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Thread: Inlet Manifold Design

  1. #1

    Inlet Manifold Design

    Moved from old tech forum


    h0tr0dder_uk
    Non Member
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    I'm considering building my own custom inlet manifold for my C20XE (2l 16v vauxhall) engine and was wondering what the fundamentals are.

    I'm think that the runners should be the same length as each other and preferably the same length as original spec.

    I'm sure there is much more to it but can't seem to find much info on the net about it.

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    Adam
    NSRA Member

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    Depending on induction (especially if using a single carb) you will find the outer cylinders will always run slightly leaner than the centre ones. If using twin carbs then each one can be centred between each cylinder for equal flow and fi... (well thats a black art all of it's own!). A lot of R&D went into the factory intakes so use those as a ball park and make educated decisions from that. Are you running a bigger cam, headers and more induction than stock? If so you could make the intake slightly longer which would move the peak hp point up the rpm range. You could also make it slightly wider to flow this extra fuel but be carefull that charge velocity can slow down to a point that the heavier fuel will fall out of suspension with the air and not atomise properly. A slightly rough as in cast texture to the manifold interior also helps this. A polished intake runner does not really help flow at all.

    My advice would be to try and utilise and modify something stock or aftermarket thats already been done unless your looking for that one off manifold.

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    Any advice or help given is actually based on having done the job, not read about doing it or Googling it.

    www.langysrodshop.co.uk Our parts are air freighted so 5-7 day delivery, The best GRP Willys body available/Rebel Wirings only UK dealer/Speedway Motors authorised dealer/Summit racing/Jegs/Hotrod parts supplied, MAC Autos, We deal with all the US hotrod suppliers even non car related stuff.
    Brake,Oil & Fuel etc plumbing stockist/Totally Stainless fastener dealer/Dolphin Instrument dealer, LMC & Brothers Trucks,
    Stainless Exhaust tube & mandrel bends stockist

  2. #2
    Adam
    NSRA Member
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    Just remembered I had this in the tech files which may or may not be of some help.

    "INDUCTION SYSTEMS
    The Manifold
    By one simplified definition, a typical automotive "induction system" is a network of passages connecting an engine's cylinders with a source of inlet air, attached to which is a metering device that regulates air or air/fuel mixtures. Such systems may include fuel injectors located downstream of the air inlet point or accommodate either carburettors or throttle body injection {TBI** units at a point servicing all passages to the engine's cylinders. That's a functional definition. In a dynamic sense, an induction system includes a range of pressure conditions that can help an engine achieve torque {or volumetric efficiency** increases beyond that obtainable without the benefit of induction system "tuning." Since an intake manifold is a network of passages connecting a throttling device {air valve, throttle body or carburettor**, you can view such passages as extensions of the inlet ports. In fact, that is exactly how they should be considered. If they are not, it's possible for each to become in conflict with the other, causing improper air {or air/fuel mixture** flow quantity and quality. Generally, at or near the inlet manifold's point of air entry, there is a throttling device. These are located where the passages of the manifold {inlet port "extensions"** meet in a common volume. That volume is often called a "plenum" and represents a region from which all manifold runners extend. Analogous to an exhaust system in which all tubes become joined in a single volume {collector**, the plenum of an intake manifold is comparable to a header collector.
    Types of Inlet Manifolds.
    If all of an engine's inlet ports are connected to a single plenum volume {chamber**, the design is said to be a "Single plane" arrangement. That means a single cavity is used to connect all inlet port passages {runners**. If some of the runners connect to a cavity different from a second cavity connected to the remaining set of runners, the design may be called a "Dual plane" design. Depending upon basic engine design {in-line, V-type, etc.**, it's possible to connect cylinders that alternate in the firing order. A typical instance is a V8 engine in which each successive cylinder is served by alternating intake manifold planes. Here's an example. Suppose the firing order is 1-8-4-3-6-5-7-2. In this case, one plane of the manifold would be connected to {serve** cylinders 1, 4, 6 and 7 while the other would serve cylinders 8, 3, 5 and 2. In terms of crankshaft rotation and inlet impulses, successive cylinders would be provided mixtures every 180 degrees of rotation. This is the commonly known 180-degree manifold. A similar arrangement can be configured for other firing order and cylinder layouts. Should the manifold design not include a plenum but provide singular paths into each inlet port, the type is called an "independent runner" or IR design. This is a typical method for fuel injected engines intended for mid- to high-rpm use, such as narrow engine speed ranges associated with sprint car or high- speed-only applications. The ability to transition from low- to mid-rpm engine operation is foregone to benefit higher rpm and sustained speeds.
    Air/fuel mixture distribution.
    An intake manifold plays a vital role in how each of an engine's cylinders receives air or air/fuel mixtures. Whether the design joins all cylinders to a common plenum {single plane** or divides cylinders in some fashion, there is a measure of "cross talk" that exists among connected cylinders, during a variety of engine operating conditions. Here's how that works. When an intake valve opens, there is a brief period of time when cylinder pressure is higher than pressure in the intake manifold. This causes an amount of combustion residue {exhaust gas** to be passed back into the manifold. Call it "reversion," "back flow" or by some other term, this reverse pressure "pulse" can affect primary inlet flow for another connected cylinder. Depending upon engine speed, load and related factors, this reverse flow of exhaust gas can adversely affect other cylinders during their normal intake cycle. As a result, air/fuel ratios present in any given cylinder at any given time of engine operation can be a combination of fresh air/fuel mixtures or exhaust gas from a companion cylinder, connected via the intake manifold. Net cylinder- to-cylinder mixture distribution, as a result, can be affected by reversion and needs to be considered in the manifold's basic design. The design or selection of an intake manifold, regardless of how fuel is introduced into the inlet path should take into account the ability to treat each cylinder as an engine unto itself. As a rule, every physical system has one or more "natural vibration" frequencies that are characteristic of the system and governed by the system. That's a technical definition of "resonance." In practice, we could say the resonant frequency of a system is characterized by a maximum condition produced from a "normal" input. An organ pipe is a common example of how a resonant condition is displayed. Based upon the physical dimensions of an organ pipe, a flow of inlet air may produce a resonant tone or pitch. Changing the pipe's dimension, given the same amount of input air, could produce another resonant point or tone. With regard to an engine's intake {or exhaust** system, it is possible to dimension a passage to accommodate specific cylinder displacements and engine speed so that a "resonant" condition helps produce an increase in total air flow {intake or exhaust**. In it's simplest form, this amounts to "tuning" an inlet {or exhaust** passage. Physical dimensions of the passage are constructed to provide a resonant tuning point {particularly relative to rpm and valve timing** at which a "boost" in flow is produced. This results in an increase in cylinder filling {volumetric efficiency** and potential gains in torque.
    Any advice or help given is actually based on having done the job, not read about doing it or Googling it.

