Ok, has anyone read the Motor Trend that has a new Beetle, Pontiac Vibe and a Mini on the cover? It is fairly new. Anyhow, you need not to have read to participate. They do a test that compared the maximum acceleration to the rpm of the engine in 2nd gear. They used 2nd gear to negate any wheel slippage. What I noted is that there was no rhyme or reason to where the maximum acceleration occured in relation to HP and torque peak.

My question is: Where does maximum acceleration occur in regards to engine maximum HP and torque? Does the maximum acceleration occur at peak HP or peak torque? Or can I not make a generalization such as this? I have a hypothesis but I am looking for opinions. Thanks

 

Supposedly peak torque, but if I'm wrong, I'm sure someone will correct me!

 

Torque would be my guess also since torque is what propells you.

 

I am no hero, but I am about one semester from an engineering degree. In layman’s terms, torque is the ability to do work and horsepower is how fast you can do that work. A good example of this is a semi-truck. They have and need tons of torque but not much hp. This is because they have a lot of work to do (weight to move) but they're not so worried about acceleration; as opposed to say economy. As you have found with the article in question, your peak acceleration isn't necessarily going to happen at either of the two numerical peaks. It will most likely happen on some type of middle ground between these two that's exact whereabouts would depend on many different vehicle specific characteristics (eg. vehicle weight, gearing, drag coefficient, traction, yada, yada, yada). This could even be slightly different between identical cars due to even more miniscule and vehicle specific details.

 

when the engine is doing as much work as fast as it can....... so bassically a mid point with torque and hp..... where it is making the most hp for its tourque... therefore the tourque would have to be higher than the hp..... but in peak power for the engine... they should be the closest hp and tq readings in numbers at any rpm.....

that mixed in with the gearing of the car creates maximum acceleration

 

so in other words tq is like potential.... and hp is how much of that potential is filled..... just like dustin mustangs said..... tq=ability to do work hp= how fast that work is done.

 

If you discount friction, peek acceleration should occur when the engine is at it's peek horsepower. An object in motion has a certain kinetic energy. Power is the rate at which energy is being transfered. The faster you transfer energy, the sooner the object will reach a certain speed.

My guess is that the fact that Motor Trend found something different has to do with the changing speed of the vehicle, transmission, etc. For instance, suppose the engine puts out 100 HP at 3000 rpm which happens to be 30 mph in second gear. The same engine at 6000 rpm might produce 150 hp, but the car is now moving at 60 mph - at double the speed the wind resistance should be quadrupled, resulting in lower than expected acceleration.

 

Oops - I just noticed an error in my first post. Kinetic energy is dependent on the square of velocity, so at a given horsepower, acceleration will not be constant. The first sentence of the previous post should read This isn't really a very useful thing to say

A side note about torque - max acceleration will occur when the torque at the wheels is at a maximum, which is not the same as when engine torque is at a max.

 

...hmmm VLich, you seem to contradict yourself:

First you said
Then you went on to say
I assume that by peek you mean peak, which could be considered the same as max, or maximum.


You also went on to say
Why is this, can you back-up this claim??

 

Not sure I understand what you're asking, but I would say in the tests you speak of, peak accel. would happen at or around peak torque b/c of it's lower speed, this would also vary with vehicle weight, over all drive ratio(I'm saying too generalized), but at peak hp is when the engine is doing the most work. ie if the vehicle speeds were the same(would have to be in different gears) peak accel. would happen at peak hp.

 

OK, many of you misunderstood my question to some extent.

Here is the scenario:


Install acceleration testing equipment
Place car in 2nd gear.
Bring car to a steady 1000 rpms with the clutch engaged
Floor accelerator until redline and note places max acceleration occured with respect to rpm of the engine.

NOW COMPARE MAX ACCELERATIONS TO THE RPMS PEAK HP AND TORQUE OCCUR!

