Airbox airflow???

16 replies to this topic
  • 5spoke

Posted December 08, 2003 - 11:06 PM


I measured the two inlets into the air-box and each measured 14.7cm2 x 2 = 29.4cm2

The intake/grab handle, in the right hand plastic, measures 23.6cm2 + all the leaky areas around the rear plastics.

Now if the 39mm dia. carb. measures 11.94cm2, why do you need to cut everything open when there is already more than double the airflow available than the size of the carbs venturi?

Without an air filter installed, I think it would help opening up the box a little, otherwise I think the difference is negligible. I’m thinking the most restrictive thing is the air-filter.

Does any of this make any sense? :)

  • TallJohn

Posted December 09, 2003 - 12:44 AM


Er - yes it seems to make a lot of sense, although I am not sure if a restricted passage can cause a disruption to the airflow therefore causing less air to be sucked in! For my 2 pence (not cents I am in the UK), although it will probably be Euro cents soon, when I took off the lid, all it did was to make it louder.

  • 5spoke

Posted December 09, 2003 - 07:48 AM


I'm leaving mine as is for the moment.

  • Indy_WR450

Posted December 09, 2003 - 10:09 PM


You may want to notice that when the seat is down in postion there is less space for air to get in. The seat acts like a lid almost sealing the 2 openings. Just take your seat off and take it for a ride standing on the pegs and you should see a difference. :) That is why most members here have some sort of holes or air box mods to allow more air in. It really helps more when hot temps or higher altitude conditions.

  • endurodog

Posted December 10, 2003 - 06:27 AM


Ok here is my take on it and thats worth about nothing :)
2 things

When the air box is open it allows more air for on demand use. The air doesn't first have to be drawn into the airbox, then through the carb. Now as the carb starts to draw air, air will back fill into the airbox but it just shortens the time by a little.

Beyond that when the airbox is opened up your only drawing through one small opening, the carb, instead of a 2. Basically when the airbox isn't open its like a long straw.

Now I could be wrong about how all this works but the bottom line is that guys with open airboxs notice an improvement in performance.

  • Treebanger

Posted December 10, 2003 - 08:58 AM


Volume and velocity,This is kinda hard to explain but to my way of thinking the stock air box is restricted to the point that it is close to collaping the filter. I noticed with stock setup dirt would collect unevenly on the filter and in some areas the dirt would really built up , Not good ,Velocity of air hitting the filter=uneven build up of dirt. Possible valve damage likely in long duration events! I have modfied my airbox for better flow and volume of air . The filter has a balanced dirt pattern to it . Which I think keeps more of the dust and dirt out . Smoother air flow better air flow.

  • endurodog

Posted December 10, 2003 - 09:40 AM


I was trying to figure it out, how to explain it. This is a good illustration of one way it works.

  • 5spoke

Posted December 11, 2003 - 05:25 PM


Thanks guys. I'm convinced and will be opening the box up with jetting soon. :)

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  • toyota_mdt_tech

Posted December 11, 2003 - 06:46 PM


I took the Roto-Zip® to mine, opened up the top area for a second snorkel just like the existing one. I didnt want to just remove the lid, this makes it easy for water to enter as its shed from the fuel tank etc. And in my state, there is plenty of water.

  • Pazzo_P

Posted December 13, 2003 - 02:40 PM


There's a new product that addresses this problem and produces a noticeable difference. I'll gather some info and report back or the local rep will see this and elaborate further.

  • DGrizz

Posted December 19, 2003 - 10:41 PM


I've had my 450 since last MarchIt's blown the rotor twice and yamaha has split the cases. I ride between 3000 and 7000 feet I have a "power now" and I have opened up the side of the airbox with a homemade stainless screen.I am running the normal mods with a 72 sj, 48 pilot, 162 main, airscrew is out 2 and3/4 turns, stock YZ450 muffler,stock needle 5th position. If you want to find out if you need more air, take the left hand cover off the side of the bike airbox. Sure helped me. :)

  • sabin

Posted December 20, 2003 - 01:06 AM


I have dyno tested wr426 with open pipe.

You get 1hp by removing the top of the airbox.
You get 1hp more by also removing the seat.

However thare ware some dscution on TT before if the holes on the side of the airbox really help. They make big numbers on dynos, but when you move maybe some vacum is created????

  • lipskid

Posted December 20, 2003 - 09:19 AM


A while back I cut my air box lid so I had a nice rectangular opening, removed the lip around the top, and intake "runner".

My seat of the pants imression was that it may have rev'ed a bit freer at wide open, high rpm's, but definately made the low end bog worse, and felt rich on the bottom. I then removed the side cover and went for a short ride, and didn't notice any difference(except noise). Dropping the needle one position cleaned up some fo the bog, but power still felt a little soft down low. After one ride I bought a new air box lid, raised the needle back up, and all is well.(I did cut the lip down on top to clear the seat base better though, because, well, I just can't help it, I gotta screw with stuff :D)

Anyway, just my observations. I can't help but think the Yamaha engineers may have spent some time working on the best way to get air around the battery. I know air box resonance plays a big part in sport bike performance. Maybe some of that knowledge went into our bikes as well :).

I don't doubt some you have had good luck with your mods, mine was a step in the wrong direction though.

  • endurodog

Posted December 20, 2003 - 05:15 PM


This stuff makes you wonder what the whirling air is doing in there.

  • 5spoke

Posted December 20, 2003 - 07:12 PM


I still think the big negative is the air filter and not the openings.

  • David_W

Posted December 21, 2003 - 09:54 AM


I have a YZ rear fender on my bike, and when I fitted it I noticed it has an air intake moulded into it where it meets the back of the seat. The standard WR rear fender butts up to the seat and almost makes a seal. I guess any addition to airflow into the airbox will help. I've got the snorkel lid removed too and rejetted to suit as per suggestions from JD.

  • CronicArt

Posted December 22, 2003 - 10:26 AM


I have never noticed any airflow restriction, but then again I have a YZ. I did however notice a big differance when I replaced my stock air filter with a high flow UNI filter. My bike did not come with an airbox cover and the rear fender as stated above has fins that allow air to flow from the rear of the seat. I noticed with the filter change that when I reved it up I could feel the air rush past my legs. (I was also wearing shorts) I know a K&N filter flows more than the UNI. I hear K&N flows about 900cfm. Changing your exhaust will give a big increase in airflow. The faster the exhaust gets out the faster new air/fuel can get in. I also don't have a battery in my way. IMO throw out the airbox lid, get a K&N filter, and a YZ fender. I bought the YZ because I wanted the performance without having to modify as much. I put a stator and headlight on. You have to take in account of where you ride. Do you want a bike for long straight flat out runs or tight wooded trails. I ride mostly tight stuff that requires more bottom performance. The close ratio tranny also helps. Thats why I decided on the YZ over the WR. 2cents


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