This phenomenon has me thinking lately how I can incorporate it into a cylinder...
https://en.m.wikipedia.org/wiki/Tubercle_effect
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What that article isn't telling you is that the divots induce spin (vorticies), which help stir up the boundary layer (stagnate air near the surface) into the free stream (high velocity). Thus the boundary layer is decreased, the drag is decreased, and the flow will stay attached (avoiding stall) at higher angles of attack.
If you really want to boggle your mind look up F1 vortex generators and see how they use little fins on all sorts of surfaces to effect things down stream. They are not used just on the front wing, but everywhere. Those air fairies get crazy.
Big transfers, why do they work for some and not others? There is a lot that goes into it other than just hogging em out. Direction of flow should start at the lowers and follow that shape and line all the way through tThe tract into the upper. I think that the entire port needs to be reshaped as a funnel which is not easy to do and takes a long time. If done right I believe they can work for some saws and some applications
I think the key to utilizing big transfers is of course balancing the intake and exhaust time accordingly but also paying a lot of attention to aiming the fresh charge into the cylinder to maximize the loop scavenging effect. Since fluids don't take kindly to being steered the aiming has to happen through the whole port, the 930 cylinder is a very good example. When I widen a transfer it's often just towards the exhaust side, maybe a little on the lower and under the bridge on the intake side to create a better funnel. It does take a long time to do, I'll often have 2 hours of grinding on just the transfer on some cylinders.
I like to imagine that the pressure inside the case is pushing a large volume of air into the transfer ports very abruptly, so smoothing and creating a larger inlet helps create a more laminar flow. Now we need to gently squeeze that volume down, increasing the velocity without getting into the transition zone to turbulent flow or cause a large growth in the boundary layer, both which are very detrimental to mass flow; this is choked flow, increase in pressure differential (RPM in this case) will not gain much in mass flow. The main squeezing happens under the bridge to about mid of the upper corner and will increase the velocity of the air, which according to Venturi's principle will reduce the pressure. With the increased velocity and hopefully very attached, straight flow there should be a nice jet stream into the stagnate exhaust gasses in the cylinder. The jet will slice the exhaust gasses right off the piston crown and crash into the intake wall. Then pool and climb the intake wall essentially filling the intake side half of the cylinder first while the remaining exhaust is forced out on the other side (loop scavenging).
Aiming and staggering the timing of the transfer port opening helps reduce tumbling inside the cylinder which will decrease the amount of exhaust gasses that are remixed into the fresh charge.
Quad ports have an advantage over duals because the divider wall provides more flow stability. The divided area helps correct any tumbling or rotation in the flow created by the inlet that might be carried in the center mass of the dual port. Essentially its a vain to help keep the flow in the laminar region.
Excellent example of what we're talking about. I never thought to start "aiming" the charge in the lowers. Brings up my next thought. Since most, not all of the flow is concentrated on the outside of the port I think shaving the inner bridge material down some would aid in reducing turbulence in the turn. How much is too much with reference to maintaining velocity? I don't know, but in my worthless opinion I think there are gains that can be found. To me, the more the path can be smoothed out the better. As long as velocity remains adequate.
I don't think grinding on the bridge will get you much. In 4 stroke porting terms you could consider that the "short side", which as a general rule you don't spend much time there because you'll probably only create a loss. Here, due to the corner, the flow is probably going to become detached and there is most likely already a tumbling pocket of air. Grinding here will only increase the pocket and the losses in free stream velocity and mass flow associated with that.
Your best bet to reduce the loss due to the corner is to increase the corner radius and create a larger straight section after the corner to regain stable flow. Grinding on the outside of the corner and up from the bridge to reduce the entrance angle will do this. But, the increase in length of the port alters the timing of the pulse, the added flow potential could counter acts this, but still transfer port timing may need to be different than what you used before for smaller ports.
. But piston design seems to be a big influence on transfers ? Please correct me if I'm wrong. The 930 has a full circle windowed piston while most of the quad port bottom fed huskies have a flat side piston I noticed. The 930's cylinder base near the ports seems to be taken far up the walls as to match the windows in the piston? The huskies on other hand are level with the base. The 930's entrance to the transfers is huge and seems hogged out but if you look at it, it gets much tighter near actual opening. Interesting. And the transfer seems agressively aimed at intake even more so than the few cylinders I've seen inside. They must've wanted the "charge" to have an easy entrance path but get tighter accelerating the flow towards the intake? The 385/390xp run very strong for there displacement, close to the 394/395 while giving up a lot of cc's. They have bottom fed transfers, while the 394/395 don't. Wonder if that is why?
