Question about BHP + Torque

[Qwert1e]

manmaths said:

Q: Is it possible to merge both the BHP and Torque figure of a car into ONE performance figure,

Yes, if it's stationary. Which is exactly the right opportunity - namely when the pub bores are trotting out their performance statistics.

[996TT02]

manmaths said:

Just wondering...

Q: Is it possible to merge both the BHP and Torque figure of a car into ONE performance figure, which would then allow 2 separate cars to be more easily compared?

The vehicles engines are comparable simply because they torque and power figures are available separately, and this would not happen if they were merged.

Having such figures separately allows you to learn about the character of the engine and thus of the nature of the performance.

Low(er) torque and higher power means that you have a revvy engine that will go only or mostly when kept on the boil.

Conversely higher torque and lower power means one that does not need to be revved so hard and will be easier to live with on a daily basis, and will ordinarily feel faster at everyday rev ranges.

If you want to compare the ultimate result of the differences, which let us not forget are only relevant when the engines are installed in a vehicle (vehicle weight etc so important) and not on the bench, you have 0-60 figures and the like. Comparing engines alone in terms of performance of the vehicle is of very little worth.

[manmaths]

hairykrishna said:

As others have said, you're using the wrong rpm. Example dyno curve;



It's clear that the peak torque is much lower in the rev range than the peak power. As for the mod question - it depends what you mean by 'increase torque'. If the torque is increased at a given revs then the power at those revs also increases. This doesn't necessarily mean the peak power is increased.

Very helpful reply and you even used the KV6 from the MGZS180.

Where I have become confused then, is every time I see one of these graphs (which are referred to and posted a lot) I'm making an assumption that both power and torque are independent of one another, when they are in fact linked.

I didn't know that power as a measurable output already factors torque into the equation. If I understand correctly - although they are linked, it's not necessarily a linear relationship - in the example of the KV6 torque tails off but bhp begins to peak.

My questions then would be in relation to the two cars I posted figures for (again just trying to get my head around it):

If torque enables an engine to produce bhp,

1) Why does the K20 manage to produce 1.36bhp per lbft of torque (1.36:1) when the KV6 only puts out 0.98 bhp per lbft (.98:1)
2) What is the typical bhp:torque(lbft) ratio for most engines? 1:1? (And do manufacturers aim for a certain ratio or relationship when considering "driveability"?)

some cursory research shows the new M3 to be around 1:1 for example.

N

Edited by manmaths on Monday 28th July 11:47

[Mave]

manmaths said:

My questions then would be in relation to the two cars I posted figures for (again just trying to get my head around it):

If torque enables an engine to produce bhp,

1) Why does the K20 manage to produce 1.36bhp per lbft of torque (1.36:1) when the KV6 only puts out 0.98 bhp per lbft (.98:1)

Don't forget, the peak torque and peak power are produced at different speeds. The relationship between power and torque is fixed at a given speed.
In the example above, the implication is that the k20 has a peakier power delivery.

[hairykrishna]

manmaths said:

I didn't know that power as a measurable output already factors torque into the equation. If I understand correctly - although they are linked, it's not necessarily a linear relationship - in the example of the KV6 torque tails off but bhp begins to peak.

My questions then would be in relation to the two cars I posted figures for (again just trying to get my head around it):

If torque enables an engine to produce bhp,

1) Why does the K20 manage to produce 1.36bhp per lbft of torque (1.36:1) when the KV6 only puts out 0.98 bhp per lbft (.98:1)
2) What is the typical bhp:torque(lbft) ratio for most engines? 1:1? (And do manufacturers aim for a certain ratio or relationship when considering "driveability"?)

some cursory research shows the new M3 to be around 1:1 for example.

As earlier, power is rpm X torque X (some fixed factor to account for units). The peak power is higher even though the torque is lower at that point purely because you have more rpm.

Torque is force, how strongly the engine twists the output shaft. If you think about it how hard a car can accelerate is how hard you can twist the wheels at a certain speed. The complication in a car is that you have a gearbox. A gearbox is a torque multiplier. If the output of the gearbox is turning half as fast as the input then you have twice as much torque at the output as you have at the input. This means that to work out how hard a car can accelerate with a gearbox you need a measure of both how hard the engine can twist (torque) and how fast it's turning the shaft at the time - hence power. Basically power is a measure of how well an engine can accelerate a car if it was given an 'ideal' gearbox.

Some confusion comes in because the power and torque figures always quoted are the peak values. They generally don't tell you a great deal in isolation. Knowing the rpm's at which these two peaks arrive at gives you some more information, the shapes of the curves more still.

[SturdyHSV]

wormus said:

this for me represents the perfect torque delivery:

Like this then?

