Bonneville America Forums Forums Technical Forums Performance Mods K&N Filter sizes -- Flow differences

So the Freak and many other applications use the RC-2890 and RC-2900, basically the same filter with a different flange offset.

http://store.knfilters.com/search/product.aspx?Prod=RC-2890

The Ghetto Freak instructions recommend the noticeably smaller RC-2340:

http://store.knfilters.com/search/product.aspx?Prod=RC-2340

How much difference in flow are we talking about? Is there anyway to quantify it? The K&N site doesn't really have that info.

In other words, if I wanted to go more compact with my air filters, would I have to rejet? I'm running RC-2890 and 2900 in the open air, no covers at all (unless it's raining).

Thoughts?

Friar, if you have custody of both examples, would a flow bench test at a auto machine shop maybe answer that question? Just guessing that is the easiest way to get a definitive answer. If a guy played with it, I used to have a flow meter tool to sync the carbs, (S U s and Stromberg s) on my Healy and a bunch of assorted TR s, add a constant vacuum source such as a shop vac betwixt the meter and tool, nah flow bench sounds better and you get real numbers to boot.

I went with the K&N RU-2750. It is a slightly larger filter, does not have the chrome tops, and does not have the offset built into the flange. And it fits with the British Customs Air Box Elimination Kit.

Basic math will tell you whether one K&N outflows the other - assuming basic shape is the same (i.e. Larger is better). On the other hand, if your motor only pulls 200cfm and your air filter supports 300cfm does it really make a difference?

I would imagine the larger filter given the same exposure to filterable contaminants over time would be the freer flowing of the two? Greater velocity through the carbs equal more power and economy and we all seek that.

There's no doubt the larger filters flow more, but is it more than we need? And I imagine the small ones would have to be cleaned more often.

Also, I have some GIGANTIC filters that I bought from a member here. They won't fit with anything under the seat except maybe brokenfixed battery relocation bracket. They won't even fit with my current custom battery box. I'm afraid they're so big they'll look ridiculous.

Didn't know that there are different sizes...I got the air box elimination kit and it came with the bigger ones...What is the flow rate of the carb ?, head ? and exhaust ?....one of them would be the limiting value....

I think if you measured the surface area of the stock uni filter, you would know in square inches what the bike needs "stock"

then you could take measurements of the freak filters and compare the numbers. Or course the variables change because you rejet with those.

But at least you can compare them in just square inches. Brand new filter flows X amount of cfm then dwindles from there with dirt. So if youre starting off big you'll be safe when its dirty.

I calculate free area of HVAC louvers and they basically go half the total square inch area for so many CFM. Of course these filters would be less than half the "free area".

At least we know that these bikes run well with the Freak filters. Get the measurements from them and see whats normal for a modified bike in surface area. And compare from there.

I got a duct-o-later sitting next to me, I cant tell you what a 3" duct could flow, but I'd need to know what kind of static pressure to look for...or velocity at least.

Anyone know?

After looking at the links and I looked at the pod's on my bike there are right and left filters (connection is off set). They have to clear the center frame member...will the ones that are not off set work?

The smaller ones are specified in the Ghetto Freak instructions, so I assume they fit. The larger ones that come with the Freak kit are the offset ones (which I'm running without the Freak brackets).

back when I was rejetting my bike and trying to keep the horseshoe attatchments I stopped at shop and the guy told me not to downsize the filters...more carburation the better

I am running smaller k&n's with nonrestrictive pipes 45/155 per calculator. Bike runs strong with no issues. I"ll try to find the part #. there are a couple of pics in my photo bucket.

Are those the RC-2340s?

Carbs RC-1920
http://www.knfilters.com/search/product.aspx?Prod=RC-1920

Crankcase 62-1090
http://www.knfilters.com/search/product.aspx?Prod=62-1090

I just got a new camera today. I'll try some pics tomorrow night