Airbox Design And Resonant Tuning Of The Airbox.

Discussion in 'Trials' started by PMK, Nov 28, 2018.

  1. PMK

    PMK Been here awhile

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    I have now seen several references from Twin-shocker regarding airbox mods to vintage bikes.

    Up front, I want to share that I have followed and applied airbox resonance and tuning on other vehicles, but never on a trials bike.

    In general, on high performance engines, airbox design can be critical. Typically though, and for the sake of discussion, the airbox inlets must already have adequate airflow capacity to handle the engines size. A tuned airbox resonance is more important on engines turning higher rpm and within a narrower rpm range. Bikes such as a road race machine or small bore mx bike, personal watercraft and sleds tend to fit this criteria well. Granted the last few years have seen improved airboots produced for the 450 mx bikes also.

    Vintage trials bikes, in my opinion tend to be ridden at low rpm and minimal throttle, with the added function of the throttle is not held steady nor in one position for any duration.

    I do believe that airbox volume in the still air portion is important to lessen engine acceleration issues from inadequate immediate airflow across the filter. Books written on the subject indicate suggested airbox volumes needed.

    Resonant tuning a two stroke, and even the four strokes brings the discussion of fuel stand off in the carb air inlet bell and extending even some short distance up the airboot. Fuel stand off is a function of resonance. The engine pulses generate a reverse wave through the carb and since not all fuel is consumed on each intake function, the resonant wave tends to carry atomized fuel back to the waves break point. The break point is a function of throttle opening, engine intake timing, and the mass of the air being moved.

    On a steady throttle operation, such as the examples posted earlier, a known setup can be made and tested to optimize resonance and fuel stand off. At low throttle settings, and changing throttle settings, resonance seems less important.

    I do consider the rubber airboot between the carb inlet airbell and the airbox important. Not in a sense to gain a resonance performance gain, but rather to minimize premix spray into the still airbox.

    Maybe if vintage trials bikes were doing those wide open moves we see the modern bikes accomplishing then possibly some airbox improvements may be a benefit. But overall, it seems most vintage bikes are not revved high nor for very long.

    Curious what others will offer. No doubt good comments, wise cracks and general foolishness, but that's ok too.
    #1
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  2. Brewtus

    Brewtus Buffoonery, Inc.

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  3. motobene

    motobene Motoing for 46 years

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    I understand everything you wrote, which was interesting, and there will be few who do. Resonant frequencies with nodes and antinodes, etc.

    Is this relevant to trials? Yes, but the extent to which it is relevant to trials, with our wide rpm range and max power irrelevance, is debatable.

    And it's easy to confuse turbulent/laminar flow with resonance. Both phenomena are not visible to us, and that leaves lots of room for high priests to spin religious ideas to squeeze the feel-good gland. That's not poo pooing the role of flow and resonance, or anyone's fine-tuned expertise. But caveat emptor applies.

    It is obvious that flow dynamics of intake systems on trials bikes matters. We know this from when things go wrong. Take the airbox-in-front era for Shercos (2010 to 2016), when they went from this simple, straight-in layout:
    2009 Carb Boot Shape.JPG

    ... to this artful layout in 2010:

    2010 Carb Boot Shape.JPG

    Some jetting hell ensued, so the part number changed from 2262 to 3177 in 2011. More jetting hell ensued (a simplification, I acknowledge). Sherco redesigned the whole setup in 2012:
    2012 Carb Boot Shape.JPG

    The last 3 model years of the airbox-in-front ran pretty well, then in 2016, after the major redesign of the 2015 Scorpa, they went back to a more common format:
    2016 Sherco Airbox Layout.JPG

    The 2015+ Scorpas and 2016+ Shercos have been quite the runners! Interesting bulge in the carb boot, eh?

    So what role do flow dynamics versus resonance play in the above evolution? It's hard to say in the fine details. One way to find out for your self is to ride your bike with the air boot and air box - even whole subframe off the bike. Put in ear plugs so your brain stem is less likely fooled by sound intensity = power. Run the same? Worse? Better?
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  4. Bounder

    Bounder ExternallyDisplaced

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    Didn't Yamaha do this way back?
    yeis.jpg
    I don't know what the volume would need to be for a given displacement but I would think it would be somewhere close to the cylinder volume. Find an old bottle from a IT175 and fill it with water and measure the volume and you will be able to find out what it is.
    It is also dependant on the bottle/reservoir being mounted above the cylinder as well from what I understand.
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  5. motobene

    motobene Motoing for 46 years

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    Some may purport to know specifically what is better and what is worse in the invisible world of flow dynamics and resonances. That gets into iffy territory.

    Doubt me on this? OK, let's look at just how different other current designs are, like TRS:
    2018 TRS Aibox Assembly.JPG

    Well TRS is a great running bike! And what about Vertigo? They have the airbox in front like Sherco used to. They certainly run well! Yes, they are fuel injected, but they still need to breathe.
    2016 Vertigo Air Box Assembly.JPG

    How about the long-running HRC bikes?
    HRC Airbox.JPG

    And what about Ossa?
    Ossa Airbox.JPG

    The point is, as soon as ideas start popping up that a particular solution or two are the absolute tits, it's good to keep in mind just how wide an array of solutions there are out there. It's VERY VERY VERY complicated, but also quite simple in that the volume of valid good solutions is quite large.

    Mistakes have been made, obviously, and there are optimizations that do work, but it is hard to separate confirmation bias from the human soul as we are hard wired to do that, particularly when effort and money are involved.

    Modeling software has sure helped engineers, but even the most sophisticated software doesn't model reality with certainty.
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  6. motobene

    motobene Motoing for 46 years

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    Yamaha often came up with innovations like YEIS. They were the first with "do not throw into the fire" monoshocks (you have to be old to know why that is funny). YEIS played theoretically on intake resonances. It wasn't some magical slam dunk or they'd've kept at that particular solution. They didn't. They considered intake resonance in other ways.

    BTW, where the YEIS bottle was (as in 'above the cylinder') has no bearing on how it works.
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  7. Huzband

    Huzband Team Dirt

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    You guys are making me miss reading Kevin Cameron every month. Keep it up, I love it!
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  8. 2whlrcr

    2whlrcr gooligan

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    I miss spring!
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  9. jonnyc21

    jonnyc21 Trials Ninja

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    I was on the same though as bene on turbulent/laminar flow and resonance after reading the first post from PMK. Just seems that it would be easy to think that gains from a change intended to improve resonance would more likely be caused by the change in the turbulent/laminar flow or improved by what PMK stated removing the issues caused by "engine acceleration issues from inadequate immediate airflow across the filter" rather than the resonance. This just seems more likely as most trials bikes are not run wide open long enough for resonance to affect any but maybe the top riders in my opinion.

    But then again I am no expert so...
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  10. motobene

    motobene Motoing for 46 years

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    Hot dang! That's flattering!
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  11. Gordy

    Gordy SUPPORTER

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  12. motobene

    motobene Motoing for 46 years

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    You brought in scavenging! :happay

    OK, I'll stop.
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  13. Gordy

    Gordy SUPPORTER

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    :lol2 Please carry on. I don't understand much of this but really enjoy trying to grasp the concepts. :nod

    The 2T KTM guys are :loco about all of this stuff. Filing custom notches in slides, adjusting powervalve springs and pre-load, different pipes, silencers, buying handfuls of jets and entire carbs. Just insane when the bike runs perfectly well stock.
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  14. motobene

    motobene Motoing for 46 years

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    All that KTM passion leads to actually spending money!

    Most of those guys aren't trials riders. They've never had the re calibration experience of trials such that much of the bullshit they think is important isn't.

    Chuckled at an MX review of 450s the other day. I see as negatives much of what they see as positives. Then again, I don't do motocross any more. Last time was 1986.

