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Old 20th December 2012, 12:53   #1
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Exclamation Is power/weight all that matters?

Mods: Couldnt find a similar post. All I could find was related to power to weight ratio and the effects of it. Do the needful in case if this is a duplicate. This is my first post so all your suggestions and corrections are welcome.

Had a lot of freetime at work and that has goaded me into posting this thread here. It has been troubling me for quite some time now. So I thought I'll get it discussed with the BHPians.

This power to weight flashes across my head everytime I have a chance to drive a different vehicle loaded(with passengers) to its limit. All of it started when I first had a chance to drive my friend's verna 1.5 vgt. I drove this verna with atleast 6 on board and the gross vehicle weight could be well above 1600kgs. If the power to weight of the vehicle is all that matters( atleast to an extent), then i should have felt the fiat linea 1.3mjd which was driven alone to be quicker/or atleast as quick as the verna. But I felt the opposite. The verna was much quicker even with 6 on board. I did the math and found out that the power to weight on the verna and linea were 'almost similar' in the above given case.

Is it the case only with me or has someone else found out the same maybe with a different set of cars??

Cheers.
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Old 20th December 2012, 18:05   #2
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Default re: Is power/weight all that matters?

In one word - no.

Power to weight ratio is a generic measure. But there's a lot more at play. Power generally is measured at full throttle , but a car that is most powerful may have weaker part throttle efficiency. Gear ratios also matter. A set of ratios set for city driving might show up poor on highways. Aerodynamics too, at high speeds air drag is the biggest force to overcome, an SUV with competitive power:weight may suffer high speed ability due to its higher surface area, assuming drag coefficient is the same as a normal sedan.

As for you example, I think a Verna with 6 on board should be fairly heavy and lose to the Linea - but 6 people should mean at least 1 was light weight kid. I'll have to look up the weights ; but then the passengers' weight may still be off !
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Old 20th December 2012, 20:03   #3
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Default re: Is power/weight all that matters?

Quote:
Originally Posted by Ricci View Post
In one word - no.

Power to weight ratio is a generic measure. But there's a lot more at play. Power generally is measured at full throttle , but a car that is most powerful may have weaker part throttle efficiency. Gear ratios also matter. A set of ratios set for city driving might show up poor on highways. Aerodynamics too, at high speeds air drag is the biggest force to overcome, an SUV with competitive power:weight may suffer high speed ability due to its higher surface area, assuming drag coefficient is the same as a normal sedan.

As for you example, I think a Verna with 6 on board should be fairly heavy and lose to the Linea - but 6 people should mean at least 1 was light weight kid. I'll have to look up the weights ; but then the passengers' weight may still be off !

Thanks for your reply, but no all were full grown adults weighing around 70-80 kgs. I have seen at many places that power/weight plays a major role but this proves otherwise. Other factors also play a role in deciding the performance altogether. I've felt the same in other cars too. A better performing car with full load is always quicker than a poorer performing car with only the driver on board even if the power/weight states otherwise. May be because I'm stuck with the notion that it can outperform the other one no matter what.

Just wanted to know if someone else has felt the same about it.
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Old 21st December 2012, 09:43   #4
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Default re: Is power/weight all that matters?

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Originally Posted by accelerate89 View Post
Other factors also play a role in deciding the performance altogether. I've felt the same in other cars too. A better performing car with full load is always quicker than a poorer performing car with only the driver on board even if the power/weight states otherwise.
The power available from the motor is one thing. The amount of it that is transferred to and reacted at the wheel contact patch is quite another. Often most of us disconnect the suspension system with the powertrain, when in fact they are very closely interlinked. In my very minimal experience with designing suspension setups, I have learned that there are a zillion suspension characteristics that decide the "performance" of a car, even in a straight line. I dont know what all those factors are, but I do know that they matter quite a bit.

One thing is the "anit-squat" characteristic built into the suspension. If a car has better anti squat characteristics, it does not pitch backward too much during acceleration. I mean to say that the transfer of weight to the rear axle is lesser if a suspension is built/designed with good anti-squat properties. This means more weight stays on the front axle , which is the powered axle. Since the front is axle is now loaded more, it is able to transfer more longitudinal forces at the contact patch before reaching its limit of traction. Thus, it can transfer more power.

This is but the tip of one of the many suspension factors that effect powertrain performance.

I'll let the experts elaborate and and discuss the rest of them

Cheers!
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Old 21st December 2012, 09:53   #5
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Default re: Is power/weight all that matters?

I agree to you on the suspension front, thats true. It plays a major role. But what I was trying to ask is if power/weight is not the final deciding factor then why does everyone brag about the power/weight ratio?

