Sesshoumaru

I decided not to do a street port



I left the stock ports intacted but I was wondering what type of texture would be the best.



For air flow/fuel atomization



here is a pic of the secondary

Sesshoumaru

primaries (even more of a pain in stock size)

Sesshoumaru

another

Sesshoumaru

last

1Revvin7

What was done on my black t2s motor was a bevel on the closing edge of the port and a backcut on the waterjacket side of the port. Figured it couldn't hurt.

Sesshoumaru

I actually took A LOT of material out.



made the transitions a lot more curved and "natural". I know it has to flow better (these are FD's) b/c below the primaries there is a big casting "blurp". Not sure what that is for but I took that out

Lynn E. Hanover

The answer is that in straight lines, or straight tubes and ducts, the smoother the better. A chrome like finish is not out of the question. There a number of considerations that generally exclude a chrome finish.



If you are flowing just air, chrome is good. A round port is the ideal. The idea is that a tube of constant cross section will cause no changes in velocity and maintain the best flow rate (generally reported in cubic feet per minute) or CFM, so, smooth first and constant cross section second. Hold your hands about a foot apart and imagine a cubic foot of air. Now look at the two ports feeding one rotor. Now think about say 300 of those cubic feet things going through the two little ports in one minute. Not likely is it?



In a normally aspirated (NA) engine, at sea level, there is a maximum of 14.7 pounds of boost available to push the air into the engine. There are some (NA)rotaries, at some RPM that can do this, and a bit better. They can ingest more than they displace. Yours probably isn't one of them. So that leaves most of Death Valley and any beach as the best place to run the engines.



Anyplace else and the free boost is at a lower pressure. But I digress.



The best is like chrome until you want the turn the flow, then even the light weight of the air in a centrifugal load tends to use only the outside of the turn. This is why you see some ports with a "D" shape with the flat part of the "D" on the inside of the turn. So the inside of the port has more area than the outside, so the air slows down a bit, (Bernulli) the centrifugal load is reduced and more of the air stays closer to the inside of the turn. Even though this part of the turn may have a slightly larger cross section, the effect is that there is less velocity change around a carefully designed turn, and less energy is lost. It makes sense so far, right?



But you want to do all of this and, have fuel mixed with the air flow. Two more problems here. The weight of the flow goes way up. And, the mixture wants to change back into drops of fuel, rather than stay in a gas (pun) like cloud. In a street car there is little problem with this. The ports are small. The flow velocity is therefore high. (Bernulli) The high velocity means that there is more energy in the mixture and it tends to stay in the gas like state. The surfaces of the port are left slightly rough.



This induces turbulence into the flow, and helps keep the fuel in suspension. Next time at the airport look at the vertical fin just in front of the rudder on the big jets.

See the line of little tabs sticking out of the fin just ahead of the hinge line. Those are turbulence generators. (vortex generators) They introduce a spin in the slow moving boundary layer moving over the fin, and Bernulli effect (higher speed=lower pressure) clamps the flow to the rudder, making the rudder more effective. On some planes there is a row of these things along the wings in front of the ailerons. Same idea.



So there is an aspect of a rough surface that is good for flow if it is restricted to the inside of turns, like the curved upper surface of an airplane wing. Adding a little energy to the the flow just before the turn keeps the flow attached to the wall of the port, (the flat part of the "D" shape) and total flow (cubic feet per minute) goes up.



OK, so what about the outside of turns. I polish the crap out of them. Make them look like chrome. Most of the flow is around the outside of the turn, no matter how much you work to improve the flow around the inside of turns. So now think about this. In most systems, drag increases at the cube of velocity. Think about an exponential curve. The faster the flow, the more important it is that cross section be constant, and a smooth surface be maintained. But we need this stuff to turn 90 degrees, and we want no change in velocity.



People in hell want ice water.



Houston we have a problem.



The flow tends to stay along the outside of the port because of centrifugal force, and that part of the flow tends to be richer in fuel droplets for the same reason.

To keep that velocity as high as possible, I polish the outside of the turn, and other than getting the shape uniform, leave the inside of the turn a bit course. The surface finish along the outside of the bowl and the radius into the chamber are more important than almost anything else in the port job. For street engines the sanded finish with fine paper is just fine. The slightly rough surface tends to reintroduce droplets into the flow and keep things nice for the computer. By the time flow gets to the closing line, there is not much left to do that can help mixture strength, but you can polish the radius all along the closing line, and improve flow without changing port timing at all. And the side seal will just kiss you the next time you open the engine. Another benefit is that there will be no detectable vena contracta at the port exit. And we all know how much that hurts.



