Bill,
I will try to convey the information I have collated over the years, in the
terms I understand it,  feel free to use the information or debunk it as you
please.

The rule of thumb to date ( for a whole range of reasons) is between 18" to
21".
I guessing, your runner lengths aren't taking into consideration the housing
inset length.

However all this is good information, as I believe their keeping the runners
thinner to increase velocity. Higher velocity provides for better ram effect
for the desired RPM.

There are two schools of thought for the PP, one is 2" runners( PL) and one
is 1.5" runners - I prescribe to the 1.5" theory. Best length to be
determined by  adjusting the length to give the desired HP at the desired
RPM.

I believe the 2" diameter runners are better for higher RPM ( used in the
LeMans engine) which provides greater flow for the higher RPM band. I don't
believe we need 2" at the RPM we are using ( max 7,500 with 2.85 ratio PSRU)
and only serves to reduce the velocity/ram effect. As there is no overlap
( with the RX8) we don't need to worry about contamination of the inlet
charge, however with a larger inlet we have a larger butterfly which
provides for less control over the opening at low RPM, therefore less
control over idle. Individual butterflies, close to the housings - for PP)
were introduced to limit overlap contamination ( in the runners) and to
provide better low-end control. The 1.5" runners and close to housing
butterflies is what Jerry is developing for PP.

I'm not fully conversant with the Renesis manifold and I thank you for going
to the trouble to provide such detailed information, which I feel will help
define a suitable 'home-brew' manifold for the Renesis.

I don't really understand which direction you are going with your
development but I hope this information can help you, even in some small
way.
George ( down under)

> >Bill,
> >There is some one currently tuning a 2" intake - I think 16" may be a
little
> >short.
>
>          I measured much more carefully just now. (It isn't easy to
measure
> length because the tubes are all tightly curved in three dimensions.) The
> primary runners are about 14 inches long and 1.18" ID.
>
>          The secondary runners join together at the 12 inch mark and then
> join with the main intake at the 19 inch length. The secondary runners are
> approximately 1.18" ID (I can't get deep enough to measure exactly. The 7"
> long common duct for the secondary runners is a pair of ovals that measure
> 2.525" x 1.16".
>
>          This may seem short, but keep in mind that the red lined is over
> 9000 RPM and the peak HP occurs at 8700 RPM.