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Info Little bit about airboxes and filters.

Discussion in 'Tech Tips' started by Murdo, Feb 5, 2018.

  1. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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  2. gregt

    gregt Active Member Premium Member

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    Interesting. As an aside, when WSB was 750 fours, it was found very quickly that for at least all the inline fours, the optimum airbox volume was 1400cc plus or minus about 50cc. That's full house race engines of course.
    He doesn't quote the volumes of std or large RC51 airboxes - which is a pity.
     
  3. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Wow that sound's small.
    Out of interest i just put a garbage bag inside my spare FZR250 3LN airbox, i managed to get roughly 5000cc's into it with a bit of room to spare, i'm guessing it would only take another 300cc to 400cc's to fill it.
    20180205_201223.jpg
     
  4. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    A few good articles I bumped across recently - check the volumes of F1

    http://www.tl1000s.info/Randys_TL1000S_Site/Airbox_design_v1_resonance.html

    http://www.tl1000s.info/Randys_TL1000S_Site/Airbox_design_v2_pressure.html

    http://www.saltmine.org.uk/randy/airboxdesign.html

    I had a thought about putting a small speaker in the airbox and using a simple computer program which scales up the frequency range to output sound and use a microphone again connected to a computer and watch the volume carefully to see where it increases as the airbox comes into resonance - then you know what it's tuned for.

    The top two articles make an interesting point about the resonance coming in just before the engine comes on the cam, with the resonance mitigating the flat spot that you encounter at that point.
     
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  5. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    I'm not sure the speaker/mic trick will work, that's a sonic resonance rather than a flow resonance. Sound waves move at a set speed for pressure (and possibly temp), whereas the flow of air is based upon pressure differentials which vary, especially due to shapes. I haven't read the articles yet, but it is some seriously complex stuff, hence the likes of F1 pouring tens of millions of dollars into aero development each year. Maybe the reed vavles used in 2-strokes could in 4-strokes, but where to put them? Upstream of a carby could allow a pre-charging effect, but you'd have to make longer intakes to have a flow effect before the reeds, which messes with flow resistance and resonance, possibly mitigating all advantages. I'll have to have a read of the articles, though.
     
  6. Mike Fulcher

    Mike Fulcher Active Member Premium Member Contributing Member

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    I agree with the comments that this intake topic is extremely complex, I personally don't believe there is any advantage using an air box unless the intake to the box has its opening in the bikes forward air stream, thereby at higher speeds of the bike there would be some pressure build up acting as a small supercharger.
    The biggest mistake most people make when any modifications are made to an engine is not getting the fuel mixture settings adjusted accordingly.
    The old and trusted method of spark plug reading can not be surpassed.
    This is simply done on a full throttle top gear run for approx. around a minute, declutch & shut of the ignition & inspect the plug (plugs)
    The plug (plugs) should have the porcelain (center electrode) coloured a tan or brown colour, if its hinting towards a dark chocolate or black shade then the engine is running too rich, if the colour is to pale heading towards white the engine is running too lean.
    There is nothing wrong with the old bell mouthed intake tubes as used in the 60's 70's & 80's particularly those with very rounded intake entries.
    Reed valves are used in 2 stroke engines as a method of induction only, reed valved two strokes tend to have a much greater RPM power range as the reeds open and close automatically depending on the crankcase vacuum or pressure, Piston ported two strokes tend to produce there power at higher RPM.
    Interesting topic guys.
     
  7. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    The airbox is a neccesity on Mikuni CV BDS and BDST equipped bike's like the FZR's
    They dont like too much airflow
     
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  8. Mike Fulcher

    Mike Fulcher Active Member Premium Member Contributing Member

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    Why? re calibrate accordingly, so are you saying an engine doesn't like a little extra boost from free extra psi from the natural environment ?
     
  9. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    It doesn't quite work that way. You aren't adding additional pressure (you need forced induction for that), you are restoring lost volumetric efficiency to as close as 100% as possible.

