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B How many horsepower?

  1. Feb 11, 2017 #1
    Salutations,

    I specialize in high performance solutions for Nissan vehicles. I've recently gotten into the top speed Texas Standing Mile competitions. The current champion is using a Ford GT with unknown power to set the record at 274 MPH from a still launch; I want to beat his record.

    I will be using a 2007 Nissan 350z, which will be engine swapped from the VQ35DE motor to the VK56DE. This will bring my current 3.5L V6 to a 5.6L V8, which a twin turbo set up will be added. Now, my question is this: how much horsepower will it take to propel a (approximately) 3,400 lbs vehicle to 300 MPH from a stand still? I will try to optimally reduce the weight down to 3,000, and the target speed is 287MPH. What kind of engine figures will I need to be achieving to make that speed?
     
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  3. Feb 11, 2017 #2

    anorlunda

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    You must specify how much time it takes to reach that speed before your question can be answered. 1 second -- impossible. 1 day -- a very small engine.

    Another critical parameter, how many hp does it take to maintain a steady 300 mph?


    The first question has to do with acceleration, the second with friction and drag.
     
  4. Feb 11, 2017 #3

    Vanadium 50

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    Somewhere around 2000.
     
  5. Feb 11, 2017 #4

    NTW

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    Assuming a frontal area of 1.5 m2, a coefficient of drag of 0.3 (a frequent value in sport cars) and an air density of 1.23, the power needed to keep the car moving at a constant 300MPH = 134 m/s against the aerodynamic drag would be:

    Power = drag × airspeed ⇒ Power = (½ × 1.5 × 0.3 × 1342 × 1.23) × 134 = 665890 W = 893 HP.

    And that, for air resistance only...
     
  6. Feb 11, 2017 #5

    Vanadium 50

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    A bunch of ~20% approximations and you get 3 digits of precision?

    The best estimator is the horsepower of cars that actually go at (or near) 300 mph. The five fastest production cars in the world average 260 mph at 1200 hp. If the only thing you were fighting is air resistance, 300 mph would take 1600 horsepower. Many of these "supercars" are more aerodynamic and lighter, which just adds to the power the OP will need.

    I think it's unlikely he will be able to get 7x the power (or even 3x the power) out of that engine. I also think that even if he did, it's unlikely that the transmission will support the sort of torque needed to reach these speeds.
     
  7. Feb 11, 2017 #6

    NTW

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    To the aerodynamic drag, you must add the rolling resistance. The rolling coefficient for tires on concrete is around 0.012 Now, for a car with a mass of 3400 lb (1544 kg) the power needed to keep the car moving horizontally at 300 MPH (134 m/s) fighting just the rolling resistance would be:

    Power = resistance × speed ⇒ Power = (1544 × 9.8 × 0.012) × 134 = 24330 W = 33 HP.

    That, for rolling resistance only... Now, the drag-associated power required plus the rolling resistance-associated power required, is 33 + 893 = 926 HP. As there are other resistances, such as engine and transmission friction, and you would also need a power reserve of –say– 500 HP, taking all that into account, in the real world, you would need an engine of at least 1600 HP.

    Or so...
     
  8. Feb 12, 2017 #7
    The 1,600hp figure sounds about right, but, I assume, that's power to the ground?

    I think I may not have been clear on certain important factors. One, the vehicle does not need to maintain 300mph, it must reach at least 280MPH within one mile, ideally 300. Getting a twin turbo V8 to 3,000bhp is actually fairly easy, on an open frame with nitro methane, alcohol, or 118 Oct. However, I will be cramming a lot of ducting into a much smaller street legal unibody.

    The recorded holder, as I found out last night, is pushing about 2,000bhp, or roughly 1,500-1,600 whp, on his Ford GT. I assume I'll need something slightly north of this figure.
     
  9. Feb 12, 2017 #8

    NTW

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    In my estimation, I accounted for the power lost to the transmission; thus, the 1600 HP is 'power to the ground', yes...
    If you have to keep that speed of 300 mph, even for the fraction of a second, you'll need the 1600 HP, for that fraction of a second...

    Good luck...
     
  10. Feb 12, 2017 #9

    Nidum

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    @anorlunda said : question has to do with acceleration, the second with friction and drag.

