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Thread: airbox intake mod

  1. #1
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    Default airbox intake mod

    Toying with the idea to run my intake hose directly into my stock airbox. I just want to alleviate some plumbing and at the same time guarantee a fresh air intake charge. I had the hose run to the back of the airbox with 45 snorkels facing frontward sitting over the filter. I even tested the intake with compressed air at the fairing openings and could feel the air come through the front of the tank. But, there is no guarantee the bike will suck all that air in as it may just flow around the edges of the tank/airbox. So, I've devised a plan to insert the intake hose directly into the airbox below the filter. Yes, it will be unfiltered air.. though they did do this on the SP5? I will use stainless mesh screen to stop small birds from being sucked in.

    My question is, will this be beneficial or will I screw up the airbox resonance and include the wrong type of turbulence? I guess your answers will be speculation but interesting to hear what others have to say.

    Robert
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  2. #2
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    i wouldn't run any unfiltered air into the airbox. if you want to run a lot of air into the engine make/modify an airbox as you see fit. i did one for an 888, it's a bit different to the 851 box, but the basis is the same. the std 888 box runs the two tubes into the front, but it's all filtered on all models.

    this one was done because the bike had 996 style throttle bodies bored to 54mm and huge trumpets that had been modified as they were hitting the std air filter. i replaced the filter with k&n pods mounted on modified muffler end caps supplied by a local exhaust maker sitting between the trumpets and throttle bodies. but then one of the trumpet mounts broke off the throttle bodies so we replaced them with original sp5 throttle bodies from gowie, making it all a bit of a moot point. but the work was done by that point.

    i just cut the front out of the original airbox and used a piece of rubber sheet to seal the top of the radiator back to the airbox so it wouldn't pick up hot air. it still ran the tubes from the front into the sides - you can see the oval holes at the front top of the sides.

    not sure what it did for power output, but as a 955 with some 37/31 valve heads from germany, g inlet and a exhaust cams and a termi spag system it made the same power as a 996sps. went well once it got itself together.
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    Brad The Bike Boy

    www.bikeboy.org

  3. #3
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    I got an extra 6hp on a tuned ST4S engine by removing the SP3 airbox, so it is quite restrictive at high revs. I haven't figured out a proper replacement yet, as my three piece carbon air box has a bit of a gap in the middle.

    Mike

  4. #4
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    Mike, post a couple of pics of the gap, I may have a solution or be able to cast a fillet to seal the gap.

  5. #5
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    As Brad has said it's not a good idea to run un-filtered air into the motor.
    Personally I don't think the standard 851 airbox is a restriction until your motor is making BIG power, I got 128 rwbhp (more than any standard mapped 996SPS despite the SPS running a fully presurised system) out of my 851/916cc motor using an early 851 airbox with the top removed, free flowing ITG foam filter with the hoses shortened and poked up under the front of the tank so the incoming air blew up into the space above the filter.

    However I have seen one SP4 and a SP(4)S with both hoses fed un-filtered into the rubber bellows over the front TB, so someone out there thinks/thought it'll work?
    But my 851/916cc motor makes more power than both of these bikes with it's completely filtered air and...
    The SP4 is actually 916cc's mapped by Foggy's race mechanic Slick Bass and the SP(4)S is a Hackett tuned 926cc motor.

    Chris Steadman did the mapping for me, but the motor made 126 rwbhp on it's 1st run.

    So you pays your money and makes your choice Robert but...
    If I were you I'd try my suggestion, as it won't cost much apart from custom mapping because it uses all easily modified but standard 851 parts and it's completley filtered which will save your topend!
    I know it worked for me and I'd certainly try/use this set up again on another motor.
    Just my tuppence worth.

    Steve R
    Last edited by Mr.R; 12-31-2011 at 05:41 AM. Reason: added info

  6. #6
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    Trying to think of a solution to a similar problem I have Robert, fitted carbon air runners which means that the air flow comes in at the the headstock but the standard airbox is high at his point. The big problem with the 851 airbox that because of the cross brace the front bellmouth uses a large rubber snorkel to meet the bottom of the airbox, therefore you cannot run pod type filters as Brad has done.

