[NiInJa]

Good explanation given by the OP !
After driving quite a few cars of varying torque and power figures, I usually tell folks to interpret/compare the numbers (Power@RPM and Torque@RPM) like this:

Assume both cars will have same weight. A difference of more than 100kg can be significant, as that would mean your car is already loaded with one average adult and one 10 year old kid, before you sit inside the heavier one.

1. High Torque at Lower RPM: At lower speeds, even with full load, the car will pull off better with lighter throttle inputs. This will be perceived by many as the car has good 'power', but it actually is not, as the same car may not achieve higher speeds. For that, we need to look at Maximum power produced.

Example: Maruti 1.2L vs Honda's 1.2L NA engine, Peak torque of Suzuki 1.2K series engine comes at around 900 RPM lesser than Hondas (both ~113 N.m), hence Suzuki cars have better driveability at lower speeds, while Honda feels lethargic at the same throttle input.

2. High Power at relatively lower (or same) RPM: While the difference between RPMs wont be too big when peak power is mentioned on brochures (usually 200~300 RPM for same fuel type), this will indicate that the car can achieve higher speeds compared to others, also it will do so 'faster' if both car's A pedals were to be floored. Think of this as a seasoned weight lifter, who can easily lift 100kg dead weight on a good day comes back recovered from viral flu and is low on energy, so he barely manages to lift about 80kg.

Example: Honda 1.5NA vs Suzuki 1.5NA Petrol. ~120hp vs ~104hp respectively at similar RPMs. Once they both hit the max possible RPMs, the Honda can still manage to put 16 more horses compared to Suzuki. Hence, the Honda will reach higher top speeds than Suzuki. Honda has more power generated if the time given for RPMs to hit max level is same for both cars, so it will accelerate faster as well. Such cars will generally feel 'lively' with a heavy foot.

3. High Torque at Low RPM, High Power at relatively Lower RPM: For commuter cars, this is the best combination, which mostly diesels were able to achieve. You get best of both worlds: excellent pull at lower speeds even with load, as well has higher top speed at lower RPMs which means lower fuel consumption. Perceived by people as 'relaxed' car, as it can do more work at lesser stress levels.

Example: 1.5 TDI from VW. Say no more!

4. Similar Torque and Power figures, but with smaller cubic capacity: This is not usually observed in cars, but highly relevant in bikes. In bikes, peak torque coming in even a little early (say 300 RPM earlier) can have a big difference in the way it responds on uphill climbing or city riding. Also, the tech used can be very different to achieve higher numbers.

Example: KTM vs Royal Enfield engines. While the 200cc KTM will have similar numbers as that of a 350cc RE, the Royal Enfield will climb uphill with less throttle inputs compared to KTM. This will again be perceived as RE being more 'powerful', it is actually not true

[Cresterk]

Originally Posted by revsperminute

Yes Sir, I'm well aware of the physics behind horsepower and torque. I'm pursuing a Bachelor's degree in Mechanical Engineering currently and believe me those two words have kept me up many a night.

EDIT: Here's me contradicting myself, humour never stood a chance against physics:
https://Youtu.be/7eZyXp1i4GU

Here is an interesting scenario, if both engines are connected using the exact same gear ratio, then it would be the engine with more torque that can move the wall. This would never happen in real life though since the engine with more horsepower due to RPM would always be geared down lower to produce more torque at the wheels.

[Cresterk]

Originally Posted by Jeroen

Thanks for an interesting explanation.



Just wondering why? Why would the type of engine make a difference?
Dyno’s are not very accurate to start with, due to how they measure and calculate the various output values. But I don’t understand why the type of propulsion would make a difference.

Jeroen

I meant that the graphs I posted for electric and hybrid cars might be even less accurate.
Finding torque/power graphs for actual cars that people here are familiar with was surprisingly hard.
The tesla graphs are from years ago and the Toyota hybrid graph is from all the way back in 2010 back when dynos weren't properly tuned for measuring electric/hybrid cars. Also I think they are averaged out chassis dyno graphs rather than the engine dyno graphs for the ICE SUVs I posted. I think only possible to use a chassis dyno to create a motor/engine combined power output graph for hybrids with power split devices since the transmission is always engaged.

[Cresterk]

Originally Posted by AYP

I am not an expert but even in a turbo diesel, I have seen the automatics shift gears close to the redline. See the videos which I have linked. The diesels(most of them) make their peak power around 4000 RPM. So, shifting at let's say 4500 RPM will cause the RPM to go down to 3500 RPM or close to it, thus keeping the car in its peak power band. Again, as I said I am not an expert but after having seen multiple acceleration videos with the automatics shifting close to the redline, I am not sure if I agree with your answer.

The DSG is shifting the gears close to 4700 RPM-

https://Youtu.be/UMjzlKWKoWs

https://Youtube.com/watch?v=-_5YZy_oeYs?feature=share4

This is correct. Automatics will shift around where peak power is produced, not near peak torque if you keep your foot pinned down. This provides fastest acceleration.

Originally Posted by IB290

Thanks for the excellent thread. I have a question though. Horsepower is also rate at which Work is done and Work is Force X Distance. And Force is Mass X Acceleration. So for similar acceleration and distance, which one is better:

1750 kg weight, 116.93bhp@3500rpm and 300Nm@1750-2500rpm

OR

1200 kg weight, 103.39bhp@6000rpm and 134.2Nm@4000rpm

Also, if peak power comes at 6000 rpm, Would it be correct to consider a de-valued peak power for all practical puspose given that we are not likely to drive at 6000 rpm?

You are correct. Unless you drive near 6000 rpm, you will be making much less power with the 2nd car. But car 2 will also be able to use way lower gear ratios than car 1, meaning overall torque to the wheels could be about almost the same if you are going up a hill or something. Difference in pulling power between the 2 cars is just 10%, despite 3x difference in torque.

Do note that car 1 will use less fuel and be more relaxed in everyday driving, if it has a decent 5+ gear auto. The torque figures make me believe this to be a turbo diesel.

Originally Posted by IB290

Thank you. Agree, the gear ratio will decide the mechanics. But doesn’t the one with higher torque at lower rpm eventually present a better platform to be exploited by appropriate gear ratio? What I am driving at is to figure out how best to compare 2 products as simply dividing the Peak power by weight could be misleading?
(Disclaimer: I am a layman in this space.)

It provides a better platform to use longer gear ratios with a lot of gears. Something like an 8 speed auto would only make sense with car 1 and would just be a waste in car 2 since it would never make enough torque to actually shift up to that ratio without the engine stalling from low RPMs due to the air resistance at higher speeds.

Originally Posted by Rakmo

Thanks for sharing this insightful information.

Although having a test drive is far better option to judge real world performance.

I wonder if there is more reliant characteristic which can help judge real world performance (taking into account gearing and Car's weight as well) from the comfort of home.

Is BHP/Ton a good metric in this regard?

BHP/Ton is a decent enough metric to compare cars of a similar aerodynamics and gearbox.

[Kosfactor]

Originally Posted by IB290

Thanks for the excellent thread. I have a question though. Horsepower is also rate at which Work is done and Work is Force X Distance. And Force is Mass X Acceleration. So for similar acceleration and distance, which one is better:

1750 kg weight, 116.93bhp@3500rpm and 300Nm@1750-2500rpm

OR

1200 kg weight, 103.39bhp@6000rpm and 134.2Nm@4000rpm

Also, if peak power comes at 6000 rpm, Would it be correct to consider a de-valued peak power for all practical puspose given that we are not likely to drive at 6000 rpm?

You are comparing 1.5 CRDe of Thar 2WD MT vs 1.5 K15B of Jimny I assume. You can read our Team-bhp reviews for both , CRDe would be awesome to have in Jimny but you can't expect it to work well the other way around.

OEMs tend to have different priorities with regards to their products, Maruti may consider Baleno RS to be their fast car while Mahindra would want pretty much everything to run fast, different customers at the end of the day.

