Porting basics

[registered user]

sstaton said he seen a engine that had alot of material removed from the crank area. i took that to meen down in the portion of the case where the crank wheels swing. i was merely stating that it doesnt make sense to me to add the extra volume in that location because of the long distance it would need to travel to reach the trans ports and cylinder. thats why i showed the photo with the green and red lines. makes more sense to me to keep the volume up higher where its travel distance to the cylinder will be much shorter. im sure you understand

[registered user]

as for the longer rod and spacer making the engine lazy. im not sure when can say that so easily. we dont know what the case compression is right now. and does the pipe not have anything to say about this extra volume ? like tfaith said earlier, i think some pipe will do much better with bigger case volume. so i dont believe its a cut and dry answer to say more volume is a bad thing. my opinion of course

[tfaith08]

camatv, I think I'm on to a few things with that same exact topic as well.  I'll PM you about them. 

 

Thinking about a 2 stroke as a whole is sort of mind numbing at first, but once you get a good grasp on the function of each little area, you'll start to see it.  THAT'S when good ideas come.  To tap on your post about the transfer aiming, I do agree that not combinations are meant to have rearward facing ports.  Out where I am (in the woods), most guys want bottom end AND top end for the open areas, which makes it difficult.  I really do try to consider the transfers in several ways, to include without a pipe, too much scavenge, too little, and just right.  From all of that, I have found that the longer the transfer length and the more rearward they face, the smoother the powerband.  It does nick a bit off of peak, bot most would rather have 80% peak power over 2500rpm than 2 more peak HP with a 1000rpm powerband. 

 

On the other hand, you have guys (like me) that just love the way their bike puts power down, but they want more.  What do you really do in a situation like that?  I guess that's why you always pipe to port instead of port to pipe.

 

The crankcase volume does bring a somewhat unexplored topic.  Most everyone thinks of the airflow as "through the piston ports" and "through the transfers", but no one thinks about the in-between.  What about the flow between the piston ports and the transfer ports?  It isn't exactly smooth.  A bit of epoxy and some more curves can really put you on top of some great numbers, but your transfers better be absofuckinglutely perfect, otherwise the power will vanish.  Tapping on that, the majority of my thought goes into getting more flow to the mains.  With Boyesen ports on a Banshee or Blaster, more air is going to reach the secondaries, as will a 250R, but what about the rears?  The 250R does a better job than most because the Boyesen ports are aimed down, which gives the air more of a chance to be in the place it needs to be for whichever port.  BUT, the problem where more air reaches the secondaries than the mains does allow for more pressure in the back of the cylinder does persist.  Does this help scavenge the cylinder or does it go out of the exhaust?  Again, this depends on the airflow through each. 

 

This is getting into my secrets here, but I believe that the secondaries shouldn't be allowed to flow to their full potential for that very reason.  90% with a careful shape is about best IMO. Additionally, I think the Banshee has a good design by having them aimed slightly upward because it does reduce the overall ability of the AFM to pile up at the boost port.  However, since the ports are very short after the last curve in the tunnel, the AFM can't be that concentrated, so it has to pile up and dissipate rather than flow.  With this, I like to either do a slight bottleneck with the big end at the cylinder OR epoxy the small radius and cut the outside ones very large.  Combined with the mains flowing at maximum speed to the back, it produces a very wide powerband.

 

Why? If the secondaries are flowing just as fast as the mains and are facing one another, pressure builds up and the flow from the boost port just adds to the turbulence.  By this time, the mains have nothing to run into but pressure.  The momentum from the mains jsut pushes everything out of the way and only a portion makes it's way up top.  Keep in mind that as the exhaust flows out of the exhaust port, it does have the slinky effect of the gasses moving.  That pressure up top makes it harder for the air to make it up top.  Since it has to go somewhere, and since fluids create a low pressure as they flow, the AFM just blends with the exhaust mixture as it passes by.  Of course, you do have the majority sitting around the boost port, snaking it's way up and you still have the pipe scavenging effect pulling the pressure out, but if you can direct that pressure ONLY up the back of the cylinder while the exhaust really vents, you'll be quite happy. 

