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Thread: Engine Power RPM

  1. #1
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    Engine Power RPM

    well the other day i was reading Motor Trend, August issue about the new G35. Since i have a 2003 and i love the VQ35DE i looked at the stats more in depth, now i've seen this before but now i really wonder. so here goes, the engines red line is 7500 RPM yet peak power is at 6800 RPM. so what does the engine do for the next 700 RPM till redline. does it lose power? i believe it does. so my question is why don't they put the Redline at say 7000 RPM so u dun just push it more and lose more power. anybody
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  2. #2
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    You'd have to look at a dyno graph to see what the power does after the peak power. It might level out or drop. They don't put the redline at 7000rpm because that could cut the speed down.

  3. #3
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    how would it cut the speed down if its not generating anymore power? wouldn't it slow the car down? although the engine is spinning faster.
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  4. #4
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    Shouldn't the engine be creating more power because of the centrifugal forces at a higher RPM? If it didn't produce more power, there wouldn't be a need for F1 cars to rev to 15,000 RPM.

    Obviously your parts need to be able to handle the shaking, but it should possibly be creating more power at an even higher RPM.

  5. #5
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    The engine is still spinning faster so it's moving the wheels faster.
    The peak power is around 4,000rpm on my truck but it still revs till 5,500. If it shifted at 4,000rpm it'd end up going slower than if it shifted at 5,500.

  6. #6
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    Quote Originally Posted by NSXType-R
    Shouldn't the engine be creating more power because of the centrifugal forces at a higher RPM? If it didn't produce more power, there wouldn't be a need for F1 cars to rev to 15,000 RPM.

    Obviously your parts need to be able to handle the shaking, but it should possibly be creating more power at an even higher RPM.
    No. Every engine is different and they have thier peak power at different places, which makes it tough to compare an F1 motor to a street motor. At a certain point the engine becomes most efficient(peak power) and once the rpms go up or down the power will be reduced because it's not as efficient. I hope that makes some sort of sense.

  7. #7
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    Remember that to accelerate you don't have to produce maximum power, just enough power to overcome the aerodynamic and mechanical drag at your present speed. That's obviously true below the peak RPM (since otherwise you could only move by revving your engine to 6800rpm and dumping the clutch), but it's also true above the peak RPM.
    As for why the redline is higher than the power peak, that's so you have a bit of a cushion so you can use the entire powerband without having to worry about sudden and abrupt engine damage. Otherwise you'd have to shift early and not get the maximum out of the engine. Of course, that's obviously more of a racing concern than a street concern.

  8. #8
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    Quote Originally Posted by coolieman1220
    well the other day i was reading Motor Trend, August issue about the new G35. Since i have a 2003 and i love the VQ35DE i looked at the stats more in depth, now i've seen this before but now i really wonder. so here goes, the engines red line is 7500 RPM yet peak power is at 6800 RPM. so what does the engine do for the next 700 RPM till redline. does it lose power? i believe it does. so my question is why don't they put the Redline at say 7000 RPM so u dun just push it more and lose more power. anybody
    As the engine speed increases past 6,800 RPM the engine power output decreases. However, there is still enough power to continue to accelerate it is just at a slower rate.

    Also, keep in mind that at the redline you have to switch into the next gear. When you switch gears the engine speed drops. By raising the redline you also raise the initial engine speed of the next gear.

    For example, if you switch gears at 6,800 RPM the engine speed may drop to 3,300 RPM. Whereas, if you waited until 7,500 RPM to switch gears the engine speed drops to only 4,000 RPM. If the engine makes more power between 6,800 and 7,500 RPM than between 3,300 and 4,000 RPM than you are better off waiting until 7,500 RPM.
    "In theory, theory and practice are the same. In practice, they are not."

  9. #9
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    Quote Originally Posted by Alastor
    As the engine speed increases past 6,800 RPM the engine power output decreases. However, there is still enough power to continue to accelerate it is just at a slower rate.

    Also, keep in mind that at the redline you have to switch into the next gear. When you switch gears the engine speed drops. By raising the redline you also raise the initial engine speed of the next gear.

    For example, if you switch gears at 6,800 RPM the engine speed may drop to 3,300 RPM. Whereas, if you waited until 7,500 RPM to switch gears the engine speed drops to only 4,000 RPM. If the engine makes more power between 6,800 and 7,500 RPM than between 3,300 and 4,000 RPM than you are better off waiting until 7,500 RPM.
    I was thinking that very same thing.