This has nothing to do with gearing or vehicle weight. (negating air friction, which I assume is low in 2nd gear as compared to total HP)

 

Max acceleration can easily be found be looking at the g force the car creates at different moments in the rpm range. To get this you will need to know a list of things including torque, hp, rpms, weight, tire diam, and others that are explained in the later.

Adjustment to develop a mean torque value: Using just the peak torque numbers will produce an exaggerated value in calculations. An engine operates at its peak torque level for only a brief moment. Conversely, using horsepower to calculate Torque values would produce a too-low value. In order to produce a more realistic estimate of mean acceleration in each gear, the following formula creates an average between the two values, to produce a mean torque value:

Torque = ((5252 x HP)/RPM)+ Peak Torque)/2

We can now develop a formula for calculating g-force acceleration in each gear by combining and simplifying the different formulas involved.

G = (Torque x Gear x 0.85) / (Rolling Radius in feet * Vehicle Weight)

acceleration requires consideration of mechanical power (torque), gearing, tire diameter, and vehicle weight. This is best calculated and represented in terms of G-force. One "G" representing actual thrust effect, or forward acceleration, in terms of a rate roughly equal to 32 feet per second per second, or 32 ft./sec2._

This will demonstrate where the car excellerates the most, when, and by how much.

 

No4NJunk, calculating average torque works to get an estimage of how the car accelerates over an entire gear, but in this particular instance it sounds like he's more interested in the peak acceleration.

I'm inclined to believe peak acceleration should occur at peak torque, since acceleration results from the application of the axle's torque to the ground as No4NJunk's second equation shows.

Does anyone have the actual figures from the magazine?

 

I do. The mini has max accel at 4050 rpm. It makes peak torque at 4000 rpm and peak HP at 6000. HMMM, follows pattern.


The pontiac has a max accel at 7100 peak torque is at 6800 and hp at 7600.

the beetle turbo max accel at 3000 rpm and peak torque at 1950 and peak hp at 5500.

If peak accel occurs at peak torque wouldn't I ideally want to hold my engine at peak torque for max accel and lowest 1/4 mile time? Then why do the CVT cars rev to max HP under WOT?

 

If you're holding it at peak torque RPM then you're not accelerating, you're maintaing a constant speed. You'd want to shift at points that maximized the area under the torque curve over the entire run.

I'm not sure about CVT, the situation would be complicated because the transmission ratio changes continuously which means that the torque multiplication through the gears changes continuously too.

Oh, and are the numbers for the Beetle right? Peak torque at 1950 sounds a little low to me?

 

CVT cars (or snowmobiles, for that matter) rev to a higher engine speed than the torque peak, using the torque multiplication of the transmission to result in greater torque output than available from the engine itself.

It's like you sacrifice some engine torque by revving beyond the torque peak but more than make up for it with the torque multiplication of the transmission.

A CVT is a good example of horsepower vs. torque. From a standing start the transmission is delivering max torque, min horsepower to the track/wheels - after 20 seconds at full throttle it is delivering minimum torque but maximum horsepower.

 

The Beetle is a turbo. That's why the torque is there.

 

Dustin Mustangs:
You're right - I thought a bit more about what I had written and I realized it wasn't very clear. Let me try to explain what I was thinking:

For a given vehicle speed, peak acceleration (and rear wheel torque) occurs when the gearing ratio allows the engine to turn at it's peak power rpm.

For a given gearing ratio, the peak acceleration and rwtq occurs when the engine is at it's peak torque rpm.

This thread is about the latter scenario, so what I had posted before really doesn't apply.

 

I am not a mechanical engineer however I would think that max acceleration would be shown by a graph of the torque curve as compared to the peak horsepower graph. I only say that from my motorcycle dyno charts. The time period where this occurs is short. The bike continues to accelerate but the point where that acceleration is the fastest should be where the max torque and max hp lines intersect. Whaddaya think?

 

So every vehicle should accelerate the fastest at 5252 RPM?

 

When the engine is doing "as much work as fast as it can" you mean peak power. This cannot lie at "a mid point between tq and HP" This is peak HP.