    www.langysrodshop.co.uk Our parts are air freighted so 5-7 day delivery, The best GRP Willys body available/Rebel Wirings only UK dealer/Speedway Motors authorised dealer/Summit racing/Jegs/Hotrod parts supplied, MAC Autos, We deal with all the US hotrod suppliers even non car related stuff.
    Brake,Oil & Fuel etc plumbing stockist/Totally Stainless fastener dealer/Dolphin Instrument dealer, LMC & Brothers Trucks,
    Stainless Exhaust tube & mandrel bends stockist

  3. #3
    Part 2

    Carburettor Systems.
    At best, carburettors are an economically driven compromise between efficient air/fuel mixture preparation/delivery and cost-to- manufacture. Utilizing a stream of essentially liquid fuel, forced into an air stream by atmospheric pressure acting on a lower inlet manifold pressure, carburettors do a poor job of atomising fuel. Droplet sizes vary, sometimes extensively. Since droplet size relates to effective air/fuel ratios {large drops require more time to burn than smaller ones**, the ability to approach uniform mixture ratios is difficult at best. Optimising ignition spark timing for a "range" of air/fuel ratios is a compromise, resulting in less than maximum power and fuel efficiency. Exhaust emissions also tend to increase. At the outset, designing or selecting an intake manifold with acceptable air distribution among cylinders is handicapped by the use of a poor "mixing valve." It is still important to use a properly designed intake manifold and one specific to the intended application otherwise the problem of cylinder-to-cylinder mixture delivery will be compounded. Use of multiple carburettors, aside from possible aesthetic value and for outright racing applications, is generally an attempt to simply increase carburettor throat area exposed to the engine at wide open throttle. Most multiple-carburettor manifolds, especially those of decades ago when this practice was popular, did not contemplate specifically improved airflow. As such, they became "mounting pads" that facilitated the use of more than one carburettor. Among the "tools" used in designing {or modifying** a carburettor intake manifold, the avoidance of sharp corners and edges is critical. These are areas where air tends to "turbulate" or form eddies, causing unsatisfactory air quality and potentially separated air/fuel mixtures. At best, mixture ratio range can be upset, leading to inordinately lean or rich mixtures in the combustion space. Ideally, air and fuel should be homogenized from the time fuel enters the air stream, continuously blended into the engine's cylinders. But if fuel enters the air stream well ahead of the cylinders and is required to follow "disturbed" air along this path, chances are better than good mixture quality will be upset before it is subjected to combustion.

    Intake manifolds & Volumetric efficiency.
    In combination with cylinder head intake ports, inlet manifold runners comprise the total inlet path. If intake flow velocity patterns are a function of runner/port cross section area, then it is possible to configure them to optimise cylinder filling. By one definition, volumetric efficiency is a quantitative comparison {typically expressed as a percentage** of physical cylinder volume to the volume of air entering the cylinder at any given rpm. For example, if an engine has a volumetric efficiency of 85% at 5000 rpm, it is said to be filling this portion of its volume at the indicated engine speed. With proper tuning, an intake manifold {certainly in conjunction with appropriate valve timing** can assist an engine in gaining v.e. levels well in excess of 100%, particularly at or near peak torque..., which also equates approximately to peak volumetric efficiency. Tuning methods, sometimes termed "ramming," have been employed for many years in both stock and racing engines. Often characterized by long intake manifold runners, contemporary tuning techniques that embody multiple degrees of freedom {multiple torque or v.e. boosts** are allowing the design of manifolds that effectively broaden and flatten torque output curves. This is particularly valuable in small displacement engines where low- and mid-rpm torque is not a function of engine size. Even in artificially aspirated engines utilizing both turbo and traditional methods, intake manifold tuning can be successfully employed for additional gains in v.e. The delivery of air or air/fuel mixtures can be significantly influenced by intake manifold design. Movement of air/fuel charges in the combustion space following intake valve closing are particularly influenced by inlet path geometry, which, of course, include the intake manifold. Whether fuel is admitted by injector or carburettor, the ability to establish and maintain its suspension with air is critical to efficient combustion. It is important that the inlet manifold's runners and inlet ports be configured as extensions, one to the other. Depending upon intake and exhaust valve location, {relative the cylinder bore**, fuel injector targeting, type of combustion chamber {in the head** and spark plug proximity and aiming, the intake manifold runner can help produce the desired type and degree of mixture motion...or can detract from its production. The complexity of turbulence modelling and that required for fluid dynamics modelling {further complicated by unsteady flow conditions in a running engine** places this type of design approach beyond the financial limits of most professional engine builders. But as with the emergence of most technologies for which commercial benefits can accrue, methods are becoming shortened and being produced on a more affordable level to include practice among others than the OEM and high-budget racing operations. The "reversion" phenomenon; When an intake valve begins to open, at the start of each inlet stroke, cylinder pressure is typically higher than intake path pressure. Residual combustion materials briefly flow back into the inlet path, until the intake stroke has progressed to where cylinder pressure equals inlet pressure. From this point into the remaining portion of the intake cycle, flow is toward the cylinder. However, physical combustion residue {exhaust gas** that passes back into the inlet path is not combustible. It will either return to the cylinder being filled or shared with another cylinder's v.e. process following somewhere in the firing order. Especially in single-plane manifolds where internal "cross talk" exists among all cylinders, the degree of reversion will impact net combustion efficiency by the dilution of fresh air/fuel charges. It is desirable to minimize reversion, unless its presence is used to diminish the tendency toward detonation. The net effect is reduced combustion temperature, which, from an emissions standpoint, reduces oxides of nitrogen {NOx**. But if optimum power is sought, the reduction of combustion contamination is desirable. Intake manifolds can be designed to help reduce reversion. Either by manifold-to-head port mismatch {intake manifold slightly smaller than inlet port** or the location of this mismatch in an area of low flow velocity, it's possible to cause reversion contamination reduction. Typically, such mismatches occur at the manifold-to-head gasket interface and should be located away from the short- path of flow into the intake port."
    Any advice or help given is actually based on having done the job, not read about doing it or Googling it.

    www.langysrodshop.co.uk Our parts are air freighted so 5-7 day delivery, The best GRP Willys body available/Rebel Wirings only UK dealer/Speedway Motors authorised dealer/Summit racing/Jegs/Hotrod parts supplied, MAC Autos, We deal with all the US hotrod suppliers even non car related stuff.
    Brake,Oil & Fuel etc plumbing stockist/Totally Stainless fastener dealer/Dolphin Instrument dealer, LMC & Brothers Trucks,
    Stainless Exhaust tube & mandrel bends stockist

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