A question about acceleration.

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Pururut

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I made a acceleration and top speed test with Lamborghini Miura P400 and Ferrari 250 GT Berlinetta. I chosed those cars because they have very similar power and torque bands. I adjusted both cars to be best at n300 class. So Lamborghini became;

342 Bhp max power
36.8 kgfm max torque
882 kg weight

While Ferrari became;

344 Bhp max power
38.2 kgfm max torque
864 kg weight

So according to these stats I expected to Ferrari having better acceleration and top speed since it has more power, torque and less weight. But the thing is Lamborghini manages to reach 336 Km/h while Ferrari manages only 299 Km/h.

How Lamborghini can be blatantly much faster while being literally worse at everything? Am I missing something?
 
Famine
Probably the gearbox settings, by the looks of that.

Also top speed depends on power and drag. Weight is almost irrelevant.
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I adjusted both cars gearbox to their optimal level. That cant be.

Drag is probably the answer but I didnt expected Ferrari to have way worse drag then miura.

Also Im pretty sure weight has impact to car top speeds because car accelerations decrease when i add weight to them.
 
Pururut
I adjusted both cars gearbox to their optimal level. That cant be.
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What is "their optimal level"? Was it the same for both cars? If so, it's not optimal and if not, that's the reason for the difference.
Pururut
Also Im pretty sure weight has impact to car top speeds because car accelerations decrease when i add weight to them.
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Weight has a very, very, very, very tiny effect on top speed.
 
Pururut
How Lamborghini can be blatantly much faster while being literally worse at everything? Am I missing something?
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What you're missing is that physics are complex. Acceleration is determined by power, mass and frictional losses (such as aerodynamic drag), while top speed is determined by power, frictional losses and time.

A car with greater mass (all else being equal) will have worse acceleration than the other car but more or less equal top speed (given a straight of sufficient length).

A car with less drag (all else equal) will initially have the same acceleration as the other car, but better acceleration at higher speeds and ultimately a higher top speed (given that the gearing is tall enough).

A car with greater mass and less drag will initially have worse acceleration than the other car, but as the speed gets higher it will reach a point where the acceleration is equal and from there on it will have better acceleration and ultimately a higher top speed.

Edit: This graph illustrates the problem by drawing acceleration as a function of speed. The vertical axis is the acceleration and the horizontal axis is the speed of the car. In this example both cars have the same power, but one car is heaver (orange) and the other has a greater drag area (blue).

The blue has better acceleration during most of the range, but somewhere around 55 m/s the orange car gets better acceleration. The top speed for the orange car is 85.5m/s and for the blue car it's 74.7 m/s.

The function is:

a(v) = P/(v*m)-k*v^2/m, where v is the speed of the car, m is the mass, k is a constant containing drag area and air density divided by two.

acceleration.png
 
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Famine
Weight has a very, very, very, very tiny effect on top speed.
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I know what you mean but that's technically incorrect:

If weight on the driven wheels goes to zero, grip goes to zero. With no grip you can't accelerate the car and that would have a big effect on top speed.
Pururut
I adjusted both cars gearbox to their optimal level. That cant be.

Drag is probably the answer but I didnt expected Ferrari to have way worse drag then miura.

Also Im pretty sure weight has impact to car top speeds because car accelerations decrease when i add weight to them.
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I don't have either car so I can't check but could you list max rpm and peak power rpm for both cars?

If you're using the max speed (auto set) adjuster it will show theoretical top speed at peak rpm. If one of the cars produce peak power lower in the rpm range* than the other you'll need a higher theoretical max speed setting in order to achieve the same top speed in practice.

*Relatively
 
Vegard
I don't have either car so I can't check but could you list max rpm and peak power rpm for both cars?

If you're using the max speed (auto set) adjuster it will show theoretical top speed at peak rpm. If one of the cars produce peak power lower in the rpm range* than the other you'll need a higher theoretical max speed setting in order to achieve the same top speed in practice.

*Relatively
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Both car have max torque at 5500 rpm and max power at 7000 rpm.

I set ferrari last gear to 365 Km/h and Lamborghini to 400 Km/h.
 
Vegard
I know what you mean but that's technically incorrect:

If weight on the driven wheels goes to zero, grip goes to zero. With no grip you can't accelerate the car and that would have a big effect on top speed.
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That depends on how you define weight. He was probably referring to the mass of the car, in which case it’s not incorrect.
 
eran0004
He was probably referring to the mass of the car, in which case it’s not incorrect.
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Weight =/= mass. Weight has no inertia and mass is not a force.

QED: technically incorrect.

I know what he meant but I think it's an important distinction to make.
 
Vegard
Weight =/= mass. Weight has no inertia and mass is not a force.

QED: technically incorrect.

I know what he meant but I think it's an important distinction to make.
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Mass is typically referred to as weight in everyday language. When you speak about the weight of a car you almost always refer to the mass of the car.

And even if he was talking about the load on the tyres, it’s still not incorrect because the weight would only be zero if the wheel is suspended in the air.
 
eran0004
Mass is typically referred to as weight in everyday language. When you speak about the weight of a car you almost always refer to the mass of the car.
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And that's why I specified that I knew what he meant. It's still technically incorrect*.
eran0004
And even if he was talking about the load on the tyres, it’s still not incorrect because the weight would only be zero if the wheel is suspended in the air.
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Funny you should mention that considering one of the cars in question is the Miura.

But still, the weight on the driven wheels wouldn't need to be zero for the car to stop accelerating before the drag limited top speed.
You need a lot of tractive force just to maintain 250 km/h. With aero lift and a light enough car...

