Gti6 Revs?

[Guest Kes]

What matters is tractive effort, this is the accelerative force produced by the combination of engine torque at a given rpm, multiplied by the effect of the gearing and the wheels to produce a linear force. Even though the engine produces less torque at higher rpm, the multiplying effect of the gear ratios almost always means that higher revs produce a greater tractive effort. Definitely in the lower gears, where the numerical difference in the ratios is quite large, changing gear at the rev limit will give the highest tractive effort. The only time it may not make sense, depending on gear ratios and the engine torque curve is for the 4th to 5th gear shift.

 

If you have a dyno graph and you know the gear ratios, it's relatively straightforward to plot a tractive effort curve and determine the optimum gear change point for any car.

 

Just as a quick example, from a graph I quickly found for an MI16 engine, and using MI gear ratios, the graph shows a standard engine produces approx 100lb ft of torque at 7250 rpm and about ~130lbft at 5000 rpm. If you change gear from 2nd to 3rd at 7250 rpm, you'll be at approximately 5000 rpm in 3rd gear after the change, so the torque is higher. Factor in the gear ratios, 100 x 2nd gear x final drive = ~820, plays 130 x 3rd gear x final drive = ~737, so more tractive effort is available if you hold 2nd gear till 7250 rpm, compared with changing to 3rd gear earlier. More tractive effort = faster acceleration, assuming that acceleration is not traction limited!

[Cameron 16]

Maybe I wasn't clear enough, I don't mean you should be short-shifting every gear! In the lower gears, 1st 2nd and possibly 3rd the torque multiplication caused by the lower gearing will definitely outweigh the torque lost at high revs, and it makes sense that you should be revving as high as possible in the lowest gears. What I mean is, where the difference in torque multiplication falls in the higher gears, i.e. 5th and 6th, and to a lesser extent 4th, it may well be better to short-shift than to rev all the way to the limiter.

[Galifrey 1]

Maybe I wasn't clear enough, I don't mean you should be short-shifting every gear! In the lower gears, 1st 2nd and possibly 3rd the torque multiplication caused by the lower gearing will definitely outweigh the torque lost at high revs, and it makes sense that you should be revving as high as possible in the lowest gears. What I mean is, where the difference in torque multiplication falls in the higher gears, i.e. 5th and 6th, and to a lesser extent 4th, it may well be better to short-shift than to rev all the way to the limiter.

 

Surely a skilled driver would change gear just before hitting the limiter? I know I do..

[Rob Thomson 6]

Most production cars have closer ratios between the high gears than the low gears. That's why reducing final drive ratios tends to help acceleration, because it brings those close ratios into a more useful speed range. It also means that at higher road speeds the maths does start to swing back in favour of changing early, but I still reckon it's advantageous to change at the red line in the vast majority of cases.

 

Just as a quick example, from a graph I quickly found for an MI16 engine, and using MI gear ratios, the graph shows a standard engine produces approx 100lb ft of torque at 7250 rpm and about ~130lbft at 5000 rpm. If you change gear from 2nd to 3rd at 7250 rpm, you'll be at approximately 5000 rpm in 3rd gear after the change, so the torque is higher. Factor in the gear ratios, 100 x 2nd gear x final drive = ~820, plays 130 x 3rd gear x final drive = ~737, so more tractive effort is available if you hold 2nd gear till 7250 rpm, compared with changing to 3rd gear earlier. More tractive effort = faster acceleration, assuming that acceleration is not traction limited!

Yep, and 'tractive effort' at the wheels is directly proportional to power at the flywheel, which is why peoples obsession with torque is so misguided.

[Cameron 16]

OH GOD NO!! Pleeeease don't start up the torque vs power argument, its been peaceful for so long!

[taylorspug 7]

:)