Recommended breathing room for velocity stacks

[z boy mn]

Hey everyone, I am replacing my air filter set-up on my Z, which has triple Weber DCOE 45s on it and need help selecting the right air horn (velocity stack).

I'm moving away from the airbox that uses a single 3" inlet and moving toward individual filters for each carb. I understand the carbs will be sucking in warmer air even with the heat shield, but some time on the dyno a while back taught me the car preferred warmer air as long as there was plenty of it.

So, I have the stock air horns (velocity stacks) that came with the DCEO 45's, which are 2.5 inches tall (outside the carb) or 65mm, and even though I got the tallest filter available, there's only .5 inches between the end of the horn and the filter housing. The horns come in a variety of lengths, starting at .5 inches all the way to stock, 2.5 inches, in .5 inch increments (except for 2.0"). I did some reading here and in the books I have, and the conclusion seems to be is that longer is better (for both air compression issues and breathing), but I can't seem to find a practiced recommendation for so how much space is needed? Most discussions recommend "1" or more" but how much is "more?"

The next size down is 1.5 inches (38mm), which would give me 1.5" of clearance...or should I go all the way down to 1" (26mm) for 2" of clearance? They're NOT cheap, so I'd prefer to just buy one set. I've included some links below:

Cheapest and widest selection of velocity stacks: http://www.carburetion.com/sitesearch.aspx?category=DCOEstack&Title=Stacks for Weber DCOE Carburetors

Example of the air filters I have: http://www.carburetion.com/Products/ProductDetails.aspx?part=99217.532

Any help or suggestions are VERY welcome!

Edited January 15, 2017 by z boy mn

[sweatybetty]

what exactly are you trying to accomplish? something that looks cool, more power, or something that filters the air?

[z boy mn]

Looking just to optimize what I have without any heroics. Filtering air is important as it's mostly a street car I'd like to last a long time.  I decided to try this route when I looked at the K&N filter at the end of the air box and it was all beat up from the engine shifting during revs and banging against the hood, etc. The dyno results were convincing, too, as I suspect the air box isn't flowing air like it should (I built it and it's basically just a box).

[gnosez]

I have used 100, 50, 30 and 10mm for my 44mm Mikunis with the 50 and 30s giving the best overall power with an ample amount of space for the filter depending on the engine (L24 & L28 with different heads and cams)

[z boy mn]

Thanks, gnoze, I was thinking the 1" (26mm) would be a good middle ground and give lots of room inside the air filter, and it's right in the middle of what you experienced, too.

I'm running a stroker motor with a ported p90 head, mikuni intake, headers, and a mildly agressive cam. I also had a crazy idea of maybe looking for a way to "make" a bigger air filter by gluing two together (a 3.5" and maybe a 2") to make a thicker one, but I think smaller horns are going to be more stable. 

Finally, I did measure the length of the runner, including carb to the head, and with the 2.5" horn it's 12"-13" (some have said that 12" is ideal)...so this going to a 1" horn till seems within the margin of what's OK. 

[RyanT67]

Jenvey (or Fox Injection in the USA) sells 40mm tall air horns at a very reasonable price.  Those, coupled with an ITG Megaflow JC100/65 air filter, would be a good combo IMO.  You would get a top quality air filter, maximum breathing, an appropriate length of air horn, and enough clearance over the mouth of the horn to not impede air flow.  

Total cost would probably be around $200 for the air horns, and $300 for the air filter and base plate.  Not inexpensive, but a good uncompromised solution.

[z boy mn]

Thanks, RyanT67, I checked out his stuff and it looks beautiful. Unfortunately, for now it's out of my price range but I did see some videos on making your own air box from carbon fiber and/or fiberglass. A fun view. 

As an update on this topic, I'm leaning toward the 1" horns as they will offer a lot of clearance and also some benefit stil. I talked to a representative at Carburetor.com and they also have a reasonably favorable exchange policy so it could be I order multiple sizes and find out which works best with the filter. I'm not sure I'll rent more dyno time with the different horns installed, but that's the best way to find out for sure. I'll try the "seat-of-pants" dyno test to start...and I have an air/fuel gauge to help monitor how the fuel is burning. 

