This can't be right, can it?

[RoyBatty]

I got the IDF 40's ready to install to the BC Coupe.
Just waiting for a waffle manifold coming from the old country.
I went to a local VW bug speed shop and bought some velocity stacks to go on my carbs.
Once I got them home and fitted them to the top of the carbs this is what I saw.
Shouldn't the transition from the bottom of the stack to the inlet of the carb be the same diameter?
Seems to me this is going to be a source of turbulent flow into the carb causing unknown tuning issues.

[So Cal Mark]

did you get stacks for 44s?

[RoyBatty]

Don't know.
I told the counter guy I had idf 40's and needed the parts for that model.

[manoa matt]

With the early Italian versions, the venturi holes in the top plate of the carb are parallel and consistent with the venturi diameter in the lower body section of the carb.

With the later Spanish versions, the venturi holes in the top plate of the carb vary in diameter. At the bottom surface where they mate with the carb body they are 40mm, but widen as you get near the top surface by a few mm. Those stacks are likely made for the later Spanish version webers.

This difference is not exclusive to IDF's and is actually more pronounced in DCOE carbs.

From the accelerator pump jet nozzle shape it appears you have old Italian webers. Search around for "true" 40mm velocity stacks. There are some out there with a radius at the top that will improve performance beyond those spun steel tube type stacks.

[RoyBatty]

Hi Matt and Mark,
I spoke to a tech Pierce that told me this is not uncommon and should not be a problem.
However, if there is a "more correct" stack to acquire, then I'm going to sell them out.
Matt, are you talking about the factory stock aluminum tops? Or another aftermarket type stack?
The other thing I am considering, it chamfering the tops to match the bottoms of the trumpets.

[carl]

I ran into the same problem with some nice cast aluminum stacks I got off eBay. They are larger in internal diameter at the base than the top of the IDFs. Next time I order stacks from someone, I'm going to ask for specific dimensions rather than "they are made for 40 IDFs". The factory Fiat IDF stacks mesh exactly with the carbs.

carl

[divace73]

I've got something similar, I'm going to make a 3-4mm aluminium spacer and dremell the difference in diameters

[So Cal Mark]

here's a pic looking down the throat of an EMPI version of the IDF with an EMPI stack; no lip

[RoyBatty]

Thanks Mark,
I'll need to measure the ID at the top of the carb bores and call you to check if those well fit the same for my idf'
EDIT:
Got the dual 40's running with these stacks on it. Due to the way the car now performs, I'm moving this a bit further down the "to do" list.
The 1600 is pulling strong all the way to red line with no hesitation or stumbles.
Looks like I was worried for nothing.

[sptcoupe]

That lip is huge and will badly interrupt airflow. That needs to get fixed or you will be giving up a good amount of flow. Hard to guess how big a decrease in flow comes from the air column bouncing off that lip, but it will be significant.

[timinator]

Hard to guess how big a decrease in flow comes from the air column bouncing off that lip, but it will be significant.

The 1600 is pulling strong all the way to red line with no hesitation or stumbles.
Looks like I was worried for nothing.

Guessing is easy. Ignoring actual experience is hard. The engine is not showing any effect of " a big decrease in flow..." which is an indication that the engine could be getting all the air it needs.