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Chasing 200 mph

by David Zatz on

The Dodge Challenger SRT Hellcat has a 200 mph speedometer. Can it actually reach that speed? Most cars can’t even come close to their top speedometer markings, after all.

Allpar’s intrepid team of volunteer analysts believes that the Challenger SRT with Hellcat engine should justify the 200 mph speedometer — indeed, it may be able to “peg” it. (Chrysler’s last four-seater capable of 200 mph was the 1969 Dodge Charger Daytona; a well-prepared but still stock car broke the barrier, thanks to the 426 Hemi engine and stunning 0.28 drag coefficient).

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Now that Chrysler has released all the specifications of the 2015 Dodge Challenger SRT Hellcat, we know what tires and wheels it will have, along with its front area, drag coefficient, and gearing. We don’t yet know its power levels but we do know its redline will be over 6,000 rpm (which is where it makes peak horsepower of “600+.”  We will have to estimate wheel slip and other losses.

“AutoTechnician” wrote that with 575 lb-ft of torque, an “oddly specific” number, the best the engine should be able to do is 635 horsepower. If torque was a little higher than 575, it could meet or beat the Z28’s 638 hp.

topspeedThe eight speed automatic drops the top gear down to 0.67, while raising first gear to 4.71. That means the SRT people could play around a lot more with axle ratios, and also puts seventh gear into play. In top gear, a Charger SRT8 with eight-speed and the same tires and axle ratio would be capable of 258 mph… in bench racing… with no wind resistance or other limiters… at 6,200 rpm. We do not expect the top speed to be 250 mph! because nobody will be driving the car in space, where there’s no air.  (Though the gravity issue could be resolved by moving the experiment close to one of the larger planets, who’s going to pay for the rocket?)

“AutoTechnician” took the aerodynamic drag rules, published aero coefficients of the Challenger SRT with Hellcat, and the Challenger 392’s published power and top speed  (475 hp, 182 mph top speed) to chart the Hellcat’s top end. The chart, for illustration only, is above/right; it shows around 198 mph at 600 horsepower, and around 214 mph at 700 horsepower. He also assumed a 5,800 rpm redline, based on Dodge’s photo of the SRT’s gauges; however, Hellcat makes peak power at 6,000, so the redline is probably a bit higher.

If one ignores drag and parasitic losses, the car should easily be able to slam well beyond 200 mph, but it doesn’t work like that. As forum member “AutoTechnician” wrote, “The power required to overcome aerodynamic drag increases with the cube of the speed (e.g. 20 mph requires eight times the power as 10 mph to push through the air.)  Keep in mind that the 638HP ZR1 can only do 205 mph. … ” The lighter 2014 GT500 has 663 horsepower and only hits 202 hp, but it doesn’t have the efficient ZF rear axle or the wide-range ZF automatic to help.

Source “Danno” wrote that the Hellcat Challenger hit 202 mph in testing, but the production models might not, as various changes were made to enhance cooling and durability.

The efficient ZF transmission may help SRT; an open question is whether HellCat will be in our conservative 600-620 hp range, or over 640 hp, as predicted by many sources.

Discussion / forums

David Zatz founded Allpar in 1998 (based on a site he had begun in 1993-94), after years of writing reviews for retail trades. He has been quoted by the New York Times, the Daily Telegraph, the Detroit News, and USA Today. Before making Allpar a full-time career, he was a consultant in organizational psychology. You can reach him by using our contact form (much preferred) or by calling (313) 766-2304


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