Porting basics

[tfaith08]

Thank you, sstaton.

[04zSpittinKlotz]

Zilla, quit being a dumbass

[registered user]

tfaith thats great your trying to help but theres a few things you should realize and probly already have. besides the site sponsors theres probly on 2 maybe 3 guys that do their own work. everyone else has likely never touched a porting tool or know what happens in the cylinder and pipe during a running cycle. zillafag for example. sprinklerman and trickcarbine are a couple more that know only what theyve read on google. your efforts might be better spent on other forums but im sure you already realized that

[sstaton1983]

Good example of why I go elsewhere to try to learn more....someone wants to discuss somthing like 2 stroke theory and operation and nine times out of ten the input is "send it out to a sponsor". When people come hear and want to learn or discuss somthing like port designs or expansion chamber design, then the flaming begins.....There is a difference between guys wanting to discuss to try to learn more, and then guys that come here trying to prove somthing...

[tfaith08]

Well, if Zilla wants to see the thread trashed, he can try to get it taken it down.  Otherwise, I'm going to make a contribution whether you, or anyone else, want me to or not.  I've been building engines since I was 11 years old and I had a multiple world record holding hot saw builder (PM me for the name) that took me under his wing and taught me a lot.  I had already quit drag racing my LT250R by the time you joined the forum, Zilla and I'm assuming you joined because you had some question about how to jet a carb and couldn't use the search bar.  With the exception of the Tecate-4, I've ported every model of 2 stroke quad there is, as well as quite a few dirt bikes, chainsaws, and kart engines.  I don't give a shit if you have 5,800 posts, don't talk to me like I'm a noob until you can make somewhat of a contribution that isn't regurgitated MacDizzy, Bell, or Jennings material.   So until then, why don't you piss off until you can decide to make a contribution to the forum (which is what we should all be here for) instead of picking apart the definition of porting.  Matter of fact, why don't you chime in and give us your experience-based guidance on how to port a cylinder?

 

 

I know that there are guys out there, both registered and non-registered (guests) that are actively looking for more information on porting, because I know I searched relentlessly for anything other MacDizzy's site when I got my start.  I read that site front to back and just about had it memorized, but there were still gaps.  I couldn't buy a book, I couldn't find any worthwhile material, I sure as hell couldn't afford a professional porting job, but I wanted to learn.  I started on every 2 stroke engine I could get my hands on and learned what worked and what didn't.  When I finally got Jennings's book, I understood it because I'd been through the process. 

 

Now, I'm crunching numbers for a TRX250R cylinder, and the owner wants the widest powerband possible with an emphasis on lower mid-range.  He's using a 36mm Keihin and PC exhaust, 22cc dome, and a short-rod bottom end. 

 

Why don't you tell me what numbers you'd use for the transfers, since I'm currently figuring those out?  Oh, and I still need an angle for the boost port as well.  This is an exercise in which I can 1) show the other readers how I use my methods, and 2) show you that you have no place to try and pick this thread apart unless you actually do know what's going on, in which case it really wouldn't matter because you lack the basic courtesies required to sustain an intelligent, constructive conversation, or at least so have you proven thus far.  In either case, you need to GTFO, ZillaFreak.

[sprinklerman]

In an effort to have a meaningful 2 stroke theory topic, why don't we pick a specific topic?

How about anyone's theory ( tested or not) on the benefits of looping the incoming transfer charge or maybe how to keep as much of the fresh charge In the cylinder while it's filling?

Just a couple ideas.

 

Sent from a van down by the river

[sstaton1983]

Aiming the transfers toward the back of the cylinder (opposite from exhaust port) should keep the fuel from dumping out the exhaust. Something i think about is crank case volume and how it effects crank case pressure.. I came across a set of cases that  had alot of material removed around the crank area and wondered how much that effected the pressure to move fuel and air up the transfers..

 

JUST THINKING OUT LOUD.... 

Edited May 7, 2014 by sstaton1983

[Deadbeat]

pretty memorable porting thread here tfaith if your interested http://bansheehq.com/forums/index.php?showtopic=163680&page=1

[tfaith08]

I tried the angling of the transfers toward the back of the cylinder on quite a few cylinders.  However, I've only ever done a port job without and then with back to back on my wife's Blaster.  I found that it didn't feel like it affected peak power whatsoever, but it did add just a bump to bottom end and midrange, but it did help on over-rev.

