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Airbox question.

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If the intake to the airbox is as big as the intake to the carb. Surely the only thing that matters is the filter?

Or does a larger opening being funnel'd into a smaller opening create that much more of a high velocity flow?

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Here is mine:

Before:

48038806.jpg

After:

48038794.jpg

I would imagine that you would be limited to the carb opening, similar to drinking something through a straw. With that said thought a larger opening should make it easier to draw it in. At least with opening up my airbox and proper jetting a noticeable difference was noted.

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i agree with ThumperTDC, i did the same thing to my airbox, really help draw in more air. you could hear the difference.

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Oh man... this is a huge debate on more threads, and variances of bikes... than you know.

As an example;

http://www.thumpertalk.com/forum/showthread.php?t=827106

I'm leaving my XR4 airbox as it came to me, with only the snorkel removed. No scientific reasoning behind my decision, just that I think it gets plenty of air... even with a pipe and re-jetted. Just my opinion, and I could be plenty wrong.

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With airbox mod,when mine was stock(jetted,pipe) 10 runs back to back with a 440.Dead even race roll on.Also dyno 3hp.This mod on xr4 is from ex honda factory rider.Also air box same on xr250.Best free hp mod for xr4

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With airbox mod,when mine was stock(jetted,pipe) 10 runs back to back with a 440.Dead even race roll on.Also dyno 3hp.This mod on xr4 is from ex honda factory rider.Also air box same on xr250.Best free hp mod for xr4

.. I hear ya. And understand there's "reasons" to MOD.

I think a more submersible bike (and a filter that stays cleaner, longer), is worth a loss of 3Hp (if the dyno was true, day to day or atmosphere same at different dyno runs). I'll lose weight, and drop a few pounds first to gain the horsepower. :ride:

You all should hear the flack I get about "not removing my backfire screen" in a 450X, compared to those who say they can "feel" a seat of the pants difference in doing so. ...I'm still laughing about that one. :thumbsup:

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.. I hear ya. And understand there's "reasons" to MOD.

I think a more submersible bike (and a filter that stays cleaner, longer), is worth a loss of 3Hp (if the dyno was true, day to day or atmosphere same at different dyno runs). I'll lose weight, and drop a few pounds first to gain the horsepower. :ride:

You all should hear the flack I get about "not removing my backfire screen" in a 450X, compared to those who say they can "feel" a seat of the pants difference in doing so. ...I'm still laughing about that one. :thumbsup:

ive always worried about water a little, but its pretty dry here in michigan so i figure it would be worth a shot. i was really happy that i did it. however i am surprised that it would gain 3HP from this

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Sure water is an issue, but look at the CRF's and other two strokes that have to have the filter inserted in through the top of the airbox...........the was why I cut mine open. There are tons of people out on the trail with these bikes and cant do a thing about the airbox being open. I for one took my chances, I would rather have more power than worry about the few times that water might get into the airbox.

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only saying it, cause i put a BST40 on my XR, and the stock airbox with only the snorkel removed. only required an adjustment on the idle jet compared to running with a pod filter.

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Depending on year, model, and what a PO did there could be zero to several "things" restricting your air intake. Here is a pic of a XR200 air box after a wet ride and you can see from the residual dirt that a lot of water got on top of the airbox.

XR200AirBox.jpg

The thing sticking up out of the air box with the three holes is what Honda calls a "duct" and its raised edge keeps water out of the air box, some models of XRs have a raised lip on the hole so don't need this duct. The duct also shields the filter from water that get into the airbox. Because of the signs of lots of water getting onto the top of my airbox I chose not to remove the duct. On some models there is another part called a "cap" instead of the "duct", sometimes called a snorkel. This further restricts air intake and I think it also reduces intake noise. My XR250 had a raised edge on the airbox opening so I pulled the "cap".

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Deep water for last two months,in Baja is every trail right now.We have no problems with mods on airbox.I did have a roll of duck tape to cover pro vents.Did not need it for any of the xrs,but the Ktm two smokes.Have a poor airbox set up,and they did stop in the water.

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