[kambites]

manmaths said:

1) Why does the K20 manage to produce 1.36bhp per lbft of torque (1.36:1) when the KV6 only puts out 0.98 bhp per lbft (.98:1)
2) What is the typical bhp:torque(lbft) ratio for most engines? 1:1? (And do manufacturers aim for a certain ratio or relationship when considering "driveability"?)

1) Because it revs higher.
2) It depends on the RPM at which peak power is developed and what proportion of peak torque is still being generated at that point.

The KV6 has a relatively low peak-power to peak-torque ratio because the torque is already falling off dramatically by peak power.

[manmaths]

Thanks for the replies guys - that makes sense.

SturdyHSV said:

Like this then?

That looks like a fun drive!

[CrutyRammers]

manmaths said:

Very helpful reply and you even used the KV6 from the MGZS180.

Where I have become confused then, is every time I see one of these graphs (which are referred to and posted a lot) I'm making an assumption that both power and torque are independent of one another, when they are in fact linked.

I didn't know that power as a measurable output already factors torque into the equation. If I understand correctly - although they are linked, it's not necessarily a linear relationship - in the example of the KV6 torque tails off but bhp begins to peak.

No, it is a linear relationship. Power = torque x revs. Torque will fall off at the top, but power continues to rise because the rpm is rising.

It may help to think about the bicycle thought experiment. You can produce, say, 80 lb ft of torque on a bike crank by standing on it. This is the same as the peak torque of a small hatchback.

Obviously you can't go as fast as the hatchback though - because you don't produce anything like as much power - because you can't pedal as fast as the hatchback's engine can turn. You can only produce that torque at about 40 rpm, the car does it at 4,000.

[anonymous-user]

SturdyHSV said:

wormus said:

this for me represents the perfect torque delivery:

Like this then?

Yep,....just like mine. I find about 600 lbs at the wheels (about 750 at the flywheel) is the limit for the Monaro. Any more and the tyres spin.

Edited by anonymous-user on Monday 28th July 13:55

[Mr2Mike]

manmaths said:

If torque enables an engine to produce bhp,

1) Why does the K20 manage to produce 1.36bhp per lbft of torque (1.36:1) when the KV6 only puts out 0.98 bhp per lbft (.98:1)
2) What is the typical bhp:torque(lbft) ratio for most engines? 1:1? (And do manufacturers aim for a certain ratio or relationship when considering "driveability"?)

If you consider that peak power and peak torque occur at different engine speeds this should become obvious. Torque is primarily determined by the displacement and volumetric efficiency, i.e. how much air and fuel the engine consumes on each cycle. This is why large capacity engines produce more torque than smaller ones, and why supercharged engines produce more torque than equivalently sized normally aspirated engines.

To make a normally aspirated engine produce high peak power, there often isn't much that can be done to significantly increase the peak torque value. Instead, the engine is designed to maintain a good level of torque at a higher RPM than normal. A K20 is still producing 90% of it's peak torque at 8000RPM. Compare that with an average 2.0L engine from a family car where torque has fallen off a cliff edge by around 6500RPM. These high revving engines will have a high peak bhp/peak torque ratio, the original EK9 Type R Civic would score 1.55.

OTOH the old lazy American V8s often produced a fairly high peak torque (simply due to their displacement) but a laughable peak power because the engines design means they were unable to fill the cylinders with much air at high RPM (i.e. volumetric efficiency falls off very quickly). These would have a low peak bhp/peak torque ratio e.g. an early 80's Ford Mustang with a 4.9L V8 would produce only 140bhp, but made 230 lbft of torque, scoring it a lowly 0.6.

A modern supercharged/turbocharged engine can produce both high peak torque and high power from a small engine. These would give a power/bhp ratio of around 0.8-0.9, i.e. peak torque a little higher than peak power.

[manmaths]

Mr2Mike said:

If you consider that peak power and peak torque occur at different engine speeds this should become obvious. Torque is primarily determined by the displacement and volumetric efficiency, i.e. how much air and fuel the engine consumes on each cycle. This is why large capacity engines produce more torque than smaller ones, and why supercharged engines produce more torque than equivalently sized normally aspirated engines.

To make a normally aspirated engine produce high peak power, there often isn't much that can be done to significantly increase the peak torque value. Instead, the engine is designed to maintain a good level of torque at a higher RPM than normal. A K20 is still producing 90% of it's peak torque at 8000RPM. Compare that with an average 2.0L engine from a family car where torque has fallen off a cliff edge by around 6500RPM. These high revving engines will have a high peak bhp/peak torque ratio, the original EK9 Type R Civic would score 1.55.

OTOH the old lazy American V8s often produced a fairly high peak torque (simply due to their displacement) but a laughable peak power because the engines design means they were unable to fill the cylinders with much air at high RPM (i.e. volumetric efficiency falls off very quickly). These would have a low peak bhp/peak torque ratio e.g. an early 80's Ford Mustang with a 4.9L V8 would produce only 140bhp, but made 230 lbft of torque, scoring it a lowly 0.6.

