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Air Intake Design / Turbulent Flow / Modifications

  1. Jul 8, 2014 #1
    Quick facts: Age (19) / Mech. Eng. Sophomore / Automotive & Physics Lover

    In an attempt to design and construct a new less restrictive engine intake system i've run into a number of questions I am not yet able to answer.

    Main Goal: To produce a new intake system and associated plumbing associated with it.

    Intake in Question: 2008-mitsubishi-lancer-26_600x0w.jpg

    Minimum Intake Width: 3'' X 3'' (9 Sq. Inches)
    Displacement: 2.4 liters (~1.2 liter volume intake per revolution assuming counterbalanced pistons)
    RPM: 6000
    Volume / Min: (1.2 X 6000) 7200 liters / min
    Cubic Feet / Min: (0.0353147 cubic feet / liter) 0.0353147 X 7200 = 254 cubic feet / min
    Cubic Feet / Sec: 4.24
    Feet / Sec Through Intake: 4.24 / (0.25' * 0.25') = 67.8 ft / sec

    1 . Correct me if my math or assumptions are wrong, but 67 ft/s seems like an awfully turbulent speed for such a small pipe and if widened, would decrease resistance / parasitic drag, no?

    2. Would applying a dry teflon grease to the new piping decrease turbulence significantly?

    3. Should the intake be as wide as reasonably possibly or am I ignorant to some physics going on here? (ie. Pulse Resonance, intake velocity leading to turbulent swirling in the piston draw, etc.) ( I feel that the air intake rate into the piston would be so quick that any initial speed would be negligible.)

    Again, suggestions / help / criticism are welcome!
  2. jcsd
  3. Jul 8, 2014 #2


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    Turbulence is your friend. A turbulent flow can have less total loss than a flow in the laminar regime. A turbulent flow into the cylinder is desired to mix the air thoroughly with the fuel.

    If you think the flow velocity of 67.8 ft./sec is high, imagine what it must be flowing into the intake ports of this engine, which will not measure 3" x 3".

    The problem with this engine layout is that the air filter is jammed against the firewall, so the air must flow from the front of the car all the way across the engine compartment, thru the air filter, and then back forward before it even gets to the engine. You might be able to reduce some of the losses by installing an aftermarket, low-restriction filter element (if one is available), but you are kinda stuck with the plumbing. It doesn't look like there is a whole lotta room under that hood to re-route or redesign the intake air flow.
  4. Jul 8, 2014 #3
    Right, well I imagined the ports would have a drastically higher velocity but would having the "supply" of air to said port be beneficial or in the grand scheme of things prove insignificant? (Via larger tubes)

    Additionally, would you be willing to elaborate on how a turbulent system would be more efficient than a laminar one?
  5. Jul 8, 2014 #4


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    Intake ducts with larger area would allow one to move more air into the engine for the same amount of pressure drop, as long as the flow remains turbulent. The particular problem you are faced with on this car is you don't have a lot of extra room under the hood to make a bigger intake duct or even provide a shorter, more direct path from the inlet to the engine intake manifold while keeping the current air filter/air meter.

    The loss in a duct where the flow is laminar is inversely proportional to something called the Reynolds No. For fully turbulent flow, the loss decreases from that of the laminar flow regime and becomes a constant w.r.t. the Reynolds No. The Reynolds No. here depends on the area of the duct divided by its perimeter, the velocity of the air, the density of the air, and the viscosity of the air. For a gas like air, you won't have a truly laminar flow regime unless the air is not moving very fast or you are trying to use a very small duct. What you will have is probably a turbulent flow regardless of what modifications you are able to make to the intake plumbing.

    Truly laminar flows are often encountered when pumping viscous fluids like syrup or heavy petroleum. To reduce losses when pumping such fluids, these fluids can sometimes be heated, which reduces the viscosity, and the piping sizes can be selected to produce a turbulent flow instead of a laminar one.

    Inside the combustion chamber, a turbulent swirling of the air is desirable to keep the mist of fuel suspended in the air before the spark is applied. This ensures more complete combustion of the air-fuel mixture.
  6. Jul 9, 2014 #5


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    Examine the existing system carefully.