[Cresterk]

Originally Posted by Samarth 619

You know, top speeds are often achieved at a rpm higher than max power rpm,

Only because the gearing is too low. If it had another gear to shift up to, it would go even faster until the car just can't go past the rpm making max horsepower, because of the aerodynamic forces being too high to overcome.

This ofc assumes a perfectly flat road with no headwind. A slight uphill and that car will slow down till peak horsepower rpm. If the curve is steeper and rpm drops below peak horsepower, then you will need to downshift, else it will just keep slowing until either the aerodynamic forces become less from the reduced speed or the hill is ascended.

Probably a typo, but do you mean to say "diesel SUV feels effortless".
And yes, I agree, indeed horsepower wins races and sells cars, even though most commuters might not use all that power..

Not a typo, it means the diesel SUVs feel effortless because they make more horsepower at idle rpm. Not just because the peak torque figure is high. Of course, this higher horsepower is only because they also make higher torque at lower rpm. However, when you are comparing engines with different idle rpms, this becomes more important.

[Cresterk]

Originally Posted by fhdowntheline

Here is the reason for proposing my argument. Back in my last sojourn in the US, I had a 130bhp 2.2 Litre Camry (ran many thousand miles) , previous to that, was a similarly powered Honda Accord, both NA. But when I came back to India, the majority of my experience has been on these small turbo charged cars, namely, the Dzire to begin with (1.3 DDIS) , then Linea 1.4 TJet, or the 1.2 TSI on the Polo. In isolation, these cars are all brilliant, but taking for example the Linea, 1.4 TJet with a full adult pax load, was fast no doubt. But I always wondered to myself recalling those Camry/Accords were effortlessly fast, they always seemed to have power in reserve.

That's because they actually don't have much left in reserve!

Nowadays with throttle by wire, there isn't much relation between your accelerator pedal position and the engine other than what the ECU decides.
https://en.wikipedia.org/wiki/Electr...rottle_control
So basically, you could be giving it 30% throttle but the ecu can decide to give 90% to the engine based on what it calculates based on load etc.

Now adding on to this:
Naturally aspirated cars also have a much more linear power curve. You press the accelerator 30%, you get around 30% power. You press it to 50% to overtake and you are getting another 20% power. In the middle of the overtake, the other car starts speeding up and you gradually push the pedal to 100% and you get the remaining 50% power.

But in modern turbo-charged cars, due to the flat torque curve during a certain rpm range, you can't control how much you are accelerating with the same precision. Especially in turbocharged diesel cars. You give it 50%, get into the turbo range and the engine might already be giving 90%. You feel like the car is really powerful during the 0-50% range (oh wow, so fast without me even pressing the pedal all that much)

But you push the accelerator to 100% and you are going to be pretty disappointed as the engine only has another 10% to give.

[Samarth 619]

Originally Posted by Cresterk

Only because the gearing is too low. If it had another gear to shift up to, it would go even faster until the car just can't go past the rpm making max horsepower, because of the aerodynamic forces....

True, of course, but practically, such a vehicle would be almost unrideable as a commuter, unless the driver/rider has discipline not to shift too high.
If it can't push beyond max power rpm in a certain gear ratio, I think it also won't be able to handle the lower rpms, or even midrange rpms? As a race car/bike, maybe it'll do fine. Road car/bike, not too confidence inspiring.

Though, this high gearing is still a great idea for a stationary engine though, something that must maintain highest speed at the cost of tractability, like a water extraction motor or generator or something, if the engine stress (if any) is not a concern.

But I must say, your understanding of the concept is phenomenal. This is some really good discussion I've ever had on this topic.

Not a typo, it means the diesel SUVs feel effortless because they make more horsepower at idle rpm...

You typed, diesel doesn't feel effortless. You're implying that it has to struggle. Hence I felt its a typo.

