Short shifting

[Vegard]

Always?? Sometimes?? Never???? So... when does it flashes???

It flashes at a suggested shift point. Some cars will benefit from earlier shifts, some from later shifts. From what I’ve seen I would guess that 2/3 of cars have the shift light set pretty well.

But having one set rpm for all gears is never going to be optimal anyway, especially for road cars.

 

[Mirror_man]

Hum... So... if we have a Dino Curve like this one for example,

At which RPM should we shif up??? (more or less)

 

[Vegard]

At which RPM should we shif up???

Depends on the gears.

If they're crossed, slightly past peak power, say 7800'ish.
If they're spread out, close to the limiter.

 

[Mirror_man]

Depends on the gears.

If they're crossed, slightly past peak power, say 7800'ish.
If they're spread out, close to the limiter.

Crossed?? Means... close to each other???

 

[Vegard]

Crossed?? Means... close to each other???

Yes, the old super close.

 

[GT_Alex74]

Well, if you really want to know when to shift, lay a grid over the power curve, and find out by how much your RPM drops when upshifting (this is probably different for each gear). Now you want to fit that RPM band on your graph to maximize the area under the power curve to get the best out of your engine.

I will take the graph posted above as an exemple. For convenience, we will consider the plain line is power and ignore the other one. Also, we will consider each gear gives the same RPM drop. I'll round the figures a bit because the graph lacks a bit of precision there.

Case 1 : upshift causes a 1000 RPM drop. Here, peak power is actually a flat section spreading between 5000 and 6500 RPM, so we can just fit in between. As that power band is wider than what we're actually using, we can shift between 6000 and 6500 RPM an not suffer any power loss. Well, if you don't care about fuel, you will want to go up to 6500 RPM because it will make a better use of the shorter gears, but if you want to save a bit of fuel, you can just shift at 6000 RPM and you probably won't really notice any speed difference. The M4 Gr.4 kinda fits that case, hence why you would see it do no pits in some C daily races when other cars couldn't without using fuel maps.

Case 2 : upshift causes a 3500 RPM drop. Now that's different, because if we shift at 6500 RPM, we will suffer a power loss on next gear from 3000 to 5000 RPM. We want to minimize that, so we have to shift a bit later. Now the power drops even more abruptly after 6500 RPM, but that doesn't mean we can't go into that zone a bit : there's still more power right after 6500 RPM than there is right after 3000 RPM. Basically, we want our shifting point and our drop point to deliver the same amount of power, that's how you'll maximize the area under the power curve. Here, that shifting point would be about 7200 - 7300 RPM, which would drop us to a bit less than 4000 RPM. You can see the engine delivers about the same power in both places, so shifting earlier or later will bring you to a place where you'd get less power. That means you will lose some speed if you short shift it in order to save fuel.

 

[Vegard]

@GT_Alex74

It's worth noting that as you change the rpm at which you shift you also change the rpm drop.
A 3rd to 4th shift that drops from 4000-3000 will also drop from 8000-6000.

I know you chose to use the torque curve for practical reasons but I think you might have accidentally explained how to get the best compromise between acceleration and fuel economy - pretending the torque curve is the power curve.

 

[KinLM]

An important fact you all seem to be forgetting is that “peak power” is also different depending on what gear you’re in.

For instance, shifting from 3rd to 4th. Making 800 engine HP in 3rd will produce far more power than making 800 engine HP in 4th. This is due to the shorter gear multiplying that engine power by a greater amount than the longer gear ratio.

To truly find the best shift point, you must multiply the engine HP by the GEAR RATIO, and then find at what point this power output is less than the engine HP multiplied by the GEAR RATIO of the next gear.

This will obviously be past peak power, as the peak power multiplied by a shorter gear ratio (and thus a bigger number) will clearly be much higher overall HP than shifting up a gear to where your RPM is not even in peak power and multiplying that by a smaller number.

 

[Vegard]

An important fact you all seem to be forgetting is that “peak power” is also different depending on what gear you’re in.