So if you are closing your eyes by now and still want to pass the quiz, here is the recap:



Mother nature cannot be fooled.



Smooth first. Fine paper finish for the street. Better with chrome outside turns for racing. Rougher short side radius, slightly flatter if there is room. ("D" shape).



Second. Constant cross section. Mother will not let you change velocity without an energy penalty. Make little patterns that fit into the runner and slide it along so that there is as little change as possible for the length of the runner. Where a shape change must be made, Keep the square inches of runner cross section the same in the transition for as long as possible. A huge port at the end of a small runner can only get the power from a timing improvement. The greater the cross section change, the more velocity will be lost. So go conservative at first. A car that runs great from 7,000 RPM to 9,000 RPM is no fun to drive to work.



Third. No sharp edges at the port opening. Break sharp edges with silicone carbide paper.



My first race is at Nelsons Ledges (near Akron Ohio) next weekend. I fired it up yesterday to get the air out of the coolant system. I got 85 pounds at idle. Eat your hearts out.



Lynn E. Hanover

ryosuke_fc

Wow. You never cease to amaze me. I know we're all very thankful for the wealth of knowledge you share with us Lynn. I'll be porting my engine for the first time soon, and this was most helpful.

Sesshoumaru

Awesome - thanks



lets me see if the student picked up on the key items



Ideal-



Keep same shape through out path

Mirror (keeps kinetic energy)

No bends (loss due to normal force -centripetal)



Real life-



To minimize loss of energy around the curve rough the inside a bit.

Not ideal path so try to obtain a equal shaped chamber





Question-



In the pic I put a pivot.

Is the reduced velocity due to the rough area meant to slows the air down so that the outside can keep "pace" with the inside reducing turbulence?

Lynn E. Hanover

In the situation where we just fire up a stock port and rev the engine way up, there are locations in the runners on the inside of the turns where laminar flow separates at random locations and interferes with the flow along the outside of the turn. So in addition to being unable to track the inside of the turn because of centrifugal force, the flow actually reduces existing flow that is still performing well.



The flat side of the "D" shaped runner is exactly like upper surface of an airplane wing. When flow velocity is high enough that it cannot remain attached to the inner (Flat part of the curved inner surface) it will separate just as it would on an airplane wing at too high an angle of attack. The added problem is that this troubled flow will then begin to choke off the high speed flow along the outside of the turn ("D") shape. So if you had a chance to design the runner instead of just grind on it in some locations, you would want a smooth constant radius turn with a flat vertical wall. A rough leading edge would help flow remain attached to the surface.



But since we need more than that amount, we can do an airplane trick like the big Boeings and leave the wall along the leading edge of the "D" shape rougher than you would think you should. This rough surface can reintroduce any stray condensed fuel droplets, and it will upset the boundary layer and add energy in the form of little spinning tornados that lay along the runner wall like a boat wake.These are air molecules moving greater distances in the same period of time as similar molecules just above them in the flow. (Bernulli) So the turbulence thus generated creates a lower pressure than the air just off of the runner wall, and that part of the flow is forced tight against the inside wall and will track right around the turn.



On the second part, An attempt should be made to maintain the same cross section in square inches where the runner shape must be changed. That is the way to keep the average velocity the same through a transition in shapes. Such as the oval runner shape changing into a "D" shape for a 90 degree turn. The will be local changes in velocity that are functions of the shape as in the "D" shaped turn. But the object is to have the average velocity through the system remain constant.



Making the port taller and narrow as in the "D" shape, slows the short side part of the flow, and removes some of the centrifugal load from the flow. (So the airplane wing does not stall).



You can go to some great lengths to make gains in the runners. However, there is a point of diminishing returns where the runners can be made larger to flow more for a given differential, but the increased runner size reduces velocity. So the aperture time remains constant and at the point of diminishing returns, the increased runner size no longer improves HP. So at that point, a slightly smaller runner would deliver the same cubic feet of fuel air at a fixed aperture time (a particular RPM), and be more responsive in street use.



So at first, nearly everything you do to the runners makes it perform better, because as you improve the runners, you allow an increase in velocity (for that particular situation). Once you get to the point of diminishing returns, nothing much else you do to the runners will help performance until you alter port timing to again increase aperture time. The length of time the intake port is open at a specific RPM. Then the program starts over again.



There is another wall that is a function of opening time verses RPM. As you can see as RPM goes up, the number of intake opening events also goes up. However, each event is much shorter as revs increase.



The tech section of Paul Yaw's web site is very clear and well written.





Lynn E, Hanover