Hi All

Recently enlarged my carbed bike's open airbox. Measuring it I found it now has at least an extra liter capacity under the filter.
I had a chopped airbox lid that only had the very top removed rather than being cut right down, so I put a spare K&N filter there and stuck it there with silicon sealant (rough prototype) . A mate of mine is looking into sourcing a schmick carbon version.

I had previously shortened the velocity stacks by about 20mm. This gave me a couple of ponies thru the range at most and a fatter torque curve. The bigger airbox with higher filter required me to open the IMS half a turn, go up one size on the slow fuel jet and raise the needle a notch, evidence to me of flowing more air, but the dyno will tell for sure. I will dyno test it sometime to ascertain if I need bigger main jets. The 165 main jets work well but richen up over the last 2000 rpm on the dyno/EGA so I'm not worried. I can feel a stronger midrange surge on the butt dyno.

With the shorter V stacks I came to the conlusion that they improved air flow in the airbox by giving them more room from the air filter (by 20mm), and the raised filter/bigger airbox gains another 30mm of space between the stacks and the filter. The stock set up has about a 30mm gap from the stacks to the filter but my set up has about 80mm. The goal was to allow the stacks more free airbox space and still air to draw on under the filter. Normally shorter stacks improve top end and longer help midrange (simplisticly), but when the shorter stacks gave more torque right thru the rev range I figured it was not about stack length but airbox size and configuration. Probably due to the very compromised airbox design on a carbed two valver. Also the inlet manifolds are so long a shorter stack wasn't going to hurt midrange, which was proved correct on the dyno. The shorter stacks helped everywhere, which I was not expecting.

I have seen examples of larger airboxes in Europe. Anyone else tried this, or know someone who has in the States?

No input, but I will be watching this thread.  Are you running an airbox or pods presently?

+1 on watching this.  Pics?

I can only think of two people that might have.

Bruce Meyers or Doug Lofgren.

I'm running the airbox with the short stacks and raised filter, so a litre more capacity.  There's a nice area of space above the velocity stacks.  I think it will utilize the volume of the front of the airbox more effectively.
I'm having trouble posting pics but I can email some picks. Feel free to post them.

Thanks, I might email Doug.

I emailed you some photos Howie.

Thanks for the info DP.

Thanks Howie.

Hmmm...  Interesting! 

Got any dyno sheets before & after?

I have dynoed the short and long stacks, and pods Vs airbox.  The short stacks worked best. The ega reading showed more airflow at full throttle. It wasn't much but it was enough to justify whittling down the stacks. But every pony counts when there aren't many. I'll photograph a dyno sheet and send it to you if you like.

The side effect of the short stacks was to make airbox removal and refitting easier. It's even easier now because I removed the annoying (and sharp) filter support struts and relocated them under the raised filter.

I will dyno the airbox soon (when there is a break in the weather). I wanted to tune it a bit street riding locally brtween the rain. Not sure yet if the main jets will change.

Did I get all of them?

Oops on the first one.  Rotate your computers folks!

Thanks heaps mate.
Cheers. I owe you a beer.

You can see in the pic a hint of the orange gasket/seal of a gutted, used oem filter. This seals the lid with it's "integrated" K&N filter. Although the filter is raised, there's still plenty of room under the tank for airflow for the open airbox.
If the dyno test is positive, it's a mod you can do for the price of jetting, a used oem filter and a tube of sealant. All you need is a sharp knife and good whittlin' skills to shorten and radius the rubber velocity stacks (worth 1-2 hp and ft/lbs thru the rev range).

So, when a e you coming to Bar Matchless in Brooklyn to buy me that beer?

Since you have an extra liter I wonder what effect closing the air box a little might create.

I haven't been to the States since '91, but look forward to getting back there. You might get that beer yet.

Maybe I could test with some duct tape.

Very interesting....  easy to do i think...

i believe freezing the rubber helps.  i might try that at some point.  have you tried the blue fcr stacks as a comparison?