    +1

    @NTW : You perhaps don't understand the difference between power needed to accelerate the vehicle and power needed to maintain a set speed .

    If you have a simple trolley sitting still on a flat surface what do you need to do to make it move ?
     
    Last edited: Feb 12, 2017
  11. Feb 12, 2017 #10

    anorlunda

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    Well, then you answered your own question.

    Good luck.
     
  12. Feb 12, 2017 #11

    NTW

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    Not the case here, since aerodynamic drag and rolling resistance should be taken into account.
     
  13. Feb 12, 2017 #12

    Nidum

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    During acceleration of a vehicle some of the power delivered by the engine is used to increase the KE of the vehicle and some is used overcome drag forces on the vehicle .
     
  14. Feb 12, 2017 #13

    NTW

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    The power mentioned above for opposing rolling resistance and the drag is the minimum power needed to accelerate the vehicle to 300 mph. That acceleration will start with a high value, and continuously decrease as the speed increases, till the target of 300 mph is reached asymptotically. If you wish a shorter time to 300 mph, you can always use the extra 500 HP also mentioned above...
     
  15. Feb 12, 2017 #14

    rcgldr

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    There's a safety issue at high speed. That Mustang GT would need modified aerodynamics to keep it stable at 274 mph.

    Here's an example of a RX7 on a Bonneville run, lifting the rear end at 215 mph, then flopping over onto it's hood (not a violent crash), the driver wasn't hurt. Skip to 2:10 into this video, and again to 3:45 (another run in another car, this time the car lifts the rear and/or the front end collapses and spins but doesn't flip).



    Another issue is reaching 280mph in a standing start mile. The fastest production cars that can reach over 230 mph take 3 to 5 miles to do this.
     
    Last edited: Feb 12, 2017
  16. Feb 12, 2017 #15

    Vanadium 50

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    Ya think? :wink:

    Clearly getting a car to go 300 mph at all is a huge challenge, and is a necessary but not sufficient condition for this race. One thing that hasn't been discussed is that getting a lot of power only helps you if you can turn the wheels faster. If it needs to go 12,000 rpm to hit 300 mph, you have a problem.

    I'm a bit surprised at the twin turbo solution. The reason for a turbocharger over a supercharger is fuel economy, which is not a big concern here. The reason for a twin turbo over a single one is to reduce turbo lag. Again, probably not an issue.
     
  17. Feb 12, 2017 #16

    mheslep

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    Which is the approximate rate work is done by drag and rolling friction on the vehicle, fair enough.

    But the losses internal to the vehicle, the actual performance of the motor at its maximum rating over time at a given temperature, power of the heat rejection system, etc, are wild guesses based on the limited information here. Starting from the know HP rating of vehicles at high speed and extrapolating up, as V50 did, is the best approach with the given information. 2000HP @300mph. And some wings.
     
  18. Feb 12, 2017 #17

    mheslep

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    No, less than 1000 HP to the ground. The rest is loss between the engine cylinders and the ground. Lots of heat.
     
  19. Feb 13, 2017 #18
    That's only partly true, and would only apply to inline turbo system on a large/small combination. A single sequential turbo can behave exactly like a twin turbo setup in that the inlet venturis can change diameter to alter charge speed at any given RPM. A twin turbo in this situation will be on isolated inlet runners, which doubles the total inlet charge. Two big turbos on independent inlet piping will double the volume of incoming air.
     
  20. Feb 13, 2017 #19
    Aerodynamics will be the bigger hurtle, and is definitely the larger factor in this situation. The current record holder used the factory body for his record with no modification to its design. The Z33 that I'll be building isn't as aerodynamically tuned for high speed and the rear end gets increasingly lighter has the speed increases, which, of course, increases the likelihood of a roll over.

    Boniville isn't the greatest example because they are running on loose lime and salt deposits, I'll be running on a retired airport runway; It's quite a bit safer.
     
  21. Feb 13, 2017 #20

    rcgldr

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    Doing a web search, I found multiple articles stating that the current "claimed record" for standing mile by a "street" car is 283.232 mph by a modified Ford GT (about 1700hp, not sure if that's rwhp or engine power), but some groups dispute this.

    Another Ford GT with about 2000 hp reached 278 mph at a Texas standing mile event. Video:

     
    Last edited: Feb 13, 2017
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