What follows is an over simplified way to figure a car's Center of Pressure (CP). Still I think it is better than doing nothing, especially if you don't have the money to hire a professional areodynamicts. A car that has it's CP behind the Center of Gravity (CG) is going to be less likely to spin around if the tires break loose. The further the CG is ahead of the CP the better in that respect, but we also need traction. In the case of Hooley's car we have gone from about a 3300# car to a 6400# car by adding weight. All of it ahead of the rear axle and to help move the CG further forward some midway in the wheelbase and some in the nose of the car. So we added weight for traction but also weight to try and keep the CP behind the CG. Before we could run the new vertical stabilizers they must of been very close or maybe the CG was behind the CP by a little. The car never has done a 360 spin.
In figuring the CP remember that if areas viewed from the side are flatter than areas with a more rounded shape they will have a higher weight than their area would indicate. Below an example of this is the large mostly flat verticals that support the wing. They will add to moving the CP rearward more so than. say the round front fenders or carb induction will in moving it forward even if the side view area of both were the same.
I started with a picture taken on the salt that was almost a full on side view but not quite so remove what was showing of the other vertical and drew a lower cut line along the rear deck and top of the car and some of the front also to get a better 'true' side view. If you take a picture get back away from the car to avoid camera wide angle distortion. Way back and zooming in is best. Also get the camera lower to the ground so you only see the side view of the car.
To get a larger model to work with (large is more accurate) I cut the picture up into 3 overlapping views and printed them at the same scale and then cut them out and made a wider mosaic from them (about 20 inches wide).
Next I cut the car out. Notice that it is a better profile after cutting on the line I talked about above.
Next the outline was laid out on a piece of flat cardboard....
... and with a cut line shears were used...
... to cut the car out.
Find something that is rather thin to balance the cardboard on.
With the car balanced we know that the area on both sides of the balancing point are the same area wise and can mark the balance point on the cardboard.
Here I laid the photo on the cardboard and things stayed pretty much the same despite some overlap in the 3 sections that were taped together. I trust just the cardboard alone better but this is more graphic as to where the CP is on the car. Again this gives equal weight to all areas of the car as seen in a side view but that is not totally accurate (re-read the fist paragraph above if you don't remember why).
This exercise puts the CP about 13.2 inches behind the CG which is good but I think it is probably a little further back due to the large influence the vertical stabilizers have on the car's CP.
Hooley weighed the car at a scale and it weighed in as above. Those numbers were used to figure the 'hypothetical' Center of Gravity horizontally (don't know where it is vertically. No one was in the car but a driver probably would of made the left weight a little more than the right as he and I are about 210 and 170.
He is also going to check this with two jacks under the frame on both sides. Moving them until the car balances on them.
NOTE: After I first wrote the above Tom Burkland contacted me and pointed out that I could use the surface area of the vertical stablizers twice while figuring out the CP as both of them saw the air while only one side of the car sees the air. With that in mind the CP on the Studebaker that is shown above is even further back. At some point I'll cut out another outline of a vertical stablizer and add it to the whole car one and rebalance and show where the projected CP has moved rearward to.
If you undertake trying to figure out the relation of your car's CP/CG choose a method or person you believe in. The above is what I feel ok with, but you have to make this decision yourself,