I'm not telling it doesnt have a role, but even the other factors like suspension etc which we tend to be oblivious of also shares a role in deciding the final performance.
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Old 21st December 2012, 11:15   #6
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Default re: Is power/weight all that matters?

Well as people have said power/weight ratio is not all that matters. In my opinion the gear ratios also plays a major part and as already mentioned suspension setup and how efficiently the car can put the power down.

Let's take an example of a car with say 100bhp/ton. If the gearing on this car is tall then it will be a comfortable highway cruiser capable of doing good speeds at lower engine RPM's. The same car with a short ratio gearbox will be a perfect city runabout but will run out of steam pretty quickly on the highway.

In your example of Linea vs the Verna. The Verna feels quicker as it has a VGT. I'm not sure if the Linea MJD has a VGT. The Verna is capable of producing good torque at just about any RPM. Also the Linea MJD engine has a lower cubic capacity than the Verna. As someone has very rightly said there is no replacement for displacement.

Last edited by vikram_d : 21st December 2012 at 11:21.
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Old 21st December 2012, 17:35   #7
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Default Re: Is power/weight all that matters?

Well, there are many parts to this puzzle. Engine power and weight are just one of them. Others in my view
  • How does the engine deliver the power? Jerky, How smooth it is?
  • The Gear ratios.
  • Suspensions (Really ), yes cos if the car feels like flying you will not be able to push, body roll etc will be because of this. In short i would be scared to push if the car does not feel planted.
  • Steering, Vague and life less steering makes the P/W meaningless. Unless you just want a straight line speed.

Last edited by mayankjha1806 : 21st December 2012 at 17:38.
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Old 21st December 2012, 19:23   #8
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Default Re: Is power/weight all that matters?

Quote:
Originally Posted by vikram_d View Post
As someone has very rightly said there is no replacement for displacement.
A very crisp explanation. This post is not to contend with anything you've said but just a different facet of the above statement. Whenever I read this statement in today's scenario, it doesn't hold water. I know you're an engineer and I'm not, so feel free to correct me.

No replacement for displacement was true a few years back but now with the advent of common rail tech and turbocharging, I would like to rephrase it as "there is replacement for displacement in the form of turbocharged inter cooling and common rail direct injection".

Consider the following vehicles. A few years back people would have been sceptical at such a scenario, but this scenario has been developing for a while and its reaching that stage where more manufacturers are going that way.

5 litre and 6 litre engines are a thing of the past and nowadays engines half or even 25% their size give that output and more. Let me show you a few Indian examples:

Toyota Corolla 1.4 D-4D - A D segment car with a small hatchback engine. Does it deliver? Yes it does!

Hyundai Fluidic Elantra 1.6 CRDi - The previous Elantra had a 2 litre. Now Hyundai reckons the 1.6 is good enough for the job and I agree.

Maruti Suzuki Ertiga 1.3 DDiS - An MPV should have a minimum of 2 litre capacity engine a few years back. But now? The 1.3 does the job.

GM Enjoy 1.3 Multijet - Another full sized MPV that's going to use the same engine with 75 bhp tune. Can it pull it off as a people carrier? I think it would!

Nissan Evalia 1.5 dCi - A commercial van turned 7 seater with a 1.5 litre engine? Isn't that a stretch? Definitely not thinks Nissan and I wholeheartedly agree!

Renault Duster 1.5 dCi - Who would have thought in the land of 2.2 litre Scorpios and Safaris that an SUV can be propelled with a puny 1.5? Renault has shown it can and in 2 different states of tune!

Renault Fluence 1.5 dCi - A spacious D segment sedan with an engine of a hatchback! If Toyota can for the Corolla, why not, says Renault!

Skoda Superb 1.8 TSi - A proper premium segment limo with a 1.8 in 2.5 to 3 litre territory? You gotta be kidding? Nope says Skoda. The Superb delivers!

Have I mentioned the Nissan Sunny, Ford New Fiesta, Mahindra Quanto, Maruti Suzuki SX4, Tata Manza, Fiat Linea, Renault Scala, or get this - the upcoming Ford EcoSport with a 1.0 litre engine? You see the pattern? Even luxury saloons are catching up with this trend now and I reckon its a great way to go. Efficient lean performing small engines will continue to rule in the era to come is my humble opinion.
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Old 21st December 2012, 19:32   #9
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Default Re: Is power/weight all that matters?

Dear Swiftdiesel, you have misunderstood what I meant. By displacement I didn't mean bigger engines. Almost all the vehicles you have mentioned are turbocharged. What does a turbocharger do? It increases the displacement of any engine by compressing air. .

