I know you are supposed to go through the traps in 4th gear but i just want to know why? Thanks

 

it all depends on your gearing where you cross the traps at. You wouldnt want to cross the finish line in an overdrive gear now would you.

 

You want to go through the traps in high gear (whatever 1:1 is). Anything less and the transmission is still trying to multiply torque through lower gears which isn't needed at the top end. If you go into overdrive, the OD gears are small and weak. They also use more HP to turn through another gear set compared to 1:1 that goes straight through the transmission.

 

thanx

 

Even if you powershift, it takes time to shift. The flow of power to the rear wheels is interupted and that slows you down. So, on that basis you want to do as few shifts as possible. OTOH, the more time you spend in a lower gear, the faster you accelerate due to the torque multiplication effect of lower gears. The best balance with a car having the broad torque curve and power to weight ratio of a 4th gen (and the majority of street and street/strip cars) is to use 4 gears in the quarter mile.

For a much lighter car with more power 3 or even 2 gears may be ideal. My friends alcohol Hemi altered uses a 2-speed Lenco, for example. A car with a narrow power band, even if powerful, may need more gears. Pro Stock cars run 5-speeds. If it's an automatic, fewer gears are needed. The torque converter helps keep the motor in the powerband. Lot's of fast drag cars run Powerglides (two speed automatics) but if they are heavier, a 3-speed (TH400) is usually used. Note that all of these transmissions shift much faster than you can shift a T56, but even so they strike a balance between torque multiplication and number of gears.

Top gear is almost always a "direct drive" 1:1 ratio so that at least one gear has minimal parasitic losses in the transmission. If it were possible, the ideal transmission would have an infinitely variable ratio that would keep the motor at the peak of the power band, slipping just enough to maximize tire grip. At high levels of drag racing, they accomplish something approaching this using clutch slippage. This is not controlled by the driver but by the setup of the clutch. In drag racing, due to tradition and the attempt to control costs, this is all done mechanically. Formula One uses very sophisticated computerized control systems for "launch control". But they have yearly budgets in excess of $100 million for the top teams.

Race transmissions have many internal gear ratios available which are chosen to keep the motor in the powerband. With a street strip car, you can't readily change the internal gear ratios in the tranny. So what you are left with is changing the rear end ratio and tire diameter to allow you to pull through the traps at the peak of the powerband in 4th gear.

Rich Krause