CFM reality - or why k&n's really suck...

   reality is a bit different than marketing approaches...

this is the formula to figure out how big of a carb you need and how
much air flow you really need...

                              CFM = (rpm x displacement)/3456

       ( 4,000 rpm * 360 cubic inches ) / 3456 =  416.67 CFM

at peak torque on an AMC 360 you only need:
       ( 1,700 rpm * 360 cubic inches ) / 3456 =  177 CFM

  So the 290 CFM carb that comes stock on the '80's vintage Wagoneers is ok...

discussion on airfilters and CFM

fsj-digest           Saturday, June 10 2000           Volume 01 : Number 885
Forum for Discussion of Full Sized SJ Series Jeeps

Date: Sat, 10 Jun 2000 00:35:03 -0500
From: "TLynn" 
Subject: fsj: Re: 16.02 mpg...

How do you compare the amsoil filter to the K&N?

- ----- Original Message ----- 
From: "john" 
Sent: Friday, June 09, 2000 23:32
Subject: fsj: 16.02 mpg...

Superdawg is fluctuating between 14 and 16.3  
average is in the high 15's.  Still haven't gotten the
amsoil air filter installed yet...

got the timing set so it just barely pings under load, if I
downshift it's fine...  so I figure I'm right on the edge of
advance... :)  

Maybe mr. carb will be able to touch it tomorrow and make it all
better. :)

- ------------------------------------------------------

  ...don't leave life without Jesus, please!
 Snohomish, WA - where Jeeps don't rust, they mold...
- -------------------------------------------------------


Date: Sat, 10 Jun 2000 00:22:52 -0700
From: john 
Subject: fsj: Re: 16.02 mpg...

At 12:35 AM 6/10/00 -0500, TL wrote:
>>Superdawg is fluctuating between 14 and 16.3  
>>average is in the high 15's.  Still haven't gotten the
>>amsoil air filter installed yet...
>How do you compare the amsoil filter to the K&N?

let's see... how would you compare a spaghetti strainer to
a fine cheese cloth?  One will let certainly let much more pass through it...

Question is, do you really want to let more pass through than
your engine is capable of ingesting anyway?  :) (k&n has some
incredible cfm specs... until the filter is placed in service, then
it drops off pretty rapidly as it clogs up, almost as quickly as a paper

When I make this comparison I'm thinking about what kind of dust,
particles and other abrasive items that are allowed to pass through
the oil-wetted gauze that k&n uses.  amsoil uses an oil-soaked foam.
I've got a video where an independent lab tested the three types of filters.
The flow rates are shown in the URL just below... but what isn't shown in
the chart is what passed through the filters.  The amsoil had nothing, the
paper a little bit... while the k&n passed all kinds of stuff...  

You can see through the gauze material quite easily, and if you clean
it improperly, which is easy to do, you can loose a lot of protection.
The amsoil foam is denser, yet doesn't clog up, holds much more and
flows more air longer.  See the chart below for a relative comparison.

 (this is an old chart, I've got a newer one around somewhere, but
the basic comparison is still valid.  I think all the filters compared
have been improved, except maybe the paper one...)

The k&n claims some incredible CFM numbers, more than the amsoil,
when the filter is fresh.  But as the k&n, and paper, filters gather
material they drastically reduce their flow, while the amsoil filter
continues to flow.  The reason for this is simple, the amsoil foam
has more avenues for air, and more capacity to hold dirt.  Regardless
of what the specs might show, it does not restrict air flow to the point
where it would make a difference.  Do the math on cfm sometimes...  it'll
point out two things, most engines are way over carb'd and your air filter
ain't gonna slow you down much... ;)

Problem is I don't know of ANY engine, short of something you'd see
on a Boeing produced item with wings, that would draw the air/cfm flows k&n
specs anyway.  An amsoil filter will flow more than adequately
for any engine out there... and if it does become a limiting factor
you could modify your air intake to stack more than one on top
of each other.  :)  Or, if you really want unlimited flow, ditch
the air filter all together. :)  (which isn't too far away from
what happens when you run a k&n any how... ;)

The bottom line is that when all the factors are considered the
Amsoil filter is by far the best value, filters the best, breathes
the best over the entire service period, is orders of magnitude
easier to clean, and protects your engine.  To clean, you use soap and
water and basically wring it out, place tack oil or regular engine
oil on it, put it back on the metal cage and reinstall...  no special
cleaners, no worries about using air or a garden hose on it...  it's durable
and replaced without the slightest hassle if you do somehow manage to damage
it!  (I've done it once... ;)

Paper is the next best for engine protection.  The problem is, 
as shown in the chart, it clogs up pretty quick and then quits flowing.
Then you have to throw it away... :(  bad for your wallet and not good for
the landfill.

