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Compression

Discussion in 'A3/S3 Forum (8L Chassis)' started by Reesy, Sep 4, 2012.

  1. Reesy
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    Reesy Well-Known Member VCDS Map User

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    [Sep 4, 2012]
    Right I don't start many threads any more as their are threads about for most things, and I know that someone from the pub will come along and spam it.

    But..

    As I have just got an AGU head, and will be doing rods soon, I want to know what are the REAL benefits from having the greater comp ratio that the AGU pistons give you?

    Will it actually make that much difference to an engine pushing out about 300bhp...

    Was having a chat with Westy about it and we can both see the difference it would have on a Big turbo car, but on a K04 will it make a difference at all?
    #1
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  3. Prawn
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    Prawn My other car is a MINI!!!!

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    [Sep 4, 2012]
    May I ask you this.....

    Find me a ko4'd car that makes 300bhp on 9:1?

    Find me a ko4 hybrid car making 340bhp on 9:1?

    You won't. Both those figures have been achieved and passed on AGU lumps with 9.5:1.

    it's the missing ingredient, the reason why no other hybrid builds seem to have gone beyond 315bhp, yet Wellys made 340.

    The ko4'd AGU that Niki mapped made 309bhp on a dyno dynamics dyno in shootout mode, where Revo st2 S3's and LCR's rarely make over 270....

    I don't know enough about the science of it if I'm honest, but I think it's down to the size of the turbos, and heat and losses etc etc.

    On a big turbo, the tradition is to run a low CR, and pile in the boost. Big turbos can do this and be well within their efficiency range. You lose some power by going lower CR, but you make up for it with more boost. So rather than 20psi and 9.5:1 CR, you get the same results at 22psi and 9:1 CR.

    On a little turbo like a ko4 / hybrid, they're often pushed to the limits of efficiency, so suddenly that extra 2psi to get the same results takes the turbo beyond it's useful point, and into the realms of heat pump.

    That's how I see it anyway, I'm more than open to be told I'm wrong, but so far, almost every A3/high CR based conversion seems to better equivalent builds that use a lower CR.
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  4. superkarl
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    superkarl MAN OF STEEL

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    [Sep 4, 2012]
    i've told you all i can reesy.
    like i said, there is no disadvantage to it. so if you can, do it
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  5. s3gazz
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    s3gazz Active Member

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    [Sep 4, 2012]
    im seriosly thinking about this too and want to buy some but for the extra 10 ponys is it werth it.

    my engine at the minis all in bits and ready to put back together started cleaning all the pistons with petrol
    just been told too to stay away from depted pistons so one option out the window.
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  6. s3gazz
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    s3gazz Active Member

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    [Sep 4, 2012]
    for me it would be easier to get some agu pistons and pop my new piston rings in them leave mine on the rods sell as one.
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  7. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Sep 4, 2012]
    I might have a set from an AEB in the garage, will need to check their condition though, drop me a PM if your interested and i'll dig them out for you.
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  8. superkarl
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    superkarl MAN OF STEEL

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    [Sep 4, 2012]
    Gazz, if you're chasing numbers and want every last bit from your set up, get them. If not bothered about the numbers and just want good power then its no big deal.

    I just think that with a ko4, and its hybrid variant, its an extra mod to give that little bit whilst running a tiny bit less boost, therefore less heat, as prawn said.

    I wouldn't stress if you can't get them, but if its convenient and cheap enough, do it.

    There isn't really much testing, just comparisons of low comp vs high comp cars, but all with different specs and maps, so a hard one to judge
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  9. <tuffty/>
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    <tuffty/> Badger 5 Edition Staff Member Moderator

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    [Sep 4, 2012]
    Higher compression will give better driveability off boost as well as helping lift torque and power on boost

    <tuffty/>
    #8
  10. s3gazz
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    s3gazz Active Member

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    [Sep 4, 2012]
    im ment to be getting some off dani now just was not shore if the dept marks would course harm as all the stuff going back on is going to be new but with depted pistons i can buy em now do i do it lads.

    karl thanks mate im after the best out of her and as tuffty said driverbilty is better too so a bonus all round really.
    #9

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