Have been searching for a day and a half for threads here and other online resources trying to figure out when secondaries kick in.

If anyone knows of a good read that explains airflow and ports and all that fun stuff I'd appreciate it

Thanks!

 

At 3800 rpm and engine under load. There's a training manual in the following link which might be of help.

Foxed.ca - Mazda RX-7 Manuals

 

Thank-you! I will look into that when I get home this evening if I have time

 

To clarify, the secondary ports are the lower ports on the outer irons. The auxiliary ports are the upper ports on the outer irons which open above about 3800 RPM. Primary ports are those on the middle iron.

 

Aaron, question...

Why are the primary port opening on the NA smaller than the secondary port openings? Am I wrong that I think that is odd since they share a common plenum at the dynamic chamber? Have I not taken a good enough look to notice that the runners are seperated in the dynamic chamber?

 

They don't share a common plenum. The primaries are smaller to keep air velocity up at low engine speeds. The 13B is like any internal combustion engine needs air velocity sometimes instead of volume.

It's why they made four barrel carbs for piston engines with secondaries that open after the engine speed increases.

 

I am actually asking about the secondaries for a 4 port engine, so thank you for the clarification Aaromcake, so when do those secondaries 'kick in'?

 

Jjwalker: I'm pretty sure that the primaries share a plenum, but the secondaries have separate runners

 

Whenever you push the gas pedal far enough.

 

I know all of that, I just never looked inside of the dynamic chamber to see it was divided.

 

Already answered I think but they primary ports are smaller because they are used at low load and idle, so the timing is conservative and the ports small to keep overlap to a minimum and air velocity up. The primary ports share a common section of the intake plenum fed by a single throttle plate. The throttle body is progressive so once you've pushed the fun pedal far enough, the two other throttle plates open and feed air to the secondary and aux ports. The secondary and aux ports have are connected by a horseshoe shaped runner fed by two throttle plates.

 

That answers that. I was unaware that the dynamic chamber was divided as I never looked down into it or poked my finger in there to see. I figured it was but then I started to second guess myself which i do a lot for whatever reason.

 

That makes sense......I should have realized that considering there are no vacuum lines or anything like that hooked up the the secondary butterflies.....just that mechanical.......dampener?

 

So here's the deal. The FC and FD 4 ports are fuel injected but the throttle plates are still controlled kind of like a carburetor. The secondary butterflies open based on the position of the gas pedal, because the gas pedal moves the throttle cable which then moves the linkage on the TB.

Remember that the size of the ports also controls the closing timing of the intake event. At low speed and low load, we want to close our intake ports earlier to get torque. On a piston engine it's the same way, where you close the intake valve earlier (when you have variable valve timing capability). At higher speeds (and also higher loads) you generally want to close the intake port later to give more time to fill the combustion chamber. So the secondary ports open up and give a longer duration and later closing timing. The mechanical linkage is a crude way to control that.

On an Rx-8 Renesis engine you have electronic throttle valve that controls flow to the primary ports. Then a vacuum operated shutter valve controls the secondary ports, so it can be controlled by computer rather than by a mechanical linkage. Then the auxiliary ports (because it's a 6 port) are controlled by an electric motor, the runner lengths and intake duct are also controlled.

So a 4 port FC is a 2-stage intake system. A 6 port s4 FC is a 3-stage intake system due to aux ports. A 6 port s5 FC is a 4 stage intake system due to VDI to change runner length. A 6 port Renesis is a 5 stage intake system because it adds the variable inlet duct at the airbox. A Renesis is basically at Porsche level complexity of the induction system, which is why it can still breathe at high rpm.

On a boosted engine you are limited by the turbo(s) more, so Mazda simplified the induction system.

 

Just a quick addition to say on the S4 (and I think S5, maybe others also), you have two sets of secondary throttle plates. The second one is controlled by linkage and opens after the primary plate opens a certain angle (as described previously). The first secondary plates are held closed via vacuum when the engine is cold for a brief moment for drivability when the second open. When the engine is warm, the first secondary plates stay open.

This is easier shown than described, lol!

 

yeah the No. 2 secondary plates, which FD's also have (controlled by a solenoid), are there because they didn't have electronic throttle to regulate engine torque. With electronic throttle you can limit torque through software control of the throttle valve.