Is there an optimum time to shift....

Type_R

Type_R

Type_R

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..I was driving today and started to think about the timing of changing up and down the gears. Its a manual car so one can change up at 2500 rpm or at 6500 rpm. Now clearly depending on whether you are aiming for economy or performance, where you change in the rev range is going to change, but where is the optimum gain in power/performance/economy for any given car?

The car in question is a MK5 Golf GTi (200 PS) remapped. What pieces of data are required to work out the optimum rpm to change up for economy or performance? How can this be worked out?
 
Type_R said:
..I was driving today and started to think about the timing of changing up and down the gears. Its a manual car so one can change up at 2500 rpm or at 6500 rpm. Now clearly depending on whether you are aiming for economy or performance, where you change in the rev range is going to change, but where is the optimum gain in power/performance/economy for any given car?

The car in question is a MK5 Golf GTi (200 PS) remapped. What pieces of data are required to work out the optimum rpm to change up for economy or performance? How can this be worked out?
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If you can get a hold of a map then you should see a dip when it reaches the higher rev range, just at the dip. Over revving is not always good for performance.
 
Just go by feel I guess. Depends entirely on how you are driving.
Under low throttle opening you're going to be shifting up sooner. Maybe you never really paid attention but you'll notice the engine will struggle to get enough air to rev higher with low throttle. Try it, keep it on low throttle and let it rev up and see how the power tails off.

If you're on full throttle then it'll be dependent on how the engine is tuned. Mainly cam really, for example a VTEC will make power all the way to the redline, my car however peaks at 5500, even though it will rev to 6750 it just doesn't go anywhere up there so i'll shift up at 5750-6000.
 
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For economy there is an optimum time to shift. Sometimes it's less economical to be shifting uber early and then having to give more throttle as it bogs down more in the next gear. It normally pays to use generally as little throttle as possible but not so slow you are using more fuel than it would have taken to quickly squirt it to get you up to a given speed if you see what I mean. For economy it is best to shift gear quickly to maximize momentum and speed, wasting nothing. I wouldn't have though this was easy to work out at all due to the many variables and different conditions/load a car can be under.

So in conclusion, yeah, just go by feel and how fast your wallet empties itself.


For performance there is always going to be a point in the rev range where you have reached and gone past peak torque. i.e. Would have been quicker to change up rather than hit the redline in each gear. Some cars this might be 3000rpm say in a turbo diesel, where as a Honda type R might be 8000rpm.
 
For performance it's a balancing act. The power output of most engines tails off before the redline, so you don't necessarily want to hang on to the redline in every gear - however with that said, changing up too early will drop you into the next ratio too soon and leave you below the powerband of the engine, (again, depending on it's characteristics) which will lose you performance.

So it varies from car to car.
 
There comes a point when near redlining when acceleration slows and noise just increases. So it depends if you like the noise or not :p

If I've got my OAP hat on I change up at 2000rpm. if I'm driving it like it's stolen it's usually just between 6500/7000rpm (limiter at 7800).
 
I feel a little compelled to post, but the Motors collective have gone over this topic a number of times before. We definitely need a Motors subject matter FAQ for things like this!

For maximum performance, the appropriate time to shift up is where the torque produced at the wheels minus drag in the next gear, is greater than the torque produced at the wheels minus drag in the current gear.

In effect, this means it is very much dependant on your gear ratios and torque curve, with a small dependence on drivetrain efficiency (measured from flywheel to the road, as the amount of drive being applied to the road is the most important thing).

If you are struggling with the concept, remember a few key things:

1) The engine produces torque at the flywheel at a certain RPM
2) The transmission and final drive gears this torque by reducing the RPM in order to increase the torque
3) One revolution of the engine transmits directly in to 1/X revolutions of your drive wheels, where X is the transmission and final drive torque multiplication of a specific gear - therefore more torque multiplication equals lower road wheel RPM and thus lower road speed

As an example, imagine you are travelling at 30 MPH in 1st gear. Your engine is producing 200 Nm of torque and it is spinning at 6000 RPM. Let's say the transmission and final drive ratio at this point is 1:1, therefore the torque at the wheels is 200 Nm. If you were to change up in to 2nd gear, the engine revs may drop to 4000 RPM. At this point the engine itself may produce 250 Nm, but to drop the RPM to 4000 RPM and retain a road speed of 30 MPH a ratio of 0.666:1 must be applied. This ratio has the effect of reducing the torque output to 0.666 * 250 Nm, or 167 Nm - less than the 200 Nm it was putting to the wheels in 1st. Therefore, the car accelerates slower. It would probably be practical in this example to continue accelerating in 1st gear until the engine is producing so little torque than the 0.666:1 gearing ratio in 2nd is overcome. I have ignored it for this example, but the drag/drivetrain loss is proportionally higher at higher RPM, so this is a (small) benefit of accelerating in lower gears.

If you have a good dyno printout and your gear ratios it can be calculated relatively accurately. Even better if you have a dyno printout with drivetrain loss.
 
PMKeates said:
post on tractive effort without mentioning tractive effort.
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So basically its tractive effort.

clip_image0023.jpg


Change gears at the speed each graph line cross, of coure you can work which rpm that is per gear aswell.
 
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