Thoughts about rev limits and peak power

[wirewick]

A question just came to mind:

Why don't manufacturers limit redlines and corresponding shift points (automatic) to best optimize peak power levels? For example, if a cars redline is at 7k and you let it shift itself during full throttle, wouldn't that take the engine out of its peak power band?

Just a thought.

[Benbay001]

What's the other option? Let it keep reving higher?

[HarryFlatters]

Because it sounds like a cow going Mooooooooooooooooooooooooooooo

[Claudia Skies]

Torque

Economy

[Toltec]

It depends what power you have at the rpm you are at after the shift. If you shift at peak power rather than after you may have less average power through the in gear rpm range.

[wirewick]

Benbay001 said:

What's the other option? Let it keep reving higher?

Have it shift sooner. To keep the revs closer to peak power.

[SturdyHSV]

wirewick said:

Have it shift sooner. To keep the revs closer to peak power.

Let's use some simple numbers and so on.

RPM - Power

2k - 200
3k - 250
4k - 300
5k - 350
6k - 400 (peak)
7k - 350

So you can imagine a little peak with a drop off after peak power yes?

Let's say for the sake of simplicity the difference between gear ratios means that the revs drop about 2k when you shift up.

So if you shift at or just after peak power, you'll drop to 4k and 300, climb back up to 6k (400) and shift again. Average of about 350.

If you instead shift at 7k, you'll drop to 5k and 350, climb back up through 6k (400) and on to 7k (350). Average of about 375.

Which do you think will be quicker?


Also, lower gears multiply the torque up by a greater amount, so even if the actual power figure was less, you may still end up with more accelerative force in the lower gear than if you shifted up.

[aka_kerrly]

Glad this isn't one of those topics where some chap knows someone who does drag racing and they always change up at peak torque so always win races!

Depends on the engine but some hit peak power an then power falls off a cliff, others can hit peak power but maintain the majority of it for perhaps another 1000 or 2000 rpm which is more efficient that changing up a gear and being lower down the power band.

[anonymous-user]

wirewick said:

A question just came to mind:

Why don't manufacturers limit redlines and corresponding shift points (automatic) to best optimize peak power levels?

er? Do you think that a large OEM would spend say £500M developing, testing, homologating and then selling a car and then think "Hmm, i wonder what rpm we should make it change gear at"?

You find that any car made in the last 20 years has very optimised shift points and rev limits, even crappy shopping cars.......

[Bobley]

I'm guessing you're referring to petrols or very sporty diesels Max? If you took my camper van to the redline would add seconds to the 0-60 time as the torque curve is going off a cliff after 3400rpm so there's no point in holding on. I just change up and I'm back on the wall of torque. The 4500rpm rev limit is just handy for engine braking on Winnets Pass!

[Mave]

SturdyHSV said:

Also, lower gears multiply the torque up by a greater amount, so even if the actual power figure was less, you may still end up with more accelerative force in the lower gear than if you shifted up.

I disagree with this bit. If the power figure is less you'll accelerate more slowly.

[jayemm89]

I find most sporty cars changing up at peak power drops you back somewhere with a bit more torque, which is useful.

Not quite the same but the owners manual for a 993 has maximum revs for downshifting without over revving the engine, don't get that stuff in manuals anymore

[McSam]

Excellent post from SturdyHSV which summarises it nicely. I've never seen any powertrain where you should change up AT peak power, it's always a fair bit afterwards, even in an extremely peaky engine that drops off a lot after - in that case, odds are the curve drops off a lot the other side too!

In most performance-oriented road cars, the rev limiter is set at the point where changing up would always be advantageous, to bring the engine back into its optimal power region. Letting it rev beyond this point is an unnecessary durability challenge, hence setting the limiter. So change immediately before the limiter.

[xRIEx]

wirewick said:

if a cars redline is at 7k and you let it shift itself during full throttle, wouldn't that take the engine out of its peak power band?

'Peak power' and 'power band' are two different things; I'm not aware of any sort of "peak power band" - power band AIUI is defined as the rev range between peak torque and peak power.

The rev point (and associated torque and power) in gear Z and the rev point in gear Z+1 is dependent on the difference between the two gear ratios. As Max_Torque said above (and he knows plenty about this sort of thing), manufacturers spend a lot of time working out the best configuration(s) of engine characteristics to gear ratios to diff ratio to wheel rolling circumferences.

wirewick said:

Benbay001 said:

What's the other option? Let it keep reving higher?

Have it shift sooner. To keep the revs closer to peak power.

Surely shifting sooner would result in revs further away from peak power in gear Z+1?

The only way to keep revs close to peak power (or any chosen point of the rev range) when changing gear is to use closely-spaced ratios; this is what we're seeing with the proliferation of 7, 8 and 9 speed gearboxes.

[xRIEx]

Mave said:

SturdyHSV said:

Also, lower gears multiply the torque up by a greater amount, so even if the actual power figure was less, you may still end up with more accelerative force in the lower gear than if you shifted up.

I disagree with this bit. If the power figure is less you'll accelerate more slowly.

If power figure multipled by gear ratio is less, you'll accelerate more slowly.

[Robb F]

Mave said:

I disagree with this bit. If the power figure is less you'll accelerate more slowly.

Gears don't do anything then?

What will get off the line quicker, 100bhp car in first or 200bhp car in 5th?

[Mave]

xRIEx said:

If power figure multipled by gear ratio is less, you'll accelerate more slowly.

The question posed is talking about change up points, so I assume road speed is a given.
Power is force X velocity, so if velocity (road speed) is fixed then acceleration is directly proportional to power.

[Mave]

Robb F said:

Gears don't do anything then?

What will get off the line quicker, 100bhp car in first or 200bhp car in 5th?

The car that is making 200bhp will get off the line quicker than the one making 100bhp.

[PhillipM]

xRIEx said:

If power figure multipled by gear ratio is less, you'll accelerate more slowly.

No, power already negates gearing effects, if you're making more power, you'll accelerate more quickly.

[xRIEx]

PhillipM said:

xRIEx said:

If power figure multipled by gear ratio is less, you'll accelerate more slowly.

No, power already negates gearing effects, if you're making more power, you'll accelerate more quickly.

Sorry yes, I was thinking of this but got it wrong:

chris182 said:

Acceleration = Force / Mass

Force = Wheel torque / Wheel radius

Wheel torque = Instantaneous Engine Torque / ( Gear ratio * Final Drive Ratio )

Instantaneous Engine Torque = Instantaneous Engine Power / Instantaneous Engine Rotational Speed

Instantaneous Engine Rotational Speed = Vehicle Speed / ( Gear Ratio * Final Drive Ratio * Wheel radius * 2 * Pi )

Combining the above and simplifying gives the following result:

Acceleration = ( Instantaneous Engine Power * 2 * Pi ) / ( Mass * Vehicle Speed )

i.e. Acceleration is proportional to instantaneous power not torque.