Question on max power @****rpm

Mister Jingles

Very amateur question here on power bands.

Correct me if I'm wrong but my understanding of an engines power band is where the most torque and power is produced ?

With diesels they produce their most torque within 1700-3000 rpm or a similar range but once they go past that there is very little purpose to continue through the revs, they produce no more power, therefore the next gear should be chosen ?

With petrols though my understanding is that there is no power range, the higher the RPM the more torque and power.

Most petrol cars will produce there maximum output after 5500 RPM. However like a diesel say after 6500 RPM, is it pointless to keep going ? Is it time to change gear or will it keep producing more power until it hits the limiter (very unhealthy I know but just put that aside for a minute) ?

I've never tried it in my car (going until it hits the limiter) but after about 6000 RPM I feel there is no more to be gained, or is there and its just me ?

challengemaster

wiki wrote:

The power band of an engine or electric motor refers to the range of operating speeds under which the engine or motor is able to operate efficiently

Specifically, power band is defined by the range from peak torque to peak horsepower (or sometimes to redline). For example: combustion engines typically generate maximum torque at perhaps 2500 RPM. The peak horsepower might be 5000 RPM. Such an engine would have a power band of 2500 to 5000 RPM, in which the engine would be very efficient.

Depends on the engine, more so than fuel type as to where that specific power band is, though.

Here's a dyno sheet from my own car, E46 320Cd.



So peak torque is produced around 2500RPM, but peak HP from 3000 to 4000. You can see the torque fall off after peak torque is produced.

The reason diesels give power/torque the way they do is because for the most part, you're dealing with a turbo. Turbo reaches full boost at ~1800RPM.

So why isn't peak HP earlier in a diesel then? Power is the product of torque multiplied by speed of rotation. Even if you have lower torque, multiplied by greater RPM you still get more power. eg. (ignoring correct units) 190*4000 = 760,000. 260*2500= 650,000

Petrol engines tend to have relatively flat torque curves that fall off near or before peak power, but for the same reason as above, the higher the RPM, the higher the power.

/edit: It's also NOT unhealthy to keep going until the redline before changing gears. So long as you do so when the car is at operating temperature, and you don't maintain those revs for prolonged periods of time. Engines are made to do that, otherwise they'd have the rev limiter much earlier

Dartz

It depends on gearing, torque and power.

The acceleration of your car is dependants on the amount of force created at the contact patches of the driving wheels, and the mass of your car. The more force, the more acceleration. Engine torque directly translates through the drivetrain to the contact patches, giving you more acceleration.

The more torque an engine has, the more it can push the car.

Power, is the rate of production of torque, and is tied in to how fast an engine is turning. The faster an engine turns, the more energy it puts out. An engine's power directly determines how fast your car can go. So, you can have a pair of engines producing 100bhp. One is big and torquey and turns slowly. One is small and light, and turns over very quickly, needing more rev's to do the same amount of horsepower.

In theory, with proper gearing, both engines will give the exact same top speed, though the engine with less torque may take longer to get there.

Complicating matters however, is gearing.
There's usually a gearbox of some sort between the engine and the tyres. What this gearbox does is create a mechanical advantage to make it easier to start the car moving without melting the clutch, while allowing you to change gears in a moving car so the engine can run at its most efficient RPM at higher road speeds.

Lets create two hypothetical cars. They're both the exact same in every way, except for the engine. In one, we put a 100bhp diesel that rev's to 4500rpm. In another, we but a 100bhp petrol engine that rev's to 6500rpm. The diesel engine'd car produces more torque than the petrol engine, but it rev's less.

Now, we drag race them.

Off the line, the diesel car will get a head start. It's putting out more force at the wheels. It'll accelerate reasonably rapidly up to 4500rpm, at which point the driver must shift to a new gear to continue acceleration. This gear gives much less mechanical advantage, so its acceleration rate drops. Diesels have to run taller gearing to go faster because the engines turn slower.

The petrol driver meanwhile, has another 2000rpm to play with before he needs to change gear. He continues to accelerate at a higher rate than the diesel car does in the new gear. But, he can also change his car before the race so that, when he's doing 6500rpm, he's travelling at the same speed the diesel is when it's doing 4500rpm.

What happens then?

Because power is essentialy the multiple of speed x torque, and both car's wheels are travelling at same speed, the force achieved at they tyres will actually be the same.

