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  1. #1
    Join Date
    Oct 2018
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    Shipston-On-Stour
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    181

    Have some fun with this RPM/Ratio/Speed Calculator

    This is fun https://www.boosttown.com/gearbox_di...calculator.php

    According to this, with my toploader and 2.88 diff I can do 95mph in 2nd and top out at 180mph in 4th. That would require a firm grip and some big boys underpants I think! Interesting little tool though.......

    Have fun......
    Dax with 1964 Dual Quad 427 and Toploader box

  2. #2
    Join Date
    May 2004
    Location
    Perth
    Posts
    4,626
    No calculation to take account of air resistance or friction from tyres and bearings though.
    Cheers, Clive

    (If I'm not here, I'm either in my new workshop or on the golf course!)

  3. #3
    Join Date
    Oct 2018
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    Quote Originally Posted by Clive View Post
    No calculation to take account of air resistance or friction from tyres and bearings though.
    Well if you're a Math graduate then there is a project for you! That would be complex. No account for all those thoughts that go through your head when you are doing those sorts of speeds either...............'Did I tighten that steering rack bolt up......' etc
    Dax with 1964 Dual Quad 427 and Toploader box

  4. #4
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    Oct 2018
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    Quote Originally Posted by Clive View Post
    No calculation to take account of air resistance or friction from tyres and bearings though.
    Well I'm no maths graduate (nor a graduate of anything else for that matter!) but thinking about it, is it right that this calculator just computes the mechanical aspect i.e. if you are doing 7000 rpm with a 2.88 diff and 255 40 17 tyres you would be doing 180 mph in 4th gear?? The issue of air resistance and tyre friction would effect the 7000 rpm, not the mechanical calculation of the speed???? In other words, you might only get 6500 rpm flat out as the car would be limited by air and friction? It's making my head hurt thinking about it. No doubt others will know.
    The real limiter is the driver! I met another Cobra owner at the Classic Car Hub (he will know who he is as he uses this forum). He asked me if my bonnet catches had ever popped. I asked why and he told me he had had a scare chasing a motorbike. The wind got under the bonnet and lifted it, popping the catches. How fast were you going I asked. As I recall it,he said 150 mph!! It was certainly of that scale. Fair play! He was a biker himself and the cobra was the only thing he had driven that was as quick. I can agree with that, but bugger, things happen fast at that speed.......
    Dax with 1964 Dual Quad 427 and Toploader box

  5. #5
    Join Date
    Mar 2010
    Location
    Hertfordshire
    Posts
    945
    I thought the reason the Daytona was invented was because the original open topped Cobras topped out at ~160mph

  6. #6
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    896
    That was fun! I got 154 mph @ 6000 rpm. But as mentioned, will my engine do it?

    I went through this years ago with model car engines and managed to find my notes. I tried it again, but with rough guesses as to some of the parameters. First of all, the major dominating factor, air resistance.

    I tried to calculate the required engine HP for that 154 mph using a formula for power balancing out aerodynamic drag, which is:

    P = ½ p V3 Cd A

    Where:

    P = Hp at wheels (in watts)
    p is the density of air, which is 1.2 kg/m3
    V is the velocity (in m/s)
    Cd is the drag coefficient (I’m assuming 0.8, which is a number for a truck)
    A is the frontal area (am guessing around 2 sq meters)

    Solving for P, I need 422.5 Hp and as I think my engine is good for 425, hitting 154 mph sounds possible. But it’s not. I have to consider the rolling resistance and the drive train losses.

    A little research found a common full-size car spec that says the force of rolling resistance is about 1.5% of the vehicle weight, although this can vary a lot depending on the car. I found this works out to over 14 Hp @ 154 mph, so I’m just going to use 15 Hp.

    I’m also assuming a 20% loss between the engine and the rear wheels.

    So;

    P(total) = Power (rolling resistance) + Power (drive train losses) + Power (air resistance)

    Sorting that out, Power (total) = ( 422.5 + 15 ) / (1 - 0.2) = 546 Hp.

    Well, I know I don’t have 546 Hp, so what is my top speed?

    Working all this backwards, I get 141 mph, a bit short of the calculator’s 154 mph.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  7. #7
    Join Date
    Oct 2018
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    Shipston-On-Stour
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    Quote Originally Posted by Eggbert View Post
    That was fun! I got 154 mph @ 6000 rpm. But as mentioned, will my engine do it?

    I went through this years ago with model car engines and managed to find my notes. I tried it again, but with rough guesses as to some of the parameters. First of all, the major dominating factor, air resistance.

    I tried to calculate the required engine HP for that 154 mph using a formula for power balancing out aerodynamic drag, which is:

    P = ½ p V3 Cd A

    Where:

    P = Hp at wheels (in watts)
    p is the density of air, which is 1.2 kg/m3
    V is the velocity (in m/s)
    Cd is the drag coefficient (I’m assuming 0.8, which is a number for a truck)
    A is the frontal area (am guessing around 2 sq meters)

    Solving for P, I need 422.5 Hp and as I think my engine is good for 425, hitting 154 mph sounds possible. But it’s not. I have to consider the rolling resistance and the drive train losses.

    A little research found a common full-size car spec that says the force of rolling resistance is about 1.5% of the vehicle weight, although this can vary a lot depending on the car. I found this works out to over 14 Hp @ 154 mph, so I’m just going to use 15 Hp.

    I’m also assuming a 20% loss between the engine and the rear wheels.

    So;

    P(total) = Power (rolling resistance) + Power (drive train losses) + Power (air resistance)

    Sorting that out, Power (total) = ( 422.5 + 15 ) / (1 - 0.2) = 546 Hp.

    Well, I know I don’t have 546 Hp, so what is my top speed?

    Working all this backwards, I get 141 mph, a bit short of the calculator’s 154 mph.
    Brilliant John! Your posts are always enlightening. That seems about right to me. I’m still not sure I would be up for testing it though, far too old for those sorts of speeds now!!! Although I love the 90 in second feeling 😀

  8. #8
    Join Date
    Jan 2002
    Location
    Marlow
    Age
    69
    Posts
    1,820
    I could pull 5800 my red line, in top I seem to remember was 154 mph. This was at Evlington all those years ago!! The only thing the arse doing 150 on the open road should pull is a prison sentence.

  9. #9
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    59
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    2,384
    Quote Originally Posted by Eggbert View Post

    p is the density of air, which is 1.2 kg/m3
    Bloody hell! Heavy air in your part of the world. Must be the cold makes it so dense.
    Cheers

    Myles D-W

  10. #10
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    59
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    2,384
    ... I stand corrected!
    Cheers

    Myles D-W

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