I know I may be asking for the secrets no one wants to talk about here but so goes it...

How thin do you cut the division between the primaries? it obviously cant be so thin as to break. What, 2 mm?

How much is enough under the coolant seals? 1 mm? 1.5 mm? 2 mm?

When do things get sketchy?

 

i would say you're fine going mms thin between the primaries, just because its an equal amount of pressure on each side... but then again, theres no reason why you couldn't just bore it out to a single hole, if you really need the flow.

 

I don't think open as to one big hole is what I'm looking for. I was just curious as to how thin both the divider and under the water seals people go.

 

On the primaries I try to keep it a sane thickness at the face where the gasket has to seal or you will just end up pinching the gasket out there and having it flap about in the flow anyways.

But then I port the runners closer to each other leading up to the transition to the bowl for a shallower bowl angle. As close as I dare without breaking through....

I don't go super thin on near the water jacket, but try to keep it a good constant radius with texture before and on the short radius to trip the flow to turbulent to keep it attached here.

If you port it to a thin knife edge here you have to think about how it is going to flow OUT of the port now into the large open area of the rotor housing.