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Thread: Light Vs heavy pistons (ie: ACL specs)

  1. #1
    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Light Vs heavy pistons (ie: ACL specs)

    who here has used the acl (mahle) pistons in their 20V? they say they arent meant to be used for a blacktop, anyone got any idea why? they seem to be unusually light for what they are and I wondering if this would make much of a difference.
    Last edited by Sam_Q; 08-04-2007 at 12:05 PM.

  2. #2
    Junior Member Domestic Engineer psychofox's Avatar
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    Default Re: acl 20v pistons

    The reason they probably say they aren't to be used for the black top is that the Cmopression chamber volume is not the same as the silver top, and that the stated Compression ratio would not be correct. The squish areas for Blacktop and silvertop are different - if you look at the piston top shape on the factory items they are quite different. I can't see any harm in putting them in a blacktop other than the Comp ratio being a bit lower.
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    yes thats what I thought, I tried to find out the deck height yesterday to see if I could shave the block to make up for it

    heres why i want them:

    standard 20v: 384 grams
    acl: 345 grams

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    Default Re: acl 20v pistons

    Interest perked

    Not much to contribute right now but interested in any further info to come out of this.

    And where'd you get them through? Happy to let me know what they cost you? PM if you want.
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    repco is a dealer of ACL, the trade price I have received so far is $790

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    Junior Member Automotive Encyclopaedia
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    Default Re: acl 20v pistons

    will the weight really make a difference? for about half that i can do new genuine rings and pistons.
    only thing i can see it making a difference in is an all out race engine.
    and if you were building one of those you have the cash to get customs ones made to your exact specs....
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    I think it would, more weight means less free reving, more losses and also more internal strain. From what I understand even 10 grams on a piston makes a difference when its goign up and down 120 times a second

    also I didnt do a proper comparasin:

    ACL: 345 grams
    4AGZE: 409 grams

    this is for a turbo engine, I don't want standard.

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    Junior Member Automotive Encyclopaedia
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    Default Re: acl 20v pistons

    ok turbo i understand.

    however im willing to bet you will not notice the difference in anything but an all out race engine where everything is being done to the last tiny nth of detail
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    yeah? I would thought that much of a reduction would make for something like a lightened flywheel. How about reduced loads on the bearings, you dont think it would make much of a difference?

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    Junior Member Automotive Encyclopaedia
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    Default Re: acl 20v pistons

    it would prob make a difference
    but enough to notice? or measure in anyway? i doubt it.
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    Default Re: acl 20v pistons

    Quote Originally Posted by Mr Revhead
    it would prob make a difference
    but enough to notice? or measure in anyway? i doubt it.
    So i read this and thought, that's a pretty decent weight saving, especially for reciprocating mass, and i reckon it just might make a respectable difference. Then i thought, i reckon it's a pretty simple calc to run for someone who's done their fair share of engineering and mechanical studies at uni. So at around 8:20am this morning (well before the first coffee of the day) i had a quick scribble on the back of my notepad...

    By 9:30am i'd had another 2 engineers tied up for over half an hour and i'd decided i had nothing to show from my years at uni except a big debt and certainly couldn't remember anything i'd been plenty competent at only a couple of years ago. Goes to show how big a difference regular use of a skill makes!

    Anyway, after discussing it over that coffee, we decided on a fairly straightforward and annoyingly obvious strategy. Apologies for the stageplay, now here's what i knocked up in between meetings round lunchtime;

    4age

    Bore: 81mm Stroke: 77mm

    delta m (^m) = 0.064kg/piston (ie change/difference in mass)


    Working on: power gain from weight saving = energy required to rotate difference in mass at given rpm

    Per piston, per revolution, starting at TDC; piston accelerates from 0 -> Vmax, -> 0, -> Vmax, -> 0, so can look at 1/4 of that cycle and multiply by 4 to get total for one revolution.

    @6000rpm (because it equates to a neat 100rev/s);

    Vmax = (angular velocity of crank)(2pie)(radius)
    = 100(2pie)(0.077/2)
    = 24.19 m/s (rounded a tad, my calcs carry through to minimise variance due to rounding)

    Now, KE = 0.5mv*2, (ie velocity squared, pardon the bodgy formatting)
    so ^KE = 0.5(^m)v*2 (i'm using ^ for delta)
    = 0.5(0.064)(24.19)*2
    = 18.7254 J for that quarter part of revolution (ie 0 -> Vmax), so for full revolution;
    = 74.9 J per revolution, per piston.

    Now, @ 6000rpm, ie 100rev/s,
    ^KE = 100(74.9)
    = 7490 J/s
    = 7490 W
    = 7.49 kW per piston.

    I got 7.5kW @ 6000rpm PER PISTON?! 30kW gain for a 64g/piston saving in a 4age sounds like a shitload... esp for ~$800...

    That's the sort of result i was expecting for overall, what's wrong with my calcs?? Is it too simplified to simply add that power value to the output? Do you need to take into account the 1 power stroke every 2 revolutions as the energy put into the piston is only every second revolution? As this is calculating an increase in energy realised at the flywheel, have i over-simplified?

    Power calculated is work done per unit time, so the 7.5kW i've calculated is per time anyway, like the quoted output for the engine. One piston saving of 64g realises 7.5kW at the flywheel??? Should this come off the theoretical maximum of a perfectly efficient engine, or off the net mechanical output after losses (sound, heat, etc)? I'm thinking i've calculated inertial gains separate from losses, similar losses still exist and don't enter into these calcs...

    I'm convinced our calcs are accurate enough, but struggle to believe the value, there's got to be something in the theory i'm missing!
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  12. #12
    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    very interesting, for only 6000 revs too, I agree it doesnt sound right though

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    Default Re: acl 20v pistons

    Bugger the numbers above, 64g/piston saving is nothing to sneeze at. Savings in reciprocating mass are definately worth chasing, and whatever the correct number turns out to be if you can justify the $$$ you will definately notice the difference.
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: acl 20v pistons

    yeah I agree, I am also thinking that. I am also wondering if it will help with fuel economy in a small way, unlikely I think.

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    Default Re: acl 20v pistons

    Ok, i just re-read up top and saw the mass difference is 39g between pistons, where the hell did i get 64g from???!!

    It still doesn't change my calcs not making sense, makes the weight saving a little less impressive though. Sorry for the miss-quote, if i can find time tomorrow (i'm onto the 3rd bottle of wine tonight) i'll edit my post above with more accurate figures.

    EDIT: Read a bit further and yes, we're talking 64g Now where did i leave my glass....
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