 

In my mind, what's happening here is that the charge is being forced toward the boost port and then forced up by the boost port.  Like you said, it keeps the charge away from the exhaust.  When the initial wave pulls the charge out of the cylinder, the AFM has to redirect because it already has momentum in another direction.  It ultimately amounts to less in the expansion chamber, making the return wave a bit weaker.  In addition, the smaller content creates a lower pressure in the pipe, which I think makes the exhaust pull out of the cylinder a bit more.  Since the charge is already looping back then up, it has to then travel from the upper region of the cylinder, giving it more distance to travel.  This makes it harder for the charge to snake its way out of the exhaust port. 

 

Also, I do believe that there exists a slightly grater vacuum between that region of the cylinder and the exhaust with rearward facing ports, so think on that.

 

As for peak, I do think it may bump it to the right by 100rpm or so, but again, this is a theoretical idea; I never felt it on the Blaster.

 

On over-rev, the return wave is only partially making it back to the cylinder before the piston closes the port.  That vacuum I mentioned earlier contributes here, I THINK.  I believe that it helps the exhaust continue to pull the charge on around because it has a further path to travel as opposed to the original design. 

 

 

 

As for crank volume, I don't have the resources to test this.  With a pipe that doesn't have a great scavenging effect, I'd like to think that a smaller crankcase would be more beneficial throughout.  Without a strong suction from the pipe, about the only way to get charge into the bottom end would be from the upward reciprocation of the piston.  Don't forget about the momentum of the charge traveling through the reeds.  If everything flows well enough, the potential exists for a slight supercharging effect.

 

Now a pipe with a great scavenging effect would take more advantage of the larger crankcase volume.  Not much more than basic reasoning is required to understand that the piston won't pull in as much or create as much pressure due to the pressure differential, but the pull from the pipe has the potential to pull mixture through the reeds and through the transfers even at BDC.  I think the limit here with a very, very thoroughly designed pipe/port combination would be where the reeds stay open for all but just after the transfers open. 

 

 

Again, this is all in my mind.  I don't have a formal education in fluid dynamics or fluid mechanics, but I have read up on them enough to answer the occasional porting question. 

[tfaith08]

https://workspaces.acrobat.com/app.html#d=5eNaltKS9ZOTGrp0FI7cGA

 

That was posted on there.  I skimmed through and agree with a lot of it, but one thing kind of stands out.  I don't like to port match the exhaust port to the pipe.

 

A smaller diameter will maintain velocity and momentum.  However, a step will prevent exhaust reversion, which we want with an expansion chamber.  I'm experimenting with a reverse venturi nozzle right now.  This will maintain the velocity, but allow the pipe reversion to take place.

 

I did notice one thing that I highly agree with.  The picture of the modified boost port that has a taper around the reed side of it is something I like to do.  There isn't much volume for the boost port to pull from when approaching BDC.  Making the transition to the boost port will aid in flow here, if my thinking is right. 

[Pasi S.]

When do you know, that you can actually accomplish a "port job"??

[n2otoofast4u]

FFS

 

Sent from my SCH-I545 using Tapatalk

[sstaton1983]

When do you know, that you can actually accomplish a "port job"??

Measure cut repeat, once you can fit your fist through the ports, then you can pour in the afm.....

[sstaton1983]

    

[tfaith08]

When do you know, that you can actually accomplish a "port job"??

 

I start with exhaust duration.  Then, I go with transfer duration based off of exhaust duration using blowdown figures.  After that, I use a version of Jennings's port-time area formulas  to find width/corner diameters.  I generally like to keep the corners at the highest possible radius as allowed by port shape and port-time area numbers.  After that, I put a piece of paper in the cylinder and use a pencil to scratch over all of the ports.  I then measure the ports and figure height from duration.  I then mark the new numbers and shapes down on the same paper and then translate it over to the cylinder(s).  Once I have all of my marks, I use carbides cut to them and then match the rest of the port shape to them. 

 

Then, I port the bottoms of the ports and make sure they line up and match the shape of the ports.  I double check all of my port measurements and once all of that is done, I switch to stones and finish all of my surfaces and call it good.