Velocity Stacks
Discussion
My KTM has a K&N filter which doesn't cope well enough with dust (off road)
No one makes an oiled foam filter for it and the paper one is restrictive and expensive
The air enters at the red arrow and passes through the filter
Ideally I'd like to add an oiled foam filter before the K&N but there's no room so I was thinking of simply laying one on top of it (glued down)
It would mean the velocity stacks are barely above it - would that affect their function or is it just their length to the butterfly that matters ?
No one makes an oiled foam filter for it and the paper one is restrictive and expensive
The air enters at the red arrow and passes through the filter
Ideally I'd like to add an oiled foam filter before the K&N but there's no room so I was thinking of simply laying one on top of it (glued down)
It would mean the velocity stacks are barely above it - would that affect their function or is it just their length to the butterfly that matters ?
Edited by KTMsm on Friday 13th June 21:31
It can all play a role, and some engines may be fussier than others.
Interesting test, and a great channel to be fair
https://www.youtube.com/watch?v=ACVkBHUupN8&t=...
Can you create an air intake somewhere that might be less prone to get dust in the first place ?
Interesting test, and a great channel to be fair
https://www.youtube.com/watch?v=ACVkBHUupN8&t=...
Can you create an air intake somewhere that might be less prone to get dust in the first place ?
I wouldn't expect the extra filter layer to cause any harm to the air flow through the trumpets. But if you're planning to add a layer of foam filter on top of the paper filter then you're adding to the flow restriction there. If I've understood the air flow correctly then the added foam filter would be downstream of the existing paper filter, so it wouldn't prevent the paper filter from clogging.
Is this a common problem with this model?
Is this a common problem with this model?
I guessed 25mm would be ok on thar test 
but I'm essentially reducing the distance under the stack not above it
The problem is that I frequently ride in very dusty conditions off-road
Enduro / MX bikes all come with oiled foam as standard and most would clean the filter after a single day event
but I'm essentially reducing the distance under the stack not above it The problem is that I frequently ride in very dusty conditions off-road
Enduro / MX bikes all come with oiled foam as standard and most would clean the filter after a single day event
GreenV8S said:
I wouldn't expect the extra filter layer to cause any harm to the air flow through the trumpets. But if you're planning to add a layer of foam filter on top of the paper filter then you're adding to the flow restriction there. If I've understood the air flow correctly then the added foam filter would be downstream of the existing paper filter, so it wouldn't prevent the paper filter from clogging.
Is this a common problem with this model?
I was going to keep the K&N because the oil would affect the paper filter Is this a common problem with this model?
Yes, it's not ideal
Generally, people either add a foam pre filter on the outside of the air box but that's a restriction as it's quite small
The more common solution is to bin the air box and replace it with an open foam intake, but the noise is horrific as I have that on another bike
The aftermarket have made a replacement foam filter for the later version of the bike
The other alternative is for me to remove the cotton part of the filter, keep the mesh and lay the foam on top of that
Have you considered making your own foam filter by cutting down a larger one? As far as I can see from the picture, the filter is just a simple flat panel. That seems like it would be simple to DIY with a pair of scissors. It may not flow any better than a new paper filter but might flow better than a clogged one. You can make it deeper than the paper filter, which will increase its effectiveness and life. It would probably also be possible to DIY a reusable corrugated one by cutting down a K&N or similar element and bonding it into the plastic frame of the oem filters. It just comes down to how much effort you want to put into it. The foam element would be easier since it'd be self forming and self sealing.
GreenV8S said:
As far as I can see from the picture, the filter is just a simple flat panel.
Apart from the two big holes in it for the trumpets! OP, which model KTM do you have? Pipercross make an oiled foam filter for the 990 SM which looks the same:https://thefiltershop.co.uk/products/pipercross-mo...
