Maxing a Car. What Happens? - Page 2

i've taken my s2000 to redline at 167 mph and it just hums on forever at that speed which is also right at the redline in sixth gear.

For the Miata example, there's a difference between a rev limiter and a speed limiter. You were hitting the rev limiter in the low gears, and either hit the speed limiter or ran out of power in top gear.

It's hard to see what happens at top speed on most new cars since they tend to be speed limited and not drag limited. If you have the chance to drive somethign with the aerodynamics of a brick and somewhat underpowered, you'll see what drag limiting will do. (One of the trucks I own weighs 5800lbs and puts about 75Hp to the ground).

You'll be cooking along at max speed in top gear, but if you depress the accelerator anymore nothing will happen except it will make more noise and burn more fuel. You wind up cruising at the point where two things occur; the horsepower output of the motor equals all resistant forces AND the vehicle doesn't have the available torque to the ground to cause anymore acceleration.

This is best shown when pulling a steep hill. You're cruising on flat ground at your max speed. You hit a hill and the vehicle will begin to slow, when it does this the speed and RPM will drop until you hit the torque peak of the motor. By slowing the rpm you will tend to lose power and gain torque (tend to, but it depends if horsepower peaks above or below 5252 RPM.) Once it hits the torque peak, the motor will level out again and you will pull the hill at that speed (properly known as the critical speed).

Now, if the motor has the ability to wind out and you were running a steep O/D this is where something neat happens. You can downshift; by downshifting you don't have anymore horsepower availble, but you get a greater torque multiplication. You now have enough torque to maintain acceleration past your previous max speed. You now cruise at a higher RPM in a lower gear to achieve a greater speed than you could have achieved in the higher gear.

That's why so many new cars make their top speed in their penultimate gear.