IDF filters

carl

True Classic
Those who read my raintray umbrella thread know I was messing with filters for my dual IDFs and finding ones tall enough to work with my velocity stacks but low enough to fit under the engine cover.

The first set of filter would fit with the raintray out. Then I got a second set from a friend

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Look pretty but don't have as much height as I would like from the upper lip of the velocity stacks to the cover.
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Here you can see the taller filter next to my new ones.

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I'm going with the shorter ones for now and hopefully I can get some dyno time someday and can see if the shorter filter has any effect on power.
 
It is difficult to tell from pictures alone, what is the difference in height between the top of the stacks and the top of the filter....as shown here:

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That looks way to close. In the recess of my memory I seem to recall you need at least the diameter of the carb throat as clearance for it to have minimal effect.
 
Here you can see the taller filter next to my new ones.

It looks like those filters are interchangeable and almost universal. And with the threaded rod attaching the tops of the aluminum ones, you could fit any number of taller filters by switching out for a longer rod. I wonder how many heights that base shape comes in? I like the look of these filter housings and I had bought a set for my Spider after my foam filter completely pulverised, but on the Spider they carbs sit too close, and the filter housings interfere... Are the IDFs spaced wider when mounted to the X?
 
Lots of options if you decide these do not offer enough space above the stacks. The filter elements are universal (so long as you get the same diameter) and come in all heights. So to maximize the available space under your engine cover you could find the best combination of components to allow the tallest elements possible. Those cast aluminum top and bottom covers are thicker than the stamped stainless ones, so using the stamped ones will offer a bit more room for filter elements. The stacks are also available in any height, so you could go shorter there. Even run without any stacks if needed. I vaguely recall there might be "offset" base plates that will sink the filter lower over the carb, but that would not help any - as that will not increase the distance between the stacks and top. If it comes down to a fractional difference, the center stud on the lower cover could be replaced with a nutsert, allowing use of a thin head bolt (e.g. truss or button head) from above (through the top). I believe you already mentioned the possibility of notching the rain tray. You could even tilt the engine slightly by adjusting the length of the dog-bone, to get the carbs a tad lower. Or even lower the bottom crossmember with short spacers to lower the bottom engine mount (lowering the entire engine) - obviously the snail mount will need to be offset accordingly. Like I said, lots of options; some simple some not, some cheap some not.
 
With that much clearance, it might be worth checking out whether or not those stacks are helping or hurting you.

On my DCNFs, it was a pretty tight fit under a 74 lid. I'm using a Sprint filter designed to go with their manifold. It was supposedly designed to max out the available height in a 74 X which is not much. When I switched to a 1500, that small change in deck height was enough to keep the lid from closing (moving the latch up solved the problem). No chance of using stacks except maybe the super short ones that look more like rings. To make more surface area for the short filter, they made one filter that covers both carbs with a large periphery.
 
Carl, I think the tallest filters would be best. But, without testing I can't say. I did select a taller filter for my racecar in hopes that it was better. I raised the cover to accommodate the taller filter.
I saw an episode of Engine Masters where they dyno tested a bunch of air filters to compare their effect on HP. The results were very interesting. Taller was not always better.
But since I doubt you want to dyno test your filters. I suggest you do some "real life" on the road testing. A seat of the pants evaluation may not be scientific, but it should be enough. When you're doing your evaluation, carry some evaluation tools like a stopwatch, tablet with an elaborate spread sheet, etc with you so you can tell the cop who stops you for speeding that you were doing science. That you're on a mission to discover the truth Might work?
 
Yes, the standard is supposedly the distance from the top of the stack to the underside of the lid should be at least the diameter of the carb throat and my current setup does not meet that. St this point I think I would rather use shorter velocity stacks. I have the filters from my 124 spider when I used these carbs (spiders have very limited space) and the filter base has built in rolled "stacks" so I might go to that. Realistically this is a suburban use poser so maximum air flow at red line is not an issue. I do want to get some dyno time but it's just to baseline the power and at least one run would be with the filter and cover off to compare. Due to the weather I have not driven the car with the current filters.
 
rolled "stacks"
There have been several tests performed on the different styles/designs of velocity stacks for Webers. For the most part the ones that have a very roundish/rolled mouth (lips?) test best - independent of length. What I mean by that is, for any given overall length of stack (RPM dependant), that style of top opening edge works best overall. But they are also the more expensive ones to get. Below are some random examples, is this like what you have from you 124?