This is for my 950 (I have 990s too) which is very similar but the holes are a different size and spacing so 990 filters won't fit
It seemed simpler to use the existing K&N as the base, the seal is thick rubber and I'd need mesh to support the foam anyway
I'm not worried about the K&N clogging up, I'm worried about it passing dust
It seemed simpler to use the existing K&N as the base, the seal is thick rubber and I'd need mesh to support the foam anyway
I'm not worried about the K&N clogging up, I'm worried about it passing dust
KTMsm said:
I'm not worried about the K&N clogging up, I'm worried about it passing dust
Do you know it's passing dust, or is this just a precaution? Foam filters tend to be less effective at filtering fine particles than paper/cotton filters. If it isn't filtering well enough, a better paper/cotton filter might be a better option.KTMsm said:
This is for my 950 (I have 990s too) which is very similar but the holes are a different size and spacing so 990 filters won't fit
It seemed simpler to use the existing K&N as the base, the seal is thick rubber and I'd need mesh to support the foam anyway
I'm not worried about the K&N clogging up, I'm worried about it passing dust
Let's be real, despite all these filters claims....they will pass dust. None of them filter as well as paper elements.It seemed simpler to use the existing K&N as the base, the seal is thick rubber and I'd need mesh to support the foam anyway
I'm not worried about the K&N clogging up, I'm worried about it passing dust
And with any setup, the more surface area you can create for any filter media, the better.
GreenV8S said:
Do you know it's passing dust, or is this just a precaution? Foam filters tend to be less effective at filtering fine particles than paper/cotton filters. If it isn't filtering well enough, a better paper/cotton filter might be a better option.
The velocity stacks were covered in it We must mix in different circles because k&ns are renowned to pass fine particles. Oiled foam filters are what every Motocross and Enduro bike uses
I don't object to paper in theory but there's also a good chance it will get wet and take a look at it and how little filter area there is
Over 100bhp through that !
KTMsm said:
Over 100bhp through that !
When you consider the area of the pleats, it's a huge area. Maybe the compromises are different for off-road bikes. I'm not a biker, and my own experience comes from road cars. If oiled foam filters are considered the best solution in this environment and you can't find one that fits, I suggest you buy filter foam sheets and cut out your own. Various motorsporty type suppliers sell them for custom applications. If you can't find generic filter foam sheets with the structure you want, buy a flat foam filter of your preferred brand and cut that up. In my experience the foam filters usually consist of a support layer and two layers of filter foam, so be prepared to do a little construction, but it doesn't seem like it should be at all difficult. Then you can strike whatever balance you want between filter effectiveness and flow restriction.
KTMsm said:
The velocity stacks were covered in it
We must mix in different circles because k&ns are renowned to pass fine particles. Oiled foam filters are what every Motocross and Enduro bike uses
I don't object to paper in theory but there's also a good chance it will get wet and take a look at it and how little filter area there is
Over 100bhp through that !
Whilst it's clearly not as simple as this..... I believe the Formula Student series operate through a 20mm restriction, and generally achieve around 100hp.We must mix in different circles because k&ns are renowned to pass fine particles. Oiled foam filters are what every Motocross and Enduro bike uses
I don't object to paper in theory but there's also a good chance it will get wet and take a look at it and how little filter area there is
Over 100bhp through that !
So that filter appears to have a lot more area than a simple 20mm hole.
Presumably people have tested, raced, dyno tested with various filters ?
What is the overall airbox design like ?
stevieturbo said:
Whilst it's clearly not as simple as this..... I believe the Formula Student series operate through a 20mm restriction, and generally achieve around 100hp.
So that filter appears to have a lot more area than a simple 20mm hole.
Presumably people have tested, raced, dyno tested with various filters ?
What is the overall airbox design like ?
Ah but what power do they achieve if the hole is then enlarged ?So that filter appears to have a lot more area than a simple 20mm hole.
Presumably people have tested, raced, dyno tested with various filters ?
What is the overall airbox design like ?
As always, it's not a one part problem, if you change the exhausts you gain 2 BHP if you change to an open air filter you gain a 2 BHP and if you remap it (or tune the carbs) you gain 2 BHP but if you do all three you gain over 10 which is approx 10% increase
I'm not trying to increase the bhp but I don't want to restrict it either and I certainly don't want loads of dust getting in - although it's probably a bit late for that dur to the previous owner
Not for years Stevie, and we are not that scientific I am afraid. . We were given a bag of some test grit and added it to the air and watched the flow rate drop. It was messing about with air filters for some machinery donkey engines. On a side note I once tried to rig up a cheap grit blaster, the vacuum cleaner caught fire after ten minutes 
Peter Burgess said:
In flow tests we found the area required for an oiled foam filter to pass the same volume of air as a paper or very lightly oiled K&N filter was 2.5 times. K&N filters are not popular with the off road cars we tune, large foam frequently cleaned are popular.
That suggests that replacing a paper filter with a foam filter the same area would increase the restriction of the filter by 2.5 times. That seems like it could be a problem. I suppose you could always try it and see whether you lose enough power to matter - and whether the intake stays clean.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff