RE: Jeep Grand Cherokee Trackhawk | Spotted
Discussion
Great motor!
Some incredible moaning "when would you use the power" seriously hand in your badge and head over to Mumsnet.
I also can't believe the comparison to an SQ7... I'm sure it feels fast picking little Olivia and Tarquin up from Oboe practice but it's not in the same league as a Trackhawk.
Stuff like this should be praised for its absurdness.
Some incredible moaning "when would you use the power" seriously hand in your badge and head over to Mumsnet.
I also can't believe the comparison to an SQ7... I'm sure it feels fast picking little Olivia and Tarquin up from Oboe practice but it's not in the same league as a Trackhawk.
Stuff like this should be praised for its absurdness.
Flipfloptrader said:
I own the fastest one in Australia. Ridiculously fun and will bring a smile to your face regardless if stock or modified.
Mine runs circa 1,100 bhp and will clock 0-100mph in 5.6 seconds.
Looks awesome!Mine runs circa 1,100 bhp and will clock 0-100mph in 5.6 seconds.
And thanks for the other owner with his write up - if I could shake your hand, I would.
Almost hit my face on the table when I saw someone put along the lines of "Why not just buy a £20k Cherokee and drop the Hellcat engine in?" Utter pleb. Almost beaten my matey who posts about his SQ7 and its great build quality...
borat52 said:
This is not a correct understanding of the physics.
I'll agree with you on the point that engine torque and wheel torque are very different due to the gearbox which is often a very misunderstood principle.
The easiest way I can think of explaining it is this. If you give me a power output and a starting velocity (assuming fixed mass) then I can tell you the acceleration assuming no drivetrain losses.
If you give me a torque and velocity I cannot tell you the acceleration unless you give me more information, ie engine torque requires you to give me engine rpm (and thus power as power is torque x rpm) and if you give me wheel torque I also need wheel rpm (hence once again power).
Another way of putting it is this 1000lbft of torque at 1rpm engine speed will be out accelerated by 1lbft of torque at 1001rpm all other things equal.
To restate, torque does not accelerate mass, power does. Torque and rpm do accelerate mass, torque and rpm being power.
Think of torque as a static force, its how much of that force that you can apply in a given time which is power which is why torque applied to a fast rotation generates more power than the same torque applied to a slow roation (again all other things equal).
Newton’s Second Law doesn’t lie. Acceleration = Force / Mass. Apply more force (higher torque) and a given mass will increase its acceleration. I'll agree with you on the point that engine torque and wheel torque are very different due to the gearbox which is often a very misunderstood principle.
The easiest way I can think of explaining it is this. If you give me a power output and a starting velocity (assuming fixed mass) then I can tell you the acceleration assuming no drivetrain losses.
If you give me a torque and velocity I cannot tell you the acceleration unless you give me more information, ie engine torque requires you to give me engine rpm (and thus power as power is torque x rpm) and if you give me wheel torque I also need wheel rpm (hence once again power).
Another way of putting it is this 1000lbft of torque at 1rpm engine speed will be out accelerated by 1lbft of torque at 1001rpm all other things equal.
To restate, torque does not accelerate mass, power does. Torque and rpm do accelerate mass, torque and rpm being power.
Think of torque as a static force, its how much of that force that you can apply in a given time which is power which is why torque applied to a fast rotation generates more power than the same torque applied to a slow roation (again all other things equal).
Your examples don’t prove true for an object with mass and therefore inertia to overcome (Torque = Inertia * Acceleration). The higher horsepower engine is certainly capable of doing more work in a given time, but that doesn’t mean it’s useful work resulting in higher acceleration.
I haven’t calculated it with real numbers, but it’s quite possible your 1lbft example is entirely unable to accelerate at all beyond 1001rpm, or it may even decelerate if the drag forces cannot be overcome. Meanwhile, the 1000lbft engine will happily accelerate from 1rpm, easily overcoming inertia and drag. However, this high acceleration will be very short-lived as it’s power limits It’s top speed.
None of this is to say though, as mentioned, that your 1001 rpm, 1lbft engine can’t be exploited with gearing ratios to achieve a torque profile that would increase acceleration for the particular vehicle under the particular requirements (you may want to optimise for 0-60 times and but have a limited top speed of 80mph, or with longer gearing you might take its top speed to 120, sacrificing the 0-60.
Edited by AndyXF on Wednesday 28th August 02:09
eliot said:
AndyXF said:
Torque is exactly what accelerates mass. Remember the equation F = m*a ? Where F is applied at an angle and at some distance from the rotational axis, torque is the result.
Power determines the vehicle’s speed for a specific torque output. P = T*w where w is rotational speed.
So torque determines acceleration while power determines the speed you can accelerate to (at a given torque).
ETA: you can of course set up the transmission gearing to manipulate the wheel torque & power based on the engine’s power & torque characteristics.
