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**[science lesson alert!!!]**

SKIP IT IF IT BORES YOU!!!

SKIP IT IF IT BORES YOU!!!

(re-post from skyroadster and another forum that came from a discussion of how to tell if top speed is possible)

As for top speed, power and drag dominate the capability over about 115 MPH (maybe slower). Roadsters, especially with short tails, are notoriously bad for drag coefficient, generally ABOVE 0.40. The Solstice is around 1.3 meters tall, and 1.85 or so meters wide - that's a huge frontal area.

Here's an equation to calculate the power required for a particular speed in MPH.

Horsepower=((frontalareasqfeet*DragCoefficient*.00 256*(MPH^3))/375)+15

.00256 is the english unit average air density. Frontal area can be simply estimated by [(Overall Height X Overall Width)*0.80] - the 80% takes into account the rounded corners. Drag coefficient is generally 0.4-0.5 for short tailed things with large negative pressure zones (like a roadster), 0.33-0.38 for something like a sedan or a cavalier, and a bit lower for a well-managed aerodynamic car.

The +15 is a ballpark estimate for chassis parasitic loads (like tire rolling resistance).

Then, don't forget to divide your answer by 0.85 - to account for the power loss through the transmission and drive train, and get your answer in engine bhp.

1.3*1.8*0.80 translated to square feet is about 20.2. Using a Cd of 0.40, it works out to:

134 MPH is about max speed for 177 Hp, for a car of this size and drag. Keep in mind the Cd is probably conservative - the solstice has an even shorter tail than most, and it's drag COULD be higher.

To go 142, you'd need 210 Hp. That's assuming you can find a gear at exactly the correct speed to get the max power.

That's how to figure out if the information is accurate. And 142 for 177 hp for a car like the Solstice is about about 30 horsepower (maybe more) shy.

Likewise, power and torque can be used to estimate 0-60 times very simply, and you can even take a stab at the 1/4 mile time and speed. You can get easily within a half second, and probably better if you know the real weight, drag, etc.

To estimate 0-60 times, take the curb weight in lbs, divided by peak horsepower, then multiply by 45%.

SKY = 2860 lbs, 177 hp (assuming same engine as Solstice). 21860/177 = 16.16 curb lbs/hp. Multiplied by 0.45 = 7.27. Solstice advertises 7.2 seconds 0-60 - so it's got to be close enough.

Equation breaks down for things more powerful than about 11 curb lbs/hp.

[/science lesson alert]

[further discussion alert]

By size I meant frontal area, not weight.

Weight only really affects top speed as a function of the parasitic loss (rolling resistance, bearing resistance...) but those can be thought of as pretty much a constant (about 15 HP at wheels).

As far as the weight and inertial effects - these are tiny when compared to the aero-drag. That's because f=ma, or a=f/m, purely a linear part of the function. A change from 2600 to 2800 lbs means a tiny change in the inertial resistance, and it's the same to go from 30mph to 40mph in 5 seconds as it is to go from 130-140 MPH in 5 seconds.

Mathematically, weight really doesn't have any appreciable effect in how fast you get to top speed (if you're close to 'terminal' top speed) because 90% of the power near terminal velocity is absorbed by aerodynamic drag.

At ungoverned top speed or 'terminal velocity', it's literally how much force is pusing the car forward (speed of the engine, gearing, wheel, velocity of the car, and ultimately power) EXACTLY BALANCED with paratisitc drag (10%, semi-constant) plus the aerodynamic drag (90%).

Since you are at terminal velocity, you speed is not changing, therefore zero acceleration, so any inertial forces are also zero.

Additionally, as you approach terminal velocity, the acceleration of the car is very low - to get up to that last 10 MPH, you're creeping along at a very low acceleration - sometimes it takes a good 10-15 seconds to gain that extra velocity. This acceleration is so low, that the power required to overcome inertia (the mass of the vehicle) during this acceleration is also very low (just a couple, if that, horsepower). It's only the amount of horsepower required to propel a car 0-60 in 600-700 seconds (not counting aero drag or parasitic losses) - a miniscule amount of power. Even if you double the vehicle weight, it's still 'miniscule' multiplied by two.

[/further indepth discussion]