 

I do not agree with this. What if I rate the HP and torque in KW and n-m? The lines do not cross at the same if the units were in HP and lb ft.

 

What does this mean? You say peak RWTQ occurs when the engine is at its peak power rpm. This is untrue.

 

Suppose you have a 1:1 drive train ratio and at 3000 rpm the engine produces 100 torque. Then you would have 100 torque at the rear wheels. If however you use a 2:1 ratio, the engine would be at 6000 rpm. Even if the engine makes only 75 torque at this rpm, wheel torque would be 150 because of the 2:1 gearing.

Because it takes energy to accelerate, acceleration (and thus torque at the wheels or force to the ground) will be highest when you are adding the most energy to the vehicle. This is when the engine is producing energy the fastest.

 

Oh Yeah!! Well said. I think the previous posting from VLich summed the issue up most concisely, better than I explained in my CVT / snowmobile posting.

Now can someone lock this thread up and get to distributing the hero cookies?? You can send mine by mail thank you, should be here in time for Christmas!!

Ho Ho Ho

 

Where did the torque and HP figures come from? Did they put the cars on a dyno or are those the manufacturers' numbers on the stand?

 

I hope you are joking!

 

I find this funny. What happens if the engine makes only 49 ft pounds at 6000 rpm? You made a sweeping generalization that does not stand. I am still thinking about your answer though. It seems that you may be on to something. Give me a few hours!

Also, you use your terminology poorly. Energy is a term for a KW-hr or a BTU. Although it can be deducted that energy is needed to produce power and accelerate a mass, I would argue that the engine is using energy to produce power.

 

Power is the derivative of energy with respect to time. As such, any time that energy is changing, you can express the rate of change as power - you can't have changing energy without power, and vice-versa.

The internal combustion engine converts the energy stored in chemical bonds into energy in the form of the spinning crankshaft and heat. The rate at which energy is converted into the rotational energy of the crankshaft is what we call horsepower; so in that sense the engine is using energy to produce some power.

 

Power isn't something that is "produced". Power is a measurement of the rate of change of energy.

 

Isn't it the rate of change of work? One of us needs to open our books back up. Maybe it is me!


Power isn't produced? Debating that is futile.

 

Are you guys working for the UN or something..........arguing over the application of a definition and such. Technically, words have no meaning. They are only sounds or letters which convey an idea to the listener from the speaker or writer. But words do not have meanings in and of themselves; it is up to the listener to decide what he or she believes the intent is of the speaker. That is why communication is usually more effective verbally opposed to written. Furthermore, I feel that emoticons will eventually remove the need for the written word.

All languages use the same basic sounds, just compiled in different orders.

 

So who votes for peak HP is peak acceleration?

 

Below is the loop I cannot seem to break. Everyone says max accel is at peak rwtq but maybe we need to compare speed! I did not write the below. I may have been wrong saying gear ratios dont matter.


One of the interesting things about kinetic energy is that it increases with the velocity squared. This means that if a car is going twice as fast, it has four times the energy. You may have noticed that your car accelerates much faster from 0 mph to 20 mph than it does from 40 mph to 60 mph. Let's compare how much kinetic energy is required at each of these speeds. At first glance, you might say that each car is increasing its speed by 20 mph, and so the energy required for each increase must be the same. But this is not the case.

We can calculate the kinetic energy required to go from 0 mph to 20 mph by calculating the KE at 20 mph and then subtracting the KE at 0 mph from that number. In this case, it would be 1/2 m 202 - 1/2 m 02. Because the second part of the equation is 0, the KE = 1/2 m 202, or 200 m. For the car going from 40 mph to 60 mph, the KE = 1/2 m 602 - 1/2 m 402; so KE = 1,800 m - 800 m, or 1000 m. Comparing the two results, we can see that it takes a KE of 1,000m to go from 40 mph to 60 mph, whereas it only takes 200 m to go from 0 mph to 20 mph.

 

Actually when discussing physics, the definitions of the words ARE very important. Calling power the rate of change of work or the rate of change of energy is essentially the same thing.

