Power to weight

[Andy.bn]

Hi there, I'm hoping someone can enlighten me a little on the following scenario of comparing the performance of 2 standard, modern city cars. They are the Hyundai i10 1.2 (which is my current car for the past 3 years) Vs the Volkswagen up! 1.0 tsi (which I'm considering for my next vehicle).
My Hyundai has a claimed 86bhp and weighs 941kg giving a power to weight ratio of 91.4 bhp/tonne by my calculations. The VW up tsi as a five door weighs 1027kg and comes with 89bhp, giving a power to weight of 86.7bhp/tonne.

The mystery is that the up is claimed to be some 2.4 seconds quicker from 0 to 60mph, despite an inferior power to weight ratio, so how is this possible? I considered a difference in gear ratios but both cars I believe are geared fairly tall for economy. Would it be just down to the characteristics of the turbo engine? It seems to defy the laws of physics!

[kambites]

One problem with the modern tendency to gear cars so high, is that you have to use considerably more of the rev range. The Hyundai may make more power at peak but the turbocharged unit will probably make considerably more on average throughout the bit of the rev range you use for a 0-60 sprint. Realistically for best 0-60 time you'll probably be launching at about 4000rpm and once you've changed up to second, dropping to around 4000rpm again; how do their power figures at that engine speed compare?

It's also possible that Hyundai and VW are weighing their cars differently - I believe VW quote "EU" weights which includes a driver. I've no idea what Hyundai quote. You'll probably also find that one or both of those figures is 0-62 rather than 0-60, which makes quite a lot of difference in low powered cars.

Edited by kambites on Sunday 18th November 21:36

[Lincsls1]

Compare the torque figures between the 2 cars, I think you'll find the turbo car produces a good deal more.
This means it will have a stronger mid range producing more power lower in the rev range than the 1.2 NA.
Power to weight ratio is a limited performance measure.

Edited by Lincsls1 on Sunday 18th November 21:40

[Lincsls1]

Compare the torque figures between the 2 cars, I think you'll find the turbo car produces a good deal more.
This means it will have a stronger mid range producing more power lower in the rev range than the 1.2 NA.
Power to weight ratio is a limited performance measure.

Edited by Lincsls1 on Sunday 18th November 21:52

[pardonmyenglish]

Lincsls1 said:

Compare the torque figures between the 2 cars, I think you'll find the turbo car produces a good deal more.
This means it will have a stronger mid range producing more power lower in the rev range than the 1.2 NA.
Power to weight ratio is a limited performance measure.

Edited by Lincsls1 on Sunday 18th November 21:52

Torque (at the crank) does not matter so much for pure acceleration. Look at the numbers of the high revving honda vtec cars. If the cars revs higher, it means it can have shorter ratios. What's matter is not torque before the gearbox but torque at the wheel.

Same power to weight ratio, same numbers, turbo or not:

http://www.zeperfs.com/duel7180-1655.htm

Edited by pardonmyenglish on Sunday 18th November 23:35

Edited by pardonmyenglish on Sunday 18th November 23:35

[Cold]

Sounds like an extra gear change is needed for one of the cars.

[Toltec]

Have you had a test drive in the Up yet? Does it feel better or worse to drive?

The number just give you a starting point, something to narrow the search a bit, ultimately there are a lot more factors than the top trump figures.

[Baldchap]

On a car like that, if you draw out the power curve, you want the one with the biggest area under the graph, not the highest spike at the very far right.

[Terminator X]

Need to see the area beneath the hp / torque graph. Don't get that from most (any?) manufacturers so you'd need to put both on a dyno to check. Alternatively the "bum dyno" is also good.

TX.

Edit - lol what he said ^^

[TwinExit]

Power-to-weight and BHP per ton are phrases that should be binned if you seriously want to grasp how capable a car is, it is about as misleading as manufacturer quoted 0-60 times.

Today's launch controlled, dual clutch boxed, turbocharged cars have cut zero-to-sixty claimed times by around 0.5 to over 1 second compared to a similar RWD/FWD manual car with the same 'BHP per tonne'. Especially 90's - 00's N/A cars with peaky power bands like the S2000, E46 M3 etc

[TwinExit]

pardonmyenglish said:

Torque (at the crank) does not matter so much for pure acceleration. Look at the numbers of the high revving honda vtec cars. If the cars revs higher, it means it can have shorter ratios. What's matter is not torque before the gearbox but torque at the wheel.

Same power to weight ratio, same numbers, turbo or not:

http://www.zeperfs.com/duel7180-1655.htm

That's not entirely correct.

Torque is the 'moment' of twisting force as 'measured' from the output shaft/flywheel as a result of cylinder BMEP.

For sake of clear comparison, a 'High revving' car with half the BMEP/torque than a turbo'd or big displacement N/A, it would need to operate at twice the RPM and the gear ratio must be twice as much to 'multiply' the engine torque to the wheels at the same vehicle speed.

The reality is very high RPM and short gearing has its practical limits on a road car.

