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stoneagegixer

Run the xr650r without air box?

13 posts in this topic

Hey Guys,

I'll try to explain a little better. When I had my old gsxr, I pulled the airbox off and put individual k & n filters on the carb. Is this possible with the xr650r? I'm not concerned with dirt since I ride it on the street only. If this is possible, what filter is there for it? This would be the best way to maximize airflow going in, how do you think the bike would run....

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Hey Guys,

I'll try to explain a little better. When I had my old gsxr, I pulled the airbox off and put individual k & n filters on the carb. Is this possible with the xr650r? I'm not concerned with dirt since I ride it on the street only. If this is possible, what filter is there for it? This would be the best way to maximize airflow going in, how do you think the bike would run....

It should be no problem finding the filter that just goes on the end of the carb. They come in many different sizes, just measure the diameter of the outside of the carb inlet and get one for that size. That is the easy part. Getting it jetted correctly is the hard part. The air box creates a slight vacuum which helps draw in the gas. The jetting needs to be richened up a lot. This usually includes a richer needle. Almost always there is a trade off. A little more top end power at the cost of low end smoothness.

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I would not think that this would work well. The air box is an intake plenum which is a container for air. Intake plenums are important for intake pulse resonance over RPM ranges. When you remove this plenum, you effectively retune the intake tract resonance for an RPM that is a function of the length from your intake valve to the carb inlet. The longer the length, the lower your power peak will be.

Also, the intake plenum works as a buffer for filtered air with respect to intake pulses. you essentially have a volume of filtered air to pull from on every intake cycle. This lowers the difference between air speeds going through the filter over the engine cycle. The difference in air speed can hinder performance.

Most crotch rockets are running 6 liter or larger air boxes. Minimum, you want a plenum 1.5 times the engine displacement for 4 strokes, 3 times for 2 strokes.

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Billahjack,

Thank you for your input. I think I understand what you are saying. Moreover, you certainly sound like you know these bikes, so I'll keep the box on. I went from sportbike to the xr, so I was making an assumption regarding tuning....

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I personally wouldn't remove the air box if you ride off-road. The 650R's air box is designed in as part of the rear fender. removing the air box will force you to remove the white flap that protects the rear shock from mud. All your rear suspension parts, (which are know to have problems anyways), will be 100% exposed to everything the rear tire throws.

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I would not think that this would work well. The air box is an intake plenum which is a container for air. Intake plenums are important for intake pulse resonance over RPM ranges. When you remove this plenum, you effectively retune the intake tract resonance for an RPM that is a function of the length from your intake valve to the carb inlet. The longer the length, the lower your power peak will be.

Also, the intake plenum works as a buffer for filtered air with respect to intake pulses. you essentially have a volume of filtered air to pull from on every intake cycle. This lowers the difference between air speeds going through the filter over the engine cycle. The difference in air speed can hinder performance.

Most crotch rockets are running 6 liter or larger air boxes. Minimum, you want a plenum 1.5 times the engine displacement for 4 strokes, 3 times for 2 strokes.

So that means that for a CR 500, you want a 1.5 litre airbox? I dunno if I've ever seen a CR 500 airbox that big... But, then again, I've never looked that closely either...

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I think the CRs have more than 1.5 liters because you are measuring the volume of the portion that the air filter sits in also.

The crotch rockets have the filters in the airbox which can be cannister type or flat plate type.

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My XR600 is running a 'modified' air box in which the left side of it is completely removed. You can hear the engine sucking air. It was done like this when i bought it and is running an edlebrock carb. Im not sure if its the airbox, the jetting, or a combo of both but im having some issues in the top end running about 80mph+ it seems to surge a lot then act as if the spark died (it looses all power and and starts bucking). I dunno what the deal is, but i do know if you go this route jetting will be a major role into how it performs.

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im running a fcr 41 pumper on my xr650l. battery is in the airbox, so i use a 4" long pod filter. clamped to the carb inlet. with the cv carb and dj kit, i would get that top end issue also. now with the fcr its perfect all the way up!

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