Buying A Car Based on Power to Weight Ratio

User210686 on 28/08/2018 - 09:30
Last edited 28/08/2018 - 09:40 by 1 other user

Hi,
I need some expert opinions as I have zero knowledge of automobiles and am now in the market for one.
When you are comparing engine power of different models, do you look at Power to Weight Ratio (W/kg) instead of just Power (kW) or Torque (Nm)?

I'm interested in the following cars:

Car A: 4cyl 2.1L Diesel Turbo, 66.3W/kg, 100kW @ 4000RPM, 300Nm @ 1400-3000RPM
Car B: 4cyl 1.6L Petrol Turbo, 81.6W/kg, 115kW @ 5300RPM, 250Nm @ 1250-4000RPM
Car C: 4cyl 1.8L Petrol Turbo, 99.3W/kg, 150kW @ 5500RPM, 310Nm @ 2300-4300RPM

Even though Car A has excellent fuel economy and looks great, its low Power to Weight Ratio kinda puts me off when thinking about future resale value.

I'll be mostly driving in the metro area on weekdays and occasional weekend trips, so it will be <10,000km a year. Do you think I should care about Power to Weight Ratio? Am I correct in assuming that lower Power to Weight Ratio means lower price everything else being equal?

Automotive

Comments

  • Nugs on 28/08/2018 - 09:32
    +2

    Torque vs weight.

    Power is a result of Torque x RPM.

    .

  • Nikorasu on 28/08/2018 - 10:05
    +20

    The only time you need to compare power to weight ratios is for traffic light drag races…
    If you aren't interested in going for a burn through the hills, then economy, safety and service costs would be more what you are looking at.
    Also, power isn't everything, torque of the Diesel will probably put it on par with the 1.6L petrol in terms of performance (pending gear ratios…)
    Looking at the torque spread also makes a difference. If you have a nice flat curve from low RPM to high over a wide range, the car will be easy to drive in peak hour (if you are a manual cog swapper kind of person…)

  • dcep on 28/08/2018 - 10:11
    +6

    i compare cars on their 0-100kmh time

    • Oz8argain on 29/08/2018 - 09:33

      that's only for drag racing right?

      • pegaxs on 29/08/2018 - 13:26

        Nah, for drag racing, you want to know their 1/4 mile times. That way you will know how fast you can get between traffic lights…

  • rompastompa on 28/08/2018 - 10:26
    +2

    I compare cars on their gear ratios

  • spackbace on 28/08/2018 - 10:32
    +1

    Hot hatches? As above, there's more to the drive than power/weight ratio

    Also, that doesn't affect resale unless it's really slow

  • HighAndDry on 28/08/2018 - 10:32
    +4

    If you care about performance, then look at 0-60 and 0-100 times. Those are far better indicators than power-to-weight ratio, just power, or even torque.

    But personally for me, performance is one of those things where once it's past an acceptable level, any more isn't really any better.

    • Rhubarb Jones on 28/08/2018 - 11:13

      Agree. For me if a car can go 0-100 in about 8 seconds I find it to be perfectly acceptable. That is by no means fast from a performance point of view but generally speaking drives well with just enough grunt for overtaking and emergency maneuvers.

  • User210686 on 28/08/2018 - 10:37

    OK Great, so I should be looking at the torque spread and kW derived from Torque*RPM/9549.
    I also found this useful website and video by some dirty old man https://autoexpert.com.au/posts/power-torque

    Acceleration and Power to Weight Ratio display an inverse correlation…

    • spackbace on 28/08/2018 - 10:45
      +8

      Just list the 3 cars so people can form a better opinion

      Better yet, have you driven them?!

      Multiple things can affect how fast the car goes etc like gear ratios, auto vs manual, if auto are they standard/cvt/dct, does it need a 'power mode' to really get going, awd/fwd/rwd, etc etc

      Don't just analyse a couple of numbers and think you know the car. You don't

      • pretz92 on 28/08/2018 - 10:55
        +7

        I think the cars23 have been left out on purpose, not something the average ozbargainer would buy.

        • spackbace on 28/08/2018 - 11:03
          +1

          Yeah I looked up 2.1 diesel and only came across MB, good research!

          All the more reason to drive them.

          I would guess MB are more popular in diesel variants (for economy) or V6/8. 4cyl gets left behind with them

        • HighAndDry on 28/08/2018 - 11:10
          +3

          How much do (used) MB buyers, since OP cares about resale value, care about the power-to-weight ratio? I'd assume things like trim level and features, diesel/petrol differences, and things like overall condition/service history/etc are much more important when comparing between similar cars.

        • spackbace on 28/08/2018 - 11:12
          +1

          @HighAndDry:

          100%, when it comes to Euro, FSH (from a dealer) is the most beneficial, and then just general condition etc.

          Diesel will be most economical, but might cost the most to maintain

        • Oz8argain on 29/08/2018 - 09:35

          lol yea, no one on ozb is dumb enough to buy a used MB

        • Micko179 on 31/08/2018 - 03:04

          @Oz8argain: did you miss OP?

  • Gronk on 28/08/2018 - 11:12

    Unless you've done something to negatively impact the power to weight ratio it won't affect resale, figures on paper won't tell you much, you should go and test drive them to see how they actually feel on the road because there's a lot more to the responsiveness of a vehicle than engine power alone.

  • Euphemistic on 28/08/2018 - 12:18
    +2

    Hardly anyone really cares about the power figures. Many care about how it drives, is it fast enough or how it handles. Many care only about tech in the car. Many don’t care about either of those and go for price.