    Does not really help, but be interested to see if anybody else has a solution (without fitting a corse type airbox bottom).

    I think the sp5 ran the tubes to the top of the airbox (above the filter) at the front - you could try an ST/MON airbox lid as this has intakes at the front which you could run the tubes to.

    Johnny
    Last edited by jr60; 12-30-2011 at 03:03 PM.

  7. #7
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    Years ago when I had a 851/907 engine in my 92' (with cross brace, early style airbox) Bruce Myers told me to just cut the top out of the airbox and pull the rear entry air hoses as far forward as possible, or make short extensions on them. Seemed to work just fine! I would not run unfiltered air into the engine!

  8. #8
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    std airbox without lid is all i would (and do) run. unless i was really getting into it (and had a heap of time to waste), then i'd make a similar thing to the above, but as i wouldn't be having to charge $95/hour i could spend more time doing it and would make a box with sides, floor and back which rubber pieces to seal against the underside of the tank and large pod filters. whether or not it'd make any difference is debateable.

    the convoluted tubes really hurt airflow, so make up something smooth and shaped to suit. make a long narrow outlet that fits up between the airbox and frame at each side (is there room?). or do what ducati recommended in the competition notes for the sbk/bott bikes and cut the front of the airbox so there's an opening through the frame rails below the front of the tank and make up a filter holder as required.

    and make the tubes larger - the fairing entries are much bigger than the tubes the black plastic pieces neck them down too. the tubes are very much the size of the times - look at the zxr, etc. taper them down if required, but the std way just doesn't look right to me. you could make up a wooden back and cast them in the silicone rubber stuff you can get. then you can sell them to the rest of us for us to all agree it makes no difference. or maybe it wll. they went to a lot of effort to change it on the race bikes, and the air entry was still lacking compared to a 916. the tubes probably don't need to be that big - look at how small the 1198 ones are, and they make a shitload of power.

    how much time do you have? you really need to log the airbox pressure to know how it's all working too, and that would change depending on gear i'd think. it'll suck a whole lot more air at 11,000 than 7,000 for a constant road speed, so in a higher gear it should have more airbox pressure. anyone else see a hole in that theory? i've wanted to log it on a bike with real ram air for a long time, but have never had the chance.
    Brad The Bike Boy

    www.bikeboy.org

  9. #9
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    Things are very tight in the front of the tank as there is no clearance for the tubes to pass due to the intake boot and tank mounts. I do agree with all as far as not allowing unfiltered air into the engine. I saw what that did to mine prior to rebuild as there was an open hole in the airbox where the crank breather is supposed to go and lots of nasty stuff in the airbox along with a completely scored cylinder (and piston) which may or may not have anything to do with. I did manage to find some tiny filters which will fit perfectly in the hose ends inside the airbox. So, if I decide to modify the airbox then I will do so with filtered air. Or I'll end up leaving it as I had it with intake hose going to the back tank/filter.

    Robert
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  10. #10
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    Some years ago someone (possibly Sigma?) wrote up the results of just removing the hoses from the rear of the airbox and it gave a performance gain, the next step of cutting large holes in the lid gave another gain, removing the lid completly and fitting a free flowing filter another.
    The hoses into the back of the 851 airbox are too long which's why the 888 'box has them fed into the front.

    As I said earlier I just shortened the hoses and poked them up under the front of the tanks, there's enough room to do this as I've done it on both my '91 and '92 851 based bikes, so they're basically the same as the later 888 'box with the top removed.

    Simple and effective does it for me and this set up is good for close to 130rwbhp.
    How much power do you hope to get from your motor?

    Steve R
    Last edited by Mr.R; 12-31-2011 at 05:59 AM. Reason: added info

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