{"id":116,"date":"2009-08-19T10:17:49","date_gmt":"2009-08-19T15:17:49","guid":{"rendered":"http:\/\/performancetrends.com\/blog\/?p=116"},"modified":"2009-09-08T14:43:58","modified_gmt":"2009-09-08T19:43:58","slug":"things-to-consider-before-top-speed-racing-at-bonneville","status":"publish","type":"post","link":"https:\/\/performancetrends.com\/blog\/?p=116","title":{"rendered":"Things to Consider Before Top Speed Racing at Bonneville Salt Flats"},"content":{"rendered":"

Top speed racing is very much like drag racing, but just on a very <\/em><\/strong>long track.\u00a0 In drag racing, it is power to weight ratio which typically determines your performance.\u00a0 However, when the track is very long, and your vehicle spends much more time at high speed, it is power to drag ratio<\/em><\/strong> which is more important.\u00a0 By drag, I mean primarily aerodynamic drag or wind resistance.\u00a0 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.<\/p>\n

To improve the power to drag ratio, you want to increase the power and reduce the drag, which makes sense.\u00a0 To go faster, you need more power and you want to make the car more aerodynamic.\u00a0 However, what you may not know, is that to go twice as fast, you need eight (8) times the power<\/strong><\/em>.\u00a0 If your 200 HP car can top out at 120 MPH, you would need 1600 HP to top out at 240 MPH.\u00a0 \u00a0(You would also need some really good tires to hold together, and good aero downforce to stay on the road).<\/p>\n

Most all racers have some idea on how to improve the engine’s power.\u00a0 Engine power can be fairly reliably simulated with our Engine Analyzer computer programs, and these can all be tested with an engine dynamometer and our Dyno Datamite software.<\/p>\n

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).\u00a0 Drag coefficients vary from a high value of about .8 for an upright rider on a vintage motorcycle, to .6 for an older\u00a0pickup truck, \u00a0to .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.<\/p>\n

To optimize the aerodynamics of your particular vehicle, you should read everything you can get your hands on.\u00a0 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.\u00a0 Typically you just make these mods you have read about\u00a0and hope for the best, because it is very difficult to measure if\u00a0 your aerodynamic mods have made any really improvement.<\/p>\n

The best way to actually measure the effect of aerodynamic mods is to rent a wind tunnel, at around\u00a0$50,000 per day.\u00a0 For the rest of us, we can preform coastdown tests.\u00a0 This is where you get your car up to a top speed, throw it in neutral and let it coast to a lower speed.\u00a0 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.\u00a0 If the coastdown times, from say 100 to 60 MPH has increased 3%<\/em><\/strong>, it means you have made a 3% improvement (reduction)<\/em><\/strong> in drag coefficient.<\/p>\n

The best way to do coastdown tests it to do several and average the results.\u00a0 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.\u00a0 From doing coastdown tests myself, I can say that this requires lots of tests and patience to get good results.\u00a0 Also, the higher the speed (not<\/em><\/strong> on public roads), the better the results.\u00a0 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.<\/p>\n

OK, so we’ve talked about the power to drag ratio <\/em><\/strong>contributors.\u00a0 But there are other, secondary effects which also have an effect.\u00a0 These effect how\u00a0efficiently you take advantage of the power to drag ratio you have to work with.\u00a0 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\u00a05 miles.\u00a0 To get the optimum top speed, you want to get to top speed quickly, to optimize acceleration at all times.\u00a0 This gets back to the drag race idea.\u00a0 You don’t have to worry about 60 ft times or pulling wheelies, but you do want to optimize your shift points.\u00a0 A quick El Mirage computer simulation showed a .6 MPH improvement on a 140 MPH car\u00a0by shifting quickly at optimum RPMs, vs “lazy” shifting at RPMs about 1500 RPM off optimum.<\/p>\n

Total gear ratio is critical.\u00a0 You\u00a0want to put the engine at it’s peak HP RPM when the vehicle reaches top speed.\u00a0 The peakier the power curve, the more critical this is.<\/p>\n

Another aerodynamic effect is lift.\u00a0 The lift coefficient\u00a0 determines how much your vehicle acts like an airplane wing.\u00a0 If you have a high lift coefficient, you loose traction at the tires and loose steering control.\u00a0 Too much negative lift coefficient, and your tires have to do more work and rolling resistance increases.\u00a0 This is another item which will require you reading up what others have done.\u00a0 Lift coefficient is very difficult to measure, but you should be aware of its effects as it has a huge effect on safety.<\/p>\n

Another detail is a hood scoop efficiency.\u00a0 An effective hood scoop at high speed produces significant boost pressure for the engine to improve power.\u00a0 For example, a perfect hood scoop at 200 MPH will produce .75 psi boost, which equates to approximately a 5% power improvement.\u00a0 However, if you have to increase the drag 10% with a big, protruding bump on the hood, it’s probably\u00a0an overall loss to top speed.<\/p>\n

To truly understand all the things which affect “real world” top speed performance, you need a vehicle simulation programs like Drag Race Analyzer<\/a> or Drag Race Analyzer Pro<\/a> which lets you modify things like we’ve talked about, which include:<\/p>\n