Question about Torque

[Zyklon]

Another thing that most people are not aware of is that you do not need to look at the torque curve to decide whether the car is "good for city driving" or "good for high rev racing".

 

Also, terms like "all the torque is available only at the upper rpm band" or "it makes 400Nm from as low as 1500rpm", these are just marketing bulls**t.

 

A simple power output curve can pretty much tell you everything. This is because you make power from rpm and torque. A car without low rpm torque will not have power at low rpm. So if you see an almost linear power curve from low rpm, it means the torque was quite constant with respect to rpm and available from low rpm range. On the other hand, if you see a power curve with an exponential increase in power at the upper rpm band, it means the torque is much higher at the higher rpm region.

[zerobim08]

  On 5/15/2015 at 11:00 PM, Zyklon said:

Power to weight ratio only tells you the acceleration.

 

In my example...both cars have the same power to weight ratio.

 

What power to weight does not tell you is the higher speed acceleration and top speed. Same power to weight, but higher total power will usually be way faster once you reach 200+km/hr.

 

In layman terms:

 

Power to weight is the same as

Pushing energy per unit mass per unit time, which is the same as

Pushing force per unit distance per unit mass per unit time

 

Therefore higher power to weight means 1kg of mass will have more pushing force.

But power to weight doesn't tell everything about acceleration from standstill. A turbo engine with same ratio usually will be quicker off the block since max power is rs reached at lower rpm.

[nazerath]

then also have to consider gearings.

[Zyklon]

  On 5/15/2015 at 11:37 PM, zerobim08 said:

But power to weight doesn't tell everything about acceleration from standstill. A turbo engine with same ratio usually will be quicker off the block since max power is rs reached at lower rpm.

 

Your statement is typical car buyer thinking.

 

Power to weight is not about taking spec sheet power divided by weight. Strictly speaking, that is max power to weight ratio and it is only valid when the engine is making max power.

 

Power to weight should be taken as the instantenous power available per unit mass of the car.

 

Therefore, if you compare two cars of the same max power:

 

Car A: 100hp at 4000-5500rpm, 1000kg

Car B: 100hp at 5500rpm, 10000kg

 

Obviously Car A will be faster. Because the power to weight ratio of car B is clearly inferior to car A when the engine is accelerating through the 4000-5500rpm region.

 

This type of question can also be taken using the energy approach by considering the work done for each engine when accelerating through 4000 to 5500rpm.

Edited May 16, 2015 by Zyklon

[chuanyan128]

3 cars for comparison,

 

Hyundai Sonata Taxi 2.0 CRDi (Turbo Diesel)

148 Bhp

Torque 305NM

0-100 @ 10.7sec.

Weight 1,660kg

 

Hyundai Santa Fe 2.4 GDI (NA Petrol)

189Bhp

Torque 242NM

0-100 @ 10.9sec.

Weight 1,694kg

 

BMW 520i 2.0 (Turbo Petrol)

181Bhp

Torque 270NM

0-100 @ 7.9sec.

Weight 1,725kg

 

Hmm.. very confused the heaviest is the fastest. The highest Bhp and power to weight has the lowest torque and is the slowest. The weakest Bhp and power to weight has the most torque and still slightly faster than the most powerful... So it's not the torque, not the Bhp and not power to weight??? Please enlighten me

[Eyke]

jit tao compare 0-100km/h times lah...

[chuanyan128]

  On 5/16/2015 at 9:43 AM, Eyke said:

jit tao compare 0-100km/h times lah...

 

Like dat ah, I boh lui buy BMW then I rather choose Hyundai Taxi better, since 0-100km/h in 10.7sec can outrun so many jap and Kor cars. Can save on fuel and some more can earn money.

[7hm]

  On 5/16/2015 at 7:14 AM, chuanyan128 said:

3 cars for comparison,

 

Hyundai Sonata Taxi 2.0 CRDi (Turbo Diesel)

148 Bhp

Torque 305NM

0-100 @ 10.7sec.

Weight 1,660kg

 

Hyundai Santa Fe 2.4 GDI (NA Petrol)

189Bhp

Torque 242NM

0-100 @ 10.9sec.

Weight 1,694kg

 

BMW 520i 2.0 (Turbo Petrol)

181Bhp

Torque 270NM

0-100 @ 7.9sec.

Weight 1,725kg

 

Hmm.. very confused the heaviest is the fastest. The highest Bhp and power to weight has the lowest torque and is the slowest. The weakest Bhp and power to weight has the most torque and still slightly faster than the most powerful... So it's not the torque, not the Bhp and not power to weight??? Please enlighten me

 

Must not forget drag coefficient and the fact that the BMW 520i is turbopetrol. It can extract more speed out of each gear and it makes the torque much earlier than the Santa Fe (like 2000RPM vs 4500RPM).

 

The taxi got more torque but less max HP. I guess the taxi reach 30km/h or w/e faster but to 100km/h the BMW gets there faster because more HP.

 

[inlinesix]

  On 5/16/2015 at 7:14 AM, chuanyan128 said:

3 cars for comparison,

 

Hyundai Sonata Taxi 2.0 CRDi (Turbo Diesel)

148 Bhp

Torque 305NM

0-100 @ 10.7sec.

Weight 1,660kg

 

Hyundai Santa Fe 2.4 GDI (NA Petrol)

189Bhp

Torque 242NM

0-100 @ 10.9sec.

Weight 1,694kg

 

BMW 520i 2.0 (Turbo Petrol)

181Bhp

Torque 270NM

0-100 @ 7.9sec.

