This is a quick right up about the difference in a 4 port 13b and a 6 port.
Let me know what yall think. Good for motor swap info if you wanted to go with a NA 13b.
S4's use exhaust pressure to activate the ports. Usually the activation occurs when the exhaust pressure reaches around 2psi or so. This means that it could activate at any RPM on the Tach, however most common occurances appear to happen around 3200-3800 rpm when under load. Remember it's exhaust pressure that does the activating. The reason for the opening of the ports is to maintain torque at low RPM's that that you don't suffer in the topend RPM's.
On the S5's they are activated using an either an electric pnumatic system or the airpump (I can not exactly remember--I think it's the Electric though). The pressure of activation remains the same and is again around 2psi. This however is usually set to activate at a specific RPM regardless of load conditions.
These ports are used as previously stated to maintain low end torque without suffering for it at top end. It's a similiar if not the exact theory behind VTEC, ECO-TEC, and every other variable valve timing out there. It's a way to have more torque at low end while not suffering at the highend of the RPM range.
As for not having a 6PI on the turbo models there would be no real advantage to it as far as I could fathom. By the time the activation would occur of the 6PI the turbo would already be making boost. Low end torque would be less of an issue since the turbo would be making boost lower than the activation point. Granted the torque wouldn't be there at idle to boost, but that distance is extremely small. The 4 port setup also is about the same size if not just a little smaller than a fully opened 6 port setup. This is primarily the reason why those who do N/A conversions usually either tie open the 6PI or remove the sleeves.
There are others that use the 6pi on turbo setups, however you would have to remove the actuators and the ACV to make the turbo fit (Not to mention the airpump). Which would mean another form of activation would need to be put in place.
all N/A engines are 6 port only the 84-85 gslse engines are N/A 4 port. All turbo engines from 87 to 95 are all 4 port. And all 12A engines are 4 port