can someone explain how your speed (MPH) can change and your time (E/T) stay the same, if you don't have any wheel spin or slipping in the drivetrain how can this happen???

considering that speed is a calculation between distance traveled and time to get there (E/T). since the distance is fixed (1/8'th mile) for instance the only variable is the time it takes to get from point A to point B, if this changes it should change in both the E/T and the MPH shouldn't it ????

please help me understand this, I read how people jet based on MPH and say forget about E/T for this tuning operation but aren't they inter-related???

 

From http://www.nhra.com/basics/basics.html :

"The start-to-finish clocking is the vehicle's elapsed time (e.t.), which serves to measure performance. Speed is measured in a 66-foot "speed trap" that ends at the finish line. Each lane is timed independently."

Speed is only measured in the last 66 feet. How you cover the first 594' can change the speed at which you cover the last 66', even though you might cover the full 660' in the same amount of time.

 

Take two cars with the same horsepower, one with 4.10 gears and one with 2.73 gears. Both will have about the same trap speed (assuming neither car is past redline during the run), but the car with the 4.10 gears will have the better ET. The car with 4.10 gears will accelerate at a high rate for a short period of time, while the car with 2.73 gears will accelerate slower, but for a longer period of time. At the end of a fixed distance they'll still be going the same speed.

 

OK, got it !!!!

I didn't realize that there was a "trap" at the end of the track, so basically, if your jetted wrong the car will be "out of breath" at the end of the track and it will slow down your trap speed.

cool, thanks

 

If your jetted wrong, it'll run outta breath the whole way down the track.

 

Can't say I'd agree with that, at least not as a blanket statement.

How the engine acts off the line can be completely different from how it acts as the R's grow. Of course, that probably applies more to AV or VS type carbs than it does to DP's. Could also apply to EFI cars.