So, I have thought this for a while:
You have these STOCK engines:
13b N/A 4 port
13b N/A 6 port
13b T2
13b-rew
13b-msp 4 port
13b-msp 6 port
You dont have turbos or original intakes. You build custom header and intake. And you have some kind of throttle body and ecu.
So, which is the best engine? Which flows best and makes best power?
And I repeat, this is just hypotetical and in theory question? ;)
WGT did a lot of work on my MX5 rotary conversion, experimenting with different intake/exhaust options. With a stock, freshly built 13B-MSP (6-port) with custom intake/exhaust/ECU, it did around 210bhp on their dyno. (standard RX8's were only 170bhp on their dyno).
A light street port by BDC motorsports and revised exhaust got that up to 240bhp.
My understanding is that the 13B-MSP 6-port is the best one without forced induction.