[GroundEffect]

Cold said:

Sounds like an extra gear change is needed for one of the cars.

Likely this.

This is never going to be a subject that is resolved easily, but you need to look at the time averaged torque at the wheels to give you a true indication of it's accelerative capabilities.

In lay terms, a lower engine torque but higher rev limit car will be able to stay in a shorter gear ratio for longer. Therefore if you take the integral of dT/dt between say 0-100mph at WOT, you'll see the benefit of revving higher for a given torque level.

Edited by GroundEffect on Monday 19th November 16:27

[TarquinMX5]

Gearing.

And aerodynamics will have a marginal effect. Not that you'd notice, of course.

[300bhp/ton]

pardonmyenglish said:

Torque (at the crank) does not matter so much for pure acceleration. Look at the numbers of the high revving honda vtec cars. If the cars revs higher, it means it can have shorter ratios. What's matter is not torque before the gearbox but torque at the wheel.

Same power to weight ratio, same numbers, turbo or not:

http://www.zeperfs.com/duel7180-1655.htm

Edited by pardonmyenglish on Sunday 18th November 23:35

Edited by pardonmyenglish on Sunday 18th November 23:35

Actually it’s the other way round. Torque matters a lot. For want of a better description, Horse Power is a representation of torque at speed.

The Honda VTEC engines produce very good torque for their displacement. Usually class leading. And the VTEC allows them to maintain this torque at higher rpm. Which is ultimately what gives you the increase in power over other engines of similar displacement.


HP = torque x rpm /5252

Essentially the same torque 1500rpk higher will result in a lot more Horse Power.

[RobM77]

The simple answer to the OP is that there's more to 0-60 than power to weight ratios. Namely:

1) Traction. This has to be considered both statically, i.e. the downward force on the driven wheels when the vehicle is stationary or at a constant speed, and dynamically, which is the force on the driven wheels when the vehicle is accelerating.

2) Gearing. You've considered the overall gearing (presumably including final drive and wheel/tyre size...), but what about the scope of each gear? For example, some cars that do 60mph in 2nd gear (like my former Elise S2 111S) avoid a gearchange to 3rd, so in their 0-60 times they save the time of the gearchange plus any negative effects of not using the revs fully in 3rd gear.

3) Torque. See above.

4) Actual power to weight. You've calculated the power to weight of an empty car. Once you add a driver, that figure for lighter cars will be affected more than for a heavier car. Also, what is the weight you're using? There are several standards for this: some are dry (without fluids or battery), some wet, some with passengers etc.

5) Test standards. As far as I know, there is no standard for testing the performance of cars. Some manufacturers give their car a 'typical load'; i.e. a suitcase in the boot and 3 passengers. For most of us, this is a gross over-estimate, as most of us travel with a coat, an iPhone and a lunchbox tops. Also, what surface is the 0-60 tested on? A nicely rubbered up bit of tarmac repeatedly used for full bore starts? Or a dusty bit of tarmac at the edge of a test track? This'll make a huge difference to acceleration times from rest.

[Andy.bn]

Hi everyone, thanks for your range of interesting replies. For me I think the signicant additional torque of the turbo engine (~ 30lbs ft Extra) must have a lot to do with it. Other factors such as weight distribution, traction and gearing are probably pretty similar.
I have driven the Up tsi and it's certainly pretty punchy in the 2000 - 4000 rpm area, way more than I'm used to, pulling well from around 1500.

[CABC]

quoted hp figures are max and not representative of normal driving as other posts have explained.

think of a real world example. a 320D vs a GT86. not from stationary but a rolling motorway start from 50mph.
which one pulls away and in what circumstances?

[pardonmyenglish]

CABC said:

quoted hp figures are max and not representative of normal driving as other posts have explained.

think of a real world example. a 320D vs a GT86. not from stationary but a rolling motorway start from 50mph.
which one pulls away and in what circumstances?

Why would you not drop a gear or two with the GT86? That's what's the 86 is made for, active driving.

[IntriguedUser]

CABC said:

quoted hp figures are max and not representative of normal driving as other posts have explained.

think of a real world example. a 320D vs a GT86. not from stationary but a rolling motorway start from 50mph.
which one pulls away and in what circumstances?

Yes I agree but in regards to the OP both cars would be going flat out, from a launch. Only thing that makes sense is that the up tsi produces its peak power for a longer time during the power curve than the i10

[Ron99]

Andy.bn said:

Hi everyone, thanks for your range of interesting replies. For me I think the signicant additional torque of the turbo engine (~ 30lbs ft Extra) must have a lot to do with it. Other factors such as weight distribution, traction and gearing are probably pretty similar.
I have driven the Up tsi and it's certainly pretty punchy in the 2000 - 4000 rpm area, way more than I'm used to, pulling well from around 1500.

Up 1.0TSI (90PS) reaches 60mph in 2nd. Most small-engine hatches need 3rd, including the Up GTi.
If you timed them to 25mph or 75mph the cars would be closer in their acceleration time.