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West Coast Challengers
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West Coast Challengers, Southern California Chapter
Dynamics of Drag Racing

(Analyzing your time slips)

By Bill Watson

     It's natural that with a group of racers, the talk commonly turns to power output. Even though the intent is good, the talk is just that -- talk. Sometimes we all wish we could get free dyno time to see just what kind of power our cars are really making.

     I would suggest that the most accurate CHEAP method out there is a trip to your local quarter mile dragstrip, which typically costs around $15 for a whole night of racing. We can argue out the accuracy and repeatability of engine dynos with rear wheel dynos with the dragway in another article.

     The paragraphs below focus on the fact that you'll use speed instead of time as your yardstick, and the point is ... YOU DON'T HAVE TO ABUSE THE CAR TO GET YOUR NUMBERS. That's the biggest excuse I hear for why someone doesn't go to the strip. You DON'T have to buy slicks, or do a neutral drop, or burn your clutch, or buy 4.10 gears, or even warm up the tires in the water box. IT'S EASY, NOW QUIT MAKING EXCUSES AND JUST GO DO IT. (Whew!)

     Like it or not, the trap speed at the end of the quarter mile is an excellent indicator of your HP to weight. If you know weight (almost every track has a scale), then you can calculate HP. It's that simple.

     Note that I have not used the term "ET" (Elapsed Time) yet. I have said TRAP SPEED, the speed of the vehicle by the end of the quarter mile. However, almost everyone involved with drag racing will discuss the ET, the elapsed time to cover that quarter mile. Yes, the two are related of course, but for all intents and purposes, here's what I hope you learn from this article:

        ·  Trap Speed will tell you about your HP to weight.
        ·  ET will tell you more about traction and your launch.

     Of course ET is important to true drag racers, because the winner is the one that gets there first. However, we're not necessarily true drag racers in our attempt to get a power estimate. Honestly, ask 10 guys at the track "What kind of trap speed are you running?" and 8 out of 10 will answer with their ET -- to one or two decimal places even. When you say, "No, no. I meant trap speed," they will fumble with a broad estimate with NO decimal places and might even have to pull a time slip out of their pocket to check. Try this question when you're at the track. It's almost funny.


     After running lots of quarter miles, it becomes clear that how well you do in the first 100 feet of the track is KEY to a good time. The last half of the track is KEY to a good speed.

     Let's use an example of a stick-shift mini-pickup that on a perfect run, gets a timeslip of 19.50 seconds at 70.00 mph in the quarter.

     Imagine that the light turns green, the truck moves two feet and the engine dies for three seconds. After restarting the engine, the driver proceeds to then complete a perfect pass. His time slip would show 22.50 seconds at 69.97 mph. The ET was 3.00 seconds high but the speed was almost unaffected, why?? It's because his racetrack was 1,318 feet long instead of 1,320, and in those last two feet this truck usually gains an additional 0.03 mph. However, the clocks recorded the long time. My point? Much of a great ET is made by a great launch.

     Now take this truck again, and the driver leaves right on the green light. However, he misses the 3-4 shift when he's at 1,250 feet. He coasts for the last 70 feet while trying to find fourth gear. Now instead of accelerating another few mph in this final 70 feet of the track, he decelerates over this distance. His timeslip -- 19.51 at 67.83 mph. Note how the ET is almost perfect (only off by 0.01 second) but the trap speed is way off (over 2 mph slow)! On a good run, traveling that last 70 feet at an average of 69 mph, would have taken .692 seconds. At a 68 mph avg., that 70 feet takes .682 seconds. That's why his ET only varied by .01 seconds, yet the trap speed was "way off." My point here, the end of the track is critical to trap speed. Shift rpm, missing a gear ... these are the big players.

     Hopefully these examples are clear. Neither of these runs are "perfect" runs, it's just that one has an error at the start, one at the finish and the results are obvious. The start of the track is a big player in the ET, but a small player in the mph. The end of the track is a big player in the mph, but a small player the ET.

     So for the weekend racer who wants to get a HP value, you don't have to buy slicks, or wish you had a limited slip differential. You don't really need to heat the tires in the waterbox or launch with huge power braking. As long as people get their shift rpm right and don't miss a gear, even a rookie will get the appropriate trap speed for their vehicle. Honing the perfect ET doesn't requires being rude to the car, buying steeper gears or slicks ... hey, we're trying to make this recreational.


     If your goal is to get a good trap speed, what are your options? More power, of course -- and less weight is obvious (but it will come out in the power calculations as no increase in power). Shift rpm chosen (auto or manual) and the time it takes you to shift (with a manual) are probably the most important tools you've got. Try different shift points to maximize your trap speed. Reduce rolling resistance by pumping up all tires to their rated pressure. Some people think that running lower pressure might help the traction in the rear though. Of course more traction will help ET, but with most street tires, running 5 psi of rated pressure will provide you with maximum traction in the first place.


     The ET clocks don't start until you've actually moved around 8 inches (this is called the rollout), so don't worry about trying to leave right on the green light. You could wait 5 seconds after the light turned green, and still get a 19.50 timeslip in our truck example above. Your timeslip does show a separate calculated time, the "Reaction Time," which in this case would be 5 seconds. That is the time from the light turning green until you rolled out of the starting zone. It's not a big thing for our discussion here.


     For the most part, a decrease in ET is accompanied by an increase in trap speed, but don't go overboard on the launch in your zest to rule the world. Just try to get smartly underway without spinning the tires much at all. Traction levels usually drop a solid 0.10 g when the tires start spinning.


