I got this from the BMW board. (1995 M3) I thought you guys might like to read it.
So I beat this guy at the stoplight, my car is so fast!
or Why Street racing is a crock to compare cars
How many times have you heard this? I have tried for years to explain to people just why a simple thing as leaving a 1/4 second later than the next guy can make a slower car beat a faster car. Or how on a rolling go, i.e. on the freeway, flooring it first can mean you will continue to pull away unless the other car is much faster. Someone finally posted to the BMW M Coupe board on bimmer.org exactly what happens. Kevin Willmorth explained it better than I have been able to and I asked his permission to post it. Below is his post (which he and I edited to create a more accurate and generic illustration):
Anyone who drag races regularly knows that between two roughly equal cars, reaction time is king. Not top end speed, nor statistical B.S. datasheets. Reaction times are what will win a race, more often than equipment superiority, unless... the two drivers happen to get away at the same instant, which is highly unlikely in a street race.
Here's how it works:
You are at the light, and car "R" lines up on you. It's a dweebs toy, and you decide to go for it. Light turns green and you put the hammer down. But, man! he's off the blocks and pulling away already! You keep the hammer down, but just can't catch him. By the time you back off at 100mph, he's like 2 car lengths away. Must have been a more powerful car!
If at the launch they get 1/4 second on you, they are accelerating 1/4 second before you from that moment forth (you cannot get the 1/4 second back, it's gone forever). That's a whopping 22 feet at 60mph, assuming exactly equal cars and drivers. (BIG assumption, esp. in street racing).
Let's look at this from the start: From the starting line... 1/4 second is a distance of 2.2 feet at 6mph (a door length), then 9 feet at 25mph (half a car length), then 16+ feet at 45mph (full car length), 22 feet at 60mph (he's pulling away!!!), then at 100mph, they got you by a total of 37 feet, or nearly 2 car lengths (you got whacked!) And that's with an exactly equal car, identical shifting at exactly the same RPM's!
But wait you say! 1/4 second difference, that's a lot! Yes, it is for a pro drag racer, who operates under a .400 pro tree, or even a bracket racer running on a .500 second tree. But for a street racer, pulling on a street light? .250 is a close pairing! Heck .500 would be a great start. You aren't really sure when to go, no yellow staging lights... It won't seem like much time, and you might even figure, "OK, I missed the launch a tiny bit, but that's not important, I'll get him!"... you would be wrong of course, very wrong...
Here's an interesting factoid. Reaction times are NOT equal in cars with identical 0-60 or 1/4 mile times! Heavy cars are generally slower reacting than lighter cars. Some suspension designs react faster than others. Short wheelbases react more quickly than long. Soft suspensions take longer to move the car forward after throttle is hammered down. Stiff cars jump more readily. Heavy wheels take more power to accelerate, (time to rethink those 20" rims!) Remember that 1/4 second reaction time is a culmination of YOUR reaction to the light PLUS the cars reaction to your giving it the boot!
Just because the car you line up on has an inferior 0-60 time, does not mean you will beat it. It depends on the cars design, mass location, and a pile of other factors. It also depends on whether you get him off the line, or he gets you.
Line up on a fast reacting driver, piloting his lighter car, and he can take you out, even if you are the faster car!
For example, let's just say that in his "slower" car (car A) he gets you by that 1/4 second at the start. You take off in car B with your 1/4 second faster 0-60. Both cars will hit 60 at the same time, but you will still be behind car A! Up to this point, car A was only pulling away. From 60 mph on, you should be accelerating faster. Since you now have an equal,(soon to be faster) speed, the gap will finally stop increasing. Once both cars have an equal speed, the faster accelerating car, (B - this is you, remember?), will start to close that gap. So when will you finally pull even? Likely around 70-75 mph, depends on the car. So you see, that small start difference makes a huge difference in the outcome. How many street races do you know that go over 75? (I know they happen, but by them most are just racing in traffic and being stupid, not racing each other by that time). What if you started 1/2 second later? 1 second later? Count out one thousand one to yourself. See just how little a difference that can be. How many times can you think of where you hit the gas 1/2 second after the other guy? The average reaction time for most people is 1/2 second. So if you wait until you see him move, that is 1/2 second before you even hit the gas! And that assumes that your car reacts as quickly as theirs!
The shorter a race is, the more critical the launch becomes. Since street races have neither a fixed set of starting lights, or a fixed distance, the outcome has less to do with the capability of the cars, than it does the arbitrary start and undefined length of the race itself. Get a head start on the opponent and cut the race short if you know you are slower. If you start late, just stretch the race to the longest possible distance and highest top speed! (We are assuming you are the "faster" car -oh and we will not be responsible for your speeding!)
Because of these factors, street races are total crap as a judge of anything, other than who got off the line first, who's car might react the best to sudden off-line acceleration, and how long two idiots are willing to keep it up in the midst of traffic and road obstacles. Just be the quickest to leave, or stretch the race out longer, and you'll win more races, as long as the cars are anywhere near close in performance!