Things to Consider Before Top Speed Racing at Bonneville

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