     Here's the formula to use to calculate HP:

     Net HP = Weight in pounds x (Speed in MPH/228.4) ^ 3

     As an example, a car magazine tested a car that weighed 3,081 lbs. without the driver (the race weight was 3,231 lbs.) The car ran a 15.7 second quarter at 86 mph. Let's plug it in to the formula:

         HP = 3231 x (86/228.4) ^ 3
         HP = 172 Net

     The vehicle manufacturer rated it at 162 Net. We come out a little high (or did the manufacturer underrate it a little?)

     Still, for such a simple formula and such a simple test, it's surprising how accurate this can be. And the best thing is -- there's no arguing the numbers on a timeslip. There are always differences between DynoJet and Mustang dyno numbers, because every setup is done by someone different and subject to error. The quarter mile is arguably the best comparison you will ever have. The only real difference to argue about is the altitude of the track! You can compare ET and mph all day and have a good discussion.


     Once you have a baseline, you should probably use a rule of thumb that each additional 6 HP will give you another mph. That's for a 3,200 lb car that runs 88 mph. If you want the real formula for different weights or speeds, here it is:

     HP for another mph above "X" speed: = Wt * (((X+1)^3-X^3) / (228.4^3))

     For instance a 89 mph quarter vs. an 88 mph quarter for a 3200 lb car:

     HP delta = 3200 * ((89^3-88^3) / 228.4^3))

     HP delta = 6.3 HP

     Once you're going 110 in the quarter, it would take an additional 10 HP to go 111 mph in the 3,200 lb. car.

     60 FOOT TIME.

     This is the standard measurement tool to evaluate your launch. It's the time that it took you to travel the first 60 feet of the track. Naturally, patterns emerge again after looking at lots of runs and of course these correlate best to time, not mph. Typically, most everyone's 60 foot time will be between 14% and 16% of their quarter mile time. If it's under 13% or over 17%, this was not your best pass.

     1/8 MILE vs. 1/4 MILE.

     After monitoring tons of good passes, patterns emerge. Typically, the mph at the quarter is around 1.26 times of the mph at the eighth, and the time at the quarter is around 1.55 times the time at the eighth. You can use these values if you only have a 1/8 mile track and get a real good idea of the theoretical 1/4 mile.


     One fact of the quarter mile is -- no matter how slow or fast your car is, the mph multiplied by the ET will pretty much be the same number every time (usually around 1400). Remember, most everyone focuses on ET so much that they'll even optimize a car for slower mph if it gets them a better ET. (Rear end gearing is one way to do this). Those guys tend to have a product closer to 1300.


     If you look at the formula again, you'll note how trap speed shows up as the cube root of power to weight. That's critical to understanding how fast one car is over another. Let's say your car does a 90 mph quarter and the guy who raced you in the other lane ran 71 mph. After the race, he wanders over to you to say, "Close race." Your reply, "I could have towed you and still beat you." (This might not be the best way to make friends, but yes, it is TRUE if the cars weigh the same)

     Do the math. (90/71) cubed is 2.04. Yes, the 90 mph car has 2.04 times the power to weight of the slower car. It has 2.04 times the acceleration of the slower car. It's just that the track is a fixed length, and in accelerating to higher speeds, you use up the track quicker. You accelerated to 90 in about 20% less time than he had to accelerate to 71, right?

     Bottom line, down where most of us run, a 3 mph difference between two cars is NOT a race. It was a clear win. There's a full 10% difference between these cars.


     Sea Level air pressure is 14.7 psi. If you go to a track in Boise, Idaho (2,850 feet above sea level) the air pressure is around 13.25 psi. That's 90.1% of sea level pressure. If the temperature doesn't change and you have an normally aspirated car, your power output will now be 90.1% of what it used to be, so you correct by multiplying your calculated HP by an extra 10.9% (1/.901, or 1.109).

     If you wish to calculate your own correction factor, here is a handy table of elevation (feet above sea level) vs. standard day atmospheric pressure (psi):

  • Sea Level, 0 feet - 14.70
  • 500 feet - 14.43
  • 1,000 feet - 14.18
  • 1,500 feet - 13.92
  • 2,000 feet - 13.67
  • 2,500 feet - 13.42
  • 3,000 feet - 13.17
  • 3,500 feet - 12.92
  • 4,000 feet - 12.69
  • 4,500 feet - 12.45
  • 5,000 feet - 12.23
  • 5,500 feet - 12.00
  • 6,000 feet - 11.78
  • 6,500 feet - 11.56
  • 7,000 feet - 11.34
  • 7,500 feet - 11.13
  • 8,000 feet - 10.91
  • 8,500 feet - 10.71
  • 9,000 feet - 10.51
  • 9,500 feet - 10.30
  • 10,000 feet - 10.11

     Yes, the detail oriented will notice that I'm ignoring lots of small effects of higher pressure ratios in the compressor, lower density air across the intercooler and even the fact that there's less wind drag at higher altitudes, and they're right. However, the overall concepts above still hold true.

     I hope this helps further your understanding about the quarter mile and some of its myths. Here's your chance to get some numbers of your own, so you'll be more accurate when you estimate your car's true power.


Bill Watson is a Sr. Engineer at Honeywell Jet Engines, and database manager of over 400 Honeywell employees' 1/4 mile runs. He has extensively studied the relationship between variables in 1/4 mile performance.


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