Ok, this came up in another thread, and since I don't know a lot about how the EFI 13b works, I got real curious.

Apparently the engine relies on backpressure from the exhaust to determine when to open two of the intake ports. I.E. it doesn't open them until there is a certain amount of backpressure. I guess blocking the ports helps low end power, and then it opens up when the revs are up and exhaust pressure is up.

I'm just wondering how significant are the advantages to this setup and maybe how does it all works more specifically?

Thanks in advance.

 

Well, the 6 port engines were designed to lower the RPM where peak torque occurs and create a wider powerband. For instance, the GSL-SE torque peaks at 2500RPM, while the 12A torque peak doesn't occur until 4,000 RPM. This is how it works:

The 12A (and other 4 port engines) have:
- 2 primary ports
- 2 large seconday ports

The GSL-SE and other 6 ports have:
- 2 primary ports
- 2 small secondary power
- 2 aux. ports (also caled 5th/6th ports), located next to the secondary ports

For NA engines, you want to keep the intake velocity high (helps with torque). This usually means small ports are better. However, a small port will choke flow and hurt power at higher RPM. The 6 port design is to get the best of both worlds. When the load/rpm is low, only the primary and secondary ports are operational (keeps velocity up). When the load/rpm increases, the 5/6 ports open. This allows more flow to keep the top end. It also influences port timing.

So, now for how it actually works. It is pretty simple really. You may be familiar with things that switch on vacuum (like secondaries on a carb). The 5/6 ports are the opposite. They operate on positive pressure. There is a tube that connects where the air injection for the main cat is. When you have more load/rpm on the car, there is more exhaust flow. This increased flow means increased backpressure. This backpressure is what opens the 5/6 ports. They are designed to open at about 2.8psi pressure. If you modify the exhaust, the 5/6 ports may open later or not at all. The RB exhaust at Mazdatrix moves the tube closer to the engine to keep the 5/6 ports working well with the decreased backpressure.

Sorry for the long post. Hope that expains it for you. If you need any clarification, let me know.

Kent

 

Makes sense, I knew about intake velocity improving torque and therefore wanting to keep your ports (and carbs and whatnot) designed for max velocity at the CFM you need, I just didn't make the connection that having ports that open later was a way to achieve that.

So now I have a question about the 12a. I've heard of a porting scheme that basically has the primaries taking the regular ports of the 12a, I -think- it was all the regular ports, and then the secondaries went to a set of peripheral ports.

Would doing something like that kill the torque at lower rpms? Like would you get a torque peak at 6k instead of 4k or something crazy like that?