ESE's works engine tuner

[senso]

Goodnight everybody, sorry for the off-topic, but its a quick question.
Does anyone here knows if I can safely open boost ports in an aprilia rs 125 cylinder that does not come with them from the factory?

[peewee]

hell maybe ill try for 12k then

[seattle smitty]

Okay, every website has to have someone who asks the really dumb questions; y'all should thank me for providing this service here so that none of you has to . . .

My dim understanding is that the most efficient, lowest drag, highest flow passage (gas or liquid) would be a straight port with a circular cross-section. Since air is HEAVY, and fuel even HEAVIER, and tend to pile up on the outside of a turn, the most effective (low-drag, etc.) cross section for the port where it makes a turn is some variety of oval (depending on how much of a turn, velocities, the particular fluid). I'm ignoring surface-finish here, important as that probably is.

Getting more specific, our engines require that as the ports get closer and closer to where they meet the cylinder wall, they have to assume some variety of fairly square or rectangular shape, with radiused corners. So, whatever might be the ideal, low-drag port shape, it will have to make a transition to the required shape needed at the point of entry into the cylinder.

But back where the air/fuel enters the transfers, and over the (relatively) long sweeping turn the modern transfer passage makes on its way toward the cylinder entry, the website dummy might guess that those transfers should be round-to-oval in cross-section . . . and NOT what we actually SEE in all the high-powered cylinders photographed on these pages. THEIR transfers are squares or parallelograms or trapezoids or anyway some shape with mostly straight sides, and very often with rather minimal radiusing of their corners. Certainly there can be "packaging" considerations, especially where working with an existing engine with inconveniently located studs and other impediments. But I see these straight-sided angularly-shaped ports being used in new, clean-sheet-of-paper engines, including singles (which presumably have lots of available space).

Thus, the question: Am I wrong in feeling that the "best" transfers would start with symmetrical oval shapes that only gradually transition to the shapes and angles of the port-entry? They don't have to be huge; indeed the better-flowing the shape, the less cross-sectional area it should need.

The sharp corners of the parallelogram-shaped transfers I see just look real draggy . . . and wouldn't that be an effect that increases with the square of velocity?

(Round/oval ports surely would call for some re-thinking of the crankcase where it feeds the transfers . . . although I think Wobbly has suggested that most of the A/F that gets transferred into the cylinder when the port windows open was already in the transfers).

[wobbly]

As long as the porting STA numbers are correct for the rpm and you have EVERY other aspect of the design spec'd to suit that rpm then
the higher the rpm the higher the power - as long as the torque holds up for long enough.
In the case of the Banshee style engine I have done designs with peak power at 11,000 and usable overev to 12,000.
With close to 100 RWHp on a Dynojet this is reliable and doable.

Re the port/duct shape.
The one thing you have not considered in the idea of an oval entry transitioning to rectangular is that the raw "flow" number ( cfm ) is only a part of the scenario.
The most important aspect is what I call " stream coherence ", that is keeping the transfer flow going in the correct direction,and keeping its shape as a coherent column.
These coherent columns ( that for example dont bend around the corner and exit thru the exhaust ducts, the main or Aux ) then coalesce together and form
Frits leaning tower concept.
Again this works best when it also is coherent in its shape and direction vector,ie leaning back.
Forming the coherent streams can only be done by having as much of the A/F mixture in the duct being directed to form a column of a particular shape as early as possible.
The side wall angles form the side wall shape of the column and the roof shape of the duct forms the top of the column and especially directs its axial exit angle.
Due to its inertia effect the flow has no option but to closely follow the outer wall, but the inner walls shape is super important when trying to maximise the discharge coefficient
thru the port into the cylinder.
Thus as we know from long experience, keeping the duct side walls consistent in angular direction with no axial twist or two dimensional directional changes, gives the best combination
of angular stream control into the cylinder,and flow coherence creating the scavenging regime as desired.

[2T Institute]

Goodnight everybody, sorry for the off-topic, but its a quick question.
Does anyone here knows if I can safely open boost ports in an aprilia rs 125 cylinder that does not come with them from the factory?

SUb exhaust in a RS 125 road bike (Rotax 122/123) ???

[F5 Dave]

I haven't seen one but usually the water jacket precludes that sort of carry on. I barely managed it in an aircooled barrel that had a jacket added.

[peewee]

if theres no stud in the way you should be able to cut the waterjacket open and weld that area. you may have to weld on a balloon shaped jacket on the exterior so you still have water flow around the new aux tunell. this all just guessing as ive never seen that particular cylinder

[seattle smitty]

Re the port/duct shape.
Forming the coherent streams can only be done by having as much of the A/F mixture in the duct being directed to form a column of a particular shape as early as possible.

If "as early as possible" is essential, then my notion was no good. Is that something that actually has been proved, or is it just the way everybody does it right now, and maybe in ten years they'll have something different?

I wonder if a part of the potential of Frits' new engine could come from getting away from those parallelogram-shaped ducts, and from ducts whose roof and floor are not formed around the cylinder concentrically but are shaped for low-drag (insofar as possible). I just don't see what's desirable about those concentric floors in particular, especially not without big radiuses in the corners. Wobbly, I really do get that this aspect of trying to make low-drag ports is not independent of a lot of other considerations, but it's the aspect I'm curious about at the moment.

If I had more brains, I could devise a software program that would draw a port, starting from where it enters the cylinder and working back upstream while making the turn and transitioning to the least draggy, smallest area (consistent with airflow-at-velocity requirements) port shapes as it proceeds toward the cylinder base (and on into the case). And if you asked it, it would give you right-angle slices of this optimal port at any point so you could see how the transition occurs. It would have to account for the weight and inertia of the gas, the fact that the flow is faster somewhere in the center of the air column and slower near the port walls, the effects of getting the gas column to make a turn (somewhat compressed on the outside of the turn), and give you a floor shape that the air could follow without stalling (golf-ball fans, take note). But, alas, I couldn't even RUN or understand such a program, which probably exists somewhere . . . .


Among today's conventional designs, somebody here had a photo of a big-bore motor I hadn't heard of (500cc twin, maybe?), and it had ports that went part of the way toward what I'm asking about. Naturally, I can't find it now, but possibly I can describe it. In particular, the inside wall (i.e. closest to the cylinder, or what car guys call the short turn) of the B transfer was square to the other three walls of the port itself, and NOT conformed to the cylinder.

[wobbly]

The inner side of the transfer duct produces the WORST result when it has ANY relationship to the bore.
The classic teacup handle shape is even more important on the short side than it is on the outer wall.
As I said the flow has no choice but to adhere to the outer wall shape - crap or superb as it may be for the flow relationship ( ie cfm )
The real trick is to create a short turn shape that promotes the best discharge coefficient thru the duct turn into the bore.
One thing Jan and Frits seem to be convinced of is the short turn and its synergy with the piston shape ( a dome ) to get flow adherence to the piston crown to
promote cooling of the pistons surface.
I havnt personally seen any evidence of the effect, with as I have described previously the flat top pistons of a Pavesi KZ2 that showed no evidence
of excess piston heating over and above a " normal " Maxter or TM setup with a 7* conic dome.
Now we have reports of flat tops making more power in supposed stock KZ2 cylinders.
Till I do the testing myself ( soon ) I believe the jury is still out.
My real problem is that the only way I have been able to get a flat top to make as good as or better power over a dome in a Honda was to use a severe toroid head shape.
This seems to be a real issue to achieve within the KZ2 rules, but I have no problem with a toroids extra surface area within the combustion space as I personally believe the heads inner surface
" should " be hotter than the transfers for example,as reducing the temp delta in this area reduces the temp ( thus pressure rise ) loss to the water.
I have for years thought that we should have two water flow regimes, one to cool the outer transfer walls ( to increase the volumetric efficiency ) and another hotter circuit to cool the head
( to reduce the heat loss of combustion due to the temp delta differential ).
KZ2 then has the added propensity to deto within the squish area, so i believe the ultimate scenario with this oddball system is to ceramic coat the chamber, but cool the squish band as effectively as possible.

[peewee]

this is just my thoughts. i know the definition of parallelogram and i dont think the rsa fits that description at all, infact the rsa B transfer opening at the cylinder base appears to have all 4 walls nearly sqaure with each other, for the most part anyways. now if you look at a modern ktm 250 or 300, those cylinders appear to be the perfect definition of parallelogram. '02+ cr250 cylinder has a shape closely resembling a paralellogram as well.

regarding the inner wall of B being sqaure with the other 3 walls, versus following the bore curvature, on the rsa you can clearly see the inner wall doesnt follow the bore curvature but rather is fairly sqaure with the other 3 walls. i cant explain the purpose but im sure frits could. i havent seen this 4 sqaure walls thing on any mass produced mx type of cylinder from honda, yamaha, ktm etc, atleast not on the larger cc engines, but i think all the race type cylinders have the walls sqaure with one another, atleast the few i have seen anyways, but then again many of them are just copies of the rsa, so of course they will have nearly identical features of the rsa



it may be hard to tell from the photo but if you were looking straight down on this cylinder, the transfer opening at the cylinder base is almsot a exact paralellogram. the full race cylinders ive seen, such as rsa, are not even close to this shape. the inner wall of B on this particular cylinder follows the round curvature of the bore as you can see in the pic

included is another pic which makes it clear. the corners of the B entrance appear almost 45* to each other. the A corners are about 90*. i forgot what im even rambling on about. time to go to bed i guess

[teriks]

...
My dim understanding is that the most efficient, lowest drag, highest flow passage (gas or liquid) would be a straight port with a circular cross-section.
...