 

My thinking for that is because the mains are at greater risk for loss out of the exhaust port, the AFM has to be screaming out of the tunnels.  The secondaries need to have the bottleneck early in the tunnel to make flow a tad difficult to get going, but flow like hell after they move.  They also need to be angled slightly upward (I like 7-10 degrees on this type). That way, the boost port has enough room to start moving and has momentum in an upward direction because the secondaries aren't piling shit up, while the flow from the mains approaches.  As the suction wave happens, the pressure from under the bottleneck is building up and screams out of the tunnel as everything else is just now dissipating.

 

BUT... if the secondaries are at a 7-10 degree angle, how can the floor be angled to get air across the top of the piston and prevent a low pressure spot which pulls the charge down AND maintain a shape that promotes flow in the direction that we want, which is 7-10 degrees upward?  If we depend on the flow from the mains to do the trick, we run into turbulence.  It does become a very difficult topic at that point and the answer hasn't come to me yet, but I'll get it to you guys when I figure it out.  Furthermore, a somewhat lower initial flow from the transfers means that there is more AFM vented from in front of the reeds (which is always good) than if it had to travel down to the secondaries.  *Keep in mind that air does have a slinky effect and momentum here.  I had to remind myself of that when writing all of this down in my notes.*  Now if travel was even to both, the AFM would try to split and create a very low pressure spot, halting the momentum of the AFM because of excess vacuum.  This is another reason why I think the secondaries should be delayed a bit. 

 

Boyesen ports may help out here some as well, but again, I haven't put as much thought into the design of these. 

 

Last thing on porting for now, I swear.  What about the area above and below the reed cage?  If air exists there, it can flow.  If it can flow, it can potentially give the reed pedal resistance and prevent it from opening while the pipe is in it's suction phase.  Again, keep in mind that air has a slinky effect and it will take time to get from the pipe to the transfers to the crankcase to the reeds.  Under an upstroke, this is a babyshit problem.  However, every incremental gain outside of that is a worthwile gain in my book.  If that area is filled with epoxy, it can be a good thing to have.  On the other hand, if the pressure is much too high to allow the reeds to open at this time, that epoxy just took up quite a bit of volume that was right there by the boost port.  Best over at Blasterforum did something like this and he said it had gobs of bottom end power.  I don't think it was a piece to a jigsaw puzzle of a build, but he did it and he said he was happy with it.  He had pics and a tutorial for it. 

 

 

 

 

 

 

I'd say that a Dremel is good for porting 4-strokes, carbs, small manifolds, transfer bottoms, exhaust ports, and reed cages.  For really good transfer work, you need a good handpiece.  My stuff is out with a buddy right now and I'm stuck with a Dremel.  To be honest, I just ported a TTR125 head, carried the shape out to the intake manifold, matched it to the carb, ported the carb, and cleaned the exhaust port up with a Dremel Chainsaver.  That one is the one to get if you must buy a Dremel.  It's small, cheap, powerful, and very balanced.  Outside of that....

 

YOU WILL NOT BEAT A QUALITY PORTING SETUP

[sstaton1983]

sstaton said he seen a engine that had alot of material removed from the crank area. i took that to meen down in the portion of the case where the crank wheels swing. i was merely stating that it doesnt make sense to me to add the extra volume in that location because of the long distance it would need to travel to reach the trans ports and cylinder. thats why i showed the photo with the green and red lines. makes more sense to me to keep the volume up higher where its travel distance to the cylinder will be much shorter. im sure you understand[/quikote]

I'm running those cases now... They look like someone done a DIY trench, but all the way across and deep enough to need epoxy...They passed a leak down

[sleeper06]

I do alot of jet ski /outboard porting . Before 4 strokes took over factory race engines had diffrent cases with added volume and these turned out to be the more desirable better running ski and stand up motors and more than one manufacturer had these options to certain people. Think about some of these ski motors putting out over 200 hp with a twin set up correctly. I have done extensive porting research read every book out there 10 times . In our shee motors I havnt found case volume to be one of the factors that yields big gains . I do believe in porting the tunnels but keeping velocity in mind. After over 100 sets I have a real good engine program for all types of riding .bringing the charge into the cylinder in an orderly fashion has shown me bigger gains than anything along with proper time area to exhaust the charge. Im no expert by far but through real world practice have found certain theories from my readings to work on diffrent platforms

[tfaith08]

So I wonder what the determining factor is here with the cases.

