Custom 930TT Shorty Headers, What would be the ideal collector ID?

[RarlyL8]

If one was to fab a set of shorty headers for a twin turbo conversion on a stock head 930 engine, what whould be the propper collector ID? Assume the engine is to produce 400HP and use stock 930 heads. I have read all the Turbo books, now I want opinions from those who have experience with 911s.

[RarlyL8]

What - all the HP junkies out playing this weekend?
Corky Bell states that the header design needs to neck down the ID from the primary pipes to the collector in order to speed velocity to the turbo. I assume these measurements will not be the same in all applications. I'm thinking 1.5" ID for the collector on each bank of cylinders is about right for a 400HP 930 with stock heads. Opinions?

[klaucke]

What do the 993TT and 996TT use? I would think that'd be a good place to start.

[RarlyL8]

I don't know. All the tech specs I have end with the 930. I figure 930 or possibly twin turbocharged Carrera information would rule out any differences in head/flow technology that would not apply to our older cars. Maybe I should start a thread titled "Who has twin turbocharged their 930 or Carrera" and go from there.

[RickM]

Maybe the answer is hidden in here somewhere Twin turbo configuration questions: Theory & Application

[RarlyL8]

Thanks Rick, I read that thread as it developed. Nothing specific popped out. This probably is not a critical item at lower power levels. Most likely there is a sweet spot where the optimum gas velocity and choking the motor converge. I'll try the experts such as Protomotive and ImagineAuto.

[VZ935]

[Lukesportsman]

Hold on, I'll go measure my 993TT headers.

[Lukesportsman]

OK, primaries are darn near 1.5" ID with collector opening 1.625. Here is the sad thing I see: The pipe above the flange appears to be larger and this no inverted cone concept.....just a step.

RarelyL8, I like the simplicity of my 993TT headers, but a custom one could easily surpass this mass produced part for power.

Without making the header excessively long, I'd like to see better merge collectors from maybe Burns or such. I have never built a collector on any of my headers that appear so half hazardly fitted. I'm just going with the idea that 993TT move out ok and this is going in a 72 lightweight chassis....who knows