Gold star to the try-hard. But it’s still a diesel - and whilst there blood in my veins and petrol in the pumps - a supercharged v8 hemi is always going to more exciting to me.Power determines the vehicle’s speed for a specific torque output. P = T*w where w is rotational speed.
So torque determines acceleration while power determines the speed you can accelerate to (at a given torque).
ETA: you can of course set up the transmission gearing to manipulate the wheel torque & power based on the engine’s power & torque characteristics.
Edited by AndyXF on Monday 26th August 00:50
I know this is PH, etc etc, but does anyone else find the line 'it's taken a couple of minutes off the school run' a wee bit distasteful?
I'm picturing his poor kids vomiting in the back while he shaves another couple of tenths off his PB, wiping out the crossing guard in the process...
I'm picturing his poor kids vomiting in the back while he shaves another couple of tenths off his PB, wiping out the crossing guard in the process...
smaybury said:
I know this is PH, etc etc, but does anyone else find the line 'it's taken a couple of minutes off the school run' a wee bit distasteful?
I'm picturing his poor kids vomiting in the back while he shaves another couple of tenths off his PB, wiping out the crossing guard in the process...
I'm picturing his poor kids vomiting in the back while he shaves another couple of tenths off his PB, wiping out the crossing guard in the process...
AndyXF said:
Newton’s Second Law doesn’t lie. Acceleration = Force / Mass. Apply more force (higher torque) and a given mass will increase its acceleration.
Your examples don’t prove true for an object with mass and therefore inertia to overcome (Torque = Inertia * Acceleration). The higher horsepower engine is certainly capable of doing more work in a given time, but that doesn’t mean it’s useful work resulting in higher acceleration.
I haven’t calculated it with real numbers, but it’s quite possible your 1lbft example is entirely unable to accelerate at all beyond 1001rpm, or it may even decelerate if the drag forces cannot be overcome. Meanwhile, the 1000lbft engine will happily accelerate from 1rpm, easily overcoming inertia and drag. However, this high acceleration will be very short-lived as it’s power limits It’s top speed.
None of this is to say though, as mentioned, that your 1001 rpm, 1lbft engine can’t be exploited with gearing ratios to achieve a torque profile that would increase acceleration for the particular vehicle under the particular requirements (you may want to optimise for 0-60 times and but have a limited top speed of 80mph, or with longer gearing you might take its top speed to 120, sacrificing the 0-60.
What I said is absolutely valid for mass, it wouldn't be valid for zero mass in so far as zero mass just means the equations all end up at zero.Your examples don’t prove true for an object with mass and therefore inertia to overcome (Torque = Inertia * Acceleration). The higher horsepower engine is certainly capable of doing more work in a given time, but that doesn’t mean it’s useful work resulting in higher acceleration.
I haven’t calculated it with real numbers, but it’s quite possible your 1lbft example is entirely unable to accelerate at all beyond 1001rpm, or it may even decelerate if the drag forces cannot be overcome. Meanwhile, the 1000lbft engine will happily accelerate from 1rpm, easily overcoming inertia and drag. However, this high acceleration will be very short-lived as it’s power limits It’s top speed.
None of this is to say though, as mentioned, that your 1001 rpm, 1lbft engine can’t be exploited with gearing ratios to achieve a torque profile that would increase acceleration for the particular vehicle under the particular requirements (you may want to optimise for 0-60 times and but have a limited top speed of 80mph, or with longer gearing you might take its top speed to 120, sacrificing the 0-60.
Edited by AndyXF on Wednesday 28th August 02:09
I'll try to illustrate by way of equations as I think from your explanation that what you understand to be force is in fact power.
F=ma, newtons law well established and I certainly won't be disputing that.
Work done = F applied x distance moved.
So at (I'm going to use round numbers but you can substute my numbers for algebra) 10mph if we we apply a force F then we actually only do half the work in 1 second compared to applying that force at 20mph (assuming the force isn't enough to accelerate the object rapidly, you could run the work done on that but you'd need the area under the graph, lets just keep it simple for now)
As power = work done / time and for any given car engine with fixed peak power output we can only do a certain amount of work per unit time as the engine does not get more powerful.
Now consider work done = F x distance moved again.
If we can only do a fixed amount of work per second (limited by the power output of the engine) and at constant F above the work done per unit time at 20mph is double that of 10mph the equations dictate that for a given power output engine, the force it can apply to the car reduces as its speed increases.
We can go a step further and bring the torque back into this now.
Given we have a fixed power output engine (lets assume it just makes peak power) and given power = torque x rpm (x constant), then we can also see that as the speed of rotation of the drive tire increases then the torque decreases as torque = power/rpm.
To restate, torque does not accelerate mass, power does. Force is not power so the F=ma equation is perfectly true but F cannot be determined unless we know both the power of the engine and the speed of the vehicle.