A modern supercharged/turbocharged engine can produce both high peak torque and high power from a small engine. These would give a power/bhp ratio of around 0.8-0.9, i.e. peak torque a little higher than peak power.

cheers for the detailed reply. All made sense to me.

N

[GroundEffect]

Think of power as how fast you're producing torque then it should make sense.

Faster you can produce torque, the more work you can do.

[Speedy11]

wormus said:

Yep,....just like mine. I find about 600 lbs at the wheels (about 750 at the flywheel) is the limit for the Monaro. Any more and the tyres spin.

Edited by wormus on Monday 28th July 13:55

Then there is something seriously wrong with your gearbox.

Torque at the wheels is

Torque at engine x gear ratio x final drive ratio x transmission loss

So for 750 lb.ft at the flywheel and using 3.08 for the diff, 2.97 and 1.78 for 1st and 2nd. The torque at the wheels ignoring transmission losses in 1st ~ 750*2.97*3.08 = 6860 lb.ft and 2nd 4110 lb.ft


Edit

Even in 6th torque at the wheels is 750*0.63*3.08=1450 lb.ft

Edited by Speedy11 on Monday 28th July 20:06

[anonymous-user]

Speedy11 said:

Then there is something seriously wrong with your gearbox.

Torque at the wheels is

Torque at engine x gear ratio x final drive ratio x transmission loss

So for 750 lb.ft at the flywheel and using 3.08 for the diff, 2.97 and 1.78 for 1st and 2nd. The torque at the wheels ignoring transmission losses in 1st ~ 750*2.97*3.08 = 6860 lb.ft and 2nd 4110 lb.ft


Edit

Even in 6th torque at the wheels is 750*0.63*3.08=1450 lb.ft

Edited by Speedy11 on Monday 28th July 20:06

I'm talking about torque at the wheels as measured in a rolling road, in 4th gear which as we all know is 1:1 ratio. Transmission losses are around 25%.

Engine has made over 800hp out of the car on an engine dyno.

[Speedy11]

wormus said:

I'm talking about torque at the wheels as measured in a rolling road, in 4th gear which as we all know is 1:1 ratio. Transmission losses are around 25%.

Engine has made over 800hp out of the car on an engine dyno.

Then your diff is broken. 4th may be 1, however you still have the diff after the gearbox so 750*1*3.08=2310 lb.ft or 1730 with 25% losses.

[GroundEffect]

wormus said:

Speedy11 said:

Then there is something seriously wrong with your gearbox.

Torque at the wheels is

Torque at engine x gear ratio x final drive ratio x transmission loss

So for 750 lb.ft at the flywheel and using 3.08 for the diff, 2.97 and 1.78 for 1st and 2nd. The torque at the wheels ignoring transmission losses in 1st ~ 750*2.97*3.08 = 6860 lb.ft and 2nd 4110 lb.ft


Edit

Even in 6th torque at the wheels is 750*0.63*3.08=1450 lb.ft

Edited by Speedy11 on Monday 28th July 20:06

I'm talking about torque at the wheels as measured in a rolling road, in 4th gear which as we all know is 1:1 ratio. Transmission losses are around 25%.

Engine has made over 800hp out of the car on an engine dyno.

What, 4th gear is always 1:1?

No it's not. Gearsets change all the time.

[Cerberus90]

Speedy11 said:

Then your diff is broken. 4th may be 1, however you still have the diff after the gearbox so 750*1*3.08=2310 lb.ft or 1730 with 25% losses.

Pretty sure he means the difference between wheel and fly power when measured on a rolling road.


For example, our TVR produces ~120bhp at the fly, which is about ~96bhp at the wheels.
With 4th gear being 1:1 and diff being 3.89, 120*1*3.89 is obviously not equal to 96.

[Speedy11]

Cerberus90 said:

Pretty sure he means the difference between wheel and fly power when measured on a rolling road.


For example, our TVR produces ~120bhp at the fly, which is about ~96bhp at the wheels.
With 4th gear being 1:1 and diff being 3.89, 120*1*3.89 is obviously not equal to 96.

No he defiantly said torque at the wheels not power at the wheels. As power at wheels = power at flywheel - transmission losses.

Edited by Speedy11 on Monday 28th July 20:50

[wedgeinald]

This is VERY simplistic but here goes...

Torque is the ability to accelerate
Power is the ability to maintain speed

Yes its more complex than that as the two are related.

You can have a high torque figure at low revs and that car will accelerate from much lower speeds.

If you have low torque at high revs, you can still have more power than at the lower revs, but less ability to accelerate.

I hope this is of some help.....