For instance, shifting from 3rd to 4th. Making 800 engine HP in 3rd will produce far more power than making 800 engine HP in 4th. This is due to the shorter gear multiplying that engine power by a greater amount than the longer gear ratio.

To truly find the best shift point, you must multiply the engine HP by the GEAR RATIO, and then find at what point this power output is less than the engine HP multiplied by the GEAR RATIO of the next gear.

This will obviously be past peak power, as the peak power multiplied by a shorter gear ratio (and thus a bigger number) will clearly be much higher overall HP than shifting up a gear to where your RPM is not even in peak power and multiplying that by a smaller number.

Gearing multiplies torque, not power.

Simplified:
1xTorque x 1xRotational speed = 1xPower
2xTorque x 1/2xRotational speed = 1xPower

Rotational speed is the speed of the driven wheels, not engine rpm.

 

[KinLM]

Gearing multiplies torque, not power.

Simplified:
1xTorque x 1xRotational speed = 1xPower
2xTorque x 1/2xRotational speed = 1xPower

Rotational speed is the speed of the driven wheels, not engine rpm.

Yes but since Power is simply work over time then the two should still be closely related.

Even if you change the above quote of mine to torque, the fact remains that the gear ratio remains relevant when choosing when is the “optimal” time to upshift.

 

[Vegard]

the fact remains that the gear ratio remains relevant when choosing when is the “optimal” time to upshift.

The difference in ratio between the two gears you're shifting between matters, yes, because it determines where you land in the rpm range. But when you use power to determine that optimal shifting point you don't have to worry about torque multiplication.
The power equation "understands" gearing in that sense.

 

[GT_Alex74]

It's worth noting that as you change the rpm at which you shift you also change the rpm drop.
A 3rd to 4th shift that drops from 4000-3000 will also drop from 8000-6000.

As I said in my post. This will definitely happen with a stock gearbox. But if tuning is available, then you can actually stage your gears in order to make it "fit" your powerband. You can also set the lower gears so you don't have too much power for some corners.

An important fact you all seem to be forgetting is that “peak power” is also different depending on what gear you’re in.

For instance, shifting from 3rd to 4th. Making 800 engine HP in 3rd will produce far more power than making 800 engine HP in 4th. This is due to the shorter gear multiplying that engine power by a greater amount than the longer gear ratio.

To truly find the best shift point, you must multiply the engine HP by the GEAR RATIO, and then find at what point this power output is less than the engine HP multiplied by the GEAR RATIO of the next gear.

This will obviously be past peak power, as the peak power multiplied by a shorter gear ratio (and thus a bigger number) will clearly be much higher overall HP than shifting up a gear to where your RPM is not even in peak power and multiplying that by a smaller number.

While this is true, in most cases, the power drop at the end of the RPM range hits the wheel power more than the gearing difference. There are some cases with road cars gearboxes where the next gear is so much longer that you want to push the current one to the revlimit to spend less time on the longer one, but generally the benefit mostly comes from dropping too many RPMs. I think the best situation to go after the ideal power band is when it allows you to save a gear change before a corner. But yeah, I never bother calculating that, at most it would change your ideal shifting point by 100 RPM, and most drivers probably don't have that much precision when shifting. Unless you have like 100% power between 4000 and 6000 RPM, and 90% power at 8000 RPM, it's neglectible, really (there are cases like that though, but you'll notice it while driving).

 

[Bruunz]

The point of my comment was that the bar doesn't flash when peak power is reached.

You are right. I even sat down and measured it too lol.

55" TV - 1080p the graph is 8" wide. In the case of the corvette gr3.

0" = 1000rpm
8" = 7000rpm

Every inch = 750rpm

0 = 1000
1 = 1750
2 = 2500
3 = 3250
4 = 4000
5 = 4750
6 = 5500
7 = 6250
8 = 7000

Peak Power is at approx 6.5" which is about 5900rpm rounded.