If you’re a gearhead, then you’ve probably wondered how much horsepower your ride actually has. And if you’re curious, we are here to guide you so that you don’t end up like the poor fella in the below video. In a nutshell, you’ll need to use a chassis dyno, which is designed to measure how much horsepower and torque is present at the drive wheels. In the below video, the unlucky guy was simply trying to figure out how many horses his pony car made, when disaster struck. The back tires on his Mustang GT 5.0 were turning at 150 mph, when suddenly one of them exploded like an atom bomb. There’s a big difference between crank horsepower and wheel horsepower, and a “rolling road” can tell you exactly what you’re working with. Check out the insanity in the below video and join us as we attempt to dissect what went wrong.
What is a Chassis Dyno?
A dynamometer is a complicated device that’s used to measure the output of something, like an engine. Automakers and engine builders use a hydraulic engine dyno to measure the horsepower and torque that an engine produces at its crankshaft (i.e. crank horsepower) or flywheel. Without a driveline load, the engine builder is able to determine the raw output, then tune the engine to meet the desired specifications. An engine dyno can also be used to ‘break in’ a motor, by subjecting it to a constant load for an amount of time that equates the desired mileage (usually 1,000 miles).
To measure wheel/rear wheel horsepower, a chassis dyno uses a set of fixed drums or rollers, which are connected to a bunch of sensors and a computer. As the vehicle’s tires spin the drums/rollers, a computer calculates how long it takes to spin their known mass. To get an accurate measurement, the vehicle’s transmission should be in the 1:1 gear (a transmission is designed to multiply power, so a higher 1:1 gear basically means the engine and the transmission are moving at the exact same speed), which is probably why that Mustang GT in the video was in 5th gear.
So why did the tire explode?
Tire Speed Ratings 101
Tires are made from layers of steel mesh and polyester (usually) chords, wrapped in a proprietary rubber compound. As you drive along, heat builds up and increases the air pressure inside the tire. Each tire is made to withstand the pressure of a specific maximum speed, based on its construction. The higher the speed rating, the stiffer the sidewall is likely to be. This not only improves the handling and lateral grip, but it allows you to explore the upper end of your car’s capability. However, tire co`ndition seriously affects its structural integrity at the limit, so any nicks, cuts, or abrasions will reduce the safe maximum speed.
Tire speed ratings are denoted by a letter, usually at the end of the tire size, before the ‘R’ and diameter (i.e. 215/50ZR17). But sometimes, it’s listed separately (i.e. 215/50R17 149Z). On performance tires, the most common speed ratings are:
H = Up to 130 mph
V = Up to 149 mph
Z = 149 mph+ (this used to be the highest speed rating, but that’s changed)
W = Up to 168 mph
Y = Up to 186 mph
The fifth-generation Mustang GT 5.0 in the video appears to be a 2011-2014 model, riding on stock 18-inch wheels. They were fitted with W-rated tires from the factory, and that’s most likely what this car was wearing.
The video claims the car had a 200-shot of nitrous, which theoretically would give it 620 (crank) horsepower for a few seconds (the Coyote 5.0L V8 is rated at 420-hp). We don’t know if nitrous was used during the dyno run, and the operator claimed that he didn’t know how fast the wheels were turning when the tire blew. But it’s very likely the wheels were turning at a much faster speed than the tires were designed to handle. Or, the condition of the tire was poor, which caused it to fail below its safe operating limit. Either way, the 150 mph speed listed in the video description sounds very suspect.