Cars We Drive!

Discussion in 'Fine Living, Home, Design & Auto' started by Bert1568, Jul 18, 2006.

  1. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    

    Was the real, nearly identical 335i vs. 335d not close enough that you want to toss around purely hypothetical situations such as you just wrote?

    1/4 mile [email protected] for the diesel, 13.5 @103.8 for the gas.

    As for your question re: 400hp, 2000 lb car?

    At 60mph, it'll accelerate at ~1.2 ft/s^2. At 100mph, .74 ft/s^2 (ignoring wind and windage losses, of course). (oops, screwed up the units. That should be 1.2 g, and .74 g, not ft/s^2).

    Any chance you could do a similar calculation from torque? I didn't think so.
     
    Last edited: Apr 15, 2012
  2. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    no
    M3 has higher HP and lower weight yet is the same speed as a car with more torque, equalize wt and the car with less HP is faster

    wrong. I know the answer, you are waaaay off
    show your work, it's a single equation, 3 variables

    a 400 HP 2000 lb car will take ~ 60 sec to go 60 to 100 mph (88 ft/sec to 150 ft/sec) or a gain of 60 ft/sec at an avg accel of 1 ft/sec-sec. really?
    my 400 HP 4000 lb car does it in 6.5 sec, lol

    yes I can knowing axle torque and tire radius (we'll assume 1' to make the math easy) what is the torque you want to use?


     
  3. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    btw
    your 400 HP 2000 lb car would take 140 sec to get 0-60 mph
    assume an avg linear accel of (1.24 + 0)/2 ~ 0.62 ft/sec-sec (like most cars in low gears)

    88 ft/sec / 0.62 ft/sec sec ~ 140 sec
    even if we assume it launches and hits 1.24 ft/sec sec instantaneously still 70 sec
    my 400 HP 4000 lb car takes a bit less than 5 sec (4.6 to 4.8)

    a 400 HP 2000 lb car would seem a bit faster
     
    Last edited: Apr 15, 2012
  4. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    

    Sigh.

    Ok, power = 400hp (and I'm going to take that to be linear over at least some range. Let's say the engine makes 400hp from 5000 to 8000 rpm, which means it makes 420 ft-lbs of torque at 5000 rpm and 262 ft-lbs at 8000 rpm.

    Let's say we're in a gear where we're doing 40mph at 5000 rpm. That's 3520 ft/min. Given an average wheel size of 24" (this is small for modern cars, but it's what cars used to run 20 years ago and makes for an easy 1 ft radius), that means that the wheel is rotating at 560rpm.

    If our wheel is doing 560rpm and our engine is doing 5000 rpm, our effective gearing is 8.92. So at 40mph we're making (420 ft-lbs x 8.92) 3750 ft-lbs at the rear wheel, on a 1 ft radius wheel, so 3750lbs of force pushing us forward. 3750 lbf = 2000 lbm x a, a = 1.875 g. At 8000rpm, the numbers are (262 x 8.92) 2337 ft-lbs at the rear wheel, 2337 lbf accelerating the car, and 1.17g.

    Obviously these are theoretical numbers, but the point being...you actually can calculate the speed of a car from horsepower, and you know absolutely nothing about the speed of a car from simply knowing the torque that it makes.

    Anyway, sorry to everyone for getting so far off track. I'm clearly bored out of my mind sitting on a rig off the coast of Equatorial Guinea at the moment.

    Back to the show...
     
  5. JohnGalt

    JohnGalt Senior member

    Messages:
    4,881
    Likes Received:
    686
    Joined:
    Feb 27, 2009
    Location:
    Ouray, CO
    :brick:
     
  6. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    

    Yeah, I made the correction above. I was using ft/s^2 when I meant to use g.

    Ok how fast does a 2000 lb car with an engine making 400 ft-lb of torque accelerate at say, 40mph?
     
