Damn, there I go again! I thought you were the OP. Stupid me (as most know).
Sorry, for some reason I must have you (lowkey) on my mind - maybe something to do with PMs LOL.
terryo, I suggest take the offer. Two should be cheaper than one, and if between you you find a good designer...
I looked for some design info but couldn't find my old notes (probably part of my unrestored
last big crash data) though I did find a B.Eng dissertation "
Engine Optimisation and Performance Characteristics for a Formula SAE Race Car" by Melinda Rachel Pink though it lacks the factors (constants etc) that I recall for extractor design - but maybe I didn't look deep enough.
As an aside, I was kinda happy to see Melinda refer to
extractors. Elsewhere I read that's an oldskool term (which IMO is fine) and how most call them headers (or hedders). But extractors is apparently an Aussie term so hey - I'm very cool with extractors.
And incidentally, it was my GURU that said the standard twincam exhaust manifold were hard to beat. Admittedly we were discussing the G200W which included the 117 G200W but I'd assume it holds for the blue (ZZR, ZZT etc) G180Ws - especially the later blues.
But we were also referring to the 400Nm G200W exhaust mod as applied to the non-Gemini Isuzus - ie, Belletts & Florians - so the clearance/interference problems that Geminis have weren't considered.
Also those discussion were back in the 1990s in case things have changed (though a few years ago I quizzed 2 manufacturers of 4ZE1 extractors that claimed "no design" - merely whatever physically fitted!!). By the 1990s we had killed the old rule that
too big an exhaust is bad - that was a hangover from pre-PC & calculator days when continuing calculations for more
peaks generally was not done.
Note too that GURU said the Isuzu twincams are unlike any other - especially wrt how they breath. Since then I have met several that "know" the same. And in retrospect I understand why my dyno tuners thought their dyno was faulty (LOL - they built racing engines but had obviously never crossed an expertly tuned G200W!).
Hence beware crap like fitting Celica cams (ie Celica profile grinds I presume). And unfortunately some Isuzu forums seem almost as bad... They question (IMO) well known & proven 20-30 year old tunes (eg, 400Nm G200Ws) but hey, they can't even get GW sparts!
I did sell some custom made Gemini GW 4:2:1 extractors which looked buddy terrific. They seemed to have similar dimensions to my torque G200W manifold & 2:1 section. I was so spewing they didn't fit my Belletts, Florians or Wasp.
BTW - most know how I push 4:2:1 for GW engines. IMO if you want high RPM power (ie, 4:1) then get a better suited modern 16V engine. The beauty of the G200W was its low end torque - ie, 300Nm@2k RPM without any engine mods - merely appropriately tuned Webers (twin DCOE45s or 48s or even larger) and the key 2:1 exhaust mod. Twice the vehicle acceleration with a st'd engine. Nice!