CAMARO6 - View Single Post - M6

draph · 01-13-2017, 02:04 PM

I got my mechanical engineering degree a long time ago and my career started in engineering project management (ie don't feel I've ever been a "real" engineer); but my gut tells me there isn't a post that got it right quite yet...

Posters so far are using engine torque, not the force of the tire contact patch with the ground to accelerate the car. Power is the measure of the speed of torque delivery to the ground - so you want maximum power to the ground for maximum acceleration.

Maximum acceleration in a car is at the power peak point in a given gear. So, what gear achieves maximum acceleration at a given speed is the gear that puts the rpm closest to the power peak- BUT, if the power curve falls off quickly after the power peak, or happens to be peak operational rpm, one might want to select a gear that puts rpm below the power peak. That might be a better trade-off than shifting to land right at the power peak, only to have to shift again a millesecond later, because the power falls off 40% or you hit fuel cutoff 200 rpm later. Arguably, cars with peaky power curves require more gear ratios to enable keeping the power delivery in the narrow sweet spot of the rpm range. No engine has a perfectly flat power curve, but big V8s and twin turbos have a flat enough power curve, you can probably be plus or minus 1000 rpm from the peak power point that you will be very close to optimum acceleration.

We can partially blame automotive press and marketing agencies for misrepresenting torque to a certain degree. On those puny 4 cylinder engines and diesels, turbos have helped add torque at low rpms, which flattens out the *power* curve (more torque delivered at lower rpms creates more power at those rpms); but focusing on torque is making people forget it's all about the *power* - speed of delivery of torque for accelerating a car.

If above doesn't help: One might notice torque ratings on some cars are now shown as "300 lb feet from 1100 to 5000 rpm". If max acceleration was all about torque, then that would suggest maximum acceleration in that particular car can be achieved using the top gear starting at 1100 rpm. That, obviously, is not true, because peak horsepower for that engine is NOT 1100 to 5000 rpm. Also, if that were the case, the car could have a 2 speed transmission, one for getting it off the line to 1100 rpm, the the other to achieve maximum acceleration and fuel economy at the same time from 1100 to 5000 rpm...

01-13-2017, 02:04 PM	#18
draph Drives: AcuraTL SH-AWD 6MT and '70 'vette Join Date: Mar 2016 Location: Indiana Posts: 46	I got my mechanical engineering degree a long time ago and my career started in engineering project management (ie don't feel I've ever been a "real" engineer); but my gut tells me there isn't a post that got it right quite yet... Posters so far are using engine torque, not the force of the tire contact patch with the ground to accelerate the car. Power is the measure of the speed of torque delivery to the ground - so you want maximum power to the ground for maximum acceleration. Maximum acceleration in a car is at the power peak point in a given gear. So, what gear achieves maximum acceleration at a given speed is the gear that puts the rpm closest to the power peak- BUT, if the power curve falls off quickly after the power peak, or happens to be peak operational rpm, one might want to select a gear that puts rpm below the power peak. That might be a better trade-off than shifting to land right at the power peak, only to have to shift again a millesecond later, because the power falls off 40% or you hit fuel cutoff 200 rpm later. Arguably, cars with peaky power curves require more gear ratios to enable keeping the power delivery in the narrow sweet spot of the rpm range. No engine has a perfectly flat power curve, but big V8s and twin turbos have a flat enough power curve, you can probably be plus or minus 1000 rpm from the peak power point that you will be very close to optimum acceleration. We can partially blame automotive press and marketing agencies for misrepresenting torque to a certain degree. On those puny 4 cylinder engines and diesels, turbos have helped add torque at low rpms, which flattens out the power curve (more torque delivered at lower rpms creates more power at those rpms); but focusing on torque is making people forget it's all about the power - speed of delivery of torque for accelerating a car. If above doesn't help: One might notice torque ratings on some cars are now shown as "300 lb feet from 1100 to 5000 rpm". If max acceleration was all about torque, then that would suggest maximum acceleration in that particular car can be achieved using the top gear starting at 1100 rpm. That, obviously, is not true, because peak horsepower for that engine is NOT 1100 to 5000 rpm. Also, if that were the case, the car could have a 2 speed transmission, one for getting it off the line to 1100 rpm, the the other to achieve maximum acceleration and fuel economy at the same time from 1100 to 5000 rpm...