    Look for airflow into a pipe that has an opening flush with a wall, change it to a trumpet to eliminate the vena contracta and so effectively expand the pipe diameter.

    Look for a pipe flow out of a wall, add a short tube extension to the pipe so as to prevent pinch of the flow by circulating side eddies.
  7. Jul 11, 2014 #6
    somewhere I have read, that tuners of high performance cars are drilling little holes (like on golf ball) onto intake ducts of cylinder head. it creates turbulent flow near the walls and results in higher flowrate. and they were measuring the results with flowbench so it was not some placebo effect..
  8. Jul 11, 2014 #7


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    That is quite possible. Sometimes polishing a surface helps, at other times the sand casting pattern can break the surface flow and so change the separation point. http://en.wikipedia.org/wiki/Flow_separation

    Your best investment in improving air intake performance will be gained by studying the fundamental principles of fluid dynamics.
  9. Jul 11, 2014 #8

    Ranger Mike

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    welcome Jeff..it is always good to see a young person interested in hot rodding...the one thing you can do is to engineer the intake to take in cold fresh air. Even better, have the intake located in a high pressure area so you can cram as much air into the intake as possible. Next hot rod step is to improve the exhaust system. reduce back pressure...these two things will provide max HP with minimum hassle..i would stick and EDGE chip or Super Chip in it to amp up the map as well...when starting out go after big chunks of Horse Power....and balance things...you can get to the point of having too much air coming in and lean out the engine...so consider the chip where you can dial in the proper fuel air mix..
    dont be afraid to ask questions...its how yo u learn..
  10. Jul 11, 2014 #9
    I guess you are looking at the inlet duct to the airbox(filter housing). I don't think enlarging a 3"x3" duct will reduce the inlet restriction any. A 67.8 f/s intake charge is not very high speed, and should not net any real pressure drop compared to the filter, throttle body or intake port.

    From the picture, I would look at the area where the air gets to the funnel going into the square duct going to the airbox. It looks like it has to go in the grill then up through those little slots in the plastic cover over the radiator. You might want to run the inlet duct all the way to the grill, or other forward opening. Try to minimize the direction changes while you are at it.

    If you can get rid of part or all of the left front headlight assembly, that would make a nice spot to draw air from.

    Hood scoops are another option.

    The easiest thing to do, would be to cut a bigger hole in the plastic cover where those slots are.
  11. Jul 11, 2014 #10


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    That would look good, but it is really not effective.
    Increasing inlet pressure and flow from ram air input is really not effective below about 200 km/hr.
    You can compute it from the equation here; http://en.wikipedia.org/wiki/Dynamic_pressure
  12. Jul 12, 2014 #11

    Ranger Mike

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    baluncore..the reference you posted is correct...for non compressible fluid..not necessarily for AIR. i maintain the cooler air will be of better performance than air subject to heat of the internal combustion engine. Any time you are moving, you are moving air. Ram air is still better than hot air if you account for rain entering and the down side of a huge bubble sticking up on the hood and associated down side of more aero drag. The days of the huge air grabber hood are classic but GONE...ducting from high pressure area like at the base of the windshield is the hot setup..pardon my pun...Like everything in life it has to be balanced.
  13. Jul 12, 2014 #12
    Before you go fabricating a new inlet tract (or consulting all the math needed to suggest what is the restriction) you should do some basic testing to find where all the restrictions actually 'live'. There is no point making a new inlet if you end up keeping the most restrictive portion, whether it be the air filter or resonating box.
    A very sensitive vacuum gauge or 1bar 5v map sensor will do to get all the results you need to make an informed decision on what to construct. Pick 5 or so points to do your testing, someone to drive the car, and a pen and paper to keep your results. Have them drive the car at a couple of pre determined rpm points at full throttle.
    You would want to test:
    in the manifold
    before the throttle body
    before obvious bends
    before resonating box
    and either side of the air filter.

