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.

Matt wrote:

> 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.

krusty kritter wrote:

> 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.

That would be 6.9 inches of water.

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

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

krusty kritter wrote:

> 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...

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

Matt wrote:
> 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.


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.

Matt wrote:
> 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.

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)

Don Stauffer wrote:

> 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.

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

G C wrote:

> Don't know on a scooter, but in my spray booth, I change filters at 2.5
> to 3" WC.

Sorry, what is WC? water something ...

> 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.

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

Matt wrote:
> krusty kritter wrote:

> > I would think that the pressure differential across your Honda's filter
> > would be extremely low at idle.
>
> Maybe I could measure a difference between a clean filter and a dirty
> one at 3K rpm in neutral.

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...

wrote:

> 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.

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