I'm assuming zero drivetrain loss and air resistance as it complicates matters greatly to introduce them but it is again power not torque that overcomes their resistance.
Edited by borat52 on Wednesday 28th August 16:37
+ 
borat52 said:
What I said is absolutely valid for mass, it wouldn't be valid for zero mass in so far as zero mass just means the equations all end up at zero.
I'll try to illustrate by way of equations as I think from your explanation that what you understand to be force is in fact power.
F=ma, newtons law well established and I certainly won't be disputing that.
Work done = F applied x distance moved.
So at (I'm going to use round numbers but you can substute my numbers for algebra) 10mph if we we apply a force F then we actually only do half the work in 1 second compared to applying that force at 20mph (assuming the force isn't enough to accelerate the object rapidly, you could run the work done on that but you'd need the area under the graph, lets just keep it simple for now)
As power = work done / time and for any given car engine with fixed peak power output we can only do a certain amount of work per unit time as the engine does not get more powerful.
Now consider work done = F x distance moved again.
If we can only do a fixed amount of work per second (limited by the power output of the engine) and at constant F above the work done per unit time at 20mph is double that of 10mph the equations dictate that for a given power output engine, the force it can apply to the car reduces as its speed increases.
We can go a step further and bring the torque back into this now.
Given we have a fixed power output engine (lets assume it just makes peak power) and given power = torque x rpm (x constant), then we can also see that as the speed of rotation of the drive tire increases then the torque decreases as torque = power/rpm.
To restate, torque does not accelerate mass, power does. Force is not power so the F=ma equation is perfectly true but F cannot be determined unless we know both the power of the engine and the speed of the vehicle.
I'm assuming zero drivetrain loss and air resistance as it complicates matters greatly to introduce them but it is again power not torque that overcomes their resistance.
Perhaps I wasn’t clear enough but I absolutely understand that torque cannot exist without power and speed. Not one of these three components(*) can exist without the other two, that is clearly defined by the equation we’ve both stated. (*)Power in this case refers to what I will call useful work done over time. Of course power can be lost as heat, which I will revisit in a second. I'll try to illustrate by way of equations as I think from your explanation that what you understand to be force is in fact power.
F=ma, newtons law well established and I certainly won't be disputing that.
Work done = F applied x distance moved.
So at (I'm going to use round numbers but you can substute my numbers for algebra) 10mph if we we apply a force F then we actually only do half the work in 1 second compared to applying that force at 20mph (assuming the force isn't enough to accelerate the object rapidly, you could run the work done on that but you'd need the area under the graph, lets just keep it simple for now)
As power = work done / time and for any given car engine with fixed peak power output we can only do a certain amount of work per unit time as the engine does not get more powerful.
Now consider work done = F x distance moved again.
If we can only do a fixed amount of work per second (limited by the power output of the engine) and at constant F above the work done per unit time at 20mph is double that of 10mph the equations dictate that for a given power output engine, the force it can apply to the car reduces as its speed increases.
We can go a step further and bring the torque back into this now.
Given we have a fixed power output engine (lets assume it just makes peak power) and given power = torque x rpm (x constant), then we can also see that as the speed of rotation of the drive tire increases then the torque decreases as torque = power/rpm.
To restate, torque does not accelerate mass, power does. Force is not power so the F=ma equation is perfectly true but F cannot be determined unless we know both the power of the engine and the speed of the vehicle.
I'm assuming zero drivetrain loss and air resistance as it complicates matters greatly to introduce them but it is again power not torque that overcomes their resistance.
Edited by borat52 on Wednesday 28th August 16:37
What I will reiterate though, is that torque (force) absolutely does accelerate mass. I know I’m a broken record, but it was written by Newton and hasn’t changed since. You acknowledge but seemingly disregard it.
“To restate, torque does not accelerate mass, power does”. No, Power / Speed does. ie Torque. Simply applying power (doing work) to a mass will not accelerate it. If the force you apply does not overcome the resistive forces (e.g friction and inertia), your power will be lost as heating.
I will admit my explanation of the two engine examples wasn’t entirely accurate, and it’s difficult to come up with a proper analogy, but I’ll try one final time to make my point:
There is a minimum amount of torque required to rotate the wheels of a vehicle of some mass, let’s say 1 tonne.
Now imagine your 1,001 rpm and 1lbft engine in that vehicle with a 1:1 transmission and a clutch. No matter how gently you engage the clutch, you will never produce significant enough torque to move the wheels. You will, however make a toasty clutch plate.
Now try that again with the 1000lbft, 1rpm engine and it will start moving with ease - albeit incredibly slowly, of course, but the point is that is was able to produce acceleration, despite the engine being capable of producing less power. The same principle applies if the vehicle was already moving (clearly in this example it’s mass would have to be much lower for this to even happen).
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