Equivalent horsepower increase: 20.Conclusion: There's a great deal of improvement to be made with the correct tonneau cover and screen combination. However, the SCTA rule book states, "The driver shall not be restricted from entrance to or exit from the car by the cockpit covering." Besides, for safety's sake, the driver needs to be able to get out quickly. Nevertheless, a new tonneau cover is in order.
Test 7: Rear FendersThe SCTA rule book dictates that the stock fenders must be in the stock location, but they can be bobbed as they are on this car. Peter wondered if extending them would help the airflow. Using the trusty foamcore, the fenders were extended down to the bottom of the framerail but it caused a 2hp increase in drag. We also tried some "fake" louvers in the trailing edge of the fenders to see if they would help, but they made negligible difference.
Equivalent horsepower increase: -2.Conclusion: Leave the fenders as they are.
Test 8: Taping Up The GapsThe SCTA does not allow the exterior taping-over of open seams, like those around the decklid. Nevertheless, we thought it would be interesting to see what effect taping the seams would have. When we taped around the decklid seams, we saw a good 5hp reduction in drag horsepower, front lift was reduced by 1 pound, and rear lift was increased by 2 pounds. Taping up the hood seams gave a far different result showing an increase of 2 drag horsepower, an 8-pound increase in front lift and an 8-pound increase in rear lift.
Equivalent horsepower increase: 5 (-2 for taping the front seams).Conclusion: Air needs to get out of the engine compartment to reduce both drag and lift so louvers would be the way to go.
Test 9: Attitude:Most hot rods like to run a little forward rake because they look cooler that way. Again, this was something that had not been considered on the roadster. Peter, however, immediately picked up that more rake means more frontal area and therefore more drag, so it was time for an attitude change. We began by dropping the front 111/42 inches from our baseline and that resulted in a staggering 10hp increase in drag and an increase in front lift of 42 pounds. The more rake we put into the car, the more we increased the drag. Consequently, we put the front back to baseline and lowered the rear of the car 11/42 inch. Eureka! A reduction in drag of 10 hp and a slight reduction in lift in both front (4 pounds) and rear (2 pounds).
Equivalent horsepower increase: 10.Conclusion: Run the car as level as possible.
Test 10: Parachute OutThe wind-tunnel engineers at MIRA had never tested a car with a parachute, so they asked us to "drop the laundry" to see what happens. What happened was not altogether unexpected in that the drag horsepower was increased to 1,062. However, the front lift increased by 253 pounds to a total of 629 pounds while rear lift was decreased by 247 pounds, resulting in an accumulative corrected rear lift of negative 55 pounds (or 55 pounds of downforce).
Equivalent horsepower increase: -1,062Conclusion: With the 'chute deployed, the front end gets really light!
Summary Of ConclusionsBy ditching the hoodscoop, changing and repositioning the headlights, installing inner wheel discs, reconfiguring the tonneau cover and screen, and changing the attitude of the car, we reduced drag horsepower from 547 (with driver) to 494. That's like finding 53 hp without touching the engine, and when horsepower levels are already high, another 50 is not an easy thing to find.