Weight or aerodynamic?
For us at the salt we have to think different as a road course car designer.
If you create downforce by aerodynamic shape - it helps by low speed - so as formula 1 - but creates a big problem by high speed - it slows you down.
If you create downforce by weight - you get (if correct positioned) a stabilizing effect, a higher final speed - but in the other way a lower acceleration.
Sum and Dave comments shows the direction a speed racer has to go.
The complete answer is:
The shape of the car need a very low cd and small cross section with a base downforce which increase only a little bit going up to high speed - Costella & Yaccouci Theorem II as a example.
Also it need enough length to smooth the airflow and a clean rear end that the shape can go properly out of the air - the #77 is excatly the opposite example
Weight on the right place to produce the necessary downforce for high speed - the way as the Golden Rod from the Summers brothers done or Tom Burkland.
The result had to be a combination of downforce - at low speed, during the acceleration, the aerodynamic of the car produce the downforce - going faster the producing of the downforce will switch over to the weight produced downforce.
To get a low increase of aerodynamic downforce going up to higher speed it is necessary that the shape is extremely clean.
Any kind of corners, bumps, airboxes, wings....which disturb the airflow, will increase the downforce going faster - but in a bad way - the airflow will be disturb by turbulences - and turbulences means less aerodynamic stabilize and less speed - and at last the airflow can't rip off clean enough from the rear end of the car.
Depends now on what kind of class you run, the solution will be different....
A roadster will be always a brick in the air
, a doorslammer can be a much smoother (aerodynamicly) racer
, a streamliner can BE designed in the perfect shape...
So, if there is the question, weight or aerodynamic downforce - at first see what type of racer you design - depends on the class you can start to think about how you can reach the goal........
Any questions, you can ask me here at the website or directly, so as some landracing.com user done it in the earlier time.
Both ways you are welcome to do it