Race Trim DCNF Manifold Question

[Rob Combs]

Hi everyone,

Hoping someone can help me understand something here - I recently found a set of Race Trim DCNF dual-port manifolds and would like to know why the large bulge or plenum area at the top of each intake runner, just below the carb flange.

Here's a link to a similar set that seems to have sold a few years ago for reference/example:

https://www.thesamba.com/vw/classifieds/detail.php?id=1819009

I'm trying to figure out if it will help swirl, cause a major slow-down in manifold velocity, or other???

I'll be using these on a stock 1600 DP for now. Not sure of compression ratio, pretty sure the cam, heads, and rockers are stock, the DCNF have 28mm venturies, 1 inch spacers between carb and manifold (for vacuum pickup and linkage geometry, spacer can be cut down if need be) and the exhaust is a tri-mil header with dual quiet packs. I have a Black Box going in this weekend so timing curve will not be an issue, just might take some tweaking.

The carb/manifold setup has not been installed yet.

Thanks for any input!
Rob