  7. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    so you can't do it with only HP (and tire radius)
    you need need the speed to calculate rpm to calc torque THEN power (as in P = T x rpm)
    lol
    my tires are 26" dia so 24" is not real small

    you can't calculate speed with HP alone, absurd, you assumed a tire size and road speed

    axle torque = 3750
    thrust = 3750/1 = 3750
    accel = 3750/2000 g or 1.875 g or 60 ft/sec sec
    you needed road speed (and wheel speed and gear ratio)
    I needed only axle torque/thrust
    that is becasue T is a primary variable and determines accel, you need rpm to calculate torque from HP


     
  8. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    I picked those numbers to make a calculation with, but they can easily be changed. The point is the engine was making 400hp, whether it was doing it with 840 ft-lbs of torque at 2500rpm or 105 ft-lbs at 20,000rpm doesn't change the outcome of it. All that was needed was the power, I simply used place holders. You can change the speed as well, and the calculation will still be valid.

    I said the car makes 400 ft-lbs of torque. That's the engine output, how do you know the axle torque? (hint: you don't).
     
  9. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    you need axle torque or wheel thrust, like measured on a dyno (and which they use to back calculate HP, since it isn't readily measured)
    unless you are talking about an old school dynomometer which turns a generator, measure current, voltage and power factor (not easily measured) take the product and divide by 746
    I gave you wheel/road HP, 400 HP, give me the axle torque or wheel thrust


     
    Last edited: Apr 15, 2012
  10. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    it changes everything
    with wheel torque/thrust I can calculate accel
    with HP alone you can't (you need rpm to calac torque and thrust, secondary varibale, it takes 2 things to get the answer)

    again
    no speed given
    all you know is:
    1' radius
    600 wheel HP
    3000 lb car
    accel?


     
  11. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    engine torque = 400
    gears = 10:1
    radius =1
    wt 4000
    x mph

    accel = (400 x 10/1) / (4000) = 1 g

    engine power = 500
    gears 10:1
    radius = 1
    wt 4000
    y mph

    y NE x

    what is the accel?

    hint, I don't need rpm, road speed, etc.
    you do
    to calculate torque/force/thrust, lol
     
    Last edited: Apr 15, 2012
  12. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    Are you under the assumption that I don't know that F=MA and that axle torque x wheel radius = F in that equation? Clearly the argument we've been having was not "if you apply more force will it accelerate faster?" and if you thought that was the case, then I don't even know what to say.

    :confused:

    We're talking about cars in this thread. As in, "the new M3 makes 414hp and 296 lb-ft of torque".
     
  13. Arthur PE

    Arthur PE Senior member

    Messages:
    728
    Likes Received:
    18
    Joined:
    Jan 22, 2012
    Location:
    Earth
    the discussion is
    which makes a car accel faster, more HP or more torque (for a closed system), torque
    can you calculate accel from power alone, no


    GTR and e60 M5
    480 vs 507 HP
    wt ~ the same 4000 lb
    0-150
    the GTR is 5 sec faster
    how so with less power?
    could it be it has much more torque due to turbocharging?

    let's summarize
    to calc accel I need wt, gearing, tire radius, torque
    you need wt, gearing, tire radius, power and road speed or rpm
    gotcha

    2 cars, identical except as follows
    one makes 10% more torque over the power band except close to redline, it makes 5% less than the other
    so the other has a higher peak HP, but less average torque
    which is faster?



     
  14. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    Why do you keep ignoring the two best examples and pull out cars with different aero shapes, different weights, different drivetrains, etc?

    VW golf TDI vs. GTI. BMW 335i vs. 335d. For that matter, BMW M3 vs. 335i OR 335d.

    You keep talking about axle torque as if cars are advertised by that number. I'm not arguing that the car putting more force to the ground accelerates faster. I'm arguing that knowing the torque at the engine tells you nothing about the force the car is putting to the ground. NOTHING.
     
  15. Kookz

    Kookz Senior member

    Messages:
    367
    Likes Received:
    8
    Joined:
    Jul 8, 2008
    Location:
    Equatorial Guinea/Suisse
    I realize from reading your previous arguments in this thread that you'll drag this on, changing the original argument to suit you, until the other person gives up out of frustration, so I'll just say I'm done posting on the subject.

    I haven't owned a car in 5 years, but I've been considering something to take my bike to track days in, and I came across this, apparently the Element is pretty huge inside. It wouldn't be fun to put in or get out, but it can't be that bad.

    [​IMG]
     

Share This Page

Styleforum is proudly sponsored by