Air filters on a pair of DCOE 40 Webers

[jbcollier]

Pretty simple calculation.  Measure the wire width, spacing, and you can work out the percentage of area covered by the wire.

[jbcollier]

Here's an online calculator to simplify things:

 

http://www.wirecloth.com/calculators/

[MV8]

10 hours ago, Christopher smith said:

Does anyone have any dynamometer or other actual flow data on the stainless mesh or for that matter any air cleaner types? What were the cars out front for all those years in D Production using for their 1340 and 1500 cc powered pre-crossflow cars?

From least to most restrictive filter (and least affected by a dirty filter): 1) foam sausage, 2) two individual mesh covered foams, 3) tall pleated cotton gauze, 4) short pleated cotton gauze, 5) pleated paper, 6) screened throats.

[jbcollier]

Agree with MV8's assessment but would add:

 

- pleated paper provides the greatest filter area, filters out the smallest particles, and has longest useable life.

- screened throats filter absolutely nothing.

- just foam can be a significant fire hazard.

- oil-impregnated, pleated-cotton, gauze offers minimal surface area and minimal filtration.

 

That all said, given a Seven's design limitations, the latter would seem to be the best compromise.

[7Westfield]

just a reflection on the "screen" filters...

I maintain a vintage FF for a friend    years ago we were at a race, and it was running hot, ambient air way up there

I had put 10 / inch screen in front of rad for rock protection

-probably 3-4 inches ahead

 

yanked it out and got 15 degrees 

[Christopher smith]

I guess the reduced throughput of a screen is the reason why the ones for velocity stacks always are always rounded out to give more surface area. Maybe not enough however. Not sure

[SENC]

It's really just a surface area thing.  If the open mouth of a ram/trumpet is about 2.5 square inches, how much is that reduced by the space the mesh wires take up?  Obviously, bigger wires and/or more of them take up more space.  I'd have to go back and measure/count mine - but just looking at them I'd not be surprised at all to find they restricted airflow 30-40%.

[Christopher smith]

I looked around for info and see that some makers of mesh caps claim 2 or 3 layers of stainless mesh with what may be a paper filter layer sandwiched in between layers  to trap finer particles. They claim no restriction but I find that hard to believe. I also went to the McMaster Carr website to see their offerings of mesh which gives nice detailed info on wire size, wire configuration and a calculated open area for quite a variety of mesh offerings. When I can get to it I will measure the actual ID of my velocity stacks inboard of the wider area where the mesh caps attach to calculate area and compare that to  the somewhat larger area of the mesh and add that to the extra area provided by the  mesh side openings since the mesh is "belled out" a bit. I can then put a micrometer on the wire and maybe count the wires per area to calculate open area to compare with the smallest cross section area inside the velocity stack. Probably a waste of time but now I am curious. it would be great to see dynamometer data if anyone has it as that is the real test and sorts out any peculiar aerodynamic effects within the ports as well as within the carb throat and the stacks and a filter or mesh screen.

[SENC]

Chris, to be sure my previous response oversimplified.  I used 2.5 inches simply based on the 40mm barrel size of 40DCOEs, and estimating the barrel of the air horns/trumpets to be approximately the same.  I guess the real question of whether there will be "enough" air is to compare the open spaces of the filter with the restricted space of the choke/venture.  ie., if you are running 30mm choke (roughly 1.2 sq inches), you'd need to make sure the screen had at least that much open space to allow in enough air or you've just moved the choke restriction way upstream.  But enough air is only 1 piece of the puzzle, air flow and velocity another.

 

The bell end on the trumpets serves the purpose of smoothing out the air flow, allowing greater velocity/flow.  Another downside to the mesch/screen filters is that they oppose that design to some extent by attaching to and covering part of the bell end. Ideally, the bell end of the air horn would have at least one inch of "headspace", whether from the facing side of an airbox or from the back of the filter (of any type).  The screen/mesh filters obviously have zero headspace, as they attach directly to the bell end (foam filter tube are generally designed with headspace and aren't snug to the bell end, by comparison).

 

I think I've seen some testing results somewhere, will look through my files and see if I saved anything, but I think the knock on the mesh filters is that they both restrict volume of air flow (as does any filter, to some extent) and disrupt the airflow and the advantages of a well-designed trumpet.

 

It's also worth noting that length matters with our Webers, as they don't have plenums and, as such, require trumpets of the right length to help reduce air/fuel mixture getting blown back out and to help get the air flow velocity optimized.

 

Most of this understanding is from reading/study over the years - perhaps @jbcollier will come along and correct/clarify as it sounds like he has substantial experience in this area. 

Edited March 29 by SENC

[jbcollier]

Substantial my a**.  Some?  maybe.

 

First, my main complaint with most online discussions:

 

There are lots of anecdotal reports like: "I did this and it went slower." or "I did that and it went faster."  Seat of the pants is not a good way to make decisions.   Each change needs to be evaluated on an even playing field.  Any changes first require recalibrating the jetting before testing.  You removed/add something and it went better/worse.  Big deal.  Did you re-jet after the change?  If not, all you are doing is evaluating different jetting!

 

I did some digging and there is not a lot of good info online.  By good info I mean changes backed up by careful testing before and after.  I did find some charts and illustrations that explain the importance of the sides/edges of an air inlet:

 

 

 

These show the importance of the edges of the air horn.

 

Now look at this side profile of a screened air horn:

 

 

Flow along the edge/lip is disrupted/destroyed by the screen.  It's basically a cork.  What you want is for the screen/filter to surround the air horn but not actually touch it.  Here's an example:

 

[theDreamer]

OK, this item just showed up on the UK Facebook group Caterham cars and parts for sale.

Discuss.

[jbcollier]

For best symmetry, the rear one should be a tad longer...

[wdb]

Heaven help that carb tuner.

[Stevensonjr]

Cutting the bonnet and nose cowl to to fit filters on DCOE 40 Webers with short air horns.