I'm only digging this up from the dead to make a technical observation and comment on why off the shelf KA extractors are crap, for those that may be in the market for them.
Working on the new airbox for my brother's DC2 'R last night, and decided to get the verniers out to measure the OD of the primaries on his aftermarket headers (i think they are JUN? that or Spoon).
44mm in diameter, inner diameter is probably only 40.8 as (i think) they are 1.6mm thick stainless tube.
Now, at 38mm outer diameter, my genie headers would have to be 2mm wall thickness if not more, as they are mild steel. that leaves us with 34mm inner diameter primaries.
Quick calc in excel using the formula for approximate mass flow through the engine, cfm = cid x rpm x ve / 3456 reveals some interesting facts.
At 8500rpm, the B18C7 in his 'R, assuming a REAL good VE of 106% (possible; engine inhales 106% in each cylinder, equivalent of 1 psi boost theoretically), approximate mass airflow out of the engine is 298 cfm.
At 6500rpm, assuming at best a VE of 93% (-1 psi vacuum), a KA24E will churn out approximately 261 cfm. Bump your rev limiter to 6750, 271 cfm; with a noss special stage 3 rom, at 7000rpm you'll be moving about 281 cfm (theoretically, your VE will probably drop quite a bit after this point though).
In any case, 38mm OD primaries in aftermarket KA headers are too small for any kind of mid to top end improvements.
Don't even get me started about poor collector design on these things, this is the most crucial aspect to obtaining a decent scavenging effect from a set of headers.