Power to weight ratio question

[dark_ruffo]

Not sure if this goes here, it's more related to a question in general than a GT5 specific question, but I just woke up with the urge to know while I'm drinking my morning coffee.

There is something I haven't fully understood about the power to weight ratio, and that's the top speed.

Say there are two cars, one with massive 500hp engine to move a weight of 1300kg (2.6kg per hp), we could say a highly tuned NSX-R, and there is an Elise on the other side, with its feather light 750Kg, with a modded engine producing 310hp (same ratio).

Quite often while playing the game I see higher powered car reach a higher top speed with a lower acceleration, while the lighter lower powered car accelerates like a mad machine then, at some point, it just suddenly dies, even if the both cars have the same Power to weight ratio.

What's the reasoning behind this? Is the higher engine power used to break the air at some point rather than just move the car mass forward, or am I missing something?

Might be a dumb question I know, but hey

 

[Zondaholic]

At a point, air resistance becomes much, much harder to overcome than weight. This is why the veyron can go 254 miles per hour when it weighs comparatively more than other supercars. It has a slippery body and extremely high horsepower, so as air resistance increases exponentially the faster it goes the veyron can continue to increase in speed and reach such high numbers.

Word.

Essentially, air resistance increases exponentially while weight doesn't increase or decrease. Drag does more at top end and weight at the bottom end to halt acceleration.

 

[dark_ruffo]

A 500.5hp () car just can't go that fast, even if it weights half as much as a Veyron. That example puts things into a better perspective than the Elise one, as there are very few cars that weight that little.

Anyway I would so pick the lighter car anyday

Thanks.

 

[mani_23]

I Think (Now thought) of it like this,
The lighter car has the same ratio, The lighter car may have less horsepower but since its pushing something lighter, its able to go fast, but eventually that engine is still the same, it still has 300hp, and eventually, that will stop working as fast and reach its limit. Resulting in lower top speed
The 500hp car has alot more power but since its heavier it may be a little bit weaker off the line or accelerating but regardless of the weight, the engine itself is still a lot more powerful, and its able to reach higher output and speeds. So it will get higher top speeds.

 

[dark_ruffo]

That was my reasoning too, but, I didn't know if it was correct, or not. But as Zonda explained, the air is what plays a mayor role in this question.

 

[Jmac279]

It's pretty straight forward. Weight has nothing to do with top speed ...

Acceleration = Force/Mass

Since we don't measure engines in terms of force, we need to convert to make it easier to understand, preferably in terms of power:

Power = Work/Time
Work = Force*Distance
Power = Force*Distance/Time
Speed (or Velocity) = Distance/Time
Power = Force*Speed
Force = Power/Speed

Acceleration = Power/(Speed*Mass)

You can also calculate Force using Torque, Gear Ratio, Final Gear Ratio, and Tire Diameter, but this is simpler assuming you don't run out of gear.

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Meanwhile, Top Speed (or Velocity) is a function of Power and Drag (again, assuming you don't run out of gear) ...

Power required to overcome aerodynamic drag = 0.5*Air_Density*Frontal_Area*Drag_Coefficient*Velocity^3

Assuming constant air density, frontal area, and drag coefficient, power and velocity are directly related. So if you want to double your maximum velocity, you would need to increase power by 8 (2 cubed).

So a car that has a maximum speed of 160 km/h with 100 horsepower and you want to push that car to 320 km/h, how much power do you need ?

The answer:

Power = 100*(320/160)^3
Power = 800 horsepower

So, basically:

Smaller Frontal Area (less width, less height) = Higher Top Speed
Lower Drag Coefficient (more streamlined design) = Higher Top Speed
More Power = Higher Top Speed

 

[dark_ruffo]

That settles it down.

 

[CivicInLake]

I'm pretty sure the point where its EXTREMELY hard to over come air resistance and aerodynamics come into huge play is around 230 MP/H or 350-360 KM/H.

Kinda why the Toyota 7 with its insane power to weight cant reach over 360 without slipstream, but easily accelerates to 400+ with.

 

[Waltermydoinher]

There's a one word answer to your question:

AERODYNAMICS. Ask anyone who's into the salt-flats and they'll tell you how much extra horsepower it takes to to "X" amount of miles-per-hour faster. It's exponential once you get up there into the triple digits.

 

[Jmac279]

I'm pretty sure the point where its EXTREMELY hard to over come air resistance and aerodynamics come into huge play is around 230 MP/H or 350-360 KM/H.

Kinda why the Toyota 7 with its insane power to weight cant reach over 360 without slipstream, but easily accelerates to 400+ with.

To increase the top speed from 200 MPH to 220 MPH, it takes a 33.1% increase in horsepower. e.g. 500 hp to 665 hp.

To increase top speed from 200 MPH to 240 MPH, it takes a 72.8% increase in horsepower. e.g. 500 hp to 865 hp.

To increase top speed from 200 MPH to 250 MPH, it takes a 95.3% increase in horsepower. e.g. 500 hp to 975 hp.

To increase top speed from 200 MPH to 260 MPH, it takes a 119.7% increase in horsepower. e.g. 500 hp to 1100 hp.

 

[Famine]

It's pretty straight forward. Weight has nothing to do with top speed...

Weight has almost nothing to do with top speed. There is a small weight component which, speed-dependent-downforce notwithstanding, doesn't change yet becomes less significant as the speed increases.

But if we already know how much power is required to do a given speed for a given car, we can work out how much more (or less) power is required to do a different speed - as you say - with a straightforward cube of the percentage speed change. Assuming you aren't carrying any passengers next time, or had a really big poo.

 

[chippy1000]

Just wondering if gear ratios could be a simple answer to the original question, or do bot the NSX and the lotus have the same gearing hypothetically?

 

[dark_ruffo]

They were two hypothetical cars to begin with