[MNBoxster]

Hi,

You'd need a wind tunnel to get absolute data. But, it's mostly about Drag, not weight or Cg (Center of Gravity).

An open car has lots more drag associated with it, especially at speed. This is because Drag, simply expressed, is due the total frontal area and the Cd (coefficient of Drag). Frontal area remains pretty much the same for the car open or closed, but the Cd differs dramatically. Think of the frontal area as the area which the air must flow around. Bigger area, the more air which must be pushed out of the way and it takes power from the engine to do this pushing.

A Closed car maintains much less turbulent airflow, while an Open Car creates a lot of drag inducing turbulents which increase proportionally to speed. The Windshield, Seatbacks, etc. create lots of vortices as air tries to flow around the open cockpit.

One way to look at this is that these turbulents (which move along with the car) essentially create, or rather mimic, a larger Frontal Area (if you looked at an open car in a wind tunnel or smoke tunnel head-on, you'd see these votices surrounding the sheet metal and any new air which the vehicle encountered must now flow around these vortices in addition to the car's sheetmetal), the car must now basically punch a bigger hole into the oncoming air in order to pass through it (not totally accurate, but it requires the same power as if it actually did). And, punching this bigger hole takes more energy or force and pretty soon, the engine cannot provide any more - it's limit is reached.

Now, a Car will continue to accelerate so long as the motive force (power produced by the engine) remains higher than the resistive forces acting against the car (aerodymamic drag and rolling resistance).

But, this resistive force does not remain constant, it is speed dependent. While drag is directly proportional to both frontal area and Cd, it is proportional to the square of velocity. In other words, if you double a vehicle's speed, you will quadruple its drag. If you formulate the problem in terms of the power required (Preqd = F*V) to overcome drag, you'll find that it varies with the cube of velocity since power is a rate (the rate of doing work).

The Drag on an open car builds (because of the ever increasing Cd) to the point where it cancels out the motive force and the car essentially hits a wall and cannot go any faster. This will occur with a closed car as well, but because the Cd remains more constant, or more constant for a higher speed, the point of equilibrium is higher than for an open car and so the resistive force does not cancel the motive force out until a much higher speed is reached. That's the theory anyway...

Happy Motoring!... Jim'99