1/8 Mile ET Calculator

How the eighth mile calculator works

Many dragstrips, and many classes on longer tracks, race over an eighth of a mile, half the quarter mile. The timing works the same way: the clock starts when the front tires leave the starting beam and stops at the eighth-mile stripe, with the speed taken over a short trap just before it. What changes is how the numbers relate. A car is slowest in the first part of any pass, so the eighth mile takes far more than half the quarter-mile time: typically about 64% of the ET, by which point the car is already at about 80% of its quarter-mile trap speed.

This 1/8 mile calculator has four tabs. The first estimates an eighth-mile ET and speed from flywheel horsepower and race weight. The second and third convert a time slip from the eighth mile to the quarter mile and back. The last turns an eighth-mile slip into an estimate of flywheel horsepower.

Every tab rests on two conversion factors, one for ET and one for speed. They are rules of thumb drawn from real passes rather than exact physics, so the calculator shows them as inputs with the typical values filled in. Everything here describes passes on a dragstrip.

What to enter and where to find it

• Eighth-mile ET and speed. From the time slip: the elapsed time at the eighth-mile stripe and the speed printed with it. On a quarter-mile slip they are the incremental figures partway down.
• Quarter-mile ET and trap speed. For the 1/4 to 1/8 tab, the finish-line figures of a quarter-mile slip.
• Flywheel horsepower and race weight. Engine power at the crank and the weight of the car with its driver, exactly as on the quarter mile calculator. Convert a chassis-dyno figure first with the whp to hp calculator.
• Conversion factors. Keep the typical values unless you have both an eighth-mile and a quarter-mile slip from the same car. Divide the quarter-mile figures by the eighth-mile ones and you have factors made for your car.

1/8 to 1/4 mile conversion factors

The factors racers use to turn an eighth-mile pass into a quarter-mile one are rules of thumb, and they do not all agree, so the calculator treats them as editable inputs. For ET they run from 1.5455, which fits a car that leaves the line gently, to 1.60, which conversion charts attribute to the NHRA, with 1.56 the most common choice. For speed they run from 1.22 to 1.28, most often 1.25: by the eighth-mile stripe a car usually shows about 80% of its quarter-mile trap speed. Neither factor comes with a published data set behind it, so treat the whole range as normal. On the ET it is about 3.5%, a few tenths for most cars. The eighth-mile equivalents the NHRA sets for its mid-range index classes fall inside the same range; for the quickest classes, where air drag dominates the back half, the NHRA uses factors above 1.6.

Physics explains why the factors sit where they do. A car that accelerated at a steady rate would need 1.414 times as long to cover twice the distance. A car pushed by constant power, with no air drag, would need 1.587 times as long, and its speed at the stripe would be 1.260 times higher. A time slip, though, prints the average speed over the last 66 ft before the stripe, and averaged that way the constant-power ratio is 1.27: that is the physical reference for the speed factor, just above the typical 1.25. Real cars start out grip-limited, close to steady acceleration, and finish power-limited with air drag building, so for most cars the ET factor falls between the two physical cases, closer to the constant-power end.

Which way the launch moves them. A car that leaves the line hard for its power gets through the first eighth quickly and tends toward a higher ET factor; one that spins the tires or leaves gently loses time early and tends toward a lower one. The speed factor tends to move the other way: a hard launch puts the car close to its quarter-mile pace by the eighth, which lowers the factor, while a gentle one leaves more speed to gain. Air drag at the top end, or a car running out of gear before the finish, also pulls the speed factor down and pushes the ET factor up.

Which way they move the results. A higher ET factor makes the quarter-mile equivalent slower, the eighth-mile ET estimated from power quicker and the power worked out from an ET lower. A higher speed factor makes the quarter-mile speed higher, the eighth-mile speed estimated from power lower and the power worked out from an eighth-mile speed higher. Power goes with the cube of speed, so the speed factor's range moves that power figure by about 15% from end to end, which the last tab shows as a range.

The best factors are your own. With an eighth-mile and a quarter-mile slip from the same car on similar days, divide one by the other and type the result in: it will beat any rule of thumb for that car.

The eighth mile ET calculator formulas

There is no separate set of drag racing formulas with a pedigree of its own for the eighth mile. This calculator takes the classic quarter-mile formulas, explained on the quarter mile calculator, and moves them to the eighth mile with the conversion factors. That keeps both pages consistent: the same car gets the same quarter-mile equivalent on either one.

1. Quarter-mile ET ≈ eighth-mile ET × ET factor (typically 1.56)
2. Quarter-mile trap speed ≈ eighth-mile speed × speed factor (typically 1.25)
3. Eighth-mile ET ≈ 5.825 × (race weight in lb ÷ flywheel hp)^(1/3) ÷ ET factor = 3.734 × (lb ÷ hp)^(1/3) with the typical factor
4. Eighth-mile speed ≈ 234 × (flywheel hp ÷ race weight in lb)^(1/3) ÷ speed factor = 187.2 × (hp ÷ lb)^(1/3) with the typical factor
5. Flywheel hp ≈ race weight × (eighth-mile speed × speed factor ÷ 234)³

The estimates carry two kinds of error: that of the quarter-mile formula itself, which is good for speed and looser for ET, and the spread of the conversion factor. Over an eighth mile the launch is a bigger share of the whole pass, so the ET estimate depends even more on traction than it does over a quarter mile, and horsepower worked out from the eighth-mile speed is the more dependable of the two figures on the last tab.

Worked example

Three cases, worked out with the same functions that run the calculator.

Typical eighth mile ET to quarter mile conversions

Common eighth-mile ETs with the quarter-mile ET they convert to at the lowest, typical and highest factors. The last column is the flywheel power per thousand pounds of race weight that the quarter-mile formula gives for the typical equivalent.

Read across a row and the spread between the columns is the honest uncertainty of any conversion. Read down the last column to size the power a car needs for a target eighth-mile time: multiply by the race weight in thousands of pounds.

Common mistakes

• Doubling the eighth-mile ET. Half the distance is not half the time. The car is slowest early in the pass, so the quarter mile takes only about 1.56 times the eighth-mile ET, not twice.
• Treating a converted time as a time you ran. A converted ET is an estimate with a spread of a few tenths. For class rules or a dial-in, only the timing system counts.
• Converting the wrong line of the slip. On a quarter-mile slip, the eighth-mile figures are the incremental ones partway down. Do not convert the finish-line figures a second time.
• Using wheel horsepower. The power tabs use flywheel horsepower, like the quarter-mile formulas they come from. Convert a chassis-dyno figure with the whp to hp calculator.
• Blaming the engine for a slow ET. On an eighth mile the first 60 feet are a big share of the pass. If the estimate and the slip disagree, look at the launch first.