Rather technical hypothetical port
 

I was thinking the other day. I do high performance head porting, and build race engines. Ive picked up all kinds of little tricks to be able to get the most out of poorly designed heads, do it every day. I was looking at the exhaust port design over all. Aftermarket porting designs make the port into a D shape, which advances exhaust port opening. Obviously this works wonders. I was thinking, that port design works because of sheer volumetric capacity. Insted of using a large D shaped port, would it be benificial to use almost like a V shaped port with a sharp opening angle, not completly through the housing? Its just a mere thought at this point, but the benifits in my mind would be a more efficient exhaust velocity pattern. The very tip of the bottom triangle of the port would start allowing exhaust gases to excape the very instant that the engine started to compress the exhaust gas again. The only problem I could think of in my mind is the possibily of weakening the face of the housing itself, along with a very concentrated blast of exhaust on a small section of the apex seal. It could have the same effect as burning an exhaust valve in a boinger engine. Ill play around with the idea more at the shop, see what the volumetric increase and hydrodynamic resistance decrease will be with the flow bench and a spare housing. Just a thought, what you guyz think?

as a general rule
gradual openings help with low end intake velocity
and abrupt openings help with top end

just a thought

thats interesting, I wasnt aware of that aspect of rotary breathing. hmmm

Re: Rather technical hypothetical port
 

Doesn't advancing the exhaust port opening decrease (low end?) torque?


-Ted

yuh when the port is partially open, you get a higher intake charege velocity for a little bit as it opens

with an abrupt opening it goes from closed to open with no size inbetween, without haveing a partial opening to speed up the charge

"Doesn't advancing the exhaust port opening decrease (low end?) torque?"

Yes it doeshamper low end torque

Allso : Reversion and intake dilusion goes with the loss of torque on a quick opening port.

If the port is opening more gradually,the above mentioned effects will be a lot less.

I have had great success with D-shape exhaust ports.

Re: Re: Rather technical hypothetical port
 

And doesn't it make the engine REALLY EFFIN LOUD?

(And people wonder why I *refuse* to open the exhaust port earlier... the stock port opens too early as it is!)

I don't think the opening/closing shape makes *that* much of a difference as far as flow is concerned. For the boingers, the valve's opening is pretty much parabolic, maybe assymetrical one way or the other depending on what the current cam lobe fashion is (fast-opening/fast-closing). From the perspective of the chamber, a peripheral exhaust port goes from closed to completely open in maybe 40-50 degrees, stays wide open for maybe 270-290 degrees, then goes from wide open to completely closed in maybe 40-50 degrees. And there's no valve in the way. :)

Incidentally, the Mazda Factory Race peripheral port housings open the exhaust only 2 degrees earlier than the production engines, as opposed to 10 or more for the templates that you can buy from the usual sources.

I cut some D shaped ports in some J-spec '81-'83 13B rotor housings. I kept the opening the same and delayed the closing. They are only a few degrees later than a T2 port, but with a smoother (rounded) closing.

The engine is going into an REPU, so I figured I'd keep the stock opening to try to keep at least some low end torque. :)

I honestly dont think anyone here is qualified to answer the first post. Its a very radical idea. I cant think of any reason it would work, but Im not an expert, and its often things like that that turn out to be technological leaps.

The opionins posted here on exhaust port opening vs closing are exactly opposite of my personal experience. 2 engines that I built, with identical porting on the intake, and with identical exhaust ports, save the opening and closing. The exhaust port was about 1/4" lower on the second engine, but the size and shape was identical. The second engine had a dramatic increase in low end power, with a broader, more progressive torque curve that started sooner and built up to peak faster. This was on a n/a engine, same car, same exhaust, intake, et c. Everything was the same except the exhaust port.

well...
 

the whole reason for this was that someone stated on this board a while ago "boinger engines have had almost a hundred years of development, a rotary hasnt had near that, and its gone pretty much unchanged, IE more rotors, bigger (same designed) ports etc. This engine is far from being the efficient power maker that it has the ability to be. Mazda will have to start working and putting money into some serious research into making this the most efficient engine around if they want to keep it. The whole idea behind the V shaped gradually opening port is that the port design itself and the flow pattern that would result would very closley mimic the natural gradually reducing, lifting, rotating motion of the rotor face itself. In my mind this port would shine at high rpm's because it would take a large air mass to increase the port velocity and make good use of the port flow at an exponential rate.

Due to the nature of a peripheral system, the only way to have a gradually opening/closing exhaust port would be if it were on the side housings.

Funny, that...

Why don't you start with the SAE papers first...
Mazda has done a lot of research already.


-Ted

All righty, as I understand it, you want something like a triangular shaped exhaust port, with the point on the bottom side. An interesting idea. I'll share my thoughts on it. ;)

It seems that you missed an important concept in it. The factory ports open the exhaust quite a good deal before BDC; 75 degrees before in most of the engines, while the chamber is still expanding! The reason this is done is scavenging... by opening the port before the mixture is fully done burning, you lose a little bit of power. However, more importantly, you get a very high energy exhaust gas leaving through the port with a signifigant enough mass to suck the rest of the gas along with it. Piston engines do the same kind of thing for the same reason, actually... there's something like a 10% bump in power vs. pumping the exhaust gas out.

So, first off, I'm going to assume the same port timing as the stock ports for the triangular port, and further assume I have the right impression of what you want to do. The initial gas flow velocity will be higher, true, but the problem is that it won't have very much mass flow, so the scavenging effect will be reduced. This scavenging is *FAR* more important on rotaries, as it's the only way to evacuate parts of the working chambers.

The only problem is that the exhaust is *already* extremely high energy, to the point where lots of thermal energy is being thrown away. That's why the exhaust is so LOUD and HOT compared to a piston engine.

The port opens as early as it does, simply because Mazda had no choice with a peripheral exhaust... open it later and they'd *really* choke off the exhaust flow unless they held it open later as well, which would increase overlap too much. As it is, peripheral-exhaust rotaries don't like idling smoothly at stoich (a symptom of too much cam in a piston engine).

Plus, we have 270 degrees to get the exhaust out vs. 180, so the pumping losses aren't as great as in a boinger.