Curiosity!

[smasht]

Just driving home from St Kilda just now and had a Ute (if i wasn't driving mine i would have sworn black and blue it was mine) fly past me like i was standing still and it got me thinking (other than if this dude will still have a license let alone be wrapped around a tree up infront).........

Does the kw / rwkw rating directly correlate to max speed (i.e more rwkw = faster top speed) or is it mainly about the speed in which the cars will get to ITS to speed?

Sorry if this has been covered :S

Cheers

Mr Curious just b4 bed time

[gcg2503]

Tautology - Guilty as charged

The act of repeating the word speed 4 times in the one sentence

;)

[Windsor220]

It relates to acceleration to the top speed. It might make a bit of a difference to top speed but thats more controlled by gear ratios, aerodynamics etc

[mikestp]

It depends on the gearing, If it reaches it top speed and is bouncing of the limiter then more power would only make it get there faster.
however if the top speed is caused by lack of torque then more power should give it a higher top speed

[smasht]

Originally Posted by gcg2503

Tautology - Guilty as charged

The act of repeating the word speed 4 times in the one sentence

;)

:( my bad, I'm sorry I'm not the best at English - fitness and sport are my fields!

[woodsy109]

No there is a lot of factors involved in determining a cars top speed. The two largest factors are aerodynamic resistance and power output. As a general rule the faster you go, the faster the air around the car is moving and thus creating resistance against going faster. So the faster you go, the more power you need to overcome this resistance.

edit: yes gearing does come into this equation however the above statement is assuming optimum gearing setup.

[smasht]

Torxteerm, mikestp, woodsy109

cheers for the answer guys.

time for bed now for me

thanks again

[madmelon]

The power absorbed by pressure drag increases with the cube of increase in speed.

eg, increase speed by 10% (=1.1) then increase drag power by 33% (1.1 x 1.1 x 1.1)

Can get detailed formulas for you if you want.

Skin friction drag is added to this but will not be nearly as significant on a car.

[Zed7fiddy]

Mate RWKW has nothing to do with it, my bike has aboout 90rwkw, and it will out perform your stock xr8 anyday, power to weight and acceleration is where its at ;)

[yift]

theoretically a dodge viper will do 400+km/h in top gear, if you had a stretch of road long enough and the conditions were optimum for it.

[madmelon]

Originally Posted by yift

theoretically a dodge viper will do 400+km/h in top gear, if you had a stretch of road long enough and the conditions were optimum for it.

...yeh if those conditions were a 100km/h tailwind.

the formula for drag force is F(d) = 0.5 x fluid density x cross sectional area x drag coefficient x velocity x velocity

F(d) = 0.5pA(Cd)V^2

for drag power, multiply by velocity again...

P(d) = 0.5pA(Cd)V^3

velocity in m/s, density in kg/m^3, Area in m^2, force is in N and power is in W

so taking the XR8 with a Cd of approximately 0.3, frontal area of about 2.5m^2, air denisty of 1.224kg/m^2 and rear wheel power of 190,000W, you can solve for a theoretical top speed of: ~74.5m/s or ~268km/h

To actually reach that speed, the drag power curve must intersect the wheel power curve at exactly that point- it often doesn't and so the maximum speed is reduced. That also neglects skin friction drag which reduces top speed further.

The falcon actually has quite a good (low) drag coefficient compared to many high end cars like Astons, Lambos and Ferraris. They're usually up around 0.36, due either to their coolong requirements or appearance requirements.

The easiest thing to change to increase top speed (apart from maybe a speed limiter...) is power output of the vehicle. This will also improve acceleration.

[mrbaxr6t]

Originally Posted by madmelon

...yeh if those conditions were a 100km/h tailwind.

the formula for drag force is F(d) = 0.5 x fluid density x cross sectional area x drag coefficient x velocity x velocity

F(d) = 0.5pA(Cd)V^2

for drag power, multiply by velocity again...

P(d) = 0.5pA(Cd)V^3

velocity in m/s, density in kg/m^3, Area in m^2, force is in N and power is in W

so taking the XR8 with a Cd of approximately 0.3, frontal area of about 2.5m^2, air denisty of 1.224kg/m^2 and rear wheel power of 190,000W, you can solve for a theoretical top speed of: ~74.5m/s or ~268km/h

To actually reach that speed, the drag power curve must intersect the wheel power curve at exactly that point- it often doesn't and so the maximum speed is reduced. That also neglects skin friction drag which reduces top speed further.

The falcon actually has quite a good (low) drag coefficient compared to many high end cars like Astons, Lambos and Ferraris. They're usually up around 0.36, due either to their coolong requirements or appearance requirements.

The easiest thing to change to increase top speed (apart from maybe a speed limiter...) is power output of the vehicle. This will also improve acceleration.

WOW what a detailed response - if we distill this are you saying the aero package on a Falcon offers less drag than that of a Ferrari? and if so can you crunch the maths to prove this? Assuming this holds then in theory it would take less power in a falcon body to achieve say 300kmh than in a Ferrari?

Have I got this right it feels wrong to me. Ferrari have been building fast cars since, well forever they would have to know what you have written and change their cars to exploit this theory.

[madmelon]

Originally Posted by mrbaxr6t

WOW what a detailed response - if we distill this are you saying the aero package on a Falcon offers less drag than that of a Ferrari? and if so can you crunch the maths to prove this? Assuming this holds then in theory it would take less power in a falcon body to achieve say 300kmh than in a Ferrari?

Have I got this right it feels wrong to me. Ferrari have been building fast cars since, well forever they would have to know what you have written and change their cars to exploit this theory.

No this is actually correct- I've emailed Aston Martin about why theirs is so high (0.36-0.38) and they said it's about the car LOOKING NICE. The Fezzas and Lambos are a bit lower than that, more like 0.33-0.34. The Falcon (in base model with Cd of 0.29) offers a more aerodynamic SHAPE than most of those sports cars. The thing they offer is a lower frontal area- they may be wide but they're generally very low.

Additionally, their aerodynamics produce some semblance of downforce at speed. I'd rather do 300km'h in an Aston than a base model Falcon.