Airbox Mods Revisited

[koko64]

Airbox mods that supply more air to heads and ports that can use them. The carbed airbox is almost an afterthought, but rearranging it to make better use of it's available volume and increasing the sub filter volume made good gains on a ported 900 motor. Didn't do much on stock heads at all.
Now days with 100 hp easily accessible from an 1100 Evo motor, this is answering a question no one is asking, but I love the character of the old 900 motors and hold hopes for another project in the future.

The stock velocity stacks are too tall and soft and are too close to the filter. Raise the filter, shorten and port the stacks (interestingly making them look more like rubber versions of Beast R stacks) and you have more airbox volume available with less eddying/negative resonance for each intake stroke. That's if the ports are up to it.
You can chop out the seal of an old stock air filter to seal your new one piece airfilter/airbox lid.

[koko64]

The short stacks combined with the raised filter create more still air space above the stacks and let the air from the front of the box make the turn and be utilized. The shorter stacks give a better taper bore so to speak to the inlet tract. Well, that's my theory.

[koko64]

Not quite finished, but as you can see, being shorter by 20mm also makes it easier to get in and out of, and locate the airbox. Looks more like a velocity stack and less like a rubber connector. By itself doesn't do much, even less for 750's which don't have such an issue with the long, narrow intake manifolds.

[koko64]

Version we will try on the Nugget. I really tried to maximize the airbox volume to try and emulate the 900/750ie airbox volume. It might do nothing, only one way to find out. Hoping the FCR39s, ported intake manifolds, ported, shortened stacks and greater airbox volume help put a bump on top of the power curve like a 750ie. The carbed and ie 750s have the same cams, valves, capacity and compression afaik, so maybe the difference is all in the intake set up.

[koko64]

I think a K&N Panigale filter would fit nicely. Should be big enough and still fit within the edges of an airbox lid cut to suit. The DU0900 filter is a tight squeeze and takes a little volume away, so a Pani filter should be even better.

Found that out courtesy of Speeddog putting me onto the K&N filter site.

[clubhousemotorsports]

Interesting stuff. I might have to play with mine to see if I get any gains. 
I am not a proponent of pod filters but I did have a customer with pods make power more like a short manifold bike. There was more than one mod done to the bike so it was not something I could say was only the pods but it got my interest.

This is not my time of year to play with my own bikes but when the slow season hits I will head over to the dyno and chop up an airbox lid or two.
What were the stacks out of?

[koko64]

The velocity stacks were the stock rubber ones that go into the airbox. I cut off about 20mm off the ends and smoothed the radius of the bell mouth area.

I found that that the K&N pods lost a pony or two of peak power and flattened out peak torque a little compared to an open stock airbox with K&N filter. I got the peak hp number back with re-jetting but not the peak torque. It made me realise how bad the stock airbox was even when it was open and that led me to start playing around with it's layout.

I noted that the "expanded" airbox did not do much to an unported 900, but made a good gain on a ported motor. The dyno chart is of a ported, hi comp 900 with FCR41s. I measured the extra volume under the raised filter and it was about a litre. This also moved the filter away from the stacks and opened up the area to the front lower section of the airbox.

I'll look for other dyno chart pics from the old thread.

I would even run pods on a high comp unported 900 and adjust the jetting for ease of maintenance, it's just that they are so loud it annoyed me.

[koko64]

K&N Pods Vs Open Airbox and K&N filter. With a rejet I got the peak power back but the peak torque was still flattened out. I don't know if more jetting could have fixed that. Not sure.

[koko64]

The gain from the airbox mods was 5 hp which was very good (the first dyno chart). The porting was done by a mate of mine with serious skills and taught by one of the best in this part of the world. The dyno operator was impressed. I wasn't expecting it and was pleasantly surprised, but on an unported hi comp 944 with FCR39s it did little if anything. After a cam change on the 944 to 900ie cams we are hopeful they might work a little better with the airbox mods, but I suspect the bad ports are still in the way.

[koko64]

Found via search function "Enlarged Airbox for Carbed Monster" thread. It has pics of the stacks and how they went into the airbox, etc. There are also dyno charts of the various mods.

[koko64]

After looking at the old thread, and the dyno charts, the power curve looks closer in shape to a 900ie than a carbed 900. The 900ie has a better airbox to my eye, so maybe there is something in that besides the differences in the manifolds and throttle bodies. There is still a drop off with the carbed 900 compared to the 900ie, as the 900ie cams should help hold the power even better. Be interested to see a chart of a ported, hi comp, carbed bike with these airbox mods and 900ie cams.