ProtoConVert

Can someone explain to me what porting means and does? Why and how exactly does it make a "brap brap" sound and does it also do the same for piston engines? What is "street" porting and what other kinds of porting are there?

lefuton

porting in a rotary engine basically means grinding out the side housing or rotor housing (for peripheral exhaust ports). what this can lead to, if you say, port it longer is cause the intake cycle to overlap into the compression cycle. this causes a lumpy idle. but leads to better breathing during higher rpm and thus more power in the upper rev ranges.

in piston motors, you traditionally change the cam shafts which have larger or wider lobes so that the valves stay open farther into the intake cycle. this also has the same effect of lumpy idle and more power at upper rev ranges.

you can however polish the ports on the heads of your piston motor too, this only gives minor gains in power as it does not make airflow any easier, only makes it smoother. by polishing the ports and making surface which the air flows over smoother, you remove the tiny vortexes as air is sucked into the chamber. the gain is minimal from this and typically only useful in racing applications.

hope that helps.

wakeech

yeah, that's the basic idea: making the ports larger.

that "lope" (as i hear it's called) is caused by really really crappy volumetric efficiency at low rpm, meaning that the chamber doesn't fill well, and thus has a hard time keeping itself running unless you turn the rpm up, where the velocity of the fuels (air+gas) is high enough to effectively keep the motor running. another factor in that sound is (as lefuton described) a larger cycle-timing overlap (the inspiration cycle starts sooner before the "end" of the exhaustion cycle, which itself is lengthened) which really really helps breathing at high flow rates and rpm, but allows for back-flow and impedance of fuel coming in at low rpm.

street porting a rotary is slightly extending the bottom of the intake port on both the side and intermediate housing down toward the exhaust port a little bit, which increases the area used during the intake cycle a little bit (the area opened up is only briefly utilized at the very beginning fo the intake cycle), but doesnt' delay the port-closure (very dumb thing to do) or introduce very much more overlap.
the next stage is the bridge-port, which is a pretty wicked port and is basically as high-performance as you can get for the street. basically, there is a thin strip of a port cut into the housing, which follows the outline of the stock port, but goes right up to the edge of where the perhipheral and side housings meet. to support the corner oil seal (a VERY important thing to not have spring out of place into your intake port) there is a "bridge" of metal left in place, creating the very thin port right on the side.
other than giving you a lot more port area, this grind also advances intake timing A LOT, and really gives the motor a nice burble, but will also hurt your emissions and mileage a lot too (just like any good performance mod will do).

beyond that are just bigger and bigger bridge-style ports, which go into the perhipheral housing itself (through the water jacket in the side housings... tricky stuff), and then onto the ultimate: the P-Port... generally, a perhipherally ported motor won't idle below 1800-2000rpm, so it's really a race-only motor (but will make like 300hp @ 9k rpm no problem).

lefuton also was right with the cams: longer duration, and higher lift equal better flow (up to the point where the piston and valve connect, and you break your motor ).
one thing he forgot though is that one can grind heads to have bigger ports (and use correctly larger valves, obviously).
the correct grinding of a head is, well, a black art. polishing it mirror smooth is often not the best solution, as fuel likes to condense on a smooth surface (not such a big problem with direct-injection), and a really thick boundary layer can build up quite easily at lower rates of flow on a very smooth surface (not good for flow), but then leaving flashing from the raw cast is (obviously) not a good thing either, so there is some kind of happy medium (i'm no expert... ask an F1 head designer).

here's the kickin'est link in existance for rotary ports:
http://www.rotaryengineillustrated.c...-ports101.html

*stretch, CRRRACK CRACK CRACK* it's nice to have something i can really say a few things about again

wakeech

forgot:
at the bottom, there is a little javascript menu which lets you view all the different port designs of the 13B through the years.

if you're interested to know what the port layout will be like on the RENESIS, it's most similar to the 1989-1991 13B NA, but not exactly: the intake ports are (i believe, just from looking at pictures) that the intake ports are located a little further down the side of the housing, close to the exhaust side ports (which look very much like the intake ports, just upsidedown and without the valved tertiary port).

ProtoConVert

so what does it sound like at non-idle, like at high rpm's... and why?

ProtoConVert

Also, when you "stroke" a rotary, or use a different triangular block thingie (pardon the laymanship), do you have to port in order to maximize response of the mod, or is that more of an obstacle?

wakeech

the higher the rpm's go, the more frequently the exhaust pulses are leaving the tail-pipe, right?? that higher frequency of pulses equals a higher frequency sound, and thus a higher pitch... basically, a high-revving, big ported rotary screams like a banshee: it sounds as powerful as it is (almost nothing is louder).

here is a link to good movies with sound:
http://www.scuderiaciriani.com/rx7/movies/index.html


um, i know it's not nice to hear, but you can't stroke a wankel motor, unless you can increase the diameter of the eccentric lobe on the e-shaft inside the rotor without weakening the whole thing too much (and i'm not sure if that would increase the rotor's torque on the e-shaft anyways).

really, the only "right" way to increase the length of the stroke in a wankel is to scale the whole cross-section of the motor larger (so you end up with a bigger oval-shaped block, and a bigger rotor, and a bigger e-shaft which with the larger e-lobe is where the longer stroke comes in).

if you were to take metal off of the rotor to try and increase displacement, it won't work: all that will do is decrease the compression ratio, as the "dead air" volume (the volume which can never be emptied from the engine... dunno what to call it) doesn't count as displaced volume, because it never empties: it's always there.
the way Mazda has increase the volume of its wankels in the past (and will almost certainly do/is doing right now with the RENESIS) is to increase the width of the rotors (which is similar to, but NOT the SAME as, increasing the bore of a piston), which has historically been in 10mm incriments.

lefuton

increasing the diameter of the eshaft wouldn't change a thing (well, more surface area for the rotor to push on), unless the center of the lobe was moved farther from axis of the e-shaft...however at that point, you are talking about entirely new housings and rotors.

wakeech

yes, that's precisely what i meant... the e-lobe would have to get larger inside the rotor, which would move the centre point of the lobe further from the centre point of the e-shaft... with rotors that have a bigger hole in the middle it could be accomplished, but there would be some very obvious strength issues with that, which is why i said (as you did) about the scaled up engine cross-section.

ProtoConVert

hmmm... what exactly do you mean by increasing the width of the rotor, and how does that increase power?

thanks for the answers

wakeech

by width of the rotor, i mean the actual width of the combustion face (the outside of the triangle which one can equate to the piston-face) is increased, along with the width of the rotor-housing (the aluminum rings which go around the rotor). yes, it's just as simple as it seems.

this increases power because it increases the volume displaced per revolution, which as i said in the "will the auto see 250hp" thread, creates more surface for the same amount of pressure to push on resulting in more force on the e-shaft.

keep the q's comin'.

babylou

Besides increased length and increased weight I do forsee one more disadvantage to increasing rotor width; volumetric efficiency will likely drop. I postulate this because an increasingly larger volume must be filled and is placed further from the ports. Did I explain this well? Let me try another way. With an 80mm wide rotor the lateral center of the rotor is 40mm from a side port. Therefore, the charge must travel 40mm, longitudinally, to fill the void. For a 90mm wide rotor the charge would have to travel 45mm. Surely, there would be some inefficiency here.

wakeech

it's true, the ports as they are won't support rpms as high for such a gross increase in displacement, which is why i'm eagerly anticipating further porting technology development...
i've got ideas, but (of course) they're probably just BS hee hee