Sizing K&N filters for HP? *** EDIT - Formula included ***
Turbo Mini Forums

So, last weekend, I took the K&N off my mini to put onto the focus temporarily. Only reason for this was that it dawned on me the focus had 120K miles, and the factory airbox is meant to be swapped out after 100K miles (stupid sealed airboxes).

The focus was also tripping the airbox vacuum tell-tale gauge just into the red on a hard acceleration - well overdue then...

I'd ordered a large K&N filter - the same one they sell in their 'whippy whoppy cold air kits - which I was going to fit onto the end of my cut/shut air intake pipe, but this didn't arrive, so I stuck the mini one on there as a short-term so I could use it.

Here is the odd thing - despite cleaning, and re-oilling the filter, the focus ran like a POS. I initially thought the oil had gummed up the air-sensor, so removed and cleaned it, but the problem was still there. After 5 minutes it threw a code for 'too rich'...

So, I took the filter off, re-set the ECU, and went for a ride. Hey presto, after a few cycles to let the ECU learn the new airflow, I had a fast(ish) focus, and no issues.

Strange thing is - I KNOW my mini makes more hp than the 2.3 in my focus, but never really appreciated the air filter may have been a little on the small side. I still don't know if it is - and it's down right now, so can't do any immediate testing. I was just wandering if it's possible to make a guestimate on flow potential based on the size - so I know if i need to shell out for another filter sometime soon....

Edited by turbodave16v on 20th Oct, 2009.

no idea on the sizing on k&ns but i had mesh over my turbo inlet for ages (no room for a filter)car was plenty fast but for some reason i found i could never get above 19psi of boost, even with the actuator disconected. suspected a boost leak, but could never pinpoint it, read a sweeping statement about mesh on turbo inlets being very restrictive on some american forum, so decided to remove mine, car felt very fast, glanced at the telltale, 30psi lovely. anyroad , only way to tell is a sensitive vacuum gauge (as you already know) or jsut try a quick pull with no filter lol.

incidently k&n oil, (or any oil) is really bad for most MAF sensors, even if you clean them afterwlrds with agood induction cleaning solution they never really recover, and while i have seen focus' with the wee gauge on teh airbox, i have never seen a sealed airbox :S

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Bet you've never seen a focus with a 2.3 either!!! The airbox was a whopping $300 from the dealer - hence looking for a cheaper solution!

So i guess by 'mesh' we're not talking chicken wire - but the stuff you use with fibreglass to bridge holes?

how did you go about re-setting your ecu, i was contimplating this for the 106, as it now suck alot more air in with the cold air feed i made for it and it has abit of a flat spot (possibly another fault)

Drives
-Ford S-max Mk2 Ecoboost
-Rover 100 VVC #2 - track project

Searching is all you need on TurboMinis

I think there is some detail in Vizard about using mesh and the poor flow effects.

No idea on sizing a K&N, if it helps my generic cone filter was stated as 400bhp rated, and is about a 3" inlet and a 6" maximum diameter of the cone.

Could just be bollocks though..

Sealed airbox, sounds a pretty gash device to me.

When I was looking for a new filter a while back I could not find any bhp figures for K&N filters

Team Racing

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Don't Avonbar have hp listed with the K&N filters they stock? At least they did on the old site. I have no idea how accurate they are but there would be no harm in inquiring where they got the info

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

So, I did something magical, and picked up the phone! And hey presto, a nice man answered my questions. Isn't technology wonderful!

Anyway, they happily told me the formula for approximating the flow of a cone filter. Apparently, the formula is not perfect, but is a guide to start with. I’ve not sat down and tried to evaluate it myself - this is just what I was told...

Tapered filters only - all dimension in inches:

(((((Base Dia + Top Dia)/2) x length) x 6) x 3.142) = CFM rating of filter.

The guy commented that low to medium stressed turbo's are just as sensitive as nat-asp in respect of power loss from a slightly restrictive filter, but in higher turbine speed applications, turbo’s are very sensitive to even a slight pressure drop across the filter. Not sure what to make of that really - as there are a lot of factors that could come into play...

K&N always go for a minimum of 50% extra over the calculated flow rating (assuming 100% engine efficiency), but going at least 100% more (double the flow) is a better target and allows for losses from a reduced airflow around the filter - such as 1/4 of the filter being against a headlight or battery box (think cold-air kit setups)

The dinky little filter I am using has only a 259CFM rating at best, so it should have worked for the focus, especially at 4000rpm where it was already spluttering.
Incidentally, the one they use for their 'cold air kit' has a 570cfm rating according to their formula - which is double the CFM rating at 100% engine efficiency at 7000rpm.
I think it was suffereing from a few different issues why it seemed to not work – a freshly oiled filter that is probably restrictive, an airflow sensor that doesn’t like oil as Evo said, and a filter that might have been a little on the small side anyways.

Either way - what is 100% obvious is that I have been running too small of a filter in my mini, so maybe Father Christmas will be nice!

Oh. Final ditty of info they supplied – you should see no more than 1.5” vacuum in your inlet tract (0.75psi suction if you prefer to think of it that way). For very hi-performance applications, 0.5” or less is an ideal target.

Edited by turbodave16v on 20th Oct, 2009.

I have a spare map sensor, I'll connect it up, I think mine is too small, drity, sqaushed and a bit naffed.

So how do you work out cfm required engine cc x max rpm /2 plus boost?

Team Racing

That's pretty much it. yes.

kn filters is a bit of a no no on cars with MAFs as they tend to suffer when they get oil in them.