This is where the Power band comes in. Generally, when we shift from a lower to a higher gear, we want to shift at the point that gives us the maximum accelerating force at the wheels (To accelerate as hard as possible). Manufacturers tune their engine's to give power in a very specific way, depending on how they're to be used.

The timing of an engine's valves, it's combustion chamber, intakes and exhausts all determine how the engine makes its power. An engine makes it's peak torque when all these factors combine at their optimum so the engine is able to get the best out of it's fuel. This is where an engine is operating at its most efficient. If we tune for low RPM torque, we get an engine that feels great for pulling off the line with a solid punch to the back, but feels like it's running out of breath higher up the rev range.

It feels that way because that's literally what's happening. It just can't breath enough through intakes and exhausts tuned for lower RPMS.

The opposite is also true. An engine tuned for high RPM power, might feel gutless at low RPMS. It might struggle to idle even.

Compare the 90bhp 1.5 tubodiesel in a Renault Megane, the the Rotary Engine in an Rx-8. Both produce surprisingly similar torque figures around 200-210Nm. (I looked it up.). The difference being, the Megane's diesel does it at 1750rpm, while it takes 5500rpm or more for the Rx-8 to start doing it. If we put them both side to side, rolling in 2nd gear, and give them both a race, the Renault will pull ahead. Up to about 60kph when it has to go to third after hitting the redline at 4000rpm (It feels like it's being strangled after 3500). The Rx continues accelerating at the same increasing rate as the engine comes on song, goes past the Renault, and keeps going on up to 9000rpm and 100kph before needing to change gear, by which stage the Renault will need to be in 4th.

One of these cars feels easier to drive around town in traffic. Another feels better cruising on the open road.

If the Renault's engine could magically be made to safely spin at 9000rpm like the Rx, however, it wouldn't produce the same amount of power because it's not tuned for it. It can't breath fast enough to make power at that speed. The less efficiently the engine can breath, the more the torque drops off. Remember that, since power is the product of force and speed, if our torque starts dropping off faster than our engine speed increases, we start to loose power.

It's the same for a petrol engines - this isn't unique to diesels. There's nothing to be gained by strangling it beyond stressing the engine parts.

If you have peak power at 5500rpm, and your engine redlines at 6500rpm, your torque has dropped off so much that spinning the engine faster isn't doing any benefit. You might want to consider upshifting. Depending on your engine's torque curve, it might even give you more torque at the wheels as the engine RPM drops to a point more favourable to its breathing.

The reason people call diesels powerbandy is because they have such a short RPM range, and are often specifically tuned for low RPM torque. You can feel the engine kick in hard at low rev's while most petrols in comparison will smoothly build power. You can also feel the engine straining harder as the rev's increase and it comes out of its optimum operating range and torque starts dropping off again. Hold on to the gear for too long, and you've come out the top of the engine's power band and are in the regime normally reserved for downhill engine braking, short engine life, or fools trying to show off. Shift earlier for better effect.

With the Rx, the effect is the opposite, you can feel the engine coming in to its powerband and it goes for a rush towards the top end, before beeping to remind you to change gears. Change too early and it falls out of the powerband and feels a little gutless. Change on time and it just keeps rushing and rushing. Change too late and bits of metal start flying.

The thing with powerbands is that you can normally feel them when driving, and tell when an engine's running sweetly or straining, or lugging. If the engine feels like it's straining or breathless, it's because it probable is. It doesn't matter what sort of engine you have, it'll still be pretty obvious to you when you're thrashing your engine for no benefit. If you feel there's no more to be gained with more RPM, you're probably right.



....... wait. How the **** did that happen at 3am?

Jesus.

I like that graph of yours, Master.

Torque is where its at IMO

corkgsxr

challengemaster wrote: »

Depends on the engine, more so than fuel type as to where that specific power band is, though.

Here's a dyno sheet from my own car, E46 320Cd.



So peak torque is produced around 2500RPM, but peak HP from 3000 to 4000. You can see the torque fall off after peak torque is produced.

The reason diesels give power/torque the way they do is because for the most part, you're dealing with a turbo. Turbo reaches full boost at ~1800RPM.

So why isn't peak HP earlier in a diesel then? Power is the product of torque multiplied by speed of rotation. Even if you have lower torque, multiplied by greater RPM you still get more power. eg. (ignoring correct units) 190*4000 = 760,000. 260*2500= 650,000

Petrol engines tend to have relatively flat torque curves that fall off near or before peak power, but for the same reason as above, the higher the RPM, the higher the power.