Yes, the circular cross section maximizes flow for a given cross sectional area. When you add the question of available space for ducts, and Wob's reasoning above, the table for sure turns towards "squareish".

[senso]

SUb exhaust in a RS 125 road bike (Rotax 122/123) ???

Yes, exhaust auxiliar port(might be wrong to call the exhaust boost ports, sorry the confusion).
There is at least 3 different aprilia rs(in fact rotax engines, 122 and 123 if I'm not mistaken), there is the earlier one with just the oval exhaust port:


Then there is the next version, with exhaust aux ports:


And the last one, with exhaust aux ports, and a small port that goes from the intake directly to the crankcase:


Sorry for the tiny pictures, that was the best I found in a 2 minute search, but they grow a bit if you click on them.

I have already made some aux exhaust ports in a couple of cylinders, but as I don't have any aprilia/rotax cylinder to see/measure to be sure, and because its sort of a common engine, I tried asking you guys if someone might know if its possible to do without welding anything.

[Frits Overmars]

Does anyone here knows if I can safely open boost ports in an aprilia rs 125 cylinder that does not come with them from the factory?

if theres no stud in the way you should be able to cut the waterjacket open and weld that area. you may have to weld on a balloon shaped jacket on the exterior so you still have water flow around the new aux tunell.

You could do all that. You could also buy the same cylinder with aux exhausts; that would be a lot simpler and probably cheaper too.
EDIT: I see that Senso has discovered the various cylinder models as well.
A thing to watch on these Rotax cylinders: the water passages upward from below the exhaust duct are tiny at best; often they are partly or even completely blocked by casting-skin. The pic shows what I mean.

[Frits Overmars]

My dim understanding is that the most efficient, lowest drag, highest flow passage would be a straight port with a circular cross-section.

I was compiling a short answer to you in my mind but then I read Wobs answers and I could not have said it any better, not even in dutch.
My short answer would have been that you are completely right about straight ports: they should be circular. But there is nothing straight about a good transfer duct, at least not at the inner curvature, where it counts. If there is, then you are giving away an opportunity to fit in a larger inner radius.

[seattle smitty]

Okay, Peewee's photo is a dramatic example. The parallelogram-shaped B ports look completely wrong, but even the square-ish A ports don't look very good (AGAIN, factors other than making a low-drag port are temporarily set aside here), at least if my intuition is worth a damn.

The A ports have very little radius in the corners, so the air flowing in the corners is dragged down badly, which tends to slow the rest of the column excessively. A round port, especially one WITH a low-drag surface, could pass a column of fast-flowing gas with a lot less cross-sectional area than those sharp-edged ports. Since the port is going to make a sweeping turn (Wobbly's teacup handle, maybe), a perfectly circular cross-section is somewhat flattened into an oval. Where do you put these oval-shaped ports? Obviously, you have to put them a little farther away from the cylinder bore, making them follow an even more sweeping turn (bigger teacup handle). This might not add a whole lot to the total volume of the ports-plus-crankcase-plus-backsideofpiston, because the ideally-shaped ports need less cross sectional area to pass the same quantity of gas (if you're worried about that volume at all).

Yep, as a sometimes-flyer of little airplanes, I do get the need to have the short turn, like the top of a wing, exactly right to help the gas column to come out of the port window properly-directed. The best cross-sectional shape for the short turn might NOT mirror the shape of the outside turn, and therefore the oval port might not be symmetrical, outside turn to inside turn. I sure would be interested to see exactly what shape results, once all the variables of the weight of the air/fuel (real racing fuel, hah!), its desired average velocity at best-power, the sweeping turn, etc, etc, were plugged in to my imaginary (??) program for shaping ports. And then what does the transition to the port-window shape look like? Must it really be a long transition, as Wobbly asserts?

As I write this, it sounds even to me like an obsession with minutiae. But getting better and better 2-stroke ports makes a difference. In our outboards, old 3-port designs (meaning two transfers, one boost port) were used until quite recently. The main producers of these engines, Konig and Yamato, probably were getting something near the maximum available from this design. But maybe ten years ago, one of the racers showed up at a national championship with a brand-new Rossi 250cc engine, and proceeded to post lap-times better than all but the top couple of 350cc rigs. Everybody there could see the writing on the wall ("Sell your old junk and buy a new Italian motor").

(I only saw Frits' answer after posting this) (which didn't speak directly to my objection about those tight square, and worse, corners . . . sir?) (As I said somewhere above, Frits, it seems to me that your new engine offers a particularly good opportunity to make room for low-drag port runners, and I doubt they would look anything at all like the ones in Peewee's photo).

(The trouble with these long posts is that I have to then edit and re-edit for ten minutes after I put them up here! Frits, as I recall, we are talking within the realm of your direct professional and educational specialty here. Is there any sort of "cookbook," meaning a technical explanation understandable by laymen, for low-drag cross-sections of gas-flowing ports that are making sweeping corners, and relating this to velocity and so forth???)