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Last Post 04 Sep 2012 11:47 AM by  Andy Hollis
SP forged piston clarification
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Mojambo
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30 Aug 2012 10:40 AM

    Good morning,

    I'm currently rebuilding my motor in my ASP STI. I'm on my 2nd ring land failure and want to replace the pistons for forged units as per the rules. I've bolded the rules below.

    The big thing I'm finding is that the weight of the forged material VS the Cast is showing to be a problem to match(forged being lighter than the cast). so if the pistons are 100% identical, dish, dome shape etc.. how can the 2 materials be matched weight wise. For this reason, I'm thinking that the wrist pin and piston must be the same weight between the 2 combined. This way there is no lightening in the overall mass of the assembly.

    If anyone has specific insight on this, please advise. I'm talking with various piston makers and so far they are all asking about the pin and piston combo meeting the weight.

    Thank you,

    Eric

    Rules:

    S. Cylinders may be rebored to no more than 0.0472 in. over standard
    bore and the appropriate standard oversize piston may be substituted.
    This overbore dimension is an absolute limit; no additional tolerance
    is permitted to accommodate wear. Cast or forged, non-stock
    pistons of the same dimensions and configuration as original equipment
    pistons may be used. Additionally the replacement pistons must
    be of the same weight or greater as the original equipment pistons.
    Replacement pistons must match OE piston configuration exactly including
    quench area. The allowance for the use of aftermarket forgings
    vs. OE castings does not permit alternate piston dome designs.
    This allowance does not permit alternative ring configurations.
    T. Rotating and reciprocating parts may be balanced but not lightened.

    JBrettHowell
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    30 Aug 2012 01:45 PM
    I don't mean to be coy, but if it doesn't say you can...

    Personally I've always read this rule as "...you can replace pistons, but the box you have to stay in is so tight, it's almost not worth it...."

    Mojambo
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    30 Aug 2012 02:30 PM
    ^^true, the more I read it, the more clear in becomes. piston to piston= exact copy and all

    I've had someone on the subaru forum indicate the same as above

    Thanks!

    Eric
    snaponbob
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    Posts:2862


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    30 Aug 2012 05:12 PM

    Agreed. Not to sound simplistic or sarcastic, I would trying to understand the cause of the failures, not the failure itself. I recall that a few years back Subaru was having this problem with the STI's due to a lean mixture just before rev limit fuel cutoff. Seems there was enough knock happening, in a fairly narrow rev range, as the engine and ECM map was heading for the rev limit that piston failure was occurring. Subaru alter the fuel map and the problem was resolved.

    Just sayin'.

    Mojambo
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    31 Aug 2012 09:25 AM
    Thanks Bob, I'll pull up some logs I have of hitting the REV limiter while tuning. We did raise the revs from 7000 to 7650, so I'll get the fueling checked up there.

    The one major factor is that the OEM cast pistons will crack under knock even at OEM power levels. Boosting more ;) and pushing it in an auto-x environment on a hot day may certainly have been a contributing factor. I was tuned for 94.

    I'll be going to E85 after the rebuild and will build in more safety into the physical motor components(forged pistons) and will tune more safety up top on the rev limiter.

    Thanks guys and good luck in Lincoln. Can't make it this year but certainly enjoyed competing with you guys last year. (was # 90/190-grey STI)

    Eric
    snaponbob
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    31 Aug 2012 12:21 PM
    Eric, if you can't substantially increase the fuel delivery volume, you will kill the engine some more. The DI engines are more of a challenge than non DI. if you have NON DI, should be easy. If you have a DI system, welcome to my world of tuning challenges due to lack of capacity.
    Andy Hollis
    Senior Member
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    Posts:6197


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    01 Sep 2012 11:15 PM
    Hint: Larger bore pistons weight more than standard bore. That should handle your material difference since there will now be more of it in the replacement. .0472 is the max, and is usually worth doing.
    Mojambo
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    03 Sep 2012 04:37 PM
    Thanks Andy. The OE spec for a subaru overbore is 0.020, so there isn't much room. I just pulled the block out, so once I measure, I'll see what I can go with for the overbore if needed.

    Thanks!

    Eric
    Andy Hollis
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    Posts:6197


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    03 Sep 2012 11:27 PM
    Mojambo wrote:
    Thanks Andy. The OE spec for a subaru overbore is 0.020, so there isn't much room. I just pulled the block out, so once I measure, I'll see what I can go with for the overbore if needed.

    Thanks!

    Eric

    While the OE spec may be .020, SP still allows a max of .0472. That means a custom piston, but that's nothing out of the ordinary for SP.

    Mojambo
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    04 Sep 2012 11:24 AM
    But from reading the rules, I was under the understanding that the allowance is 0.0472 unless specified by the OE. which in my case is 0.020
    snaponbob
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    04 Sep 2012 11:34 AM

    "When in doubt, don't."

    Andy Hollis
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    Posts:6197


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    04 Sep 2012 11:47 AM
    Mojambo wrote:
    But from reading the rules, I was under the understanding that the allowance is 0.0472 unless specified by the OE. which in my case is 0.020

    Here's the allowance for Street Prepared. There is nothing here which restricts you to the OE max bore diameter. Note that the word "standard" as used in these rules is as defined in section 12 "standard part".

    S. Cylinders may be rebored to no more than 0.0472 in. over standard bore and the appropriate standard oversize piston may be substituted. This overbore dimension is an absolute limit; no additional tolerance is permitted to accommodate wear. Cast or forged, non-stock pistons of the same dimensions and configuration as original equipment pistons may be used. Additionally the replacement pistons must be of the same weight or greater as the original equipment pistons. Replacement pistons must match OE piston configuration exactly including quench area. The allowance for the use of aftermarket forgings vs. OE castings does not permit alternate piston dome designs. This allowance does not permit alternative ring configurations.

    It is common practice in SP to use custom forged "clone" pistons from vendors such as JE sized to that .0472 max overbore.

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