Choosing an air filter
 

I'm tired of my current self-destructive pre-turbo intake setup. Yeah it doesn't like heat. Also I like to hear the turbo spooling. It gets nice cool air but I don't want to find out if a turbo likes deteriorating rubber. Please excuse the dirty engine bay.
Attachment 212836
Attachment 212837
Attachment 212838
Attachment 212839

So I've been looking at the K&N filters, nice big selection, but amazingly I don't see a filter that has a 60mm flange that is angled. So that seems to leave me with 57mm and 62mm, specifically one of these...
http://www.knfilters.com/search/prod...x?Prod=RU-1720
http://www.knfilters.com/search/prod...x?Prod=RU-1770

Do I get the 57 or the 62?
Are these filters big enough to support 250ish HP?

I don't care about hot air from the radiator.

I would say anything is better than the current setup. I take it you plan on going turbo-AFM-filter in a straight line? Depending on the size of the filter you might have clearance issues between the hood and near the radiator fan.

If you could, take the time to measure things and make a sweep back toward the firewall like alot of people do. But realistically, anything is better than what you have right now.

Vash-

Air filter is going straight on the turbo. I should prolly add something to my sig. I can't for the life of me find any 60mm pipe.

Is that like the tubing that they use for washers or dryers?

Nah, you can get this stuff from Autozone, Orielly's, whatever your local auto store is in the ricer section. It definitely doesn't stand up to turbo applications.

Im sorry but I LOLd at the tin foil/kragen intake pipe set up. It may sound crazy but I personally see merit in a plastic intake tube(like the company that used to make iceman inkaes used) plus PVC type tubing isnt expensive and comes in a huge amount of sizes and shapes. Idk though, metal seems to be how most people do it

Adhesive insulation tape ftw. I had to do that to keep it from melting.

PVC will get brittle with heat and I've never seen it in the right size. I can't say I haven't used it for a cold air intake on my old Eclipse gs though.

Well its prob not PVC, i dont know my plastics very well lol.

A mom and pop exhaust shop will bend you some tubing for your intake if you want a single piece crazy intake setup like that. If you want to do a intake with a cold air box heatshield i suggest getting a pretty small filter like what you see on Saml01's car and the filter offered in the BEGI kits.

PVC is not a good option, it doesn't withstand that kind of heat very well and it emits some kind of vapor that is bad for O2 sensors I believe.

Ah thanks. Well the question now is... My turbo inlet is 60mm, should I get a filter with a 57mm flange or a 62mm flange?

Looks like AC and PS is out, I'd clock the compressor and mod the charge piping to fit. Leaves plenty of room for an intake when you're done. :)

Look at dwights car. I think you will like the airfilter he is using. We can get some piping and make something of it :)

If it were me, id do a 45* coupler at the turbo, length of pipe, 90*, then the filter tucked up over the lower rad hose in the wheel well.

The turbo intake is about 2.5" so a coupler and this should bring it over to the shock tower and than add filter.

http://store.summitracing.com/partde...5&autoview=sku


PS: nice intercooler.:)

I used to have that same hose :giggle:
Attachment 212834

I currently use a K&N RU-5111 and route it to the wiper cowl via 2.5" tubing.
Attachment 212835

https://www.miataturbo.net/forum/showthread.php?t=16531
My cowl CAI setup. Very similar to what SamS did above.

OP asked about a filter supporting 250whp...
Does/can the size of the filter affect anything??? I'm using a pretty damn small one, its KNN but I think its for a MC or snowmobile or something....

I know he did, and there are two K&N filter part numbers listed in the link I posted, the one I used and the one Sam used. Here is the one I am using
http://www.knfilters.com/search/prod...x?Prod=RD-0720

According the the K&N sizing guide this is large enough for 250 HP.
If the filter is too small, there will be a higher pressure drop across the element that will effectively choke the turbo. It is not an all-or-nothing thing, but gradual. Go here for sizing formulas:
http://www.knfilters.com/filter_facts.htm#SELECT
The way I did it is I doubled the engine displacement to simulate doubling the volumetric efficiency under boost.

Short answer is, get the biggest one you can fit in the available space WITH room around the outside so the air can flow into the filter. See my CAI thread for an example of what NOT to do with allowing space around the filter.

