Porting past flange diameter

Discussion in 'Performance Mods' started by velzie, Mar 24, 2014.

  1. velzie

    velzie Member

    Is there any point in porting the exhaust port diameter greater than the hole in the exhaust flange? Similarly, what about the intake? I bought a Z extension but the flange hole is much smaller than the intake port (and smaller than the original intake tube). Will the benefit of greater length outweigh the smaller diameter hole?Thank you!
    Last edited: Mar 27, 2014

  2. HeadSmess

    HeadSmess Well-Known Member

    think about it. a smaller hole will flow....less air possibly?

    ahuh. yep.

    now, another thought...what is streamlining?

    yup. making things all smooth and curvy so that there isnt any turbulence. why?

    hmmm. to save energy, by maintaining a smooth airflow.

    whats this got to do with porting?


    you dont want ANY sudden changes in diameter, you dont want ANY lumps bumps or protrusions, you definitely dont want ANY restrictions on the intake...

    smooth as a babies butt is the generic term. not polished, mind you. big difference!

    the length of a pipe isnt going to do DOODLY SQUAT UNLESS all the ports and fittings are shaped to ensure smooth, non turbulent airflow.

    this means a dremel, a few files, some patience, and figuring out how to match ports.

    enhancing flow will do far more for performance than just throwing on items that cost money and dont match, therefore, dont work.
  3. jaguar

    jaguar Well-Known Member

    "porting the cylinder diameter greater than the hole in the exhaust flange?"

    the word "porting" is commonly used to refer the change the size and shape of the cylinder ports (intake, transfers, boosts, exhaust) so that you can't "port the cylinder diameter" but you can bore it out to a larger size if you have a larger piston to fit. Either way, your question makes no sense at all.
  4. velzie

    velzie Member

    The typo has been fixed. HeadSmess understood.
  5. max350

    max350 Member

    Bigger is not always better, the only "porting" needed in a china engine if you are not knowledgeable in tuning, The thing to do is to smooth and remove burrs in intake, transfers, boosts, exhaust and make a "jacka "in the piston to open the intake completely

    ("jacka" piston is an old Swedish word for that kind of tuning)
  6. velzie

    velzie Member

    I appreciate the responses, but the question was answered by HeadSmess.
  7. max350

    max350 Member

    Then you can close the thread for further response
  8. velzie

    velzie Member

    I wasn't aware that was a thing.Bro, do you even forum?
  9. HeadSmess

    HeadSmess Well-Known Member

    typo? :confused: ?

    there can always be addendums...

    best way to match ports...bit of blank cardboard, mount over port. paint flange with something. carefully mount, and get impression on cardboard. then you know where to trim flange. to get the port, rub the (thin) cardboard with a pencil... from memory the shape is approximately a 22x17mm oblong, with 6mm radius corners... vaguely... :) oh. exhaust.

    intake was something like 17x12...? with a 30 degree tilt... gotta try and match that, smoothly.

    if you really dig deep you find just about every bit inside can do with some smoothing off and matching... 8 grams of aluminium shavings later :)
  10. ImpulseRocket89

    ImpulseRocket89 New Member

    I have no intention of digging this thread up, but I wanted to interject a little engineering and fluidynamics in to answer the OP's question.

    You can open up the port size to the same diameter as the primary tube on the exhaust manifold flange, and yes in basic principle it will flow more gas. However, that does not mean you will increase it's effective volumetric efficiency, and here is why... (and so it is known, this is mostly 4 stroke theory in practice here)

    Almost all engine exhaust manifolds are designed to have primary tube I.D. larger than the port in the head. This larger diameter creates a ridge with a flat wall at the face of the port inside the tube that does two functions. First, with a high pressure and volume of gas coming from the port, that sudden expansion in volume creates low pressure zones that help with filling the immediate volume of the pipe. Secondly, that low pressure zone creates a swirl effect around the wall of the flange inside the tube and creates a vortex, and that vortex around the low pressure zone then becomes a high pressure barrier. That barrier effect actually prevents some exhaust reversion from going back into the cylinder, contaminating the soon to be fresh intake charge with "dirty" air.

    If it helps you understand it better, think of it like the reverse opposite of having intake manifold runners a larger diameter than the head port. They create a high pressure zone that restricts flow into the chamber. Thus, port matching the intake removes all restriction into the head and giving a smooth and clear path to the valve. In the case of the exhaust, you kind of want the opposite effect between the manifold primary tube/runner and the head. The restriction at the port actually allowing better exhaust escape velocity, and causing a restriction for exhaust gas to flow back into the head.

    In short, if you were to increase the exhaust port size to a point where it matches the flange side, it would be smart to actually increase the manifold and tube diameter at the head.

    Now, With 2 strokes it can be a little different, because of things like expansion chambers where the idea is to try and reverse some lost intake charge into the cylinders, and having reliance on port size and position to determine how the engine runs and where it makes its power vs. a camshaft and valves of a typical 4 stroke. This is a grey area to me because I have not researched it fully. Even with said ridge of gas to reduce reversion, it would not prevent the returning exhaust wave pulse from putting some of the lost intake charge into the cylinder again. If it is more beneficial to have the flange port size be smaller than the exhaust side would really come down to the unique aspects of 2 strokes and the effects of too free flowing of an exhaust can have on chamber temps. In truth, just going off of the basic intake and exhaust flow charecteristics of a 2 stroke engine, in some ways it seems you would want the opposite of the principles behind a 4 stroke to work in your favor, especially on the intake side.Since 2 strokes are relatively unfamiliar to me, this is my disclaimer due to ignorance on the subject.

    Hope that helped you understand or make sense.
    Last edited: Apr 19, 2014
    velzie and FurryOnTheInside like this.
  11. velzie

    velzie Member

    I've always wanted to sneak in on (or possibly register for) fluid mechanics classes, I have all the pre-reqs!

    I will eventually experiment with my header length later this year and will now have a greater consideration of header ID.
  12. ImpulseRocket89

    ImpulseRocket89 New Member

    I should let it be known that most of that theory is coming from a 4 stroke world. In all honesty, in a 2 stroke it would seem more beneficial to kind of work it inside out and backwards from the ideal setup of a 4 stroke. (I have subsequently edited my previous post to reflect that)

    Explanation. On the intake side, having the port actually larger than the diameter of the intake manifold tube from the carb would create that same reversion preventing vortex effect, and in effect would reduce the amount of intake charge pushed back out during cylinder down travel. And having the manifold flange and exhaust tube being smaller than the exhaust port to some degree would help limit how much fresh intake charge was pushed out of the cylinder (and regulate chamber temps) during the exhaust cycle. 2 stroke engines, by nature, are far more sensitive and reliant on port shape, size, position, etc than any 4 stroke, because they rely on those for the same function as multiple parts on a typical 4 stroke.

    It would be kind of fun to experiment with different combinations and record the results though. 2 strokes are still a relatively new world to me, but there is a lot of good info out there that I am reading up on.