pressure drop across air filter

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.

 

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.

 

That would be 6.9 inches of water.

Maybe I could measure a difference between a clean filter and a dirty
one at 3K rpm in neutral.

 

Ah, I don't usually use Google Groups, but I just tried it and I see
what you mean. I will look at some of those links ... thanks

 

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.

--
Gopher 33 28 19N 112 01 49W
'77 CB750K '78 CB750K
'00 ZG1000 '96 Ducati 900SS
**********pull 'mychain' to reply***********
("I've abandoned the idea of trying to appear a normal, pleasant person.
I had to accept myself as I was, even if no one else could accept me.
For the rest of my life I would continue to say precisely the wrong
thing, touch people in the raw and be generally unpopular. I had a
natural gift for it" W. F. Temple)

 

In a carbureted system, a plugged air filter acts about like a choke.

 

Sorry, what is WC? water something ...

I'm thinking maybe I can fill a clear tube with ATF and strap it to the
handlebars.

 

You could probably get reasonable measurements up to the first torque
peak around 4500 RPM and again at 9000 RPM. The midrange flat spot air
flow reversal of an engine might make the differential pressure look
very low
or even non-existant...

 

Doesn't look like they make one for CJ360T. I emailed them last night
and am waiting for a reply.

 

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

 

Getting an U-tube to work is simple and accurate.
The rest of the test is not, I think.

I have never seen any proof that K&N is broadly 'better' than anything else.

'Better' entails a lot of criteria. 'Better' at what?

You need to have a constant and measurable flow of air, for delta P to mean
anything at all. Low delta P doesn't mean too much about filtration
efficiency,
if you are interested in that, and if not just dump the filter and suck in
anything
that comes your way.

Filters often become more efficient at removing small particles when they
become least efficient at passing air.

 

I just ran such a test.

I have the left and right used filters, and I'm trying to determine
whether to replace them, as they run about $45 each off the shelf. I
was able to buy a NOS right filter on ebay for $16.50 delivered. I
tested all three of these filters.

I used a 2.0hp 6.0A model QPS20 wet/dry shop-vac attached where the carb
would be.

In "Experiment 1" I found that both the used filters developed a
differential pressure near 40mm H20 and that the new right filter
developed about 19mm H2O.

For "Experiment 2" I made some improvements in the vacuum setup (cleaned
the shop-vac, used a shorter cord, straightened the vacuum hose). Then
the DP was about 58mm for the used filters and 44mm for the new filter.

I could use some help in interpreting the data ...

 

Well, even the 58mm DP is less than a tenth of a pound, it's actually
about 1/3rd of the DP that would cause a pressure alarm across the big
K&N filter at the cogeneration plant. And your operating conditions are
nowhere near as dusty as the ash removal system I described.

As long as you now have a new filter, you should be able to see if the
right hand cylinder still makes the spark plugs sooty...

Then try NO filter at all and see what happens. If the cylinder stops
carbon fouling, you know it's an intake side problem...

 

WC water column, inches water. I forget the amount but it's something
like 1 inch mercury is 124" water.

Domestic gas pressure and filter differential are the only things I know
measured in WC.
--
Gopher 33 28 19N 112 01 49W
'77 CB750K '78 CB750K
'00 ZG1000 '96 Ducati 900SS
**********pull 'mychain' to reply***********
("I've abandoned the idea of trying to appear a normal, pleasant person.
I had to accept myself as I was, even if no one else could accept me.
For the rest of my life I would continue to say precisely the wrong
thing, touch people in the raw and be generally unpopular. I had a
natural gift for it" W. F. Temple)

 

Not if the float chamber is vented to the area between the filter and
the throttle valve. It only acts as a choke if float bowl vented to
ambient atmosphere.

 

That is a couple of inches of water. I am trying to remember what
atmospheric pressure is in inches of water. Seems to me it is abut 16
feet, right? That would be 192 inches. If so, those filters are not
lowering pressure at intake by all that much, as would be expected.
Most air cleaners really are quite efficient devices.

 

You're seeing a velocity squared effect...

The increase in delta p can be explained by the dynamic pressure of the
air molecules striking the filter medium at a higher speed.

dynamic pressure = 1/2 density X velocity^2

dynamic pressure is in pounds per square foot

sea level air density of dry air at 59 degrees F is 0.002377 slugs per
cubic foot

(a slug is a unit of mass density, multiply a slug of air times 32.2
and you get the weight of a cubic foot of air---it ain't much

velocity is in feet per second

 

The static pressure of one foot of water is 0.433 inches...

So 40 millimeters times 0.0394 = 1.576 inches

1.576 divided by 12 = 0.131

0.131 X 0.433 = 0.056 PSI

 

I agree...Pressure alone doesn't tell the whole story. Do you think he
could adapt a second
u-tube to measure the dP parameter of airflow through an orifice plate, and
correct it accordingly?