    What is important for CV's is a constant supply of still, smooth air, hence the airboxes are absolutely critical for proper running. The capacity of the airbox is important as as the flow rate. Bigger is generally better, but you have resonances and depreciating returns to worry about

    All ram air systems do is lessen the low pressure effect in the airbox as close to atmospheric pressure as possible. Or at least that's my comprehension of it.
     
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  10. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Agreed Linkin. To have a forward facing intake will only make a positive pressure over about 160mph (250Km/h), so pretty useless on a road bike.
    Mike, the velocity stack/trumpet on old bikes work because they are (mostly) not travelling fast enough for the air stream flowing past the mouth of the intake to cause any turbulance. The passing air at higher speeds will actually cause a partial vacuum and cause some of the fuel to be 'siphoned' out. See about halfway down the page here,
    https://www.knfilters.com/filter_facts.htm#WORD.
    For most road riding the airbox working as designed is the best compromise.
     
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  11. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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  12. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    It would be interesting to get a ZXR250 on a dyno and feed the ram air in the left fairing somehow - I suspect one would see more than 40/45HP. Would explain why they feel faster than the other 250 4's I've ridden
     
  13. Andych

    Andych Well-Known Member Premium Member

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    There are gains to be made with Ram Air but... the complete system has to be designed around it... as shown in the article.. Kawasaki have a balance tube to the float bowls to equalise inlet pressure and bowl pressure or it wouldnt work..
    And... they also say under 120 MPH the gains are minimal at best...
    Great for the drag strip though...
     
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  14. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    • Ram air is not as simple as pressurising the airbox. The carbs need to vent to the same pressure as does the fuel tank otherwise the fuel is going to be pushed away by the pressure difference. Many of the superbikes that fitted Keihin FCR carbs and add ram air airboxes as part of their kits, had the carbs fitted inside the box. An easy way to address the problem I mentioned.
    • Air box resonance is not a simple science. http://www.planetsoarer.com/resonator/ResonatorsAcoustic.htm and http://www.calsci.com/motorcycleinfo/Airboxes.html
    • For tuning, spark plugs are read at the very base of the porcelain. In most cases you will need new plugs and a magnifying glass and light. The stupid pictures of spark plug reading that you see in Clymer manuals and the like are just that, stupid.
     
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  15. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Remember that with the forward facing intake that the faster you go through the air, the faster the engine goes and the more air the engine sucks in to the carbs until you are going fast enough that the air used by the carbs is less than what is coming through the forward intake and you will get a small (usually 1psi above atmosphere or less) increase in pressure.
    Spark plugs always read the colour at the base of the porcelain, not top of electrode.
     
  16. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    So, the ZX9 is getting 8 millibars (0.11psi) at 70mph (112Km/h) and 20 millibars (0.29 psi) of increased intake pressure at 120mph (194Km/h). Hmmm nice, but hardly practical or relevant for our 100Km/h road rules.
     
  17. Mike Fulcher

    Mike Fulcher Active Member Premium Member Contributing Member

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    Yes, I was mostly talking about race bikes, a lot of the quickest road race bikes were fitted with ducting from the front of the fairings to the air boxes which according to my understanding definitely increased horse power at high speeds.
    Anyway this has started an interesting forum, modern technology is hard to keep up with for us old fogies.
    I have noticed that a lot of the modern high performance 4 stroke engines are now using computer controlled inlet manifold lengthening & shortening according to the RPM of the engine, I guess this is to do with the resonance theories you are referring to.
    Whats your thoughts on this?
     
  18. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Those that don't have ram air will also use ducting to draw cooler air from the front rather than the hot area above the engine. The colder the air, the more dense and hence more oxygen. It does not have to go directly into the airbox.
    Have a play with this http://www.csgnetwork.com/relhumhpcalc.html A 40 degree F increase in temp results in a 10% loss in power. It is not trivial.
     
  19. Mike Fulcher

    Mike Fulcher Active Member Premium Member Contributing Member

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    I agree totally, isn't that why you can get a lot more horsepower from a Turbo charged engine by fitting an after cooler radiator?
    whats your thoughts on water injection that was first used as in WW2 aircraft engines to give an instant power boost in emergency situations?
     
  20. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Sometimes those road rules are more like guidelines :D :D :D
     
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