[Cresterk]

Originally Posted by Samarth 619

True, of course, but practically, such a vehicle would be almost unrideable as a commuter, unless the driver/rider has discipline not to shift too high.
If it can't push beyond max power rpm in a certain gear ratio, I think it also won't be able to handle the lower rpms, or even midrange rpms? As a race car/bike, maybe it'll do fine. Road car/bike, not too confidence inspiring

Ah but at lower rpms, you are going slower meaning less aerodynamic resistance to overcome.



Velocity is squared, meaning a small change in speed can have a drastic difference.

Also as a real life example : the 8 speed bmw auto or the 9 speed mercedes 9gtronic, the highest gear is only used for cruising. It's also impossible to go to redline in the highest gear unless you flash the ECU to get rid of the speed limiter and you are going downhill.

https://www.youtube.com/watch?v=pzwWlsZtvKo

Check how the car upshifts after getting to top speed so it doesn't hold it at redline.

But I must say, your understanding of the concept is phenomenal. This is some really good discussion I've ever had on this topic.

Thank you, I spent quite a lot of time researching and thinking about all this because the usual arguments just did not make sense with my personal observations.

You typed, diesel doesn't feel effortless. You're implying that it has to struggle. Hence I felt its a typo.

"Your 400nm diesel SUV doesn't feel effortless taking off in 3rd gear at idle just because the high torque numbers mean that horsepower is irrelevant..."

The doesn't corresponds to the 2nd part of the sentence.
If I said 'does' then it means that I'm agreeing horsepower is irrelevant. This sentence structure means that your SUV feels effortless not because of the higher torque, but because of the higher horsepower generated lower down. I agree that it can be a little confusing so I will edit it for clarity.

[alpha1]

Engine torque/power curves tell only half story.

One needs to add the effect of gears and wheels to produce a set of torque (or bhp) vs rpm curves for each gear super imposed in order to deduce the drivability, load pulling abilities, or sprinting etc capabilities of the vehicle.

[speedracer05]

Is there any way to correlate engine efficiency with Torque and BHP figures in the RPM plot?
Is tapering or lower of torque figures at higher RPM an indication of engine running at lower efficiency?

[Cresterk]

Originally Posted by alpha1

Engine torque/power curves tell only half story.

One needs to add the effect of gears and wheels to produce a set of torque (or bhp) vs rpm curves for each gear super imposed in order to deduce the drivability, load pulling abilities, or sprinting etc capabilities of the vehicle.

Correct, hence why I gave the example of a 16 ton rated truck with just 132hp and all the further explanations I provided in the comments

[Jeroen]

Originally Posted by speedracer05

Is there any way to correlate engine efficiency with Torque and BHP figures in the RPM plot?
Is tapering or lower of torque figures at higher RPM an indication of engine running at lower efficiency?

It all depends of your definition of engine efficiency. In a car fuel efficiency is more relevant which is measured in km per liter or something similar.

When you talk about engine efficiency, what parameters are you looking at?

Jeroen

[VCheng]

Originally Posted by Cresterk

[b]

All enthusiasts whether they be of the car, bike, airplane, boat or lawnmower variety has heard the phrase "Horsepower sells cars but torque wins races"

What if I tell you that this very phrase itself was made up by some clever salesman or advertising agency to sell cars and mislead people?

.....


In conclusion: Horsepower absolutely does win races and sell cars.

Thank you for that good explanation.

Over a lifetime of buying and enjoying cars and bikes, I generally have found that engines with nearly equal (or comparable) figures for both horsepower and torque (the more the better) tend to suit my driving style and usage the best.

Engines that tend to produce much more of one compared to the other tend to be more specialized.

[CarML]

Originally Posted by Cresterk

Note: This post is intended to explain a concept to beginners without much of an engineering background so it might be a bit too oversimplified for the mechanical engineers among you.

.

Thank you for this very informative thread. What an apt example of the old Tata truck!
It would be interesting to understand the following as well.
1. How are gears used to best leverage the parameters to provide optimal performance for a given car?
2. How do engineers tune the same engine to produce different power figures? What are some of the judicious decisions taken while doing so?
3. How does the equation change dynamically for hybrid vehicles?
Thank you again.