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Thread: 86 Pistons Pics - Would you use them with a supercharger or turbo?

  1. #11
    Veteran Vance's Avatar
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    the brz sti coming out in 2013 will not be turbo according to subaru


    Quote Originally Posted by ae86 View Post
    ive come to the conclusion dan has more STi's than a Subaru Dealership, yeah i went there

  • #12
    Moderator Sam-Q's Avatar
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    That is the single best piston design I have seen for any factory engine. It has multiple really good features built into it. No wonder they can handle such a high compression ratio.

    Any have a combustion chamber pics?
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  • #13
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  • #14
    Moderator Sam-Q's Avatar
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    thanks Matt/ Now I can see why this can run such high compression. This would be a very detonation resistant chamber. I can explain many of the different features shown here if people want.
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  • #15
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    Yep I'd appreciate it...

  • #16
    Senior Member briigo82's Avatar
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    So would I. Explain away...

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    alright if I remember I will write my observations on the weekend
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  • #18
    Moderator Sam-Q's Avatar
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    alright where to start...

    now this is going to be a long winded summary of info I read or I am working out myself from what I can figure out. If someone can add something or correct me then go ahead.

    Base piston and pin design:

    - This design of piston is unlike the convention in that it's what's refered to a "slipper" design. This doesn't feature a piston skirt all the way around but rather has it at full length where it has the forward-aft movement and not and the sides where there is trivial rocking force. The skirt design therefore intersects the pin support right where the end of the pin is. Using a conventional design extra material has to be add not only to the sides of the pistons to distribute the force into the pin area but also the piston crown as the distance between it's supports is a greater span. The reduced skirt contact area also reduces total drag on the engine.

    - Art has gone to the extra trouble of removing material wherever it wasn't needed, small areas like between the gudgen and the crown has an indent all to remove material.

    - The bottom of the piston has the shape follow the countour of the gudgen pin support, this reduces any un-neccesary skirt length which further reduces weight and also drag.

    - Although pretty much standard these days the oil ring grouve has drilled lubrication holes, some earlier more primitive piston designs had a slot which is quite a weakness.

    - Features a special coating on the contact surfaces of the skirt.


    Crown shape and chamber:

    Now this is a tricky one to explain. I am not familiar with the progress of detonation in a direct injection engine as it tottaly changes one of the initial causes.

    Alright- ideally the combustion chamber would be the smallest sphere possible with the ignition in the dead middle. However this has to be altered to suit the other conditions of the engine. The basic theory is the same though- the smaller and more isolated the chamber is the better. When a combustion chamber is larger it increases the surface area of the combustion event. This then causes more heat loss, less proper burning and on externally injected engines encourages uncontrolled preigniton and dual flame front collision from the outer edges to the main firing. This is one of the big reasons why 5 valve engines have quite the disadvanatage compared to 4 valve chambers. The chamber is narrowed right down with the use of the quench zones which I will get to shortly. The combustion zone recess in the piston has also been shaped in a triangle shape which is quite clever. This is matched to the path of the fuel discharge of the fuels direct injector. So the theory would be that the fuel is pushed evenly and completely across the entire combusion zone.

    Now the really sneaky trick with the crown design is the angled quench areas. This is what was previously F1 tech only. If what I read is correct Cosworth was the first company to introduce this in a mass produced engine (motorbike). It's quite a task for a manufacturer to impliment this feature as the piston needs to match a 3d shape of the chamber to a very tight clearence. The gains however in such a design are clear. In a conventional chamber the quench zones are the flat machined areas of the head, the gasses are pushed out into the chamber area and are cooled down- hence the name. However with an angled quench zone the air is pushed directly into the middle of the head- right where the spark plug is. This would also stop the injected fuel overshooting the chamber and being unburnt in the quench area or the exhaust valve pockets increasing emmisions.


    There... comments anyone?
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  • #19
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    Thanks for that sam...
    Ae86 Drivingclub - Toyota AE86 Car Club where AE86 | KE70 | Toyota 86 / GT86 | Subaru BRZ | Scion FRS owners

  • #20
    Senior Member ill-minded's Avatar
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    I feel like a big nerd for understanding all of that and having prior knowledge of tapered squish areas. I've actually got a technical paper done by Mitsubishi on the effects of tapered squish pistons.

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