Airfilters and pressurised airboxes...

[Gobert23]

First of all a little bit of context. I'm not seeking the 'ultimate' in performance, but merely wishing to try and clarify my understanding (and settle an arguement). My understanding (I don't say 'facts') may be completely wrong, so I put in to the (informed?) masses of the PB forum ('I'm doomed')

My understanding is that pressurised airboxes (airbox systems that can get to greater than atmospheric pressure, from the assistance of 'ram-air'?) are sensitive to 'volume'.

Air filteration (e.g. air-filters) are important in keeping contaminants (e.g. crap) out of an engine's internals.

There's arguments for various filter materials, styles and placement, but my recent discussion has revolved, specifically, around the use of 'filters' on a Duc' 916 (with Corse carbon-fibre airbox and matching Corse airtubes*).

I current run a pair of Pipercross filters, that've been modified to fit at the point where the intakes meet the airbox (near enough the same as the road bike), but I'm told that this 'is all wrong' and I should be using the 'JHP' (John Hackett Performance) style filter (this sits over the throttle bodies, centrally, within the airbox).

The conflicting information seems to be about compromised airbox volume (the JHP filter would 'appear' to take up more space), but then do the individual filters restrict 'flow'(?) You get the idea?

There's also the argument that no filters at all will provide the 'best' result, but I digress...

Is there a simple answer as to what the 'best' filter placement/material/design/solution is?

G

*They're actually early 'road' specification are are slightly compromised by fittings for indicators, but they do have larger inlets that the stock/OE road items.

[originalracingsnake]

Can you dunk the two filter types in a jug of water and measure the displacement once soaked? You need a small, efficient filter that flows and filters. Trouble is filters that filter, tend not to flow the better they are at filtering.

Placement of the filter inside the airbox would only matter if the filter altered the way the air got into the TB, by obstructing it.

You could in theory, place a filter in the nose of the airtube leaving the airbox completely free. You could fit fine gauze filters over the TB's too, but they would not filter well on a road bike.

I'm always amazed at the junk that builds up in the airbox snout of the K5. Flies, gravel, dust and seeds mostly. I'd say the ebenifts of various filters are to be disputed, and only side by runs on a dyno will tell you which works best for your bike.

[fran9r]

Is there a simple answer as to what the 'best' filter placement/material/design/solution is?

No.

But I'm interested in the questions -specifically about airbox volume. I'm not 100% if it's talking about before or after the filter that is measured, or even if it matters. No tuning book I've seen specifically answers this question, which suggests they haven't tested it.

As for the filter placement, I would have thought it would matter. If they are over the throttle bodies rather than in the airbox inlet (away from the throttle bodies), surely the resonance is tuned by the distance the filter is away from the TB, as opposed to the airbox lid?

The Durbhan approach seems to be make the airbox as big as possible...don't know if there is an optimum, but would be intersted. And the difference between that and turbod applications.

[Stephenrc45]

From what I've found anything on or near a bell mouth is a bad thing. They are very sensitive to flow and even injectors to close to them can compromise their effectivness.

I would personally use a filler in the air intake on the Ducati if I had to use one.

[matt_the_blat]

There was a very long and detailed discussion on this subject, around this time of year about 2 years ago (may have been on the old .com forum).

Foo asked the original question, I explained some basic physics about what can and can't happen and millemille contributed a lot from his practical experience. And everyone else got stuck in and we had a right good technical discussion

Firstly, airboxes do not work by pressurising the air. They don't and they can't. To get any kind of significant pressurization you've got to be doing at least 200mph and it's an inverse square law e.g. halve your speed and you only get a quarter of the effect. Read this article (written by me a few years ago now) if you want to know more

I have no doubt that a well designed airbox can give performance benefits, it's just that we shouldn't believe for a moment the way they work is as simple or straightforward as pressure recovery. I suspect it has a lot more to do with getting the resonance of the airbox "in tune" with the engine and the speed at which the inlet valves open and close, for whatever performance you're after e.g. mid-range, top-end etc.

Have fun,

Matt

[gurninman]

Just read your article..... quality stuff, mate

[fran9r]

On the shower injector later model bikes I used the pipercross filers in the air tubes.

I was thinkin along the lines of filters in the air tubes...how deep is the filter? I was thinking it would be thicker that a normal panel filter...good, bad or daft? Would this slow the air speed too much, hence a panel filter would be better?