[sudeepg]

Originally Posted by IB290

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?

I guess, it depends. Based on what the OP said with the truck, the choice would also depend on what gearing is applied to the automobile with these power plants.

[IB290]

Originally Posted by sudeepg

I guess, it depends. Based on what the OP said with the truck, the choice would also depend on what gearing is applied to the automobile with these power plants.

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.)

[OffRoadFun]

In simple math:

Power = Torque x Angular velocity (RPM of the engine) (1)


What gives a feeling of "acceleration" is Torque which is proportional to "angular" acceleration (how fast wheel accelerates (not spins)).

HorsePower (HP) is what dictates how fast a can eventually accelerate at higher speeds as angular velocity (= RPM) is understandably higher at higher car speeds

Since the product of Torque and speed is constant (=maximum rated power of the engine), the torque starts to go down once speed is high, that is why it is difficult to accelerate at high speeds, or the car runs out of breath!

That is why its not the peak torque which is important, which many engines may show. It is the band of RPM over which that peak torque is available is important and that is what ultimately decides car's characteristics like 0-100 time etc.

You may notice trucks have massive torque but they just run out of breath at high speeds which means they are great for lugging goods at lower speeds but can not attain high speeds with high load which is what is needed from them.

And what we call tuning is nothing but auto makers playing with the Torque vs RPM band to come up with multiple flavors of engines of same displacement (loosely speaking)

[OffRoadFun]

Originally Posted by AYP

Excellent thread mate. I have contempolated this for a long time, i.e. the RPM at which we should change gears for maximum acceleration.

Now as far as a petrol is concerned, you can go all the way to the redline for maximum acceleration(do correct me if I am wrong). However, I am still not sure about the diesels. I have 2 cars at home-

1. Figo 1.5 TDCi- 100 PS@3750 rpm and 215 NM@1500-3000 RPM

2. Jetta 2.0 TDI- 140 PS@4200 rpm and 320 NM@1750-2500 rpm

Can you please suggest the best RPM to shift at for maximum acceleration? My apologies if the answer is already present in your post and I am asking the same thing again. Also, is there a way to determine the best RPM to shift gears for max acceleration after looking at the power and torque figures and the RPM at which they are generated? Or do we need graphs to determine this?

Thanks.

Although it would be best judged after looking at the Torque vs Power vs RPM graph but in general if you want to maximize acceleration, you need to maximize torque so a shift should be done right before the RPM when Torque starts to drop which would 3000rpm for Ford and 2500rpm for Jetta. I am assuming the power curve is flat in both the case at till that point.

This is precisely the reason, programmed modern dual clutch beat any human in any car as of today.

[OffRoadFun]

Originally Posted by fhdowntheline

Excellent explanation. I would request OP to also comment on the difference in the performance scope for small CC high bhp/torque engines vs larger capacity engines. It is my (rather uneducated) view that a car with a 1.5 litre 160 bhp/250NM engine and fully loaded with passengers will struggle ever so slightly to maintain steady cruising in the higher speed regime (100 kph plus) than a fully loaded car with a 2.0 litre 150bhp /200NM NA engine. Talking about petrol of course.

No, what ultimately determines the highest speed and stability at higher speeds is peak power , not the volume of the engine. Given the data and assuming the graphs are identical across RPM except for the peak torque and power mentioned, 1.5L will be performing better than the 2.0L at every speed (rest all being same like weight etc.)

Depending on the design, vibration etc. may be more for smaller engine but performance wise, it will win!

[AYP]

Originally Posted by OffRoadFun

Although it would be best judged after looking at the Torque vs Power vs RPM graph but in general if you want to maximize acceleration, you need to maximize torque so a shift should be done right before the RPM when Torque starts to drop which would 3000rpm for Ford and 2500rpm for Jetta. I am assuming the power curve is flat in both the case at till that point.

This is precisely the reason, programmed modern dual clutch beat any human in any car as of today.

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

[OffRoadFun]

In general, one wants to maximize Torque at a very high level which is Power/RPM. This is the chart of a similar engine for 2.0TDI I found from web.



From this, you are probably right the peak of P/v may not coincide with where torque starts to taper off.

Another thing to keep in mind is every gear shift (whether DCT or manual) comes with a cost of reduced RPM by at least 1000 RPM (and 200ms time lost in shift at the very minimum) on the graph so it may make more sense to first go higher on RPM graph and then shift the gear to make up for that and take a bit of hit on torque, again a kind of optimization/trade-off.

