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Top speed racing is very much like drag racing, but just on a very long track. In drag racing, it is power to weight ratio which typically determines your performance. However, when the track is very long, and your vehicle spends much more time at high speed, it is power to drag ratio which is more important. By drag, I mean primarily aerodynamic drag or wind resistance. In addition to aerodynamic drag, there is rolling resistance from tires, driveline losses, but the higher the speed, the larger the aerodynamic component of overall drag.
To improve the power to drag ratio, you want to increase the power and reduce the drag, which makes sense. To go faster, you need more power and you want to make the car more aerodynamic. However, what you may not know, is that to go twice as fast, you need eight (8) times the power. If your 200 HP car can top out at 120 MPH, you would need 1600 HP to top out at 240 MPH. (You would also need some really good tires to hold together, and good aero downforce to stay on the road).
Most all racers have some idea on how to improve the engine’s power. Engine power can be fairly reliably simulated with an engine simulation computer program, and these can all be tested on an engine dynamometer.
The biggest contributors to aerodynamic drag are the vehicle’s frontal area (silhouette of vehicle when viewed from the front) and it’s drag coefficient (a rating of how easily the vehicle slices through the air for it’s frontal area). Drag coefficients vary from a high value of about .8 for an upright rider on a vintage motorcycle, to .6 for an older pickup truck, to .4 for a modern aerodynamic sedan, to .35 for a modern sports car, to an incredibly low .15 of “pencil shaped” land speed record cars like the Blue Flame.
To optimize the aerodynamics of your particular vehicle, you should read everything you can get your hands on. The basic shape has a large effect, but subtle things like windshield moldings, vehicle rake (lowering the front end), underbody protrusions all add up to huge improvements. Typically you just make these mods you have read about and hope for the best, because it is very difficult to measure if your aerodynamic mods have made any really improvement.
The best way to actually measure the effect of aerodynamic mods is to rent a wind tunnel, at around $ 50,000 per day. For the rest of us, we can preform coastdown tests. This is where you get your car up to a top speed, throw it in neutral and let it coast to a lower speed. For this to be accurate, you should use the same stretch of very flat road, and do the test in both directions to minimize the effects of wind and slight grade of the road. If the coastdown times, from say 100 to 60 MPH has increased 3%, it means you have made a 3% improvement (reduction) in drag coefficient.
The best way to do coastdown tests it to do several and average the results. It is also best to use some type of data logger so you get lots of accurate data and the driver can concentrate on driving. From doing coastdown tests myself, I can say that this requires lots of tests and patience to get good results. Also, the higher the speed (not on public roads), the better the results. There is also software which can separate how much of the coastdown drag is from the tire rolling resistance and how much is from aerodynamic effects, and come up with actual numbers, like your drag coefficient is .322.
OK, so we’ve talked about the power to drag ratio contributors. But there are other, secondary effects which also have an effect. These effect how efficiently you take advantage of the power to drag ratio you have to work with. For example, top speed tracks vary in length, from Maxton’s Monster Mile at just 1 mile, to El Mirage’s 1.33 miles, to Bonneville’s legendary 5 miles. To get the optimum top speed, you want to get to top speed quickly, to optimize acceleration at all times. This gets back to the drag race idea. You don’t have to worry about 60 ft times or pulling wheelies, but you do want to optimize your shift points. A quick El Mirage computer simulation showed a .6 MPH improvement on a 140 MPH car by shifting quickly at optimum RPMs, vs “lazy” shifting at RPMs about 1500 RPM off optimum.
Total gear ratio is critical. You want to put the engine at it’s peak HP RPM when the vehicle reaches top speed. The peakier the power curve, the more critical this is.
Another aerodynamic effect is lift. The lift coefficient determines how much your vehicle acts like an airplane wing. If you have a high lift coefficient, you loose traction at the tires and loose steering control. Too much negative lift coefficient, and your tires have to do more work and rolling resistance increases. This is another item which will require you reading up what others have done. Lift coefficient is very difficult to measure, but you should be aware of its effects as it has a huge effect on safety.
Another detail is a hood scoop efficiency. An effective hood scoop at high speed produces significant boost pressure for the engine to improve power. For example, a perfect hood scoop at 200 MPH will produce .75 psi boost, which equates to approximately a 5% power improvement. However, if you have to increase the drag 10% with a big, protruding bump on the hood, it’s probably an overall loss to top speed.
To truly understand all the things which affect “real world” top speed performance, you need a vehicle simulation program which lets you modify things like we’ve talked about, which include:
Actual engine power curve through entire RPM range
Drag coefficient and frontal area (and possibly lift coefficient).
Transmission and final drive ratio
Hood scoop efficiency
Tire type (to estimate rolling resistance)
Shift RPMs and shift type (fast, slow, power shift, etc).
You also want to read up on what ever you can find on top speed racing.
Kevin Gertgen is the owner of Performance Trends and writes all the software. Visit Drag Racing Software to view this and other products or visit Things to Consider at Bonneville for this and other articles on Motor Sports.