Velocity Stacks Options

[neyen14]

I am using dual Weber 40 IDF's. 96mm cylinders c35 scat cam

#1 The Original "velocity stack" is a straight shot down the throat of the carb.

#2 Shorter shinier velocity stack is for 40 - 48 IDF and leaves about a 1/8" lip at the top plate of the carb

#3 The taller less shiny velocity stack says it is for DRLA & IDF 36 - 40mm but leaves about a 1/16" lip at the top plate of the carb

I started to widen the inside of the #1 Original and thought it better to have a straight shot down the throat then it would be to have any bit of a lip.... however, I was recommended to use the non original velocity stacks and don't worry about the lip, they've "found it doesn't affect performance".... which seems counterintuitive to the ideas of porting, polishing, and general flow etc

1. Do you think opening up the original velocity stack and having the straight shot is worth it?

2. Should I not worry about the lip?

3. Is there a stack that would fit the 40 IDF specifically without covering a range?

[neyen14]

Here's an above view with each

[Front yard mechanic]

Can’t you get taller air cleaners and use the tall stacks

[neyen14]

Can’t you get taller air cleaners and use the tall stacks

The height of the stacks isn't the problem.... it's the inside diameter of the stacks compared to the inside diameter of the opening of the "top plate" or opening of the carb.

[Jack Standz]

There are at least two different diameters on venturis for IDF carbs, so there might be velocity stacks with different diameters. See if your favorite carb vendor can set you up. (IMG:style_emoticons/default/smile.gif)

[neyen14]

There are at least two different diameters on venturis for IDF carbs, so there might be velocity stacks with different diameters. See if your favorite carb vendor can set you up. (IMG:style_emoticons/default/smile.gif)

Good morning! Thx you Mr. Standz (IMG:style_emoticons/default/lol-2.gif). I think I'm all set with the venturi at least until I see how it runs. The issue is the area well above the venturi. It was the vendor that told me not to worry about the lip created by the wider inside diameter velocity stack (I tried to show in these picture). The stacks seem to be only sold like a one size fits all 40mm -48mm or 36mm - 40mm..... same with the air cleaners now that you mention it lol
Thank you

[Jamie]

Here's an above view with each

The flow dynamics of velocity stacks goal is to create a straight column of air induction into the carb throat without interruption. Any deviation of the straight induction usually creates turbulence, and that reduces input flow. For a full on high RPM race engine that disruption might be significant, but for a street engine the reduction of intake air flow would not likely be notable. The design of an air cleaner for the intakes would be of concern, as any restriction there would be of more relevant for race engines with greater intake air flow requirements, less so for a lower RPM street engine. (IMG:style_emoticons/default/driving.gif)

[stownsen914]

You don't want to have a ledge/lip at the velocity stack / carb interface. As mentioned above, it disrupts the air flow and kinda negates the benefit of a velocity stack. You should be able to find stacks with the correct ID for a reasonable price.

[neyen14]

Here's an above view with each

The flow dynamics of velocity stacks goal is to create a straight column of air induction into the carb throat without interruption. Any deviation of the straight induction usually creates turbulence, and that reduces input flow. For a full on high RPM race engine that disruption might be significant, but for a street engine the reduction of intake air flow would not likely be notable. The design of an air cleaner for the intakes would be of concern, as any restriction there would be of more relevant for race engines with greater intake air flow requirements, less so for a lower RPM street engine. (IMG:style_emoticons/default/driving.gif)

Thank you for this feedback.... pretty much the way I understand it. Even though I have no plan to race it I'd still like to have it be all around as efficient as possible. I'm gonna continue widening the inside diameter of stacks that came with the carb being that they line up perfectly... the go from there.

[neyen14]

You don't want to have a ledge/lip at the velocity stack / carb interface. As mentioned above, it disrupts the air flow and kinda negates the benefit of a velocity stack. You should be able to find stacks with the correct ID for a reasonable price.

Thank you, and correct, I definitely don't want the ledge. As I mentioned above I'm going to start by widening the id of the stacks that came with the carb.... I was hoping someone can share stacks that they've found that fit well.... I bought two sets and they don't fit well at all (that when the last vendor said it doesn't matter)

Whatsoever, thank you

[zig-n-zag]

Flared-top velocity stacks were available for the early 40IDF but only for a short time before replaced with the die-cast version as you have.

Those flared-top velocity stacks were steel, flash chromed with an inside ID of 39.8mm and 64mm tall. The carb top opening of the early carb is 37.5mm, so it’s already a mis-match.

The later version of the 40IDF had a larger carb top opening of 42.5mm. Most people used the flared velocity stacks from the 44IDF carb, but these stacks had an inside ID of 44.9mm, and were 56mm tall. The 44mm carb has a top opening of 46mm.

IN contrast, the 48IDF has a carb top opening of 46.4mm and the velocity stacks for those carbs has an ID of 45.9mm and are also 56mm tall.

[stownsen914]

It sounds like the stacks you've gotten so far are being sold as suitable for various sizes of carbs. So not even intended to be an exact match. There are certainly stacks available to be a more exact match. It may make sense to confirm the ID at the top of your carbs, and talk to vendors who can tell you the exact ID at the base of the stacks they are selling. The stack ID at the base can be a touch smaller than your carb, you don't want them larger as you know.

[davep]

I would consider making an intermediate plate to fit between, and taper from the stack to the throttle body. This would add to the stack height as well as eliminating the lip.

[Literati914]

Flared-top velocity stacks were available for the early 40IDF but only for a short time before replaced with the die-cast version as you have.

Those flared-top velocity stacks were steel, flash chromed with an inside ID of 39.8mm and 64mm tall. The carb top opening of the early carb is 37.5mm, so it’s already a mis-match.

The later version of the 40IDF had a larger carb top opening of 42.5mm. Most people used the flared velocity stacks from the 44IDF carb, but these stacks had an inside ID of 44.9mm, and were 56mm tall. The 44mm carb has a top opening of 46mm.

IN contrast, the 48IDF has a carb top opening of 46.4mm and the velocity stacks for those carbs has an ID of 45.9mm and are also 56mm tall.

That's freakin' maddening !! Seriously, why wouldn't you, if you're going to the trouble to produce an item, for use with something that was traditionally and still regularly used in a competition setting, not make it as exact as possible for an intended application? OK, the idea is so that it would fit more than one unit, ..to only be acceptable in a street setting.. sucks.