Drag Strip Math: Finding The Deeper Meaning In That Timeslip

• July 15, 2021
• Story By
  Mike McGlothlin

There is more than meets the eye with that tiny piece of paper you collect at the end of the drag strip. With a timeslip you can see how quick your reaction and 60-foot times were, how quick your overall elapsed time was and how much speed you picked up before rolling across the stripe. But beyond what’s printed on paper, you can accurately extrapolate what your elapsed time and trap speed would’ve been on a longer track (ex: eighth-mile to quarter-mile conversion) and even determine how much horsepower your vehicle is sending to the wheels (i.e. rwhp or whp). All you need for the latter calculation is your race weight and access to a reputable online horsepower calculator. Then you’ll never need to visit a chassis dyno again. For quarter to eighth-mile and eighth to quarter-mile conversions it’s even easier. All you need is the E.T. and/or mph number and a calculator.

Keep scrolling to find the deep value that’s hidden in your timeslip.

Reading the ‘Slip

Most of us know the basics of what’s printed on a timeslip, but for newbies we’ll point out the points of data collected and present on a typical one. Reaction refers to each vehicle’s reaction time. The 60 ft stat displays in seconds how quickly each vehicle made it to the 60-foot mark. The 330-foot elapsed time can be found on the next line, followed by the eighth-mile E.T. and then the eighth-mile trap speed. On a quarter-mile track, the list will continue on with a 1,000-foot E.T. and conclude with quarter-mile E.T. and quarter-mile trap speed. Each of these data points can tell you a lot of things, such as where you need to improve as a driver (usually reaction time or the 60-foot) or whether or not your car is performing as well as it should.

Eighth-Mile To Quarter-Mile E.T. Conversion Factor

When you don’t have a quarter-mile track at your disposal, there is a way to guestimate what your vehicle would run in the 1320 based on its eighth-mile elapsed time. Simply take your eighth-mile time and multiply it by 1.57. For example: a 7.70-second eighth-mile means you’ve got a vehicle capable of going 12.0 in the quarter-mile. In some instances a conversion factor of 1.56 is more accurate, but either one will get you pretty close. Should your quarter-mile timeslip not display your eighth-mile elapsed time for whatever reason, you can simply divide your quarter-mile E.T. by 1.56 or 1.57 to get an accurate idea of what it was.

Eighth-Mile To Quarter-Mile Trap Speed Conversion Factor

Just like for E.T., there is a way to accurately deduce what your quarter-mile trap speed would be based on the eighth-mile info you’ve gathered. For a lot of vehicles, multiplying eighth-mile trap speed by a factor of 1.25, 1.26 or sometimes even 1.27 yields great results. Full disclosure: the 1.25 and 1.26 conversion factors suffice most of the time for street-driven diesel trucks participating in drag racing, which is what yours truly spends most of his time at the track indulging in. Aerodynamics definitely play a role here, too, especially on 7,000 to 8,000-pound trucks during the course of a quarter-mile race.

RWHP Based On E.T.

Automakers and a lot of bench racers like to talk crank (or flywheel) horsepower numbers, but at the track the only ponies that matter are the horses that make it to the ground. Luckily, a trip across the scales to find your vehicle’s weight (with you in the driver seat) and your elapsed time is all you need to get a close idea as to the kind of horsepower you’re making. Numerous online horsepower calculators allow you to input these two data points to find your rear-wheel horsepower (rwhp, or whp for wheel horsepower). For its preciseness, we prefer the automotive calculator at WallaceRacing.com.

RWHP Based On MPH

To get an even more accurate picture as to the kind of horsepower your vehicle is making at the wheels, plug your trap speed and race weight into the online calculator of your choice (again, we prefer the Wallace calculator). Trap speed is more representative of horsepower for several reasons, but one of them is the fact that it isn’t reliant on traction like a good E.T. number is. Notice in the photo above that the rwhp number from E.T. is lower than the rwhp number from MPH shown here. The info displayed here and in the previous image were from the same eighth-mile pass, where a 7,200-pound 4x4 Ford Super Duty ran a 7.55 at 91 mph.

The Exception To Horsepower Calculators

One thing to remember about horsepower calculators is that they represent an average, whereby an all-out pass (100-percent fueling) is made from start to finish. In higher horsepower applications, where traction wouldn’t be possible with 100-percent of its horsepower in use on the launch, power has to be pulled out of the start, then poured back on once the vehicle is moving (and applied in a way that doesn’t blow the tires off). A lot of diesels capable of 2,500 to 3,000hp use roughly half that power on the launch, then progressively ramp up power at key points throughout the acceleration process until maximum power is being made. Sometimes that doesn’t happen until mid-track.

High-Horsepower Disclaimer

It’s important to keep in mind that, for all of the above math we’re talking street cars, trucks and SUV’s. Higher horsepower and dedicated racing applications change things a bit. As just one example (of many): when converting eighth-mile to quarter-mile numbers keep gearing in mind.

More specifically, if a vehicle is geared tall to optimize its performance in the eighth-mile and is already in overdrive before eclipsing the ‘660 mark, it will likely be out of gear in a quarter-mile scenario—so your eighth-to-quarter mile extrapolation won’t be accurate.