Power: The rate at which work is done expressed as the amount of work per unit time.

Work: The transfer of energy from one system to another, especially by the application of a force.

If you're getting hung up by power and kinetic energy vs. torque, work the problem from both sides. Imagine a car of a certain mass that has a certain torque at a certain RPM (regardless of whether it's peak or not). Calculate how many HP the car has at that same speed, then try to calculate the acceleration in two different ways, one from torque and one from power.

 

Let's get one thing straight here. Torque is where the motor is making it's peak efficiency, not power! Power is horsepower and that's work over a peried of time. A motor that makes peak torque has nothing to do with accelerating a vehicle. It's about the horsepower. A good example could be deisel trucks. They make peak torque just off idle (compared to gas) yet you don't see them shifting at 2500rpm to pull into traffic! They shift at like 3500 or more if they need to get moving fast (acceleration).
Theory says that max acceleration occurs at peak hp. Think of the equations of work (energy required to move an object from point a to b). HP is how much work can be done.
Power = Force * Velocity . Let's just say you own an Audi with that new invinite gear tranny. Your best economy would be shifting as low as possible. Your best efficiency (distance vs gas consumption) would be keeping the engine at peak torque. Your best acceleration would be to keep the engine at peak horsepower. I'm sure I made a burp somewhere but everything that's explained should make sence.
There is a kit that's called the hydra-rev and it's made for racing, especially drag racing. What it does is keep your lifters on the cam lobe. After peak horsepower and high revs the valve springs don't have enough time to keep the lifters on so valve float occurs and hp goes down the toilet. The hydra-rev has it's own springs that keep this from happening and you'll notice that the horsepower curve will look a lot smoother after peak hp. Almost like a mirror in most cases. This is so that you can shift well enough after peak hp and fall back into a higher hp/rpm. The idea is to not shift with the area under torque curve but hp curve! If peak hp is made at 5500rpm, and if you shifted at say 6000rpm and fell to 4500rpm you could be faster than if you shifted at 5500 and dropped to 4000. It all depends on many variables but that's is a classic example of when and why to not shift at peak hp and especially not at peak torque.
Just remember that torque is how efficient your motor is per revolution while hp is that force vs distance vs time.

 

What theory is that?

 

If horsepower accelerated the car, then you'd be able to accelerate at the same rate in any gear. We know this isn't true because cars pull harder in first than they do in overdrive. (Not because of wind resistance either, because you can easily see it happening on a chassis dyno where there is no wind resistance.) This is because the engine's torque is multiplied by the gears in the transmission and rear end.

 

In the scenario described (2nd gear from 1000rpm to redline) I think peak acceleration of the vehicle should fall at the same rpm as peak engine torque. The mini's numbers and even the pontiacs numbers agree if you assume some inaccuracy in the measurements. The beetle is the one that's really confusing. I haven't seen a dyno graph, but I think it has a very flat torque curve up to about 4k.

I'll bet that if they put the same vehicles on a dyno in second gear, they would find the peak torque at the same place they found peak acceleration.

 

you're not understanding what im saying..... what i mean is... say your peak hp is 500 @ 6000 r's.... and peak tq 560 @ 4000 r's....... say at 6000 ur making ur 500 hp but only 390 lbs of tq......your mid point is going to be something like 400 hp and 420 tq at 5000 r's for example that would be like a max acceleration b/c they are close numbers... tq is still higher... and its b4 your peak hp.

 

Wrong.
Horsepower is a calculation that has torque in it's formula!!! What part of this is so hard to understand???? So yes gears have a lot to do with it but no matter if you're in 1st gear or 6th, the car will accelerate fastest at peak hp. I can't make this any more clear except to send you reading through my physics books and or websites with complimenting descriptions. If that's the case then here, read what is included on this site and it should clear things up with the whole gearing confustion. BTW, that's kind of why I included the Audi trans that doesn't have gears! It was to neglect any kind of confustion with torque multiplication.
http://www.allpar.com/eek/hp-vs-torque.html

 

I think you need to read your physics books more than I do. You should be able to make it much more clear by deriving a function for acceleration based on power.