    End of the day. Drive them and buy the one you can afford amd like the most.

  • redvaldez on 28/08/2018 - 12:29
    +1

    My current turbo diesel 4WD has a power to weight ratio of 64.3 kg per kw. My old 4WD, which was petrol, had a power to weight ratio of 70.2 kg per kw.

    If you looked at this figure only you'd think the petrol was a better performer, but in reality the turbo diesel is far nicer to drive and feels a lot more punchier, although admittedly it's coupled with a more intuitive automatic with a few extra gears.

    And despite my current car having a not dissimilar power to weight ratio to Car A, it's a pretty pleasant car to drive. It's not a rocketship, but it doesn't feel lacking either. I'm not missing my old V6 sedan as much as I thought I would (which, for interest's sake, had a 105.1 kg per kw power to weight ratio).

    Moral of the story: figures on paper only tell you so much - you need to take them for a drive.

    • Cliche Guevara on 29/08/2018 - 10:10

      Check your units. They make no sense. You probably mean W/kg.

      • redvaldez on 29/08/2018 - 10:17
        +1

        Whoops… shows you how little relevance it has to me then

  • Masterpupil on 28/08/2018 - 13:10
    +1

    Agree with others above that say take it for a drive.

    Diesel engine torque delivery can be quite surprising as it tends to come all at once, particularly if there is a turbo, but sometimes can be kind of laggy. The economy can be amazing.

    I would expect Car A would practically be much better than car B simply because of the torque the economy, plus the engine is likely to last longer (although in a euro not guaranteed). Servicing on a diesel may be higher however, depending on the complexity of the engine.

    Gearing will make a huge difference. The diesel will generally be heavier in the front though.

  • pegaxs on 28/08/2018 - 13:40
    +1

    There is no way that Car B produces 250Nm of torque @ 1250RPM…

    That being said, comparing these cars on that basis is pretty pointless.

    A diesel engine has a lot more torque and produces it a lot lower in the rev range. It's "power" appears lower because it produces this torque so low down. Power is a factor of torque x rpm / constant. And power isn't the ability of the engine to work, torque is. Power is just a measure of how much work (torque) the engine can do over time.

    As others have suggested here, take them all for a drive and buy the one you feel is the best. The subset of used car buyers that take "power to weight" into account when buying a second hand car would almost be zero. Features and safety are what sell cars…

  • Illian on 28/08/2018 - 16:39

    Like others said, 0-100 times will tell you everything you need to know when it comes to power. And in the case of a 4 cylinder car, nobody is gonna really care either way, I would go for the more powerful because it generally means more responsive (safer in a situation where you need to respond quickly), but I also would choose mostly based on my own preference for how it looks. Also, I know this is OzBargain, but I say this all the time, resale value is a non-factor, the car is for you, not whoever comes next, you don't make suggestions to your gf/bf so they're more attractive for the next person they date!

  • User210686 on 28/08/2018 - 17:51
    +1

    I agree. Thanks! I'm primarily focusing on aesthetics now.

    • spackbace on 28/08/2018 - 17:56
      +1

      May as well, they're good looking cars!

      Just be mindful of all the factory options if looking at used cars and make sure you're getting the features you want (redbook can be your friend here, showing what options are available on that model).

  • manu on 29/08/2018 - 08:02
    +1

    I assume Car A is C200 CDI. I have been driving C250 CDI and it is the best car I ever driven. It's the Car Of The Year 2011.
    C250 CDI has same engine but different turbo, filters, tune. Power: 150Kw@4200rpm Torque: 500Nm@1800rpm. Twin-turbo diesel with no turbo lag due to low & high speed turbos. 7 speed auto with different program modes.
    Would cover 1500km in a single tank between Sydney Melbourne. Usually around 5.5 l/100km suburban drive. Or low as 4 l/100km on highway. Still does 0-100 in 7 seconds in sports mode. On highway it keeps the distance (Distronic/active cruise control), emergency braking and keeps lane (Lane keeping & active blind spot correction). What more to ask?
    You need to drive all of them yourself and assess.

    • brendanm on 29/08/2018 - 10:07

      C250 does not have "high and low speed turbos", it is a compound setup.

      • pegaxs on 29/08/2018 - 13:28

        no turbo lag due to low & high speed turbos

        Top kek right there… :D

  • DarkOz on 29/08/2018 - 08:42

    I think it's best if you tell us your budget and we can recommend the best car to suit your requirements. Mind you some of your requirements are contradicting as if you want a fun drive and power it's not going to be fuel efficient. I think you are looking for a warm hatch, something in between a vanilla hatch and a hot hatch.

  • Oz8argain on 29/08/2018 - 09:37

    Don't get too carried away with the numbers. Compare which one FEELS fast. If you look at Matt Farah on youtube, he compared a fiesta ST with M3.. guess which one came out on top?

  • HeWhoKnows on 30/08/2018 - 16:21

    Dont worry about this mumbo jumbo
    Its meanless on its own
    There are far more important things to consider.

    Go and drive the cars and buy the one that feels best to drive
    That way you will be both happy and satisfied you made the right decision

  • 203132 on 24/10/2018 - 14:10

    Something that you didn't mention…are you buying new or used?

    If you are buying a used diesel it will typically be more expensive outright but you will need to drive a lot to offset the cost difference in maintenance and fuel.

    You'll want the extra power for weekend highway trips for overtaking and getting on to the highway, where I would look at the time taken to go from 80 to 120 km/h

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