    From there, you can confidently assign an accurate % to each of the sections you have tested and appropriately fix each one, neglecting the non restrictive ones. You may even be able to radically improve the inlets breathing efficiency whilst making the thing look stock.
  14. Jul 12, 2014 #13


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    For most purposes, air can be treated as an incompressible fluid as long as its velocity stays below about 0.3 Mach, which is approximately 325 ft/s at sea level. The velocity computed by the OP in his intake duct is well below this figure.
  15. Jul 13, 2014 #14


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    An alternative approach might be to work out how much air is actually needed. Then consider that the airflow might be deliberately restricted to indirectly limit fuel consumption and maximum power. That may be to indirectly protect engine components, such as the pistons from high temperatures at high RPM. The engineers knew what they were doing when they designed the original intake, the filter and ducting, into the available space. They had access to the archive of records and analysis of failure modes for similar vehicles over a wide range of conditions.

    It is easy to throw out the original “engineers compromise” system and replace it with a simple system, only then to find that the original had a hidden benefit you had not considered. You will then, after each failure, need to progressively modify your solution, until after 20 years, it looks something like the one that came with the new vehicle.

    Maybe analysis for the purpose of understanding would be a better approach.

    Make a list of all the requirements of the air flow system. Consider rain, hail, snow, heat, icing, dust, bird and wasp nests, etc. How will you melt the snow that reaches the air filter, how will you remove the melt water, by evaporation or drainage? How will you stop hail stones from perforating the filter and letting the accumulated dust through?
  16. Jul 13, 2014 #15
    Cooler air is always better, of course. Every production car I have seen that was made in the last 30 years has an intake system that is designed to draw in cold air(outside/ambient, as opposed to underhood/heated). A few do have a flap to allow warmer air from the exhaust area in during warmup, but they all draw air from outside the engine compartment, and most of them are designed to do so quite efficiently. There may be some slight compromises to reduce induction noise with some resonance tubes or chambers, but power is not significantly restricted by the inlet tract size on any stock car I have seen. If you go and double the size or RPM potential of a car's engine, then the stock inlet ducting might start to restrict the engine.

    I mean, really, do you think a 3" square duct is limiting a 2.0L stock engine that might rev to 6500rpm, when plenty of race cars are making 4 times the power output of that engine on 5.8L engines spinning up to 9500+rpm through four 7/8" holes that together have a cross section of 2.4 square inches, and have much more turbulent flow, since it is a flat plate with sharp edged holes with no radius or taper to bring the air into or allow it to recover from the restriction.

    What I don't get is all these so called "CAI(Cold Air Induction)" intakes I see, particularly on Honda Civics, or similar "modded" cars that have an open element filter in the engine compartment, usually in direct stream of the radiator fan. Sometimes they have a metal partition with some weatherstripping around it to make a poor attempt at blocking the hot air, but most of the time these are drawing just as much hot air as cold air. A factory sealed air box does a much better job of getting plenty of cool air. If hot air can get through, it will. Even if there is only a small gap, the intake air temperature goes up significantly.

    They do make a lot of noise(which I like), so you might think it is faster by driving it. The reusable cotton and screen filters do have less resistance to flow,but also allow more dirt into the engine. Oiling them helps them stop more dirt, but a little too much and the oil will damage MAF sensors, and they still don't catch as much dirt as a factory filter. I haven't really noticed cars with functioning HP filters get significant wear from the dirt, but I definitely see MAF sensors contaminated and causing poor performance from the excess dirt that can get through. Much more so on poorly designed systems that allow some air to completely bypass the filter with poorly sealing attachments.

    If you can get the oil quantity right, a reusable filter in a stock air box is usually the best performance you will get on a stock engine. If you can eliminate any of the resonance tubes and block any holes left in the main ducting, or replace a resonance chamber with a piece of pipe it would get you that induction noise we all love, and maybe gain a tiny amount of power, or maybe loose some, without having to buy a high dollar Hot Air induction that will definitely loose power.
  17. Jul 13, 2014 #16
    In the picture below, I have shown with a red circle the possible restriction for cold air. The slots I mentioned before there are the only way for cool air coming in the grill to go up before it gets to the radiator or condenser, and into the funnel to the inlet duct. My suggestions for this car, from looking at the picture below are to.

    1: Block off the passage to the resonance chamber that is circled in yellow to help increase noise, and possibly help airflow a little.