Weight 1,725kg

 

Hmm.. very confused the heaviest is the fastest. The highest Bhp and power to weight has the lowest torque and is the slowest. The weakest Bhp and power to weight has the most torque and still slightly faster than the most powerful... So it's not the torque, not the Bhp and not power to weight??? Please enlighten me

 

The magic component

ZF 8 speed.

[Zyklon]

  On 5/16/2015 at 7:14 AM, chuanyan128 said:

3 cars for comparison,

 

Hyundai Sonata Taxi 2.0 CRDi (Turbo Diesel)

148 Bhp

Torque 305NM

0-100 @ 10.7sec.

Weight 1,660kg

 

Hyundai Santa Fe 2.4 GDI (NA Petrol)

189Bhp

Torque 242NM

0-100 @ 10.9sec.

Weight 1,694kg

 

BMW 520i 2.0 (Turbo Petrol)

181Bhp

Torque 270NM

0-100 @ 7.9sec.

Weight 1,725kg

 

Hmm.. very confused the heaviest is the fastest. The highest Bhp and power to weight has the lowest torque and is the slowest. The weakest Bhp and power to weight has the most torque and still slightly faster than the most powerful... So it's not the torque, not the Bhp and not power to weight??? Please enlighten me

 

Read the post above yours. The answer is inside.

 

It is not only about peak power. It is also about how long you can make the peak power. Did you know that the 520i actually makes 180hp from 4500rpm all the way to 6500rpm? That means that whether you are at 4500rpm or at 6500rpm, just put your feet down and you get the full 180hp. Coupled with the closely ratioed, lightning quick and efficient zf 8hp, the 0-100km/hr on the 520i actually happens at the rated 180hp output from 30-100km/hr.

 

Let's not forget that BMW tends to under-rate their actual output...on the other hand, my personal experience with kimchi brand is that 1 kimchi horsepower is not as strong as 1 german horsepower.

[Beehive3783]

  On 5/16/2015 at 12:13 PM, Zyklon said:

Let's not forget that BMW tends to under-rate their actual output...on the other hand, my personal experience with kimchi brand is that 1 kimchi horsepower is not as strong as 1 german horsepower.

 

You know why? This is because the kimchi horse eat kimchi, whereas german horse eat sausage.

[chuanyan128]

  On 5/16/2015 at 12:13 PM, Zyklon said:

 

Read the post above yours. The answer is inside.

 

It is not only about peak power. It is also about how long you can make the peak power. Did you know that the 520i actually makes 180hp from 4500rpm all the way to 6500rpm? That means that whether you are at 4500rpm or at 6500rpm, just put your feet down and you get the full 180hp. Coupled with the closely ratioed, lightning quick and efficient zf 8hp, the 0-100km/hr on the 520i actually happens at the rated 180hp output from 30-100km/hr.

 

Let's not forget that BMW tends to under-rate their actual output...on the other hand, my personal experience with kimchi brand is that 1 kimchi horsepower is not as strong as 1 german horsepower.

Thanks. You must be a bmw supporter.

 

I guess kimchi done too many plastic surgery, their horses are partly fake.

[Mustank]

  On 5/15/2015 at 2:28 PM, Zyklon said:

Many people confused by diesel torque and what not...all from marketing bulls**t or confused individuals spreading the wrong idea...or was it topgear that said something along of the fastest you can reach and how fast you can reach the fastest.

 

You CANNOT use torque as the sole indicator of acceleration. You need to use power which is the function of torque and rpm. This means that assuming both cars are at full throttle and both are equally aerodynamic...

 

Car A: 200Nm torque, weighs 1000kg, engine running at 2000rpm

Car B: 200Nm torque, weighs 2000kg, engine running at 4000rpm

 

Layman will tell you that Car A is faster. Of course ma, at 2000rpm already 200Nm torque. Some more car A is lighter leh. WRONG.

 

Actually both car A and car B will have same acceleration at this moment.

 

In the similar manner, if Car A is a diesel hatch making 200Nm from 1500-4000rpm and weighs 1000kg. Car B is a 2000kg limo with a high reving petrol engine, making 200Nm from 3000-8000rpm. Both will actually be equally fast in acceleration if drag and drivetrain lost are the same. The difference is in their power and top speed will definitely differ.

 

Don't believe me? Draw out the free-body diagram and write their equation of motion.

 

car b jiak you jin dang

[Mr_b20]

acceleration speed is a combination of so many factors, not so straight forward.

 

but the most important are :

 

1. weight

2. horse power

2. torque

4. powerband

5. gear ratios (to match the above)

6. traction

7. aerodynamics (at higher speed)

[Vhtfhwlego]

The thing about BHP, BHP at engine or at wheel?

[Cephas]

  On 5/19/2015 at 1:26 AM, Vhtfhwlego said:

The thing about BHP, BHP at engine or at wheel?

 

steering wheel?

[Lowrider]

most likely it's on paper

Edited May 19, 2015 by Lowrider

[1fast1]

I've made very long post(s) about this (search), but here's the Cliff Notes:

 

1) Torque (torque/weight actually) makes a car feel fun on the road.

 

2) HP (HP/weight actually) makes a car actually fast on the track.

 

3) If you're always driving perfectly (optimally) for performance as typically happens on a track day, HP is much more important than torque, which explains the 0 to 100 timings in the post above.

 

4) If you're not always driving perfectly (as typically happens on a public road), a high torque lower down in the rev range allows you to cover for that.