[Subscribe to Daily Digest] |
The desired result is acceleration, with the object being to minimize the time-to-speed and/or time-to-distance. Instantaneous acceleration is force (torque at the wheel divided by the moment arm) minus drag, the difference divided by mass. Time to speed is obtained by integrating the acceleration curve. Time to distance is the obtained through the double integral.
Now here it gets interesting. Wheel torque is engine torque multiplied by the effective gear ratio, (minus transmission losses). With an internal combustion engine, there's zero torque at zero speed, so you have to do something to get started - otherwise, you just sit there for all time. Engines have a torque peak (or with our turbo's engine electonics in gear, a plateau with a fall-off), and a little higher rpm above that, a power peak. Then there's a red-line, the maximum allowable engine speed based on the mechanical limitations of the design. For most modern production engines, it's valve float. **Digression...For the Saab engine, it's been demonstrated that at rediculously high rpm (like, for example, when a one-lap driver catches 2nd instead of 4th on a down shift) the valves float right into the pistons but the rods and pistons stay together.**
When you get to the top end of the engine's power band, if you want to keep accelerating, you need to shift. Now everything stays pretty much the same, except the effective gear ration drops. Optimum acceleration occurs when the wheel torque in the higher gear, just after you've shifted, exactly equals the torque in the lower gear just before you've shifted. Shift too soon, and you've lost the advantage of the lower gear ratio. Shift too late and you're subjecting yourself to a lower acceleration rate for some period of time. Hence the attraction of a higher number of more closely spaced gear ratios, even for our turbo engines with their broad, flat torque bands.
So back to the original question, which form of torque curve is better? For racing, it's the curve with the greatest area under it between the entry point (the rpm the engine is at when you shift into the next higher gear) and the shift point. For "the fun of it", you might prefer a late torque peak, b/c with drag increasing with speed, that keeps a more uniform acceleration level through the gear, which might be more fun. But now we're aruguing matters of taste...
posted by 24.166.84...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