Thanks for the show of pics and info. I did the math on the sizing and found that even the small filters I listed can support the power, barely.

97.7 x 7000 / 20,839 = 32.8 sq in. required for minimum surface area.

RU-5111, 4.5" x 5.75", slight taper to 3.5" at tip.

4.5 x 3.14 x 5.75 = 81.24 -.75 = 80.49 sq in.

this is not factoring the small taper, but I doubt it will lower 48 sq in of area.

Brain I don't agree with your # for displacement. I believe this formula is intended to calculate a maximum acceptable volumetric air flow rate per unit area (flux?) based on displacement, RPM, and an assumed volumetric efficiency for a naturally aspirated engine. With a turbo, say running at 15 psi, the flow rate through the filter would roughly double for the same displacement and RPM. So, for a 1.8 liter, I think the displacement should be ~ 110 cu. in. x 2 = 220 cu. in.
Or ~ 195 cu. in for a 1.6 liter.

The same volume of airflow is drawn in regardless of intake manifold pressure. The air is compressed; your cylinders don't act like balloons. Your displacement NEVER increases, cap bold italic. If the VE increases, use a multiplier of 10 - 20% or so.


Regardless, the formula I used (provided from K&N) is simply relating volume and rpm to minimum surface area.

Dude, I KNOW the displacement never increases :) BUT, the flow through the filter does increase for a turbo versus naturally aspirated. No question. The pressure drop across the air filter is a function of flow rate. So if the flow rate increases, the pressure drop across the filter increases. No question. The point of the K&N formula is to help keep this pressure drop within reason by specifying a minimum size filter element area for a given airflow (determined by engine displacement and RPM) based on a NA engine.

Since the formula does not allow one to input flow rate or peak engine power directly, one has to compensate for the increase (turbo) in airflow somehow. I estimated this increase by doubling the displacement in the formula. One could just as easily multiply the whole thing by the ratio of boost to ambient pressure. Hence I used 2X for 15 psi of boost (atmospheric at sea level is roughly 14.7 psi.). This is conservative, that is, 15 psi boost is not going to exactly double the flow, but it is a worst-case estimate. It is certainly more than a 10-20% increase in VE.

If you do not compensate for the increase in flow from the turbo, the filter selected based on the formula is going to be too small per K&N's guidelines. Seems obvious to me, maybe I am not explaining this correctly. Hopefully this explanation makes more sense.

You are absolutely correct. Sometime in the past year, K&N dumbed down their sizing formulas. From what I recall of the older equations, which I have here circuitously scribed into an excel macro that I stupidly provided no documentation, it could be approximated just as you describe.

I used to use a cylindrical paper filter for a late 80s Prelude. While the area was about triple what K&N suggests as the minimum size by their current N/A-based calculation (granted a K&N obviously is more free-flowing than paper, but regardless), and still satisfactory using their old equations, it definitely didn't flow enough up top, or in vacuum. Switched to an appropriately sized K&N, and off-boost response is better, and top-end isn't joked (at least on the intake side) anymore.

The pleated K&N filter material will flow 6.03 cfm of air per square inch, says them.

if this is the case, the RU-5111 I posted above should flow something like 450CFM.

the volume air flow at 7000RPM on a 1.6L at 2PR would be 395CFM give or take.


I was incorrect before, im silly like that. however, on the dyno, I've seen a car with a 2" intake tube and a tiny little motorcycle filter make 190rwhp at 10psi or so. As a test I had him remove the intake tubing all together and run it open at the turbo, as i thought it might be choking him off up top...there was no difference in power output.

Interesting... Always good to hear real-world test data like that. This would seem to indicate the K&N formula is conservative.

Ok so realllllllly sorry for the ignorance, but ever since i can remember formulas/equations and me just dont mix...I just cant seem to ever get the same answer twice...

This is what I'm using...
http://www.knfilters.com/search/prod...?Prod=RC-70040

Am I choking my chicken?? Eggh I mean turbo

Did you happen to see both AFR logs for comparison? I'd be curious to see if he went leaner up top without the intake.

IIRC it did go leaner....here's the two runs, and yes we corrected the lean mid-range.

Bryce why dont you try this. I found my old filter set up.