    2: cut out the purple shaded area to let air come from the grill area into the inlet duct, and

    3: Seal off the orange outline around the incoming air to the hood by building it up with some kind of compressible foam rubber to keep hot air out.

    Depending on what it looks like under the area that is to be cut out from the front panel, it may be a good idea to put a deflector of some sort to help direct more air upward and into the inlet. be careful not to block off too much airflow to the radiator though.

    Attached Files:

  18. Jul 13, 2014 #17


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    I beg to differ. If the air is adiabatically cooler then there is no advantage to be gained.
    I do agree that it would be silly to direct hot air to the intake, when more dense cool air is available.

    The resonance chamber is there to prevent flutter of the airflow in the inlet duct. Those oscillations may choke the duct airflow and create flat spots at some critical speed. Any extra noise will be accompanied by a reduction in the smooth flow of air. If you want noise it would be better to modify the exhaust.

    It will get enough air as it is without a deflector to ram air. Why might you want hailstones, small rocks, small birds and big insects to be directed up into your clean air intake?

    Why increase the airflow to the engine if it is now running a stoichiometric mix?
  19. Jul 13, 2014 #18
    Ah, yes of course, but the only way you would have adiabetically cooler air going into the engine would be to cool it by purposely creating a restriction to cause a pressure drop, so yes cooler, lower pressure, less dense air is not better,

    The resonance chambers are there mainly for noise reduction. I doubt an opening in the side of a smooth duct will increase airflow. The noises they cancel out are usually not caused by flutter, but the natural frequency of the duct combined with the throttle blade turbulence, and the valves themselves. I don't know if you noticed, but in my previous post I had stated that you may gain or lose a tiny amount of airflow by doing so, so I am not really suggesting it as a performance mod, more of a sound thing. The duct is plenty big enough to support the airflow needed with or without a little flutter and have far less pressure drop than any air filter does anyways. Increasing exhaust noise is more expensive and easy to get dreadfully wrong, leaving an annoying drone at cruise speed.

    I was just trying to add a possible suggestion that might help keep air that is backed up in front of the radiator with some heat already picked up out by directing the airflow and keeping it separate from the radiator airflow. It kind of depends on where exactly any heat emitting exchangers lie under there. You notice it wasn't even part of my 3 suggested steps, and I agree that ram air is generally a net loss proposition, especially under speeds of 100mph, but generally there are better and worse areas of the exterior or interior of a car's airflow to draw air from at any speed. And what about gravel, insects, birds or whatever? anything that makes it through the grill, up and into the airbox will be stopped by the filter and be collected in the bottom of the airbox. I seriously doubt any configuration you could come up with that draws from the grill opening would collect enough stuff to impede airflow in 20-30k miles of driving on normal paved roads. How fast do you drive in a hailstorm anyways?

    What? that doesn't even make any sense. The mixture is not affected by adding more air. The engine management will add however much fuel is needed depending on guess what: the airflow, as well as load, temperature, etc.
    Last edited: Jul 13, 2014
  20. Jul 13, 2014 #19


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    autodoctor911: You are really advocating slightly increasing the airflow to a stock engine in order to increase the maximum RPM, where the engine will always be most inefficient?

    The OP was concerned with reducing imagined turbulent flow and perceived inefficiencies. If the OP needed significantly more power at higher RPM from a stock engine, then polishing the air ducting is not really the sensible solution.

    If it ain't broke, don't fix it, or you will just break it.
  21. Jul 13, 2014 #20
    Oh, I agree that it would be best for efficiency to do absolutely nothing to this engine but normal maintenance.

    I thought it was the OPs intention to try and get more power by redesigning the intake system for more airflow at the maximum rpm.

    If you have read what I have said, I am not suggesting that any of this is practical, or should be done to this car, only trying to suggestions that would be better than what most people do, which is to install an open element filter in place of the factory airbox. I always said that any gains would be minimal, if at all.

    And the airflow to this engine is not what limits the rpm to which it can rev, so it would not increase maximum RPM. If it does allow some small gains in airflow, mainly by using a filter that is more efficient at flowing air and less efficient at keeping out dirt, the gain would be less pressure drop at or near maximum RPM within the intake system, allowing more air to be pulled in at those speeds, which would make slightly more power.
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