    They were down on two brands for having power that didn't spool up so fast, yet few bikes are as awful to ride on rough single track as amped up motocross bikes. All for the pleasure of having your teeth rattled out with over-stiff suspension, and stalling the motor while precariously suspended on baby heads next to a 200-foot dropoff!

    Eeeeeee - thuh.... aeeeeyah!
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  15. Gordy

    Gordy SUPPORTER

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    Yeah, a full blown modern MX bike would be hard for me to get my head wrapped around. My only salvation would be altitude! I go to the races and those full-on factory 450s have to be pumping close to 70 HP running on race fuel at sea level. They sound awesome but I really can't admire enough the skill it takes to handle one of them. :bow

    I tune my KTM 300s for tractability. Overall power is not a problem with those bikes. They are scary fast when you want to.
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  16. PMK

    PMK Been here awhile

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    Laminar flow over a surface vs non laminar flow over a surface. Laminar flow can be difficult to obtain and then can be even more difficult to remain laminar on many surfaces that change shape.

    Carb bodies, never looked into it, but I suspect they are not laminar flow. Simply too many bumps and projections, then the slide, so it seems unlikely.

    As the air enter the intake, we must agree the filter is certainly not laminar by any means.
    Possibly, depending upon length, diameter, diameter changes, possibly the airboot has some ability to bring laminar effect back to the airflow prior to the carb.

    With reversion, I doubt laminar flow would be possible. Simply too many turbulences happening at incredible speed.

    Twin-shocker initially brought this entire concept up in other posts. Without doubt, there are times when a resonance tuned intake is a means to fully optimize the fullest amount of power.

    I was always understanding that for engines operating at varied throttle settings there were two accepted opinions.

    One is placing a proper size air filter at the inlet of the airbox. Again an airbox of proper volume and a workable shape. These still airbox designs act similar to the Yamaha boost bottles in some ways, allowing the airbox to "store" a large volume of air at higher pressure. This high pressure air will rapidly enter the airboot and carb, helping to draw fuel from the float bowl, and allow good acceleration performance.

    Second is when a larger airfilter is utilized, allowing less pressure drop across the filter itself, and having an ability to "store" air behind the filter itself. If space is not an issue, often these are common.

    Boost bottles such as what Yamaha introduced, may have been a means to improving engine response with a greater duration of vacuum. Boost bottles were plumbed in between the carb and cylinder. Ironically they came about quickly and disappeared about as quick. Possibly, since these were introduced at a time when single shock suspension was truly being sorted out, maybe airbox criteria was far less than ok and the boost bottle helped the engine draw air in quickly on throttle opening.
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  17. PMK

    PMK Been here awhile

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    KTM can certainly sell to the public, out of the crate high horsepower two strokes for MX. Sometimes though, they do miss the target. Almost as if they are at a different shooting gallery. Granted I do not have a new KTM, but my 250EXC was not jetted well and did not respond well to jetting changes. Ultimately, I removed the cylinder head after checking squish clearance and measured the squish band width with the head removed. Without going crazy for perfect suish settings, I cut the head to be within accepted standards for squish band width and clearance.

    That simple mod cleaned up the jetting and allowed the bike to be jetted easily. Stock pipe, stock silencer with SA, stock powervalve spring and setting. Without doubt, I spent far more effort getting the PDS suspension dialed in.
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  18. 2whlrcr

    2whlrcr gooligan

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    I didn't touch my KTM airbox at all. I did have to replace the reeds, because all 17 KTM's came with junk reeds. And I did switch to the green power valve spring, because I'm a pussy.
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  19. Gordy

    Gordy SUPPORTER

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    :nod
    I have modified heads on both 300s and have no jetting woes at all.
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  20. PMK

    PMK Been here awhile

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    Agree, but that kind of contradicts your previous post mentioning the run perfect stock.

    Did you mod the heads yourself or have someone do them for you?
    #20