 

JPrevost, this is from the page you referred me to as well:

 

Why did you quote one part and not read the rest of it? You're starting to **** me off.
If I must make you look stupid then here it goes.
Let's play the quoting game and see if you can follow;
"Unfortunately, *engine* torque does not tell you the full story. What matters is the torque *delivered to the tires*, including the effects of the transmission."
"Shift at the redline, not at the torque peak!"~that one is outlined in red so you wouldn't miss it (obviously you did)
"Shift to maximize engine POWER, not engine torque!
This is *exactly* the same as saying "shift to maximize transmission output torque". But it's a little easier to apply."
"You'll find that a car running at peak power at a given vehicle speed is delivering the maximum possible torque to the tires (although the engine may not be spinning at its torque peak). This derives immediately from first principles in physics."
"A car accelerates hardest with gearing selected to stay as close as possible to the engine *power* peak..."
"You can determine optimal shift points by graphing horsepower vs. velocity or transmission torque vs. RPM. Engine torque alone will not determine shift points."

So now I guess I made a mistake. When I said "theory" I should have said "fact".

 

I did read the rest of it. The focus of the article is on determining the optimal shift points to get the greatest possible average acceleration (lowest E.T.) over a 1/4 mile run. That isn't relevant to GoFasterFirebird's original question which was when does peak acceleration occur across the RPM range in a single gear.

My intention is not to **** anyone off. This is a discussion, not an argument.

 

Sorry for getting hot headed. My apoligies for going off on you like I did. I just hate it when I'm quoted from one of my sources and made to look bad. Again, sorry, my fault for taking it the wrong way. I'm glad you're more mature than me or this would have gotten ugly.
The idea for having a motor with a flat torque is so it can utalize a wide spacing of gearing. The new idea of having no gears could lead to much smaller engines accelerating just as fast as a bigger motor. Personally I find it more fun to bang through some gears (yes I've test driven the Audi but my Mom went with a Volvo ). Nothing like having a M-22 in a track car or a T56 in a daily driver.
This was a very good way to show how torque is misleading. If you get anything out of this I'd hope it's that torque is just a part of the system, utalizing it with tranny gears and engine speed is how you apply the horsepower to the pavement.

 

Newton's Second Law says that the acceleration (<B>a</B>) of a body is directly proportional to the force (<B>F</B>) acting on a body in the direction of motion and inversely proportional to the mass (<B>m</B>) of the object:

<B><OL><LI>a = F_net ÷ m</OL></B>
<B>F_net</B> is the sum of the forces in the direction of travel of the car. These are the forward propelling force exerted by the tires on the road, the rearward force from the rolling resistance and friction, and rearward force from drag through the air. We'll ignore drag and rolling resistance for now.

Torque (<B>&tau;</B>) is a force applied perpendicularly to a line from the front of application to the center of rotation of a body, and is equal to the perpendicular force times the distance from the point of application to the center of rotation (<B>r</B>):

<B><OL START=2><LI>&tau; = F_p × r</OL></B>
We can rearrange equation 2 to solve for the tangential force:

<B><OL START=3><LI>F_p = &tau; ÷ r</OL></B>
In our application, we can use equation 3 to calculate the force which the tires apply to the road given the rear wheel torque and the radius of the tires. It is the force equal and opposite to this that the road exerts on the tires which accelerates the car. Combining equations 1 and 3 gives an equation for the acceleration of the car neglecting drag and rolling resistance as mentioned earlier:

<B><OL START=4><LI>a = &tau; ÷ ( r × m )</OL></B>
Equation 4 shows that acceleration is directly proporational to torque, so that in two situations where the other factors (mass and radius) are the same, the case with the greater torque will have the greater acceleration.

