I've been doing some research into the accuracy of P-box/D-Box 1/4 mile times compared to actual dragstrip 1/4 mile times and this is what I've come up with so far. First, let me briefly explain how a dragstrip measures time:

There are infrared beams located at the beginning and end of every dragstrip. The two beams that we're concerned with are the Staging Beam (or "Starting Beam") and the Finish Line Beam. The Staging Beam initiates the Starting Tree when your front tires block the beam. The Tree then counts down and gives you a set of green lights telling you to go. After you start moving forward and the back of your front tires unblock the Staging Beam, the track's timers start counting down. Once your front tires cross the Finish Line Beam at the end of the track, the timers record your time from start to finish. Pretty simple.

The Trap Speed measurement (or mph achieved at the end of the track) is not quite that simple. It doesn't measure your exact speed when you cross the Finish Line Beam. Instead, it takes the average of your maximum speeds between the Finish Line Beam and another beam called the Speed Beam located 66' behind the Finish Line Beam (this distance has been altered over the years to make Top Fuel dragsters appear slower to insurance companies.)

The P-Box, on the other hand, starts measuring your E.T. as soon as you start moving forward from 0 mph…so the P-box E.T. will never exactly match the track's measured E.T...since you need to physically move past the Staging Beam for the track's clocks to start. Now, the variance between the P-box E.T. and actual E.T will depend on how hard you launch your car. The harder you launch it (the quicker 60' time you get)...the faster your front tires will move past and unblock the Staging Beam…and the closer the E.T.s between the P-box and actual will be. But they will never consistently be the same.

There exists a feature called the "1 foot rollout" on the P-box, where you can set the P-box to start measuring your time after you roll forward 1 foot...but it never matches up with the actual beams on the track (at least, it's never worked for me). The P-box also measures your actual maximum speed at the end of the run, rather than giving you an average.

So...as you can see, it makes sense that the two will never be exactly the same. It doesn't mean the P-box is inaccurate, it just uses a totally different method of measuring your speed than a dragstrip does (I believe that the method dragstrips use to measure MPH is inherently inaccurate).

Anyway, after looking over numerous timeslips and comparing them to actual P-box times, it looks like P-box 1/4 mile runs show anywhere from .3 to .6 seconds slower than actual timeslip E.T.s, because the P-box starts measuring E.T. sooner than the dragstrip clock does...and 1-2 mph faster than actual timeslip MPHs, because the P-box doesn't average the MPH over the last 66'. It shows the actual maximum speed, which is always higher than the averaged speed.

So, for example, if you run 12.0 @ 125 using your P-box...that run could have been anywhere from 11.4 to 11.7 @ 123-124 on an actual dragstrip....depending on how hard you launch the car (and how much traction you get).

Anyway, I just thought some of you might find this interesting. Anyone that has more direct comparisons between P-box runs and actual dragstrip runs, feel free to chime in. I'd like to find out if others are seeing the same thing that I am.

 

Does the length of the car also alter the 1/4 ET? If so, comparing like cars is more accurate.

 

I wonder if we try many P-box's on one car...Will we get same numbers on all P-box's on same car?

 

The beam is blocked by the front of the front tires, and is then unblocked by the rear of the front tires. Length of the car doesn't matter.

 

Great info Scott. Thanks!

 

Thank you for the info Divexxtreme! I was wondering why you don't take P-box's 1/4 runs. I thought the data maybe inaccurate. But now I know you just wanted to keep consistency.

 

Exactly, bro.

 

Any one?

My friend Proto 997tt ran 11.2sec on pump gas....So it should be high 10's

 

Interesting read. Thanks for that Scott.

 

is it also that different if you check the times manually with the driftbox software to exclude some errors of preliminary time start count?

the dbox and pbox have (of course) built in limitations of 0.1 sec and additionally a bit more error if moving with slow speeds like start or stop, because the calculate speed and position with a mixture of doppler GPS-Speed and differential GPS-Position information. the latter is more precise the higher your speed is.

 

Scott,

Did you compare the differential in the Pbox to actual 1/4 times when the 1 foot rollout was enabled on the Pbox? If so , what was the difference in recorded times with the feature on as compared to off? ie, how much difference did it make to your original observation of .3-.6?

 

I never got it to read any differently. It still gave me the same .3-.6 errors.

But heck, maybe I didn't use it/set it up correctly?

 

if you compare the Pbox's and dragstrip's 60' time, can you see if most of the difference is already in there?

 

I'll be listing p-box trap speeds in my sig.

 

http://www.performancebox.co.uk/down...CHvPB_Test.pdf
[media]http://www.performancebox.co.uk/download/GTECHvPB_Test.pdf[/media]