On most engines changing the paper element will increase your fuel economy.
I've been running amsoil filters since 1983, the reason I decided to
become a dealer, and while I tend to neglect my vehicles a bit, you know
like not bothering to clean the air filter on my olds diesel for three
years because the stupid thing was supposed to have died already... ;) and
when I finally did remove the bugs, weeds and all the crudded on dust,
it didn't make a bit of difference in performance or mileage...  which is
remarkable considering how drastic the performance was btwn paper and the

When you consider the cost of the filters it makes sense to go with
an amsoil filter too.  First off they're lifetime guaranteed (not only
a million miles... ;) and then they cost about half of what a k&n costs.
Prices vary on the various sizes, but if memory serves me correctly (which
I don't trust very much... ;) it seems that one filter I looked into
was about $10 for a paper, $25 for the amsoil and about $45 for the k&n.

I ran across a situation once with a Mercedes where the cheapest, lowlife
brand of paper filters I could find was about $45, while the amsoil 
filter was $20 or $25!!!  k&n didn't have one for that application.

I've been forced to buy a k&n once because there wasn't an amsoil one...
after cleaning it one time I fixed my problem by taking some amsoil filter
material and wrapping the k&n. ;)  Superdawg came with a k&n filter on
the holley... first thing I did was fabricate an amsoil filter to go
over the top of it.  :)  Absolutely no change in performance or mileage
by placing that "restrictive" material over the k&n... ;)

I'm sorry to have gone so long on the reply... but I'm kind of passionate
about the air filter thing.  I truly believe it's the best value in the
auto parts world.  It filters the best, flows the best over the entire service
period, is easy to care for and is affordable to buy.  

Oh yeah, the performance issue,
the amsoil filter improved my 0 to 60mph time tests in that olds Diesel I
mentioned before from 26 seconds to 19 seconds!!!  A 5.7L Diesel tends to draw in a lot
of air...  I've not experienced such a gain in any other vehicle gas or
diesel...  but that SOLD me on the amsoil filters so much that I signed up as a Dealer
and I've had an amsoil filter on almost every car I've owned since then... :)
You know, I think I may still have the air filter from that olds...  :)

I finally killed that olds in 96... 230,000 miles on it...  and they say
the 5.7L diesel wasn't any good... it'd still be running if I hadn't "fixed" the transmission...
rofl... I found out that Diesels don't like going from drive to first at 65 mph...
oops... I guess I put that little fork thing on the wrong side of that one little
part...  drat...

hey, for grins, here's the formula for CFM,

                              CFM = (rpm x displacement)/3456

for example, the maximum torque of an AMC 360 is around 1,700 rpm, so let's
run the numbers at that RPM...

(1700 x 360)/3456 = 177.0833  CFM   say, what?  so you mean to tell me that
the 290 cfm carb that came stock on the 81 Wagoneer was adequate??? ;)

still, that seems kind of low... so let's bump the number up to say 5,000
rpm...  (don't try this at home kids)

(5000 x 360)/3456 = 520.833333333333333333333333333333 CFM 

Of course, you'd probably design your setup for your maximum torque because
that's where you'd have the best volumetric efficiency...  (well, that's what this book
I've got here says ;)

Now, if you look at the k&n specs, you might see a cfm rating of maybe 900
to 1000cfm...  and they'll claim that the amsoil filter only flows at 700cfm or something
like that...  and a paper element at 400 cfm or thereabouts (again, those numbers are
from memory and from a k&n catalog...  I really don't care about the exact numbers because,
as you can see by doing the math, it doesn't matter...)

Anyway, my point is we don't need that much steekin' cfm no how.  

ok, one more... (can't help myself)
the 4.0L has peak torque at 2,500 rpm...  (2500 x 242)/3456 =
175.057870370370 cfm

      no, wait... one more... (gotta check superdawg out)
 stock 4.2L for 83, max torque at 1,800 rpm (1800 x 258)/3456 = 134.375 cfm...
    (see why a 500 cfm holley on that puppy was a bad idea?  and why my mileage
        went from about 11 to 16?  :)

I really don't care if you buy an amsoil filter from me or not.  I just
hate to see friends pay so much for so little in a k&n.  The k&n is
way overpriced for what they are, and if you live where there is dust you may
actually be shortening the life of your engine... the amsoil oil-soaked
foam breathes plenty, much more than paper, and that's what the guys that designed the
engine used in the first place... I hate to idea of throwing the paper elements away and not
being able to clean and reuse...  and I really don't like cleaning the k&n
filters...  sure, they seem to work ok...  seems like the guys that have a k&n like 'em
(except me)... but most of 'em have never had an amsoil filter either...   (if ignorance
is bliss, then some of us must be in paradise... ;)

(the formula came out of the auto math handbook by hp books)


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