Reading a drag racing time slip: reaction time, ET and trap speed

Every pass down a drag strip ends with a strip of paper, or its on-screen equivalent, listing a handful of times and two speeds. Most people look at the elapsed time and stop there. That number matters, but it is only one of three separate measurements on the slip, and it is not the one that decides who won.

The lines on a slip

NHRA-style timing puts beams across the track at fixed distances and records when each car breaks them. A typical slip for a quarter-mile pass looks like this. The figures are an illustrative example for a car running in the elevens, not a real run.

The 1,000-foot line is there because NHRA's Top Fuel and Funny Car classes have raced to 1,000 feet since 2008.

Pink Slip Kings: Drag Racing is built around the same measurements: you watch the starting lights, choose a launch RPM and time your shifts.

Reaction time and the tree

The Christmas tree has two common sequences. On a full (sportsman) tree the three amber lights come on one after another, 0.5 seconds apart, and the green follows 0.5 seconds after the last amber. On a Pro tree all three ambers light together and the green comes on 0.4 seconds later. Reaction time is the gap between the start signal and the moment the car leaves the stage beam.

Slips print it in one of two ways. Many show it with the tree delay already subtracted, so a perfect light is 0.000. Others measure from the last amber, in which case a perfect light reads 0.500 on a full tree and 0.400 on a Pro tree. Check which scale you are reading before comparing numbers between tracks.

Leave the stage beam before the green and you get a red light: on the 0.000 scale it shows as a negative number, such as −0.012. In eliminations a red light loses the round, except in the rare case where the other driver commits a worse foul, such as crossing the centre line. Because the car has to roll a short distance before it clears the stage beam, good drivers start moving fractionally before the green appears.

ET does not include reaction time

This is the point most often missed. The ET clock starts when the car leaves the stage beam, not when the green lights. The reaction time has already been recorded and is not included. A driver who sat on the line for half a second after the green can still post the same ET as one who left perfectly.

So in a heads-up race the winner is not the car with the lower ET. It is the car that reaches the finish line first, and that is reaction time plus ET:

The right-hand car was 0.050 seconds quicker down the track and still lost by 0.050 seconds, because its driver gave away 0.100 at the tree. At 120 mph a car covers 176 feet a second, so that margin is about 8.8 feet at the stripe. Winning with the slower ET is called a holeshot, and it happens often enough that experienced racers watch the R/T line as closely as the ET.

Trap speed is not average speed

The quarter-mile speed is not worked out from the ET. It is measured separately over the last 66 feet before the finish line, from 1,254 to 1,320 feet. The eighth-mile speed uses the same 66-foot window ending at 660 feet. At 120 mph the car crosses those 66 feet in 0.375 seconds, so the trap speed is essentially the car's speed at the finish.

The average speed over the whole run is much lower. A quarter mile in 11.200 seconds averages 117.9 feet a second, about 80 mph, against a trap speed of 120. The difference is the time spent getting going: in the example the car took 1.6 seconds, 14% of the run, to cover the first 60 feet, which is under 5% of the distance.

The dyno, telemetry and race time slips in Pink Slip Kings are the places to look for these answers.

Reading where time was lost

Because the slip separates the parts of the run, you can read it as a diagnosis rather than a single score.

1. R/T is pure driver. A slow or red light has nothing to do with the car.
2. The 60-foot is the launch. Wheelspin, a bog from too few revs, or a launch that overwhelms the tyres all show up here first, and every hundredth lost in the first 60 feet carries through to every later split.
3. 330 feet and the eighth mile show whether the car kept pulling through the early gears. A good 60-foot followed by a weak 330 points to a problem after the launch, such as a missed or slow shift.
4. Trap speed is mostly about power against weight. Two runs with the same trap speed but different ETs usually differ at the start, not at the top end. A higher trap speed with no better ET suggests the extra power is being lost to wheelspin.

Comparing two slips line by line is the quickest way to see what a change did. If a tuning change improves the 60-foot but the trap speed is unchanged, it helped the launch and nothing else. If the trap speed rises and the 60-foot stays the same, you added power the car can already use.