High rpm power loss problems, regards to air filters

I always heard it had to do with how the air flows better with the filter base on. Kinda like a very short velocity stack (like on the webers-- those metal horn looking things on top). Some people think that w/o a filter the motor will have more power, but I think it's one of those counter-intuitive misunderstandings.

BTW, I love your site. I had bookmarked it a long time ago-- the vids are cool.

--Tim

Could be the air gets better flow efficiency through the old filter, like at high rpm the air resonates properly and flows more than 100%, whereas with the other setup the tube is too long and causes slower resonance.

Don't know the tech terms, But it has to do with the air pressure
flowing over the top of a unsheilded carb.
A sheild of some sort will beak up the airflow allowing the eng to draw air in.

PB

sounds like the bonnet is restricting it

an easy way to see if your filter is undersized is to do a run with the filter removed from the inlet pipe,, (just don't do it down a dirt road) if the flat spot clears up,, the filter is too small,,, if you still have the flat spot then the your home made air intake is the restrictor... that is the on the car, lets try it method.

If you're one to try and figure out what you will need before you do it, and know it will work, all you have to do is the math:
So how much air does your engine need?
Engine CFM=(CID x RPM/2) divided by 1728
(351 x 5500(guessing at your redline)/2) divided by 1728
(1930500/2) divided by 1728
965250/1728
Engine CFM=558.6

the filter CFM formula is the H (heigth)x L (length) x 6
or substitute the C (circumfrance) for the L

so in your case with the 14 inch round filter, you need to figure out the C. Which is done with geometery. C = pi x D (diameter).

C = 3.14 x 14
C= 43.96
guessing that it's 2 inches high, the numbers plugged in comes out:

filter CFM = 2 x 43.96 x 6
filter CFM = 527.5

so the math shows that 14x2 round size of filter would be just slgihtly too small..

using the same formulas, you can figure out what your conical filter flows..

The other half of the formulas that are used are the Effective Filter Area Required, (EFAR). The formula for this is:
EFAR = (CID x RPM) / 25,500
EFAR = (351 x 5500)/ 25,500
EFAR = 1930500 / 25,500
EFAR = 75.7 square inches

and then the size of your filter is:
C x H = EFAR
43.96 x 2 = EFAR
87.92 = EFAR provided by the filter

This is probably more complex than most worry about,, but if you want to do it right, I gave you the roadmap for it!!!! and if you made all the way through that, you deserve a banana or two..

Sounds to me like his motor's leaning out on the topend with the long pipe and conical filter. Run it again with it on then , immediately pull a few plugs and read them. This will tell you exactly whats happening.

I noticed the same situation when I was having carb problems awhile back. I have a 14 inch drop base air cleaner. I was adjusting the carb with the air cleaner removed but when I replaced just the base of the air cleaner, the idle speed would increase in rpms, so now I adjust the idle speed with the air cleaner base installed. Must be the flow of the air into the carb as others have mentioned.