Re: low-flo filters

[Scott Fisher <sefisher@cisco.com>]

Chris Dyer writes:

> [...] they're cheap and any 3 year old can install them.

Yup -- any older than that, and their hands are too big to fit in the
space around the air filters on our cars. :-)  

In general, air filters are easily frobbed because they're the first
link in the chain and therefore there's not usually a lot you have to
take off to get at them.  But that's a silly reason to change something
on a car.  The *real* reason you change something is because it's the
*weakest* link in the chain.

> But from everything I've experienced & read (including many magazines' 
> dynometer readings) these things essentially do nothing.  

Hi-po air filters got a reputation in the Sixties and early Seventies
with U.S. V8s.  On many of these cars, you had a 400-cubic-inch (call it
6-liter-plus) engine sucking air through a 2-inch (call it 50mm) hole. 
On many such cars, the air filter was an impediment to performance, and
removing it or replacing it (or just cutting a bigger hole) made an
easily measured difference, because the air filter really was one of the
"weakest links" in the system.  For example, the K & N air filter on a
'65 Chevelle Super Sport removes a large metal hatbox with a 2" hole,
and replaces it with a 17" metal disc that has a circumferential filter
about 3" *high*.  HUGE difference in available airflow, and the effect
on 1/4 mile times is measurable, even on an otherwise stock engine. 

But *is* the air filter the weak link in a typical I5 engine?

It certainly *looks* like the weak link, because it's got that teeny
little intake hole going into the filter box, not much bigger than a
single intake tract in the plenum.  But to give a quick reality check to
the dimensional suitability of the stock intake... I'll bet it's the
same size as, or larger than, the combined area of the throttle body's
butterflies.  And it's *certainly* bigger than the curtain area of a
single stock intake valve.

> HOWEVER, if going "full bore" on hp enhancements, these filters can be a 
> "necessity".  An engine is a pump, so if your adding all sorts of engine 
> goodies to make a better pump, one needs more air, hence the lo-flow 
> filter.

Exactly -- it's a pump, and more to the point, it's a *system* which
pumps air in and out, with various contaminants on each side.  All the
parts have to work together; often, even if you *do* put in a properly
functioning high-performance part, it may have no effect or it may have
the reverse effect, because it throws off the balance between the other
components in the system.  That was often the case with headers, which
change an engine's breathing characteristics so much that, unless
compensated for by mixture, timing and possibly cam changes, they can
yield an engine with drivability problems that far outweigh any actual
performance gains.  System, system, system.

The best way to find out the weak links in an engine is to take each
component out separately and put it on a flow bench.  Measure the
airflow through each component individually -- from the kewl cone-shaped
air filter to the coffee-can sized exhaust tip -- and then see which one
flows the *least* effectively.  Then work on that one.  Then the next
one.  Then the next one, and the next one, and then sooner or later you
start again because you'll run into a new least-effective component. 
But that isn't something you can sell in a magazine or over a Web site,
and it isn't something a three-year-old can install with garden tools.

That's not to say it isn't worthwhile to try to make cars go faster.  On
another enthusiast list I used to belong to, we had purists who would
look down their noses at anyone who wanted to change *anything* that the
Factory, In Its Infinite Wisdom, had chosen to put on a car.  That's
just silly; every car, from the R8 to the Hyundai Elantra, is a box of
compromises, whether they're dictated by budget or rules or track length
or buyer demographics, you choose what you give up and what you want to
get in return.  And some of us want to have a car the way *we* want it,
not the way that the Factory (or more likely these days, the Factory
Marketing Department's Focus Study Groups) decided it should be
delivered.  Especially to the U.S., but that's another rant...

So, does anyone have any hard data about the flow restrictions in an I5
air filter box?  By hard data, I mean "We put the filter box from an '87
5k on a flow bench and measured 240 cfm, then we put a Brand K filter in
it and measured 390 cfm."  (And *where* did I just see the conversion
factor for cfm to bhp?  That is, the way of guesstimating that a 10%
increase in airflow would mean an x% increase in horsepower?  I remember
that it's less than 1:1...)

--Scott