High rpm power loss problems, regards to air filters

Discussion in 'Classic Mustang Specific Tech' started by daveoxide, Dec 12, 2003.

  1. I have a 67 Mustang with a mild 351w underhood. I have recently been experimenting with different air filters, mainly a cone style filter adapted to connect up to a carb and the original stock style 14" round filter.

    I built up a little piece out of sheet metal that goes over the top of the carb (edelbrock 1405) and it has a 3" diameter inlet tube to it. I then connected a 3" diameter flexible tube to the inlet and ran the tube to a conical air filter located on the fender well. The conical filter has the same effective filtering area as the standard 14" filter I have always been using.

    Funny thing happened with the conical air filter setup, at high rpm's, past 4000rpm, it would surge and just basically lose all its umph. I thought "hmmm, I wonder if its restricting air flow?" I disconnected the "carb bonnet" and tried it again with no filter..... did the same thing, surge/loss of power above 4000rpm. I went back home, figuring I must have broken something, and put the old standard 14" filter and its assembly back on the motor and went out and tried it again. Goes like a rocket all the way to 5000rpm! Huh? I can understand that my design might do more harm than good, but shouldn't no filter be better than a standard 14" round filter?!??!

    What is going on? I am very curious as to what is happening under the hood. If anyone has any past experience or knowledge of this subject, I want your insight! I am just plain dumbfounded by what happened. And I repeated the tests atleast 3 times with each setup, and all the times with the conical filter and no filter it ran out of steam at 4000rpm, but with the stock filter it flew to the moon!
  2. 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. :nice:

  3. 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.
  4. :shrug: 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.

  5. 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
    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..

    :banana: :banana:
  6. 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.
  7. The effective flow of the 14" (it is actually only 13" in diameter :shrug:) is 612cfm by the use of your formula's (the filter is 2.5" tall, so your were close). The flow of the conical filter (it does have a smaller area than the standard filter) is 546cfm. The standard filter is paper, whereas the conical is a reusable "high flow" type, so the cfm of both could be very similar.

    Also, I try not to rev past 5000rpm :( , I need the thing to last and plus it just starts making more noise than power past 5000rpm :p.

  8. I have an O2 sensor in the exhaust, and I have a mixture meter that I hooked up too, but alas, the darn O2 sensor has bitten the dust :(, so no mixture reading for me.

    But I somewhat agree with you, I think it is going lean, like it is flowing too much air. But I shall get another O2 and slap it in there and get a reading with the mixture to see what is happening.
  9. 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.