Another of the DOC articles / Making an air filter

Technical help not Dutton or model specific

Another of the DOC articles / Making an air filter

Postby snowy2 » Tue Oct 12, 2010 1:51 pm

Making an air filter

I know that air filters are not too expensive to buy, but I had tried a K+N and two Piper cross air filters and didn't like either of them for various reasons. (The K+N was old and misshapen, and needed replacing, the Piper cross air filters split away from the base (twice). So I thought that I would look at making one.

I needed something to act as a frame for the filter medium which I decided would be foam air filter from a diesel Citroen (C15 van or Visa saloon they cost about £8.) the “frame” was a chip pan basket with the handle cut off. This makes a circular air filter about 7” diameter and about2 1/2-3” tall. I wanted the air filter to weather proof so rather than covering the whole thing with foam and then messing about with expensive air filter water proofing spray. I made a top for the air filter by using a plastic bowl of a suitable size from the kitchen and using the base as a mould, It was made in fibreglass into which I also moulded some ally reinforcing mesh. It does reduce the potential surface area for the air filter to work from but I figure a 3” high 7” diameter air filter will be enough, especially as it will be poking into the air flow above the bonnet.

The C15 van air filter is a foam tube about 6” long and 4-5” internal diameter, at first glance not good for our purposes. But being foam it will cut easily to any shape and it comes with a glue line already, so cut the foam along this glue line and lay the filter foam flat. It will form into an oblong shape made up from two layers of foam, the outer side will have a coarser texture. Cut both layers into two half's parallel to the longer length. Thus you will have two strips of foam about 3” wide and from memory 12-15” long. Using super glue, join them end to end to make one long double thick foam strip. (don't glue the layers together) More on what to do with this later. But you will need to keep and use both layers of the foam together.

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The Chip pan basket doesn't on it's own make for air filter frame, it needs to fixed to some sort of base, this was made from some ally sheet of about 1/8” thick (checker plate will do) Lay the basket on the plate upside down and draw round the basket rim and give it about 1/2” extra, then cut it out with a jigsaw. The hole in the centre of this plate will need to be a fairly close fit to the carb top and the holes drilling to fix it to the carb body. But this will be fairly easy to mark out and cut with the jigsaw again.

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To secure the basket to the plate at various points around the rim, where the basket meets the plate drill pairs of holes inside and outside of the point where the rim contacts the plate and secure with small cable ties, half a dozen will be fine. Then you will need to fit the foam to the outside of the basket, do this by wrapping the filter foam round the outside and then pulling it slightly to “stretch” the foam, if you over lap the foam and cut through both layers at the same time the join will be a perfect fit once glued. There will be no need to secure the foam to the basket as hopefully you will also be making a “lid” as I have done to hold the whole together.

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You will have a slight taper to the air filter when assembled, it being about ½” wider at the base to the top. To secure the top to the air filter I used two long 6mm bolts which go from the base to top, through two holes drilled to suit, and held on with nyloc nuts. You shouldn't over tighten these nuts as the wire chip basket support will easily crush. (although not in normal use) the base of the bolts can be secured in place permanently if they are threaded for their entire length by adding another suitable nut, fully tightened up to the ally base.

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Once the whole is ready to be fitted to the car, the air filter will be a bright yellow in colour, you can, if you wish colour it a differently, by a light spray from a touch up can. And once it is ready to fit you will need to lightly soak the filter element in engine oil and wring it out as dry as you can manage without damaging it. The air filter will within a few thousand miles be black with trapped dirt what ever you do. Performance when fitted to my car does not seem any different than any of the more expensive air filters I have fitted in the past. But I prefer the appearance of my home made filter.

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After a year of use I decided the filter element required replacing, a very easy thing to do now, and at a cost of only £8.
Legerra 2.0L pinto (currentcar)
Phaeton S1 project car (1 sold!)
Special loosely based on "locost"
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snowy2
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