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Thread: Relation between ET and trap speed in drag racing

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    Brazilian Consultant Redliner's Avatar
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    Relation between ET and trap speed in drag racing

    Ok, this will probably turn into a huge discussion, but I will ask anyway.
    We have a drag racing forum and someone mentioned the relation between ET and trap speed, saying that the lower the ET, the lower the trap speed, but couldn't explain why, and I also can't understand how isn't the speed higher
    Could someone please give me a good explanation or point out why he is wrong?

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    the lower the ET, the higher the trap speed. but the thing is, it varies because of traction, gear ratios, and how the car is driven.

    example:

    Marty Ladwig and his Cobalt SS Pro FWD dragster did an ET of 7.344 with a 200.17mph trap speed back in April at Richmond. in the finals of that event, Marty posted up an ET of 7.634 with a trap speed of 169.36. as you can see, the trap speed went down considerably but only lost .3 of a second in ET.

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    rides with Rebecca Black.. in the back thedguy's Avatar
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    elmo187 is right.

    The trapspeed is an indicator of how fast the car is capable of. I recall reading that a version of the e46 bmw m3 can run low-13's. A USDM wrx sti or evo 8 can also run 12's times. If you look at their trap speeds you can see that it has everything to due with traction/chassis setup. The M3 has a higher trap speed, if they could get more traction off the line it would have no trouble beating those evo's.

    Chrysler actually published a list of how fast a car should be able to go given a certain trap speed. I have a copy of it somewhere but to find it would take me an eternity.
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    Quote Originally Posted by thedguy View Post
    elmo187 is right.

    The trapspeed is an indicator of how fast the car is capable of. I recall reading that a version of the e46 bmw m3 can run low-13's. A USDM wrx sti or evo 8 can also run 12's times. If you look at their trap speeds you can see that it has everything to due with traction/chassis setup. The M3 has a higher trap speed, if they could get more traction off the line it would have no trouble beating those evo's.

    Chrysler actually published a list of how fast a car should be able to go given a certain trap speed. I have a copy of it somewhere but to find it would take me an eternity.
    that was my post in some other thread, you got a memory like an elephant, kudos for that i cant even find that thread anymore.
    Last edited by MadCow809; December 15th, 2007 at 3:56 PM.
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    Trap speed is an indicator of horsepower.

    It's true that if you get a better launch, you're at higher speeds earlier. In road racing, the earlier you can get on the throttle the more speed you'll have going into the next corner. Road racing is just drag racing between corners.

    The thingi s, if you look at say, a graph of the speed of the car through the whole 1/4 mile. It increases VERY quickly at first, and then it starts to level off. It levels off enough to the point that you can actually get a rough estimate of how much horsepower (rough estimate, but most ways of measuring horsepower ARE rough estimates aside from bolting an engine up to a dyno. Chassis dynos aren't all that accurate) a car has. Things like weight and frontal area/aerodynamics have an effect also, but that's why it's more of a rule of thumb thing.

    For instance, I have a friend with a 1990 Camaro with a hopped up little 305 in it that has 243hp and 243 tq according to a chassis dyno. Those numbers may be inaccurate since he has a really loose torque converter (drag race thing) that skew with chassis dyno readings and makes them look worse, but they help ETs.

    He regularly traps that car at about 102-104 mph at the 12.60-80 range. Last time we were at the track there were some awesome looking hopped up new Porsche 911 Carrera Turbos. They just couldn't get that car to launch off the line at all. They either spun the wheels or bogged it every time, and they were doing 12.70s. The difference from my friends camaro with the little 305 in it and these porches was that my friend was trapping at 102 mph, these porsches were trapping at nearly 130 mph.

    So that's a good example of how mph can be a good rule of thumb indicator for horsepower. Because even if you get a TERRIBLE launch, the speed graph over a 1/4 mile run levels out near the top. So if you ruin your first 10 or 20 feet, you still have another 1300 to go, get the idea? By the time you get to the end, you're not accelerating very fast anymore, so the speeds will be relatively consistent most of the time even if your times are not. Thats because trap speed is determined more by how much hp your car has than how good you are at driving it.
    Last edited by InfernalVortex; December 17th, 2007 at 11:07 AM.
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    Thank you! Now I can make more sense out of this.

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    Quote Originally Posted by Redliner View Post
    Could someone please give me a good explanation or point out why he is wrong?
    A relatively concise reason why he's wrong.

    The trap speed is how fast you're going at the end of the quarter. Your ET is how long it took you to get there.

    Speed = distance / time. Increase speed. Keep distance constant. Time drops.

    In words, you have to go faster to travel a set distance in a shorter time. So your ET should be inversely proportional to your trap speed.


    As people have said, the actual numbers vary depending on the car. Stock AWD turbo "rally cars" tend to have great ETs for their trap speeds compared to 2WD cars with bigger engines, since they get plenty of traction off the line but peter out as their relatively small engines and bad aerodynamics kick in at higher speeds.

    Of course, if you start ramping up the power in those AWD cars their trap speeds will increase as their ET drops so the above rule still applies.

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