Finally, I found an article about air flow hat wasn't exactly helpful to a non-engineer but an interesting read on fluid dynamics as connected to air horns. I'll attach it.

RET_Bellmouth_Sept.pdf

Edited January 17, 2017 by z boy mn

[rossiz]

i'm a little late to this thread, but a key point when thinking about air horns is that they are designed to assist w/air flow and really only work optimally when that air is still (in a big air box). you will have very different conditions driving at speed with air rushing past them (through the filter elements) horizontally than when on a dyno. i spent quite a bit of time testing various configurations on motorcycles and found that just about any change in the airflow/turbulence made a measurable difference. for example: on a ducati, the carb air horn on the front cylinder points straight forward and sits behind the front tire. i was not happy about the prospect of filth flying off my tire and being gulped into my carb, so i made a round aluminum plate and mounted it 1" in front of the horn off spacers - immediate measurable drop on the dyno, even though the free air around the plate was larger than the diameter of the horn. worst possible application is exposed horns w/metal screens - aside from poor filtration, the screens cut free air by quite a bit. i did tests w/multiple sizes of screen and was shocked at how much restriction there was - power loss was significant and the carbs needed to be re-jetted for the revised flow.

so that's why open horns inside a big air box (bigger is better) w/a single filter at the inlet is best for performance - provides a large volume of still air to allow the horns to do their job. you can get fancy with ram-air scoops that pressurize the air box at speed as long as they are ducted to keep the air inside the box as still as possible.

of course all of this is pretty "academic" b/c the performance gains/losses i'm describing are pretty small in relation to the power/weight ratio of a car vs. that of a bike... but i figured i'd pipe in.

[z boy mn]

Rossiz, thanks for your update. I've been working on this since the posts began (well, mostly thinking). And, I've come up with some ideas to try. Overall, though, my thinking has landed on the following (supported by your own dyno runs):

1) Longer (i.e. stock) air horns/velocity stacks are best...both for resonance, runner length, and air conditioning (streamlining, compression, air speed, etc.). This was confirmed by a few folks I know at Redline, Top Performance, etc. and others who like to ponder fluid dynamics. So, decision 1: keep the 2.5" horns.

2) More air is better. I looked into the generic air filters which are 3.25" deep and the K&N ones which can be 3.75" but both sit really close to the air horn (.5" or .75"). This seems to suggest that there would still be a restriction, especially at max RPM) made worse by the environment inside the air filter as valves open and close. As the opening of the air horns is 2.5", I'd like the same space between the end of the air horn and the filter surface. In plumbing, that's how you'd do it, yes? It's true that air "turns" more quickly than water as it's lighter, but it's the same idea. Decision 2: more space is needed.

3) The turbulence thing you talked about is definitely an issue, as well as filtration, so in addition to lots of air, I need to make lots of it available, protect it from the heat coming up from the header, turbulence, and keep it relatively cool. I have a heat shield over the manifold already, but decision 3: add some kind of heat shielding.

I've attached my latest heat shield and have a plan to build a bracket to stack two air filters on top of one another which will give the horns the 2.5" of space I think they need. I've got the heat shield finished I CNC-ed from aluminum (see photo) and added heat protective shielding on the bottom. I will work on the brackets next. More when I'm finished!

 

Edited March 6, 2017 by z boy mn

[madkaw]

I vote air box !


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[z boy mn]

Hey Madkaw, I'm not surprised with that work of art in your engine bay. That was similar to the set-up I had earlier (not quite so shiney, though), but part of the problem is the piping that ran through the radiator support area kept getting damaged because the engine shifted. In one case, it cut right through the aluminum pipe. In other "drafts," it destroyed various other types, too.

So, do you have solid engine mounts on your engine from shifting so much? Maybe the silicone protects the aluminum? Or, did you make the hole larger?

I also like your linkage set-up...I'm re-examining mine, too. Do you have a close-up of that?

Edited March 6, 2017 by z boy mn

[madkaw]

I sold the carbs so I don't have any pics handy of the linkage .
Never noticed any issues of piping movement . I used silicon couplers thru the radiator core so it probably absorbed the movement - no solid mounts .
Still have the CAI- wall art for now.



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