Using this observation, consider the cases of a car operating at its power peak, and the same car operating at its torque peak. As a practical example, lets use the data for a 2002 Z06 (because they're so cool):

<B><UL>
<LI>Peak torque: 400 lb ft @ 4800 RPM
<LI>Peak power: 405 HP @ 6000 RPM
</UL></B>
In order to compare these two cases we need to calculate the torque of the engine at its power peak. For this we use everyone's favorite equation relating torque and horsepower:

<B><OL START=5><LI>Torque = HP ÷ RPM × 5252</OL></B>
Doing the calculation tells us that at peak power the engine is making <B>354 lb ft @ 6000 RPM</B>. This is <i>less</i> than the peak torque, so the car's acceleration is <i>greater</i> at peak torque than it is at peak horsepower. (Notice that there's no need to take gearing into account since it's the same in both situations).

Throughout this (and presumably in GoFasterFirebird's original question), we've been talking about <i>instantaneous</i> acceleration in a single gear. Considering average acceleration over the length of a drag strip is a far more complicated proposition.

 

Is not HP torque over time? If that is the case doesn't max acceleration occur at the point where they are the same(hp+torque).G0D how I wished I had payed attention in physics. Why by the way can't I say the word G0D- hey i just got it by the heathens. just goes to show I don't know crap about this, but it's interesting as heck.

 

Ok, just for fun then:

Power (<B>P</B>) is the rate of work (<B>W</B>), or work per unit time (<B>t</B>):
<B><OL START=1><LI>P = W ÷ t</OL></B>
Work is a force (<B>F</B>) applied over a distance (<B>d</B>):
<B><OL START=2><LI>W = F × d</OL></B>
Substituting equation 2 in to equation 1 gives:
<B><OL START=3><LI>P = F × d ÷ t</OL></B>
In my previous post, we determined that a torque (<B>&tau;</B>) applies a tangential force at a radius (<B>r</B>) of:
<B><OL START=4><LI>F = &tau; ÷ r</OL></B>
Substituting equation 4 into equation 3 now gives:
<B><OL START=5><LI>P = ( &tau; × d ) ÷ ( r × t )</OL></B>
A point at radius (<B>r</B>) from the axis of a body rotating through an angle in radians of (<B>&theta;</B>) will move a distance (<B>d</B>) of:
<B><OL START=6><LI>d = &theta; × r</OL></B>
Substituting equation 6 into equation 5:
<B><OL START=7><LI>P = ( &tau; × &theta; ) ÷ t </OL></B>
A rotating body has an angular velocity in radians per unit time (<B>&omega;</B>) of:
<B><OL START=8><LI>&omega; = &theta; ÷ t</OL></B>
So therefore equation 6 becomes:
<B><OL START=9><LI>P = &tau; × &omega;</OL></B>
Since there are <B>2&pi;</B> radians in a full revolution, we can derive the angular velocity from revolutions per unit time (<B>S</B>):
<B><OL START=10><LI>&omega; = 2&pi; × S</OL></B>
Substituting into equation 9:
<B><OL START=11><LI>P = 2&pi; × &tau; × S</OL></B>
Of course we've been using SI units throughout, but if we convert Watts and Newton-meters into Horsepower and Torque, we'll get:
<B><OL START=12><LI>HP = Torque × RPM ÷ 5252</OL></B>
Which should look a little familiar.

 

ok everyone put their physics book away.
my vote is for fastest acceleration occurs at peak torque.


yes hp= power.

suppose we had two engines, both making 300hp.
engine one is a 454 making 300 hp at 4000
engine two is a japed out 2.0 liter making 300 hp at 8000

which engine would accelerate(0-60 ) a 3400 pound car using a th350
and a 3.00 final drive faster???????

the 454 would obviously because it makes more torque.the 2.0 liter would need real real low gear and a higher stall convertor to match the 454 acceleration.

now lets say the 2.0liter was stuck using the 454 drivetrain ,and it wanted to match the acceleration of the 454.well the only way to do this is increase the hp .in order to increase the hp for the given 2.0 rpm range u need to increase torque.


dave