Again, as I said it is probably much more complicated than what I mentioned and definitely for best acceleration, touching the redline makes much more sense as you mentioned.

[Rakmo]

Originally Posted by Cresterk

If you want to compare engine performance, the best option is to go test drive it yourself.

If you want to do it from the comfort of your home, then a horsepower/torque/rpm graph is a good option that tells you actual meaningful data. Maximum value of torque by itself is useless. Acceleration timings can also help since it also takes the gearbox into account.

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?

[Cresterk]

Originally Posted by fhdowntheline

Excellent explanation. I would request OP to also comment on the difference in the performance scope for small CC high bhp/torque engines vs larger capacity engines. It is my (rather uneducated) view that a car with a 1.5 litre 160 bhp/250NM engine and fully loaded with passengers will struggle ever so slightly to maintain steady cruising in the higher speed regime (100 kph plus) than a fully loaded car with a 2.0 litre 150bhp /200NM NA engine. Talking about petrol of course.

It would be impossible to say without also knowing at what RPMs this peak horsepower and peak torque is produced. The horsepower and torque produced a smaller turbocharged engine is not inferior to the hp and torque produced by a larger N/A engine so it won't be struggling unless you maintain a very low rpm at the wrong gear. If you put your foot down in either, the turbocharged car will be faster even on hills on account of having 10hp more.

But here is an interesting part about your scenario that you haven't asked. The smaller turbo-charged engine is more likely to use more fuel than the bigger N/A 2.0 engine if you drive it constantly in the RPM range where the turbo produces boost.

Originally Posted by SuperbNavigator

This is an excellent thread.
Is it possible to have similar comparison of current mainstream diesel engines in Indian market?
If there is data elsewhere it would greatly help to have horsepower/torque/rpm graph of Fial 2.0 vs Mhawk vs Hyundai diesel vs Toyota fortuner diesel

Thank you

I found these torque and horsepower graphs from random websites, including other posts on teambhp, by scouring the internet. If you can find graphs from the newer cars, I would be happy to help you analyse it. Sadly, I do not have access to a dyno nor the cars.

Originally Posted by AYP

Excellent thread mate. I have contempolated this for a long time, i.e. the RPM at which we should change gears for maximum acceleration.

Now as far as a petrol is concerned, you can go all the way to the redline for maximum acceleration(do correct me if I am wrong). However, I am still not sure about the diesels. I have 2 cars at home-

1. Figo 1.5 TDCi- 100 PS@3750 rpm and 215 NM@1500-3000 RPM

2. Jetta 2.0 TDI- 140 PS@4200 rpm and 320 NM@1750-2500 rpm

Can you please suggest the best RPM to shift at for maximum acceleration? My apologies if the answer is already present in your post and I am asking the same thing again. Also, is there a way to determine the best RPM to shift gears for max acceleration after looking at the power and torque figures and the RPM at which they are generated? Or do we need graphs to determine this?

Thanks.

If acceleration is your sole concern, you would be shifting up around the max power range.

Plug the values into the rpm, torque formula to get the actual horsepower values.

1. At around 3750 rpm.
If you shift at 3000 rpm (peak torque) instead, your engine is only producing 90hp, meaning the remaining 10hp is remaining unused.

2. At around 4200rpm.
If you shift up at 2500 rpm, your engine is only producing 112hp, meaning the remaining 30hp is unused.

If you want to accelerate the hardest possible, you must utilise the engine to 100% of it's power rating.



I am a bit overwhelmed with the replies but I will slowly keep replying to everyone throughout the week whenever I get free time.

[Cresterk]

Originally Posted by revsperminute

Wonderful thread. In layman’s terms:

Attachment 2465298

As amusing as it is, I'm afraid it is not true.

How much you drag the wall depends almost completely on momentum which is dependent on the speed + weight of the car. So whichever car is heavier and hits the wall faster, will drag it further.

But for the sake of argument, let us imagine that the car's don't get damaged from hitting the wall and the driver keeps the throttle floored throughout.

Well, then it depends on the gear ratio.
The car which hits the wall in 6th gear will be faster meaning a lot more momentum, but it won't be able to drag the wall by engine power.

The car which hits the wall in 1st gear will be travelling slower but keep the accelerator pressed down and it will keep dragging the wall.

If both cars hit the wall in 1st gear, then the car with lower gear ratio will drag the wall further. eg: If one of them is an SUV with 100hp/150nm with low range gearbox engaged and the other is a sedan with 110hp/200nm, then the SUV will be the winner despite the sedan having more horsepower and torque.