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The distance from the top of the stacks to the bottom of the lid cover was 24mm, not the 38mm of the top of the carb venturi.

Just for grins I grabbed all my IDF filters and bases.

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Upper left is the "too short" filter base that we had been discussing above.
Upper right is a cork gasket that goes between the carb top and a filter base. Doesn't do anything for me as it does not affect the clearance of any velocity stacks.
Lower left is a filter base with built in velocity stack. This is what I used on my 124 spider setup.
Lower right is a generic IDF shorty stack that would go on top of a filter base. Can't use this with the upper left filter as the long retaining bolts would interfere, plus the stack inner diameter is much larger than the carb venturis so would cause a step.



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Decided to go with my old 124 filters with integral stacks. Distance from top of stack to bottom surface of the filter cover is the same distance as the diameter of the carb venturis at the top, 38mm. The stacks I was trying to use with the previous two filter housings have a 42mm diameter so they caused a step from the velocity stack base to the carb body top.

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All done and like many of my projects I came full circle back to my starting point! By the way, when I had the DCOEs on the rat I had huge filters and tall stacks because the assembly was located in the rear trunk.
 

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This is what I did on top of my IDF's. Its a 3D printed trumpet. The one on the left is my first rough idea, the one on the right is a later effort with a rolled over bell mouth. These are 18mm tall which gave me a tuned length between 3500 and 4000 RPM that you can feel. They sit under a 55mm tall rectangular filter.
 

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Hey Carl, I saw a Weber air filter modification that might be of interest. It allows better air flow when the filter assemblies aren't as tall as you'd like (due to height restrictions). They opened the tops and added a filter surface under them:
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Maybe something along these lines could be done if you want more air flow.
 
Off topic, but in the photo I just posted about the air filters, there are a couple of other interesting items. The engine oil breather can (top left corner), and the inline fitting on the coolant overflow tanks top hose. I wonder what that inline fitting is; one-way check valve, filter, quick disconnect?
 
Those screens look to have about the same filtering ability as the ones they put on velocity stacks. Looks like the housings are just used to hold the top screen in place. On my car, there is not much room between the top of the air filter and the lid so I don't know how much it would help (if at all), but I see that the rain tray looks gone on the photo so that could make a difference, particularly if the slots in the lid line up with the throats.
 
Doc, did you notice the Monza style fuel filler?
Those top screens look useless and I read somewhere that bare screens actually impede air flow.

If my FAZA snorkel works to my satisfaction then filter height won't be a problem.
 
Those top screens look useless
Ya, I wasn't suggesting to use just a screen like in the photo. But it gave me the thought that the tops of the filters could be opened up and a real filter element (a flat panel) could be added across there. In other words just like the sides, but also on top to allow more breathing where the air horns are too close to the cover. I have several sets of Weber filters; some have the K&N style elements and others have a foam element. The foam ones have a metal screen type panel that supports the foam element. That could be done on the top - a open mesh panel with a foam filter material over it, then the top cover over that. But I agree that the snorkel will eliminate the height restriction issue.
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If you search I'm sure you can also find a flat piece of the K&N type element that could be used as well.
 
On one of my spiders I had Uni pod filters, one on each IDF velocity stack (which were just vertical columns with no taper) . They self conformed to the underside of the spider hood. Tapered stacks make them difficult to use.
 
It's kind of interesting, foam filters have pretty much been the standard with off-road vehicles for decades, like dirtbikes and sandrails. But never really gained a lot of popularity for performance street cars. They certainly exist but are way out numbered by the K&N type. Vice-versa, the K&N type have lost popularity for off-road use due to inadequate filtering capacity in the harsh environment. Both are washable and reusable, and both seem to last about the same usable lifespan. However the foam ones are usually MUCH less expensive. As to which filters better, there are tons of tests that "prove" both are better than the other. Personally I think foam works better (at least as I found on all of the off-road vehicles I've had). However there are significantly more street applications available in the pleated gauze (K&N) type, so it is much easier to get them for a particular car.
 
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