In drag racing circles turbo cars are given a multiplication factor of 1.6. This means that in a 1000cc turbo car you can cram in 1600cc worth of fuel and air. That is increasing displacement my friend.

Last edited by vikram_d : 21st December 2012 at 19:36.
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Old 21st December 2012, 23:20   #10
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Default Re: Is power/weight all that matters?

Quote:
Originally Posted by vikram_d View Post

In your example of Linea vs the Verna. The Verna feels quicker as it has a VGT. I'm not sure if the Linea MJD has a VGT.
Yes, the linea also has a VGT.

Last edited by GTO : 23rd December 2012 at 13:42. Reason: Removing broken quote tag
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Old 21st December 2012, 23:34   #11
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Default Re: Is power/weight all that matters?

Taking my last post further. F1 engines from 2014 onwards will be 1600cc V6 turbocharged. Those will define what small engines with high displacement can do. I for one am looking forward to the revival of the petrol engine.

Edit: So whoever said there is no replacement for displacement was so right and so ahead of his times. :P

Last edited by vikram_d : 21st December 2012 at 23:36.
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Old 21st December 2012, 23:36   #12
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Default Re: Is power/weight all that matters?

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Originally Posted by swiftdiesel View Post
Toyota Corolla 1.4 D-4D - A D segment car with a small hatchback engine. Does it deliver? Yes it does!

Maruti Suzuki Ertiga 1.3 DDiS - An MPV should have a minimum of 2 litre capacity engine a few years back. But now? The 1.3 does the job.

Renault Duster 1.5 dCi - Who would have thought in the land of 2.2 litre Scorpios and Safaris that an SUV can be propelled with a puny 1.5? Renault has shown it can and in 2 different states of tune!
But I'll have to disagree with you on the corolla altis, it delivers but not as expected from a 16lakh car. A 1.4ltr VGT doesnt do the job as expected everywhere. Performance is one of the drawbacks of the D-4D.

In the case of ertiga and duster the power/weight plays its share. Both these cars weigh around 1250-1300, and that is only as heavy as a diesel linea. I hope that explains it. But plonk the 1.3 ddis in an innova and the outcome could be pretty terrible.
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Old 22nd December 2012, 02:28   #13
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I think what is most implant in a car is if it is a purist product or a collaboration of multiple conglomerates. A car is pretty much like a living organism. The brands that develop their own power trains/engines, usually have better ride and handling than brands that borrow engines or technology. Worldwide, Nissan-Renault is known as the worst brand ever as the cars neither have the European build quality and handling nor the Japanese agility.
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Old 22nd December 2012, 08:35   #14
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Default Re: Is power/weight all that matters?

Power to weight is one of the most important descriptions of how powerful a car is, or any vehicle for that matter. It's for this reason that superbikes (liter class) can outrun most supercars, at least in the 0-100 dash. It's simply power-to-weight coming into play. Power-to-weight is often quoted by manufacturers at the peak value, but the actual value may vary in use and variations will affect performance.

accelerate,
To answer your original question, when a car is loaded with people & you're gunning it, your engine characteristics change. There ECU of your car will squirt more diesel, may increase the rail pressure, & your turbo will be producing more boost.
Keeping all other variations similar, the Linea will have lesser load on the engine, as only the driver was driving. Even at higher rpm's the Linea engine will not be as loaded as yours, due to lesser stress on the engine, therefore less fuel is squirted, less rail pressure, less turbo pressure.

This is also the reason why rolling road (dyno) results which are super high, CANNOT be duplicated on the road. On tghe dyno, the wheels & engine are moving against a resistance (the roller on the dyno). For turbo powered cars, this means that the engine is under high load, so more fuel is injected & spark timing advanced. This results in more power. Doing the same thing on the road will not result in the exact same performance, because the engine is not as loaded as it was on the dyno.
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Old 22nd December 2012, 09:54   #15
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Quote:
Originally Posted by accelerate89 View Post

But I'll have to disagree with you on the corolla altis, it delivers but not as expected from a 16lakh car. A 1.4ltr VGT doesnt do the job as expected everywhere. Performance is one of the drawbacks of the D-4D.

In the case of ertiga and duster the power/weight plays its share. Both these cars weigh around 1250-1300, and that is only as heavy as a diesel linea. I hope that explains it. But plonk the 1.3 ddis in an innova and the outcome could be pretty terrible.
Very true. None of the engines I mentioned can exactly replace a higher displacement engine because of the longer bore and stroke. It's a compromise accepted by the buyer since the car will not be driven at it's peak power all the time. I've already mentioned elsewhere on this forum if I would like a higher capacity engine- definitely! Will a smaller engine do the job- yes with some compromises. It may not tick all the boxes but for me it ticks most of it if not all.
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