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Thread: SC250: Airbox Mod

  1. #1
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    SC250: Airbox Mod

    A minor project while winter ticks on...

    I was exploring how to add an inner fender and started looking at the stock airbox to decide to stay with the O&M or go with a K&N filter. I think you need the fender if you go with an aftermarket air intake. It is too easy to slop a lot of grime back into the air intake area + it dirties up the backside of the engine. I'll post the inner fender under a separate post if I stay with the factory airbox.

    For now, the factory airbox wins out on noise control + does a nice job of air filtering for engine longevity. In looking it over, I am a bit surprised at the restrictive design. The throttle body rubber boot to air box is quite large in diameter, 1.5" or so. To my surprise, the airbox has a tube that all air must pass through (pic 1, tube outlined in brown). This tube is a little over 0.5" in diameter on the inside, my pointing finger will not go very far into the intake side. I know we only have 244cc's, but when you are screaming at 7-8,000 RPM, it would seem you are sucking through a small diameter straw. So, it seems plausible that if you can cut the tube out and graft if a larger inside diameter tube, you can help the engine breath easier. This, combined with an aftermarket exhaust should help power production at higher RPM.

    Also, the outside airbox cover for many of the engines in this family look the same if you are 125, 250, or even 500cc's. I can see possibly using the same cover for the 125 and 250, but would be surprised if the 500 also used the small tube cover. Tomd, you have a 500cc motor; the next time you pull the airbox cover, look to see what is the diameter of the inner tube.

    Thoughts?

    Jerry
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  2. #2
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    Great to see that im not alone in the line of air box mods, currently i am running on a K&N filter on my sc300 and it is heaps better in the mid and top range of the power band. The same could not be said with the low end torque on the bike as it doesnt seem to be as "punchy" as it use to be during a traffic light grandprix. The reason the K&N is still on is because most of my riding involves the freeway and the awesome induction roar it makes.
    If i am not mistaken, the stock air box is desgined with noise emissions and low end torque in mind as the scooter was designed ideally for town use which involves a lot of stop start traffic.

    It would be great if the stock air box could be modded for both low end and high end range of the powerband. Any thoughts out there?

  3. #3
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    That was my thinking and the reason for playing with the stock airbox. I do think the volume (perhaps shape) of the airbox is there to help power at lower RPM. When you are running high RPM air flow is everything, so the K&N wins out because its a shorter/wider path for air.

    I think if you can mod the small tube, you increase the air flow potential to the airbox so it shouldn't hurt low rpm power (torque) production.

    Jerry

  4. #4
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    An interesting Vespa thread on their airbox mod attempt

    http://modernvespa.com/forum/topic60894?page=1

  5. #5
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    Yeah, ive seen that thread before, and came to a conclusion that the airbox on the vespa is not the same as that on the sc. However, this thread might be useful, http://modernvespa.com/forum/topic89030?page=1 go through it and let us know your thoughts on the matter. Cheers!

  6. #6
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    You are right, this was a much more relevant thread as they have the same airbox.

    Comments:
    1. I would not do the scoop route; that was someone else's try at getting more air to the unfiltered side. Water and other contaminates is the issue.
    2. I thought about the plate approach, it would be simple enough. But, I think I'm going to go with a tube if there is room.
    3. I think you still need a tube on the unfiltered side, about 1-2" long. There is a reason the factory set up has that. My first reaction is to help "aim" the air flow into the airbox cavity. Without it, the airflow would be turbulent. In the end, this might relate to noise more than performance.
    4. air inlet to the airbox, I think this could be opened a bit more. I think you can do this and route a flex hose from that end to the front somewhere but not directly open to the front to ingest water, but somewhere you can clean air vice air from engine area.

    These previous threads do seem to suggest noticeable power/performance can be had for not much $.

    Jerry

  7. #7
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    Based on the other threads, I figured it was easy to pop the tube out - it was. You just have to loosen the metal clamp and twist and pull. You can see the space to work with. I will make area measurements so I have compatible holes (that is, there is no sense in making big cavity holes inside the airbox if you do not enlarge the air opening in the cover on the end) (pic 1).

    Just for scale, I took the tube to see what size coin just passes through the smallest diameter in the tube. A penny will get through, but a nickle sits on the top of the tube. This is the end that air is sucked into 1st heading further into the airbox (pic 2). Think how small that is compared to the rubber boot between the airbox and throttle body! Can you imagine running flatout breathing only through a straw...

    Jerry
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  8. #8
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    Higher intake vacuume usually relates to better atomization of gasoline and increased fuel economy. In the 70's at the onset of the first gas crises many people used in-dash vacuume gauges to monitor their intake manifold vacuume so that they could adjust their throttle position to keep their engine vacuume as close to 20 inches or more as was possible.

  9. #9
    apriliaforum expert Buckeroo Bob's Avatar
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    A simple butterfly valve inside a larger intake tube actuated by a shaft with a paddle in the outside wind stream might work. The faster you go the wind pushing against the paddle blade would cause the butterfly valve to open with speed. A light spring or proper weighting would cause the valve to return to the closed position as speed (wind) decreases. A picture would help, eh?

    Truthfully, I probably don't know what I'm talking about. :-)
    Last edited by Buckeroo Bob; 01-24-2012 at 08:56 AM.

  10. #10
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    That or an Iris type apature could be an option which would work in effect like having a staged 2bbl carb. Small apature for starting off/low speed and the large one for running at high speed.

  11. #11
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    I like the thoughts expressed here, the only question now is how to do this on the cheap. I shall give it some thought as the mods should include this possibility.

    Jerry

  12. #12
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    Quote Originally Posted by spacetiger View Post
    I like the thoughts expressed here, the only question now is how to do this on the cheap. I shall give it some thought as the mods should include this possibility.

    Jerry
    A variable apeture would have to be able to stay free in unfiltered air so on reconsideration an Iris or even a flat slide operated by a vacuume pot would probably be prone to sticking from the dust. A vacuume operated butterfly with a penny sized hole in the middle maybe something to consider.

  13. #13
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    Rockynv,

    I think Bob's thought would be a more simple way to go. 2 separate tube paths to the unfiltered side, with one of them containing a simple butterfly valve. It does not have to seal air tight. This approach might allow you to run an even smaller "primary" tube to help lower RPM torque.Just have to work out a fool proof actuation scheme.

    An old over/under 20 guage shotgun/22 caliber rifle comes to mind...

    Jerry

  14. #14
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    A diverter like the intake preheat on a car/truck? Two tubes and a butterfly would be more complex than one tube and a butterfly with the small hole in it. When closed it would be in the low rpm mode with the flow going through the small hole in the butterfly and open would be for high speed.

    The tube is listed in the AF1 store as: AIR CLEANER BOX-VEHICLE MANIFOLD - 829543 $39.99

    Is it made out of gold? I was thinking that I would order a spare in case I messed it up but that is more than double what I expected for the hose. Maybe if they called it a hose it would be less than $10.00 but call it a manifold and it is almost $40.00.
    Last edited by Rockynv; 01-25-2012 at 05:25 AM.

  15. #15
    apriliaforum expert Buckeroo Bob's Avatar
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    Name:  intake valve.jpg
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Size:  16.4 KBHere's my picture, crude as it is. But, it can generate some feedback and suggestions on how to modify (or scrap) Ha!




    Had to scratch my head a bit to navigate the new picture posting procedures.

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