lacks compared to the second car.

Clear as mud? :smash:



[Modified by semvhu, 11:08 AM 4/8/2003]

 

The idea is to maximize rear wheel torque at all speeds. A CVT is a perfect way of doing this.

With real life engines and transmissions you would plot rwtq for each gear to determine optimum shift points, you can't determine optimum shift points from engine characteristics only, you need to consider what happens when you shift to the next higher gear.

The correct question to ask is how to build an engine, tranny, rear end combo that maximizes rwtq at all speeds.

As they say there is more than one way to skin the cat.

 

And where do you get the most RWTQ for a given speed when you can choose any gear ratio you desire? The peak of the engine POWER curve. :smash: :crazy:

And the discussion continues. :jester

 

The only reason the discussion continues is because you haven't read this link. Anyone who takes the time to look at it will see what's going on with torque and horsepower. Come on guys, READ THIS LINK.
http://vettenet.org/torquehp.html

 

Was that for me? I read the link and agreed with everything in it. I didn't think anything I'd posted disagreed with what was said. If I am in error, please point to what I've said and I will correct myself.

Thanks!

 

well, that link totally puzzles me... for someone who seems to understand the physics so well, his last statement borders on the ridiculous. i mean, he's pointing out that hp is the most basic measure in every argument he makes, but then he keeps insisting on calling it "torque at the wheels," or " wheel torque at a given car speed."

i guess it's semantics, just seems crazy to me to define something in a roundabout way.

that statement would much more correctly be said:

"it's better to make more hp, and gear accordingly."

what he said is akin to saying:

"my favorite dessert is butter, eggs, flour, oil, baking soda, and sugar, all mixed together, and baked in an oven. mmmm... that eggs flour soda sugar butter is good."

i mean, come on! what do you think hp is? it's the most basic measure of the performance capability. my numbers prove that a car's acceleration at a given speed is determined by hp. sure, you can take the torque and do all the calculations (if you know enough other variables) and find out how hard you're accelerating.

or, just take the hp. it's this magical thing that somehow encompasses all the gearing differences, earth's rotation, moon tides, zodiacal increment...

semvhu, you're on the right track. since you got that much, let me take it a step further:

thrust is constant, power is constant. if thrust is constant, then the time to accelerate a given amount is the same no matter how fast you're going. to go from 1000m/s to 2000m/s takes the same amount of time as from 10000m/s to 11000m/s. (ignore any relativistic effect.)

but, the difference in KE is bigger in the second example, since energy is a function of the square of the velocity. if the time to complete the velocity change is the same, that means there was a bigger energy change per time. which means, power is increasing.

where is the extra energy coming from?

 

I don't have a problem with his last statement. I think he's writing for a basic HP-101 crowd and he wants to state it in a few ways. Torque at the wheels or torque at road speed is a way to take all the gear reductions into consideration.
I think this text is a fairly painless way for folks to learn about HP/Torque. From time to time I see these threads, with guys spouting all kinds of voodoo about HP and torque. If they would only read this text they would see the science of the measurements.

 

"difference in KE is bigger in the second example"
What is the difference in the KE delta by adding 1000ms? Isn't that what you are asking?
Add 1000ms to a high speed or a low speed and how much KE have you gained in each case by adding 1000ms?

Isn't this the way to look at it"

 

right. if the ship's mass is 1000kg (assume constant mass, an infinitely large and massless fuel supply). KE = 1/2 * m * v^2

so in the first case, deltaKE = [1/2 * 1000 * 2000^2] - [1/2 * 1000 * 1000^2]

in the second case, deltaKE = [1/2 * 1000 * 11000^2] - [1/2 * 1000 * 10000^2]

1.5x10^9 in the 1st case, and 1.05x10^10 in the second case. my question is, then, if the ship takes the same amount of time to accelerate in both cases, but the ship's rocket power is constant, where is the extra energy coming from in the larger deltaKE?