Airbox or separate Filters? An age old question.

[Nasher]

All, an age old question.

Airbox or separate Filters? An age old question.

My 1997 Carbie M900 is fitted with a Dynojet stage 2 kit(Well that’s what the info in the file says, I’ve not checked) and two huge pint glass sized K&N Filters, which look cool, sound nice, and the bike appears to run OK except being a little bit rough low down in the rev range. The engine is effectively 900SS spec with earlier V2 big valve Heads having been fitted, and it has standard headers with Remus Ducati Performance silencers.

All this was done before I brought it, and I’ve not ridden a standard M900 Carbie, so I’ve no idea if it’s an improvement or not.



I’m currently also building a 900SS Mongrel with a 966 CCW Big Bore kit, and have been looking at Dynojet kits for the standard carbs.
I’ve noted that Dynojet don’t list a stage 3 kit for the M900/900SS which is specified for use with separate filters, and the Stage 2 kit is specified to be used with a standard or modified airbox.
In fact they say that where a Stage 3 kit is not listed it’s because they have found there to be a benefit in using the standard airbox.

I’m probably going to use a standard airbox on the SS with a K&N panel filter, both of which came with a dismantled 600SS Donor bike I brought, and chuck a Stage 2 kit in the carbs.

It does highlight that my M900 is not as suggested is best practice by Dynojet.

I’m well aware that most cut the top off the standard airbox on both the M and SS 900, and because of the above I’m considering sourcing an airbox assembly for the M900 and making a clamp plate to run it without a lid.

Does anyone have any real world experience to share that might persuade me it’s worth going to the bother of doing the work to convert the M900 back to an airbox?
Might I expect some more low down grunt etc.

Thanks

Nasher

[Nickj]

Probably not much advantage to an airbox that you'd notice.

[Dukedesmo]

Despite not having one on my bike, I believe an airbox is generally better and bigger (airbox) better still. If I could devise a suitable airbox for mine, I would.

I don't have one because the split single carbs don't easily lend themselves to one (it would need to be very large and an odd shape) but during dyno testing, removing the filters over the stacks freed up 4 or 5 extra hp, presumably because the filter disrupts flow around the intake that an airbox wouldn't.

[Jez900ie]

Originally Posted by Dukedesmo

Despite not having one on my bike, I believe an airbox is generally better and bigger (airbox) better still. If I could devise a suitable airbox for mine, I would.

I don't have one because the split single carbs don't easily lend themselves to one (it would need to be very large and an odd shape) but during dyno testing, removing the filters over the stacks freed up 4 or 5 extra hp, presumably because the filter disrupts flow around the intake that an airbox wouldn't.

I don't know much about how these things work, save for the standard theoretical info most people have. I must say I am impressed at 4 or 5 hp increase by changing air filtration.

[Nasher]

Thanks Guys.

I'm unsure if I should go to the bother, but might buy one anyway and see how I feel.

Nasher.

[Dukedesmo]

Originally Posted by Jez900ie

I must say I am impressed at 4 or 5 hp increase by changing air filtration.

Problem was that it lost power with the filters fitted and I couldn't run without as the vertical cylinder carb intake is directly behind the rear wheel and facing forwards!

It appears that the filter was affecting the way the air flows around the intake stack. I have since fitted longer stacks with K&N filters which definitely filter better and although I've not re-visited the dyno I think I have improved things (also changed the exhaust pipes at the same time) but even if it didn't I can't run without filters.

An airbox would eliminate the disruption caused by the filter being in contact with the stack/intake and a bigger airbox would (likely) more closely replicate running without filters.

Certainly on race bikes they used to fit larger airboxes, along with other 'stuff' to make more power although the gains are likely to be very small...

[Mr Gazza]

Originally Posted by Dukedesmo

....the vertical cylinder carb intake is directly behind the rear wheel and facing forwards!

Can't work out where your carb is DD... I am assuming you mean the front wheel?..

I remember reading (sampling) Phil Irvin's "Tuning For Speed" many years ago, although much of it is still very relevant.
There is a section on the shape of the carburettor intake. The shape of this is critical according to Irvin. We used to call this a Bellmouth, but I believe they go as trumpets or stacks these days? (Shortened from Velocity stacks I assume?)

