2013 Plastic versus Metal intakes (WRX vs STI)
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This is a discussion on 2013 Plastic versus Metal intakes (WRX vs STI) within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; What is your opinion of the plastic intake (WRX) versus the metal intake (STI)? It kind of freaked me out ...

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    Registered User chipset35's Avatar
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    2013 Plastic versus Metal intakes (WRX vs STI)

    What is your opinion of the plastic intake (WRX) versus the metal intake (STI)?
    It kind of freaked me out when I looked at the cars today at the dealership.

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    Registered User cflip2x's Avatar
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    Im guessing your talking about the Intake Manifold not the Air Filter intake.

    The WRX's have had the plastic intake manifold since 08. I agree it is a little sketchy at first but I have not heard of any major problem with it. It can even be used in a rotated intake manifold setup. One of the drawbacks is that you can not use STI TMIC and 02-07 WRX TMIC
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    Registered User jimbo058's Avatar
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    Plastic stays relatively cool compared to the heat buildup in metal. This helps keep the intake charge cool. Some of the older CAI's, notably Perrin, had metal tubes which became notorious as "Hot Air Intakes" until they changed their design to plastic tubing. I had one of the older designs, and it used to get so hot from underhood temps that you could hardly touch it !

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    Registered User chipset35's Avatar
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    So basically the more expensive car (STI) has the inferior intake material?

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    Registered User Heide264's Avatar
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    Quote Originally Posted by chipset35 View Post
    So basically the more expensive car (STI) has the inferior intake material?
    Depends on who you talk to.

    Seems that the composite manifold in the WRX is lighter and holds less heat.

    Then again, I would take the STi manifold over the composite one any day.

    I don't think I've heard of them failing for what it is worth.
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    You would think that the STI intake manifold would only have the advantage for looks or if you decide to port and polish the manifold. If you are not going big power then the plastic manifold is completely fine. Nothing to be worried about... I think

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    So a Performance cold air intake is really not a good idea?

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    Registered User Heide264's Avatar
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    Quote Originally Posted by v-rex View Post
    So a Performance cold air intake is really not a good idea?
    I believe this thread is talking about the differences between the trims "intake manifold"... not "air intake"... so no, this thread does not infer anything about a cold air intake for better or worse.
    Quote Originally Posted by Trainrex
    He was throwing balloons filled with sulfuric acid and shrapnel at the swat team. They finally had to take him down with rubber bullets.
    2011 STi Build Log
    -Part 1 - Reading, Data Logging, and Analyzing Data
    -Part 2 - Turbocharger 101 & Basic Boost Control
    -Part 3 - EcuFlash, Experimental Defintions, and a Drive By Wire Intro

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    This topic has been covered on multiple occasions. Please use the search bar. Furthermore, I am having this thread moved into the Engine Modifications subforum where it belongs.
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    Quote Originally Posted by chipset35 View Post
    So basically the more expensive car (STI) has the inferior intake material?
    Aluminum is a more attractive material, IMO. The STi is based off the older intake manifold, while the WRX uses the newer intake pioneered by the Legacy GT. Recall that the WRX used the same aluminum intake manifold as the STi (albeit silver as opposed to crinkle red). Subaru has likely used the organic composite intake manifold for the following reasons:

    1.) It is cheaper to manufacture with advanced composite molding plants
    2.) The organic composite has superior heat retention (and consequently heat resistance) to the outgoing aluminum design. This negates the need for Phenolic spacers as used in the previous aluminum design.
    3.) Depending on the mold, Subaru can achieve much smoother airflow with less sidewall resistance compared to cast aluminum, which would require significant polishing to achieve the same flow.


    The downsides are as follows:

    1.) The composite is more pliable, and as a result is more susceptible to damage.
    2.) Organic composites or "Plastics" are viewed as "cheap" and as a result, unattractive.

    To me, the pros outweigh the cons. Furthermore, with the price argument, I believe that you will see composite intake manifolds on both the WRX and STi on the next iteration of the model (should there be a distinction between the two).
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    OMG WTF MainFrame's Avatar
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    Intake Manifold Flow Bench Results

    Stock 07 STI Intake Manifold
    Runner 1 284 CFM
    Runner 2 280 CFM
    Runner 3 280 CFM
    Runner 4 274 CFM
    Average 279.5 CFM
    Runner to runner max variation 10 CFM or 3.6%

    Stock 07 STI Intake Manifold w/TB

    Runner 1 303 CFM
    Runner 2 304 CFM
    Runner 3 306 CFM
    Runner 4 299 CFM
    Average 303 CFM
    Runner to runner max variation 10 CFM or 2.3%



    Plastic Intake Manifold w/TB

    Runner 1 293 CFM
    Runner 2 283 CFM
    Runner 3 283 CFM
    Runner 4 291 CFM
    Average 287.5 CFM
    Runner to runner max variation 10 CFM or 3.5%



    EJ25 SOHC Big Plenum Short Runner Intake Manifold

    Runner 1 306 CFM
    Runner 2 303 CFM
    Runner 3 303 CFM
    Runner 4 306 CFM
    Average 304.5 CFM
    Runner to runner max variation 3 CFM or 1%



    EJ25 SOHC Big Plenum Long Runner Intake Manifold
    Runner 1 304 CFM
    Runner 2 304 CFM
    Runner 3 304 CFM
    Runner 4 300 CFM
    Average 303 CFM
    Runner to runner max variation 4 CFM or 1.4%



    Forsa Industries Rear Entry Sheet Metal Intake Manifold
    Runner 1 329 CFM
    Runner 2 326 CFM
    Runner 3 314 CFM
    Runner 4 295 CFM
    Average 316 CFM
    Runner to runner max variation 34 CFM or 10.3%

    Forsa Industries Rear Entry Sheet Metal Intake Manifold w/TB

    Runner 1 352 CFM
    Runner 2 343 CFM
    Runner 3 324 CFM
    Runner 4 324 CFM
    Average 335.75 CFM
    Runner to runner max variation 28 CFM or 8.3%



    Cosworth Intake Manifold

    Runner 1 303 CFM
    Runner 2 299 CFM
    Runner 3 299 CFM
    Runner 4 314 CFM
    Average 303.75 CFM
    Runner to runner max variation 15 CFM or 4.8%

    Cosworth Intake Manifold w/TB
    Runner 1 333 CFM
    Runner 2 318 CFM
    Runner 3 318 CFM
    Runner 4 314 CFM
    Average 320.75 CFM
    Runner to runner max variation 19 CFM or 5.9%



    Process West Intake Manifold
    Runner 1 349 CFM
    Runner 2 356 CFM
    Runner 3 360 CFM
    Runner 4 374 CFM
    Average 359.75 CFM
    Runner to runner max variation 25 CFM or 6.9%

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