[mayd85]

Vehicle's propulsion system has to work against various opposing forces like surface friction, friction within the drivetrain itself, air (it's appears much thicker at high speed and big size), gravity etc. Some of these forces doninate at lower speeds (e.g static friction) others at higher speeds (e.g. aerodynamic drag, rolling friction).

Torque is essentially the force which the engine generates in order to propel the vehicle at the desired speed against all these forces.

In typical sense, i.e. in engine specifications, Torque is meant to indicate the force generated by engine to propel the vehicle from standstill till moderate speeds.

Now, as the vehicle speed increases, the opposing forces increase almost exponentially. So, in order to increase the vehicle speed, the engine has to generate force more than the opposing forces which increase exponentially (didn't I say that earlier?).

If you look at the torque speed curve of an engine, you'll find that the engine is not able to generate high torque at higher rpm. Well, that's just how engines are. But hey, once vehicle is in motion, one does not need so much torque to keep it moving, right? So, the torque produced by engine, even though small, may still be able to overcome the opposing forces and hence, may be able to increase the speed of vehicle or keep it going at the same speed! That's where the term POWER comes in. Mathematically, power is just torque multiplied by rpm. So, when you see the number mentioned against power in the vehicle specs, what you are essentially looking at is the ability of the vehicle to keep going at higher speed, overcoming forces of nature and maintain whatever top speed it's capable of (Note, peak power and top vehicle speed may not always be exactly correlated, because sometimes speed is artificially limited for some reasons.)

[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. Maybe its the state of tune that also matters. Even more recently, when I checked out the City vs the Corolla Altis (1.8), on a highway run, the Altis was effortlessly fast, the much lighter City, was only adequately so. I am referring to the ability to accelerate through those 70-100, 80-110 kind of transition requirements that we often encounter on our highways.

[revsperminute]

Originally Posted by Cresterk

As amusing as it is, I'm afraid it is not true.

How much you drag the wall depends almost completely on momentum which is dependent on the speed + weight of the car. So whichever car is heavier and hits the wall faster, will drag it further.

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

[Samarth 619]

If I had to explain it in least words, I would say that torque is a force, not necessarily causing movement. I push the wall with a certain amount of torque, (it will obviously not move) yes it has torque.Now, the interesting part, power is actually adding that missing "movement" to the torque, movement which is rpm.

Torque is important because it pushes the vehicle forward.
RPM is important because it directly decides the speed.

Both are important when measuring potential of an engine, the force and the speed.
Hence, power is a hypothetical spot on the rpm band, where the product of "rpm X torque-at-that-rpm" is the highest. Thus it denotes maximum power achievable from the engine.

You know, top speeds are often achieved at a rpm higher than max power rpm, but the power is actually low there. The engine won't lurch forward further at those speeds... yes, it means the power is on meltdown, torque gets so low there.

Originally Posted by Cresterk

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, its because the high torque numbers mean it produces higher horsepower at lower rpm.
In conclusion: Horsepower absolutely does win races and sell cars.

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..

Originally Posted by IB290

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.)

All depends on what rpm you upshift and downshift, and what kind of user you are (commuter, power driver, track racer, etc.).

You're right in a way, Peak power/ weight is a great metric for track racers, but not so much for a commuter. Reason being, the racer revs hard in first gear, escapes the lower rpm, and then upshifts at 7500 rpm (for example). Thereafter he never sees low rpms again (unless he stops) as he downshifts at 5000 rpm and upshifts at 7500 rpm. He remains in this 5-7.5 krpm zone. So, he's always going to be on a boil, low end torque is irrelevant for him.

The average commuter might never touch 7500 rpm, so the higher power and high rpm may be useless for him.
For the average commuter, torque in the lower rpm's make for a great learner vehicle which doesn't stall easily on launch, doesn't require much gearshifts, and pushes forward without raising the revs high, etc.

Often, engine with wider bore and short stroke make more high end power, and lesser low end torque. (Example: KTM Duke 390)
And Longer stroke engines make more torque at low rpm, but can't rev too high. (Example: Royal Enfield 350cc)
The above are single cylinder motorcycle examples as I'm more of a biker. Multicylinder engines often have higher overall potential in both power and torque.

[IB290]

Originally Posted by Samarth 619

For the average commuter, torque in the lower rpm's make for a great learner vehicle which doesn't stall easily on launch, doesn't require much gearshifts, and pushes forward without raising the revs high, etc.

Many thanks Samarth for the explanation in a simple way. Since many here would fall in commuter category, it would really be weight vs low end torque to watch out for. The gearing will play its part as well.