Velocity stacks and filter heights

[ps2375]

I posted the same thing over on the samba, but some here may not go there and vice-versa.

So, currently my IDF's have 2.25" velocity stacks with 3.25" airfilters. Leaving only 1" between them and the lid. I know the underside of the lid is getting "washed" by fuel, because last year when I first got the car back on the road I had plasti-dipped the chrome bases and lids black cause they looked better. I had also coated the insides of them to cover all the chrome edges better. After driving some, when I removed the lids for some reason, the undersides were clean.
Today I went to local VW part shop to ask what was tallest filter I could fit under my deck lid w/o standoffs, nobody had an answer. I was pretty sure that 6" filters would be a no go. So, tonight I made a 1.75" cardboard spacer to see if that and my 3.25" filters would fit, the cardboard was a bit more than the 1.75" and it seems that a measured filter height of 5.25" will just fit on 1/2 side and the same will easily clear on 3/4 side to deck lid(and even clear my engine harness connection above carb on that side).

What kind (if any) benefits can I expect with the added room above the stacks? I know the air will have a better entry into the stack opening, and this will translate to what? Better cylinder filling, better throttle response?? Just looking for a better understanding of what makes a carb tick and how to optimize what I have.

One of the resources that the parts shop called and asked what a good minimum distance between top of stack and lid said that 2" was his min distance and 6" was even better(I think the latter was partially in jest).

[FJCamper]

Hi ps2375,

Add all the room you can to the space between the top of the velocity stack and the top plate of the filter. Giving it the room you described, your carb will tune normally.

Velocity stacks actually manage "stand off" which is a fog of fuel and air rising out of the carb throat as the side effect of "reversion," which is one reverse pulse of force outwards for every intake pulse of air-fuel entering the engine. Weber's are non-plenum carbs and need velocity stack to work right. A plenum is a space below the carb that acts to dampen reversion. Big V8's with 4bbl Holley carbs have special plenum manifolds. That's why Holley's are not velocity-stacked like Weber's.

You may have noticed that 44's come out of the box with short stacks, usually under three inches. By the book, such short stacks would run the power band higher, actually suited for a racing engine, if the cam and heads were matched up to the stacks. The actual reason you get short stacks is limited filter height.

If I remember correctly, dyno tests (Jeff Lain, Kaddy Shack) showed that stackless Kadrons actually gained power with a filter housing deck above them. The presumption was the carb standoff was dampened by having a roof to bounce off, whereas with no overhead filter deck, the standoff went higher straight up, and leaned the mixture.

You saw the effect of pooling standoff fog, which contains gasoline, in your own filter housing. It dissolved the Plasti-Dip material.

FJC

[ps2375]

Thanks for the reply, I'll bed adding the height in week or so, I haven't been told of any adverse effects from adding the space, only benefits. I won't be expecting any noticeable gains, other than maybe some additional volume from the throats(of which I am perfectly happy to live with).

[ps2375]

So, here's where I'm starting from.


I had a set of tops and bottoms already on hand, so I didn't need to source any. Here they are tack'd together.


Then I had a friend weld them along the seam.

[ps2375]

Which after some cutting and grinding became this.


Which allowed this.


And then this.


I'll be getting the "rings" powder coated and then they can be installed. They will then have about 3" of room between the top of stacks and the lids. Should be plenty.

[ps2375]

Got them done and installed. The sound seems to be a bit throatier. And it might drive a little better, but that could all be in my head. Only had a short drive in it so far.

[tattooed_pariah]

so what was the verdict? Did this help any noticeable amount?

My 40IDF's currently look like your starting point, I never ran velocity stacks before and I've noticed it's been a nightmare tuning them since putting the stacks in..

[ps2375]

so what was the verdict? Did this help any noticeable amount?

A little more sound from them and not too sure about power. But no real detriments.

Fortunately for me, the carbs have been tuned for this motor for a long time, so I haven't had to mess with them much. I have closed the idle mixture screw some, that fixed the overly sooty exhaust and I might go another half turn in, as the exhaust still shows to be rich at idle.

The motor made 114whp at a recent dyno, and the one run I did with the lids off didn't show any improvement, nor dramatic loss, but the plot was buggered. What there was of it seemed to be consistent with the others.

I would think the stacks should make the carbs more consistent and easier to tune.