  10. #10
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    Quote Originally Posted by coolieman1220
    well the other day i was reading Motor Trend, August issue about the new G35. Since i have a 2003 and i love the VQ35DE i looked at the stats more in depth, now i've seen this before but now i really wonder. so here goes, the engines red line is 7500 RPM yet peak power is at 6800 RPM. so what does the engine do for the next 700 RPM till redline. does it lose power? i believe it does. so my question is why don't they put the Redline at say 7000 RPM so u dun just push it more and lose more power. anybody
    well you have to understand why the gears are there and when proper shift points are, as well as what the "redline" actually is

    i'm not gonna explain the first point here cuz it would take too long but i trust everyone knows (we're all petrolheads right ?? ^^)

    anyways, the proper shift point for say 1st gear to 2nd gear would be when the torque at the wheels in first gear at said RPM would be equal to the torque at the wheels in 2nd gear at the new corrosponding RPM

    if you shift before this point, then you could have accelerated faster by staying in the lower gear longer, if you shifted after this point, then you are bogging the engine down and you could have gone faster by shifting earlier

    the redline is a limit the manufacturers place on the engine to prevent it from reving too high ... why would you want to stop it from revving you ask. It's because the internals of the engine have a limit to how fast they can move before they snap, each engine is different which is what causes some engines like diesels to have such low redlines and F1 engines to have such high redlines. By strengthening the conrods and pistons you can handle higher piston speeds and thus run at higher rpm before reaching the "limit". Ofcourse there are much more variables to take into account but this is the "major" one.

    Now back to the question you asked (sorry bout the rambling btw). You're right in that the engine is losing power (making less power to be precise) past peak, but if the drop in power isnt very significant, then you can space the gears in such a way that you can use those few extra hundred rpms before the engine reaches redline, this gives you a bigger power band and a more flexible engine.

  11. #11
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    Quick summary:

    1. At more RPMs, more speed. It's a given linear relationship for every gear. The rate of acceleration changes accordingly to available torque.
    2. If you shift at peak power of a particular gear, you could fall out of the meaty part of the power band of the next gear, which will deteriorate you overall acceleration run.
    3. Optimal shiting point depends on each motor, but I would say that rev limit would be a good bet on most cases. The engineers know what they are doing, you know?
    Zag when they Zig

  12. #12
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    Good explanation here.....

    http://www.modularfords.com/forums/r...ion-41307.html

    see the article there in .pdf format

    Quote Originally Posted by coolieman1220
    well the other day i was reading Motor Trend, August issue about the new G35. Since i have a 2003 and i love the VQ35DE i looked at the stats more in depth, now i've seen this before but now i really wonder. so here goes, the engines red line is 7500 RPM yet peak power is at 6800 RPM. so what does the engine do for the next 700 RPM till redline. does it lose power? i believe it does. so my question is why don't they put the Redline at say 7000 RPM so u dun just push it more and lose more power. anybody

  13. #13
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    things that prevent engine revving higher:
    1) gas flow (is valve timing, engien porting geometry suitable?)
    2) piston inertia (can reciprocating components move any faster, this is moment of inertia)
    3)friction - this is the big one, mechanical friction cause mechanical power drain which maybe why power drops after point of max. power.

  14. #14
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    Quote Originally Posted by jediali
    things that prevent engine revving higher:
    1) gas flow (is valve timing, engien porting geometry suitable?)
    2) piston inertia (can reciprocating components move any faster, this is moment of inertia)
    3)friction - this is the big one, mechanical friction cause mechanical power drain which maybe why power drops after point of max. power.
    Indeed. In the older fuel injection systems,the carburetor couldnt handle to inject the fuel properly at higher rpm's. The timing went bad and the power dropped and emissions rised. The first injection systems had that problem too. But i nthe current generation of commonrail/motronic system that is completely solved

  15. #15
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    Quote Originally Posted by drakkie
    Indeed. In the older fuel injection systems,the carburetor couldnt handle to inject the fuel properly at higher rpm's. The timing went bad and the power dropped and emissions rised. The first injection systems had that problem too. But i nthe current generation of commonrail/motronic system that is completely solved
    Thats why people messed with cams and timing on carb'd motors.

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