 

In theory, if you had a cylinder with porting configuration "A" and then cut another cylinder from a different application to the same proportions with the same bore and stroke, rod angle, head design, timing advance, pipe, reed, carburetion capabilities, filter and filter location, etc. it should produce an identical powerband.

 

Take a Blaster for example. With it's 66mm bore, if it was possible to stroke it to a 72mm (from 57) with a blank cylinder modified to the same proportions as a 250R and everything as described above, it would effectively be an air cooled 250R engine IN THEORY. Now there is obviously the possibility of doing this to make an engine the same as a watercraft. The reason I say that is because somewhere along the line, the case volume has to be altered to become an identical setup before optimum power is achieved.

 

So with that, the case volume dimension has to have a set of determining factors here.

 

The first that I think is triple transfers on watercraft. (does your 200hp configuration fit this bill?) That should create a greater velocity because of the same amount of pressure difference between the reed and cylinder, but the smaller port size due to the area of 2 port dividers rather than one. Also, the angle between the reed and the transfers, as well as the distance from the reed to the case and the transfers, would play a part here if I'm thinking correctly.

 

Now with a Banshee (or any engine), what would be the point where one would need have the case volume increased? I don't mean to imply that the goal here is to increase case volume, but I'd like to know when it would become beneficial to do so. I can come up with only a few times where I believe that it would be a good idea. One is when the increase comes as a byproduct of streamlining the flow from the reed and to the transfers. *This is one of the two places where my recent focus has been.* The other is when an engine is designed for a wide powerband and a pipe designed to pull enough AFM is fitted.

[RollinRhino]

Okay, lets first figure out the basic layout first.   Is this a case reed engine?   Case volume matters a lot on a case reed engine btw. 

[RollinRhino]

Look at any 80's or 90's 2 stroke motocrossers from the Big 4.   Case Reed induction was ultra efficient which is why a stock YZ250 made around 50 hp.   Even more amazing is a YZ125 which makes somewhere around 40 stock.   Both are case reed induction engines.   Less restriction and no fuel pooling in the valleys on those.   The powervalve also was a huge factor in the curve and delivery of power.  

[sstaton1983]

Reeds are a restriction in the intake.Rotary valve motors seem to be more efficient, to bad they take up so much room.

[registered user]

honda tried rotary valve on a mx bike. they must not of liked the results

[tfaith08]

A rotary valve can make some good power and they are very tunable, but a reed is more dynamic when talking about cylinder pressure.

 

And I do like how a lot of the modern, remaining high performance bikes have case reed designs.

[tfaith08]

Does anyone have any thoughts on Boyesen port aiming/dimensions?

[registered user]

if your refering to a honda, some of them the reed cage sits in so far it blocks off most of the boyesen port so im thinking its defeating the purpose of even havin them ports. ive heard of people moving the cage back alittle but i havent tried it myself. hard to say without knowing what engine you talkin about.

[tfaith08]

I'd say to lets keep it to Banshee engines, but I don't see why other applications couldn't be cited as well. I mean, once you start using epoxy in excess, a lot of porting configurations become possible. I'm looking at getting another set of Banshee cylinders and adding epoxy to the outside of where I want to cut the Boyesen ports.

 

I was looking at the extra 250R jug that's on my bench and I'm trying to see what the original design was going for. It's obvious that the engineers were going for a top down approach, but for what reason? Was it to aim the ports down that much or was it because the mixture was easier to get to flow in that direction? Design constraints? Why? I can see that aiming them down will allow the mixture a chance to make it to the mains. I can also see where aiming them up a bit higher might make the mixture want to pile up on the front wall of the secondary, so I don't know the reason. I know what I think about them, but I'm trying to get some constructive discussion going.

 

I'll tell you something that's pretty cool for you smokers (of cigarettes, of course). Take a cylinder and blow smoke through some of the transfers while holding the cylinder up to a light and watch what happens. It's cooler than you'd think it would be. If anyone doesn't understand some of the flow concepts that have been discussed thus far in this thread, go do that and you'll see.

[trickedcarbine]

Used to think guys with flow benches were just stoner hippies with insence. Then I learned what smoke sticks were for....