So, I'm finally porting my engine, and have run into an issue of sorts. When porting the intake, I don't know how deep to go, if that makes sense. When looking at the iron, there are two different materials, the chrome outer surface, and cast iron beneath. I've been trying to find more information, but can't seem to come up with any as to whether or not I need to port down into the cast iron, or just take the upper level of material off and blend the cast iron in.



This is the best picture I can find of what I'm talking about, you can kind of see the difference in material.

Any help is appreciated, thank you.
~Devon

 

no difference in material, its all iron. the port runner is cast, and then the port opening is machined, so there is sometimes a step.

 

So just go all the way in to the bottom of the bowl, then?

 

Yes.... you need to blend everything into the bowl. Rough out your shape... almost to your scribe with the carbide burrs. Then start blending the bowl transition with grinding stones. I do most of my work with the little gray ball stones. I also like to knife edge the opening edge of the port (the edge closest to the runner). Most of the work in porting is in the bowl and smoothing the transition into the closing edge. Just think in terms of airflow.... any sharp edges will disrupt airflow and kill port velocity.

 

someone should do the CCs test like they do with piston heads. It would be interesting to see the difference. My question is this though, when the average Joe ports his motor and get's it all fancied up with a **** eating grin on his face, does he skip the intake manifold and exhaust manifolds? To me it makes sense to port EVERYTHING, not just the motor, or am I wrong?

 

^ Nope , you are not wrong - I port match everything as well.

 

Port matching is important.... but if you HAD to skip port matching, it's better to have a smaller opening flowing into a larger opening, to prevent turbulence and reversion.

 

a friend of mine just cuts the port opening bigger, and leaves the runners and manifolds 100% factory, and it actually works really well. it seems like you can go wrong porting the runners and stuff if you don't know what you're doing

 

I think it's better to not touch the manifold and port runner, just change the port itself. Unless you know exactly what you're doing you'll probably do more harm than good. The port runners are already huge and cause a drop in velocity where the manifold meets the iron plates. Port matching the manifold moves the velocity/vacuum drop farther away from the engine, causing more fuel vapor to drop out of suspension and condense on the sides of the ports.

Also, making everything shiny and smooth looks nice, but it's actually hurting airflow. I don't finish the ports with a stone, do it all with a rough carbide bit. If you can it would be a good idea to sandblast the ports with a rough grit to get a nice textured surface. I think factory and race teams still extrude hone them to get the rough surface. All the little imperfections in the surface create a small amount of turbulence that actually increases airflow and helps keep the fuel suspended in the air. The big step up in size from the small intake runner to the large port does the same thing, opening up the intake manifold slightly will probably help a large port, but go too far and you will lose power.

 

porting on a rotary is more about the timing of the flow, then the increase in CFM. were in a piston engine, porting the heads, ONLY affects the AMOUNT of CFM, and intake timing is done all via camshaft.

That being said, porting the runners/ port matching the manifold, can/will increase CFM.

 

With all the being said ^^^ I will not be opening up my 12a for any sort of porting until I do some further research.