Every time I read a message on Google Groups, I get a list of websites
that Google thinks might be applicable. I could spend hours reading all
of those sites...

But, from my experience working with filters in a cogeneration plant, I
remember that we had a large K&N filter in the ash disposal system of a
biosolids-burning waste heat heat boiler. The biosolids were dried
sludge from a sewage digester, and the stuff still had about as many
BTU's per pound as wood, according to the engineers.

But it left an ash, and that had to be vacuumed out of a baghouse
without running the ash through the impeller of the vacuum, so the ash
went through a cyclone separator and the big K&N filter that you could
stick your head inside only had to deal with the lightest particles
that the cyclone separator didn't remove.

There was a differential pressure alarm that was supposed to tell us
when the filter was plugged up. That would happen when the pressure
differential reached 0.25 PSI.

I would think that the pressure differential across your Honda's filter
would be extremely low at idle.

I believe it can get up to a few tens of inches of water.  That is, of
course, not a lot considering the value of atmospheric pressure measured
in inches of water.

That is why I don't ordinarily believe these claims of fantastic
horsepower increases with a fancy aftermarket air filter.  Even removing
it altogether shouldn't amount to that much of an increase.

Likewise, this stuff about how dirty air filters affect fuel milage
bothers me.  Since the eighties epa regs required carb float chambers
vented not to atmosphere, but to plenum between air cleaner and carb.  I
believe modern FI also measures ambient air pressure there, so fuel is
NOT SUCKED into manifold by pressure drop across air filter.

Don't know on a scooter, but in my spray booth, I change filters at 2.5
to 3" WC. This is where I loose the required air velocity (100fpm)
across the face of the booth. The easy way to check is to measure a new
filter and an old clogged one. If all you want is to find the lowest
restriction, you don't need a baseline to start.
The DP is really only important at normal operating load, however.

  >Newsgroups: rec.motorcycles.tech,rec.autos.tech
  >I would like to be able to measure the pressure drop caused by an
  >air filter.  Then I could compare different makes and models of
  >filters, and compare a new filter to a used one.
  >My air filter has a peculiar configuration that lets me put the
  >open end of a clear plastic tube on the inside of the filter while
  >the engine is running, without modifying the filter.  So I can set
  >up a U-tube of water to measure the pressure difference.
  >What would be a typical pressure drop (say at idle) for a 1970's
  >four-stroke engine that uses a paper filter?  Or where could I find
  >specifications or other data so I can know what I should expect?
  >Any units are fine: mmHg, mmH20, psi, atm, etc.  The vehicle of
  >interest happens to be a '76 Honda 360 motorcycle, but general
  >information is welcome.

If what you are interested in is what's happening when you're riding,
do the test on the road, in realistic conditions.  Air flow through a
360cc engine at no load is so low you'll play hell getting any useful
numbers.
Run your tubes to a handlebar mounted manometer and go down a
deserted road (with a friend riding chase).

I'd try to get a differential pressure gage called "Magnahelic" (sp?)
for this job - much easier to read its needle.

If it's only data you want, do it on the bench.  Suck air through
various filters with a vacuum cleaner, powered through a Variac if
you want variable speed.  Instrument for pressure drop, volume of air
flowing through.

If you just want the bike to run better, get a K&N (small version of
what Krusty had in his power plant).  When I was in the business,
they made them to fit your Honda.  Far better than the stock paper
grit-strainer.

Frankly, I don't understand the fuss about a Honda 360 - it's about
the most boring bike I ever rode - not much power, dull handling.
Maybe the brakes were pretty good...

Tom Willmon
near Mountainair, (mid) New Mexico, USA
(In the '70's, owner, Sun Cycle, Rockville, MD)

Net-Tamer V 1.12.0 - Registered

Ok, I must be missing something basic here....

Why not just use a run of the mill cheapie dial vacuum gauge from a
cheap tune up kit and measure it like 'ported' vacuum?????

At idle it is going to be really low, open it up and you get 'manifold'
vacuum.  Change the shape of the air filter and the ported will change
radically.  The manifold vacuum is the one I would be watching....

Or if you are really serious about playing around, just drill a hole and
tap a nipple into each manifold so you get the manifold vacuum at all
times?

Other than that, the vacuum cleaner will give you an idea of the
differences, but in real life, well....

I keep track of my gas mileage and when it starts to climb, I start
checking things like filters......

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's