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Thread: am i right?? gear change q's. sciency types needed

  1. #1
    www.5lab.co.uk
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    Question am i right?? gear change q's. sciency types needed

    right, i know some of you like putting science theories into practice on cars, so am i right in saying this on another forum? its in relation to taking cars to the 'pod - the engines in question have peak torque at ~4,500rpm, and peak power at ~5,900 - stock volvo b200e redblocks for those who want to know..

    5lab wrote:
    purely out of interest - where were you boys changing gear? just that the peak power on a b200e is at 6000 rpm so i would have thought you'd be best to change at ~6500 rpm..

    SteveP wrote:
    Around 5500-5700rpm... theres not alot of point revving to 6500rpm, peak power may be at 6000rpm.. but peak torque isn't!

    5lab wrote:
    yes but there is a reason why you mesure a cars power in power, and not torque. you see, when you change gear, your torque goes down - so that in 5th gear you can never get as much torque as you do in 4th. however, your power remains the same - because while at 3,000 rpm in 3rd gear you have more torque than at 3,000 rpm in 4th, you're wheels are going the exact amount quicker in 4th to compensate, providing the gearbox does not create more friction (or similar) in 1 gear than another (which it probably does but to negligable effect).

    so, you should apparently change gear at the exact point where the engine speed in your old gear (say, 3rd) provides the same amount of power as the engine speed in your next gear.. you can work it out using the little 'mph per 1000 rpm' table in your owners manual and a torque graph. seeing as your car slows down a tiny bit every time you change gear, you should probably compensate by changing a tad later, as your next engine speed will be a tad lower

    of course, this can be tricky as the power curve often drops off very quicky, yet rises slowly - so while this can be the most efficient way of changing gear, if you get it late by a small amount it can make a big difference to your accelleration.
    i think i'm right, but not 100%. any thoughts?
    hughlunnon@yahoo.com | I have sigs turned off..

  2. #2
    Va Va Voom Lowe's Avatar
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    The way I work it out is to look at the torque band and work out where the most amount of torque is spread. I then change gear so that when the next gear up engages I'm at the bottom of the torque spread... This was ideal in the Clio since the torque was all mid to top end

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    I would agree with you, but only in theory.

    I think the best place to look for data on this would be within the F1 or Rally trades

    That, or post it on the VVOC (http://www.vvoc.com) or Vectra-Sport (http://www.vectra-sport.com) forums

    Stu

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    F.A.S.T. Butuz's Avatar
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    But the engine produces the same amount of turning force (torque) regardless of which gear your in.

    I can see where your coming from tho.

    Butuz

  5. #5
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    butus, the engine does but not at the wheels. gearing increases or decreases torque, and does the opposite to the number of rotations - this is why its easier to pedal a bike in 1st gear, but you go slower than if you were in 5th
    hughlunnon@yahoo.com | I have sigs turned off..

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    VTECmeous Vimeous's Avatar
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    Erm....

    I've never set out to drag race but in general I send the 'ole needle round until it gets to a point where, when I change up the revs are just above the vtec change over point.
    In my case that means just nipping into the red (8.1-8.2k) and you'll land in and around 6k in the next gear. It's only done rarely though as 7.8-8k (8k red line) will still land you pretty much ontop of the 5.6k changeover.
    I assume this is why many peeps running my engine use after market ECU's to drop the changeover point to 4.8k to give them more headroom high-up the rev range.
    Last edited by Vimeous; 05-04-2005 at 12:57 PM.
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  7. #7
    Now with added sobriety Rave's Avatar
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    Power accelerates cars. Basically you want to change gear to maximise the amount of power the car is producing, so you want to change gear once you're past the car's power peak at the point where the engine would be producing more power at the revs it'll drop to when you change up*. There is no set point; if the ratio between the gears differs there'll be a different optimum change point for each gear. The maximum torque point is a red herring (unless it happens to coincide with maximum power, which is unlikely). Power is what matters.

    Cars will always accelerate slower in higher gears because a) it takes more energy to accelerate a car by a set amount the faster it's going, and b)wind resistance becomes more and more of a factor.

    * Edit: having reread your quote I see that this is what you're saying. Not quite sure what you're driving at with the first paragraph about 3000 rpm in 3rd and 4th etc. though.
    Last edited by Rave; 06-04-2005 at 02:50 PM.

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