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    log style vs. equal length manifolds

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    Rick Ears

    Posts : 145
    Join date : 2009-02-25

    log style vs. equal length manifolds

    Post  Rick Ears on Sun Mar 01, 2009 4:05 pm

    There are two different types of turbocharger manifolds; cast log style and welded tubular style

    Manifold design on turbocharged applications is deceptively complex as there many factors to take into account and trade off
    General design tips for best overall performance are to:


    • Maximize the radius of the bends that make up the exhaust primaries to maintain pulse energy
    • Make the exhaust primaries equal length to balance exhaust reversion across all cylinders
    • Avoid rapid area changes to maintain pulse energy to the turbine
    • At the collector, introduce flow from all runners at a narrow angle to minimize "turning" of the flow in the collector
    • For better boost response, minimize the exhaust volume between the exhaust ports and the turbine inlet
    • For best power, tuned primary lengths can be used

    Cast
    manifolds are commonly found on OEM applications, whereas welded
    tubular manifolds are found almost exclusively on aftermarket and race
    applications. Both manifold types have their advantages and
    disadvantages. Cast manifolds are generally very durable and are
    usually dedicated to one application. They require special tooling for
    the casting and machining of specific features on the manifold. This
    tooling can be expensive.
    On the
    other hand, welded tubular manifolds can be custom-made for a specific
    application without special tooling requirements. The manufacturer
    typically cuts pre-bent steel U-bends into the desired geometry and
    then welds all of the components together. Welded tubular manifolds are
    a very effective solution. One item of note is durability of this
    design. Because of the welded joints, thinner wall sections, and
    reduced stiffness, these types of manifolds are often susceptible to
    cracking due to thermal expansion/contraction and vibration. Properly
    constructed tubular manifolds can last a long time, however. In
    addition, tubular manifolds can offer a substantial performance
    advantage over a log-type manifold.
    A design feature that can be common to both manifold types is a " DIVIDED MANIFOLD" , typically employed with " DIVIDED "
    or "twin-scroll" turbine housings. Divided exhaust manifolds can be
    incorporated into either a cast or welded tubular manifolds.

    The concept is to DIVIDE or separate the cylinders whose cycles
    interfere with one another to best utilize the engine's exhaust pulse
    energy.
    For example, on a
    four-cylinder engine with firing order 1-3-4-2, cylinder #1 is ending
    its expansion stroke and opening its exhaust valve while cylinder #2
    still has its exhaust valve open (cylinder #2 is in its overlap
    period). In an undivided exhaust manifold, this pressure pulse from
    cylinder #1's exhaust blowdown event is much more likely to contaminate
    cylinder #2 with high pressure exhaust gas. Not only does this hurt
    cylinder #2's ability to breathe properly, but this pulse energy would
    have been better utilized in the turbine.
    The
    proper grouping for this engine is to keep complementary cylinders
    grouped together-- #1 and #4 are complementary; as are cylinders #2 and
    #3. Because of the better utilization of the exhaust pulse energy, the
    turbine's performance is improved and boost increases more quickly.

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