The bellmouths used to just be bare naked out in the breeze in the olde days, but the shape is just as important whether it is in an airbox or inside a clip on filter.

Nashers filters look like there is enough room inside for a nice bellmouth?

An airbox or plenum chamber does a lot of work (worth a whole chapter). It modifies the air flow into a nice ready supply of clean air, at the correct pressures and volume, all lined up nicely for the carb(s) to easily digest.
There's an awful lot going on in there, which, without going into any detail, contrasts vastly from simply snatching whatever air is passing in the turbulent slipstream.

I reckon if you can afford the filters some kind of shelter, so that they draw from relatively still air, (without robbing the heads of cooling air) and make sure there is room in the filter for a nice bellmouth without fouling it, then you should be on the money.
My money would be on an airbox though.

[Flip]

Originally Posted by Mr Gazza

We used to call this a Bellmouth, but I believe they go as trumpets or stacks these days? (Shortened from Velocity stacks I assume?)

The bellmouths used to just be bare naked out in the breeze in the olde days,

Like this Mr G? (Less bath plug of course)



Nasher, they way I understand it is that pod filters are not so good with CV carbs because they won't allow them to remain constant due to them no longer being in still air- I know there are plenty that claim the carbs still work- which they may do (just maybe not as well as they might with an air box).

However if you had a set of FCR's then the story would be very different and the benefits much more apparent- remember when all four cylinder bikes ran proper slide carbs?
The story goes that CV carbs were only invented to stop the 'less delicate' rider from flooding it when starting or preventing the bike from bogging down when given a fistful of throttle at too low engine revs.

[Nickj]

A little bit of my retirement package is going into a 748R, savings seem a bit pointless (possibly an on-call osteopath too) Now they had the bigger airbox which fits into the frame which isn't the regular trellis.
Probably a good idea as the motor is a real air gulper with big valve lifts and the above intake shower injectors would work less well with even more turbulent air.
CV's do prefer more constant pressures so a box makes a bit of sense and the volume can be used to do a bit of pulse tuning but it's maybe more effective looking at the stack length and clearing any steps in the intakes above the carb or injectors.

[Dukedesmo]

Originally Posted by Mr Gazza

Can't work out where your carb is DD... I am assuming you mean the front wheel?..

Vertical cylinder carb is a sidedraft and the intake points just to the left of the headstock so in the 'firing line' of the front wheel, horizontal is a downdraft and points directly upwards under tank.

Originally I had the supplied (blue) Keihin bellmouths which doubtless work well but the only way to fit a filter is to use a foam 'sock' filter which robbed power at the top end but had to be used as the carb would be sucking in all the debris from the front tyre.



Sock filter fitted;



Since then I located some longer stacks which are less 'bellmouthed' and longer but have a thicker end that a filter can be clamped to, the (K&N) filters are much wider and so don't disrupt the flow like the sock filters and also filter better.



K&N filter fitted



For old school bellmouths, I have open intakes on the Guzzi with just a large mesh to keep stones etc. out but due to the design these face backwards so are OK (as long as I don't ride it on the beach).



Cleaned up;

[Dukedesmo]

Originally Posted by Nickj

A little bit of my retirement package is going into a 748R, savings seem a bit pointless (possibly an on-call osteopath too) Now they had the bigger airbox which fits into the frame which isn't the regular trellis.

Something like this;



EVR large airbox with TPO 'Beast' velocity stacks on my 916, shame you can't see it when the tank's on because it's automotive porn...

[Nasher]

Thanks guys, all interesting stuff.

Looks like I need to try it.

Nasher.

[Albie]

Well if your talking carby bellmouths my weber


21615982_10214505675820794_6741138046333610996_n by Andy Patterson, on Flickr

21761585_10214505676740817_7715350678126040658_n by Andy Patterson, on Flickr



LOL

[utopia]

I thought those meshes on the bellmouths of Guzzi carbs were to stop them sucking your nuts in while you were getting your knee down ....?
You know, in the Italian style .. shorts and trainers.

[Nasher]

Originally Posted by Albie

Well if your talking carby bellmouths my weber

How Twee