Update, new TB is about finished, hope to have it in the website in another month or so...
Comments and details below courtesy of GroupA/Skunk2 Engineer Clinton Yee:
I'd like to thank Bill Schenker for his extensive track and dyno testing of our Pro-Series throttle body on his Miata. Off the top of my head, he's been running it for over 18 months in his SOLO car.
This TB was made specifically for the Miata 1.8L and was designed with various levels of guidance and support from Bill Schenker, Oscar Jackson and Brian Goodwin. It's always a pleasure working with you guys
The Miata TB is part of our Pro-Series TB line up. It uses the same shaft and bearing system that has had zero shaft breakages over the multiple thousands of units that have been installed, which include OEM and built 8k+ RPM redline Hondas/Acuras.
As far as sizing goes, our Honda/Acura TBs range from 68mm up to 74mm and are installed on engines from 1.6L to 2.4L that make 95 to 300+ Hp, normally aspirated.
For Miatas, the 64mm inner diameter (2.5" ID) size was chosen to work especially well with the less restrictive JDM flat top manifold (59mm opening). The TB's 64mm opening has about 18% more cross sectional area than the OEM 59mm manifold opening and about 18% less cross sectional area than a typical 2.75" (70mm) Miata intake pipe. So the TB's 64mm ID provides a nice even flow transition going from intake pipe (70mm), to TB (64mm), then to OEM intake manifold (59mm).
It will also work well with the standard US manifolds (also 59mm opening).
In terms of power, do no think of a larger TB as a power "adder", think of it as a power "un-corker". Meaning that other supporting mods are necessary to realize power gains. A larger TB will not make power itself on a stock Miata. Unless breaking a TB shaft is a concern, our larger TB is not a recommended first mod.
This TB will be available in about 6 weeks from long time miata.net sponsor, http://www.good-win-racing.com
Group A Autosports
Here's a picture of an early prototype: