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Automotive Intake Manifold Turbo flat 4

  1. Jan 20, 2012 #1
    Just a introduction to my project, I have designed and building a 69 Volkwagen Bug with a Version 8 subaru Sti Engine, The pan is built with tube braces and coil over suspension, I built a tall back bone frame for strength and for the rest of the frame to be lower for ride height and suspension. The car is street able

    I have just begun looking into designing a manifold, for two reason, Im using a race style brakes so I do not need the big vacume line for booster, and most of the other components on the original Intake Manifold is unneeded for my application,

    The car is boosted with a 16G turbo, Slightly larger then stock turbo, and I have aftermarket cams with a large duration (Turbo cars usually do) Duration is 272

    My current design is going to be made out of Aluminum and Tig welded The Plenum to have a volume of 350 Cubic Inch, With 7 inch runners with 5 degree taper going into Flange, Inside the plenum I will put velocity stack for each runner facing opposite side of the other 1/4 inch away from wall of Plenum

    I am just interested in seeing other peoples ideas, and concerns. I would like to know how I can use the kinetic energy waves from valves closing, to help force air into the chamber

    I have not done a project like this, and I am just in the beginning stages of the design

    I want something focused on mid range power, car will be used for Auto Cross and Time attach and driven on the street.
  2. jcsd
  3. Jan 21, 2012 #2
    For autocross you want to get the best throttle response and least turbo lag. Long intake runners help with both; you should try to get the total intake tract length in the 12-14 inch range (from plenum to intake valve). Make sure the transition between the runners and the port doesn't have more than about a 10 degree path change.

    Sounds like you're using the SDS intake building guidelines; yes?
  4. Jan 21, 2012 #3
    I am not sure what the SDS intake manifold guidelines are but I am interested into reading about it
    Also would airflow better on a smooth surface or a rougher surface such as a sand blasted surface
    Last edited: Jan 21, 2012
  5. Jan 21, 2012 #4
  6. Jan 28, 2012 #5

    jim hardy

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    would you guys post a pointer to where i could learn physics behind intake manifolds and cams?

    got a type 4 (1600 from porsche ) for experimental aircraft
    need to peak it in 2600 RPM range where a propeller still has some efficiency

    turbo to not turbo?
    Last edited: Jan 28, 2012
  7. Feb 14, 2012 #6
    Last edited by a moderator: May 5, 2017
  8. Feb 27, 2012 #7
    I'll preface my questions by mentioning that I am more knowledgeable with naturally aspirated motors, and not so much with forced induction.

    Could you explain how the long runners help reduce turbo lag? I know the rule of thumb that long runners (generally) help increase torque at lower RPM's, and that shorter runners (generally) increase horsepower and top-end power, but at the expense of low end torque.

    I also thought that throttle response (particularly with NA motors) is quite influenced by the distance from the throttle body to the intake valves, and the size of the plenum. (Assuming it is a single throttle body on plenum intake, and not individual runner throttle bodies). With a turbocharged motor you want to keep the boost piping short & minimal/smooth bends, don't go with oversized pipes, and don't oversize the intercooler.

    Your project sounds pretty interesting, and will certainly be a blast in that VW Bug.

    While larger duration cams certainly help let more air in, be sure they have little overlap of the exhaust and intake timing. Being that the Subaru motors are almost always forced induction and have a big following, I would be confident that an aftermarket company wouldn't put out a poorly designed cam set, but it's worth noting.

    Those kinetic energy waves would be considered Helmholtz pulse tuning. This is the practice of constructing the intake runner so that pressure waves will time with the intake valve timing to begin airflow into the cylinder sooner, and "squeeze" a little more air in at the last moment before closing. I mention this because Helmholtz tuning is primarily focused on naturally aspirated motors. I certainly may be incorrect, but I imagine that there may be very little if any gain in tuning the pulses when you are forcing the air into the cylinders in the first place.

    An intake plenum size of 350 cubic inches sounds massive for the Subaru motor. Is the motor a 2.5L? I believe the rule of thumb is to have the plenum about the same volume as the motor displacement, or somewhat smaller. I will check my references and get back to you on this, but it's what first comes to mind.

    I'll also check on the runner taper, but I believe five degrees is at the upper limit of what's recommended. I think two to three degrees taper is the average. I'll check on this and reference articles - I may be thinking of NA motors anyhow...

    I suppose my last comment would be to research what the aftermarket racing companies offer for performance intake manifolds. While they are likely incorporating some OEM features into the design, you may find that you are trying to reinvent the wheel, and your efforts (and money) could be put to better use. The great thing is that the Subaru motors have a well-established performance path, so you don't have to break ground trying to get power from it. Have a look at what is proven to work.

    Good luck with your project. I'll be back to clean up my post and add links to articles as I pull them up.
  9. Mar 1, 2012 #8
    If you want to get crazy too the turbo manifold can be tuned up to the turbo. Will help with rpm range needed
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