Pururut
Nope.
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Then the reason must be a difference in aerodynamic drag. PD has a rather poor track record when it comes to modeling that part of the physics model accurately.



*Famine himself describes technically correct as the best kind of correct.
 
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It's called weight in GT Sport, and as that's the basis of the topic and the forum we're in...

(I do think it's odd that real car manufacturers refer to weight rather than mass, and never include the words "equivalent force" after kg/lb; even McLaren)
 
What can affect top speed when you're using the exact same engine and gearbox :
- body shape (drag)
- tyre width (friction)
- camber and toe (friction)

Now, those two cars might have similar peak power / torque, but the engines can still be very different :
- max RPM (allows the use of shorter gears to reach same top speed)
- peak power RPM (for top speed, the later it is, the better, if power drops for 1000 RPM then you'll be likely to hit a drag wall there)
- general power curve shape (some cars have 90% of max power in a 500 RPM range, others have that on a 3000 RPM wide band, and that can make a hell out of a difference)

Another very important thing is the impact of the transmission as a whole, disregarding gearbox ratios. Power figures provided by manufacturers are generally taken at the flywheel, but then a significant amount of energy can be lost in the gearbox, driveshafts and differential : all these elements are subject to friction. FF, MR and RR cars, with everything being in the same place, suffer less from that than FR cars which need to have a long driveshaft to connect front and rear transmission elements. 4WD cars are obviously even worse in that department. Technology levels and manufacturing precision can also help a great deal in reducing frictions in the transmission components, so that must be factored in as well.

Miura being MR, 250 GT being FR and older, the former would probably get better uncorrected figures on a dyno run (what we call power at the wheels). In the end, figures are just that, and nothing will replace actual measures and testing.
 
I made the test again. For removing any problems about the gearbox I set both cars to 400 km/h. This time I will test the max speed at Ssrx downhill.

Lamborghini reaches 347 km/h at ssrx downhill and drops to 338 km/h straight away.

Ferrari reaches 305 km/h at ssrx downhill and drops to 295 km/h at straight away.
 
GT_Alex74
Now, those two cars might have similar peak power / torque, but the engines can still be very different :
- max RPM (allows the use of shorter gears to reach same top speed)
- peak power RPM (for top speed, the later it is, the better, if power drops for 1000 RPM then you'll be likely to hit a drag wall there)
- general power curve shape (some cars have 90% of max power in a 500 RPM range, others have that on a 3000 RPM wide band, and that can make a hell out of a difference)
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-Both cars have very similar power and torque curves and exact same max power and torque Rpm values at 7000 and 5500 rpm. Both cars max rpm is 8000 and min rpm is 1000.
 
Vegard
But still, the weight on the driven wheels wouldn't need to be zero for the car to stop accelerating before the drag limited top speed.
You need a lot of tractive force just to maintain 250 km/h. With aero lift and a light enough car...
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Just around 1,5 to 2 kN, which is what a 16 bhp Fiat 500 can produce at 25 km/h.

200 kg is enough weight to produce that kind of grip.
 
eran0004
Just around 1,5 to 2 kN, which is what a 16 bhp Fiat 500 can produce at 25 km/h.

200 kg is enough weight to produce that kind of grip.
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So, to conclude:

If he meant mass he was technically incorrect because he said weight.
If he meant weight he was technically incorrect because weight can have more than a very, very, very, very tiny effect on top speed.

Yes, weight is used instead of mass in everyday language.
No, it's not a good idea to use it instead of mass when discussing classical mechanics.
 
There are two main factors affecting top speed , first is the gearing , even if you max it out on the last gear it doesn't mean it can reach the speed being shown.
Secondly , the weight also plays a small role.
 
Ah yes, the old FR vs MR battle. I can't afford either of these cars, but engine positioning is very influential. A modified Ferrari Dino vs Modified with similar numbers) FR Coupe would produce similar results I think.
 
Pururut
-Both cars have very similar power and torque curves and exact same max power and torque Rpm values at 7000 and 5500 rpm. Both cars max rpm is 8000 and min rpm is 1000.
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Aero efficiency. Also Ferrari didn't build cars for aero efficiency. He built them to look good.
 
Forgetting for a moment that GT is absolutely awful at working out top speeds, it's pretty obvious that one of these cars is more slippery than the other.

20190503155319.jpg
20190503155328.jpg


With all things that GT measures being equal, the difference you've got left is aerodynamics.

More power does not always equal more speed. Why is the fastest car at Baku powered by a Renault? Why are Ferrari and Mercedes not top of the charts?

upload_2019-5-3_16-1-54.png
 
daan
Forgetting for a moment that GT is absolutely awful at working out top speeds, it's pretty obvious that one of these cars is more slippery than the other.
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Surprisingly, I can't dig out drag coefficients for the two cars, but the Miura's frontal area is 17.95 square feet compared to the Ferrari's 20.81 square feet.

Even if they had identical everything else, the Ferrari would need 13% more power to go the same speed - or the Miura can go roughly 5% faster for the same power.

Plumbing all the numbers into my calculator suggests the 250 GT has a drag coefficient of 0.365 and the Miura is 0.293.
 
Even though weight doesn't make much of a difference, remember that the front of the Miura got lighter as it got faster.
 
Could it be that the Miura has a little more lift coefficient? But the power curve of the engine does tell much of the story, as well as the gearing.

Definitely agree that there are lots of environmental factors that contribute to the top speed and acceleration of those two cars. Weight does play a part but mainly for stability reasons, and not really much on top speed and acceleration.
 

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