/edit: It's also NOT unhealthy to keep going until the redline before changing gears. So long as you do so when the car is at operating temperature, and you don't maintain those revs for prolonged periods of time. Engines are made to do that, otherwise they'd have the rev limiter much earlier

not quite. Turbos vary due to needs. One might start boosting at 1500 and give boost till 3000. Another might start at 2500 and keep going till 5000.


Id call a power band a area of the revs where you have a noticeable hike in power.

like a 1.4 petrol has no hike its a smooth rise. So you wouldn't say it has a powerband

wheres my gsxr the powerband is 10-16k. Theres a jump in power.

modern diesels do feel powerbandy as its nothing till 1500 and has a burst till 3k

tossy

Jesus. wrote: »

I like that graph of yours, Master.

Torque is where its at IMO

Diesel driver?

Jesus.

tossy wrote: »

Diesel driver?

Ha! Actually no believe it or not! But the other half has a wee diesel Rio and I do prefer it out on the Motorway to my yoke.

Na its just that I'm not a "Revver". Some fellas like revving their engine hard but I like just going along nice and aysey with the minimum of fuss. For everyday driving for the likes of me, I think the torque figure is more important than the bhp.

Cleveland Hot Pocket

Jesus. wrote: »

Ha! Actually no believe it or not! But the other half has a wee diesel Rio and I do prefer it out on the Motorway to my yoke.

Na its just that I'm not a "Revver". Some fellas like revving their engine hard but I like just going along nice and aysey with the minimum of fuss. For everyday driving for the likes of me, I think the torque figure is more important than the bhp.

You need a v8 engined car so you can do both..

tossy

Jesus. wrote: »

Ha! Actually no believe it or not! But the other half has a wee diesel Rio and I do prefer it out on the Motorway to my yoke.

Na its just that I'm not a "Revver". Some fellas like revving their engine hard but I like just going along nice and aysey with the minimum of fuss. For everyday driving for the likes of me, I think the torque figure is more important than the bhp.

Only pulling your leg, this thread doesn't need to descent into a petrol v diesel debate.

Cleveland Hot Pocket

tossy wrote: »

Only pulling your leg, this thread doesn't need to descent into a petrol v diesel debate.

I don't know what you're torquein' about

CiniO

Mister Jingles wrote: »

Very amateur question here on power bands.

Correct me if I'm wrong but my understanding of an engines power band is where the most torque and power is produced ?

With diesels they produce their most torque within 1700-3000 rpm or a similar range but once they go past that there is very little purpose to continue through the revs, they produce no more power, therefore the next gear should be chosen ?

With petrols though my understanding is that there is no power range, the higher the RPM the more torque and power.

Most petrol cars will produce there maximum output after 5500 RPM. However like a diesel say after 6500 RPM, is it pointless to keep going ? Is it time to change gear or will it keep producing more power until it hits the limiter (very unhealthy I know but just put that aside for a minute) ?

I've never tried it in my car (going until it hits the limiter) but after about 6000 RPM I feel there is no more to be gained, or is there and its just me ?

Looks like for me, you are asking at what revs to change gears, to obtain maximum acceleration possible.

To find out, you will need chart of your torque (or power - doesn't matter which as one can be calculated knowing the other), and your gear ratios.

To accelerate as quick as possible, you need to maintain highest torque on your driving wheels through the acceleration.

So basing on torque (or power) chart of your engine, and gear ratios, you can calculate torque on the wheels.

It might be a bit work, but that will give you exact answer at what revs to change gears to obtain maximum acceleration.


In you example of diesel having it's max torque in lower revs range (1700-3000) it's obviously true, but it doesn't mean it's not worth accelerating up to higher revs. All because what you need is highest possible torque on driving wheels - not on the engine shaft.

So in example you might be at 4000rpm on second gear, or 2500 rpm on third gear.
Torque on engine shaft will be higher at 2500rpm.
But torque on driving wheels might be higher on 2nd gear at 4000, then on 3rd at 2500. If that was the case, it is worth to rev it to this 4000rpm on 2nd or possibly even higher.

As I said - all can be calculated fairly easily, just knowing torque chart and gear ratios. Most likely sweet spot revs to change the gear for next one, will be different on each gear.