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Head Flow - A Deeper Understanding

  
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Head Flow - A Deeper Understanding

 
tuffnuff tuffnuff
Moderator | Posts: 7827 | Joined: 12/09
Posted: 09/22/13
08:03 AM

Let me say that Racers are pretty fickle people, they abandon things and people that don’t work for them. So one must consider that this fellow is doing something right even if at a distance from the mainstream thoughts on this subject. There is something to be said about not using a velocity probe as simply placing it into the air stream upsets the flow and disturbs the measurable result. This gets into the old scientific argument that the very act of observing a system upsets that system and what you “see” is not the natural system. However, I like to use velocity probes but do appriciate the problems they introduce and understand the position of guys that rather read gross flow instead of local flows. Yes the aftermarket tends to give up low lift flow in favor of mid to high lift flows. This works for a competition engine where lifts tend to be .5 inch or more but a “warmed up” street engine geared to turn low revs at cruise wouldn’t care for this approach as it reduces low and mid RPM torque and pushes both the torque and power peaks up the rev range. Which is OK if you have the gears to sustain the revs. Otherewise the engine will be slow as it has to wind its way to the torque peak, especially in higher gears or overdrive. So to a large extent the intended purpose of the engine will dictate how you go at the ports. This is to say that the ports, the cam and compression all need an agreement  that supports the vehicle weight, aero drag ratio, gearing and unique track or competition qualities. In-so-far as wet flow and atomization are concerned, in many ways these things take care of themselves within a competition engine where high operating speeds result in extremely turbulent gas flows within the ports and this tends to keep the mixture well homogenized. As the RPM levels drop and port velocity slows the flows become more laminar and centrifugal force may throw fuel out of the mixture. At this point understanding wet flow characteristics become important as now the port needs to be designed to remix the fuel and air streams at some point ahead of or within the combustion chamber to achieve proper atomization prior to ignition. At least this is true of carbureted or TBI engines. Fuel injected engines where the fuel is introduced at relatively high pressure either behind the valve or directly into the combustion chamber don’t have the same problem of maintaining long distance atomization within the mixture. But much care has to go into placing the fuel stream relative to the air stream to achieve proper atomization with these injection systems.
Some things that work on one engine don't necessarily work on another. If you have a copy of David Vizard's book "How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads" he takes you through the pretty general stuff that's applicable to most anybody's heads. His writing is actually well founded for improving the street engine that also sees some competition. But it really doesn't cover the discoveries and secrets of building competition heads. He does cover building your own flow bench very well. My homemade bench of many years ago was quite similar except it used a furnace fan as the vacuum source. Cranked all the way up this cleared the area of small children, dogs and cats, pulled the carpet off the floor, laces out of your shoes, stuff like that. Of course in reality you have to adjust vacuum, thus flow, to simulate conditions at various lifts. The big problem is that it’s difficult to use a fan to duplicate the real dynamics of gas flow within a port being subjected to variable suction which is dependent on piston position and velocity in the bore, reversion effects caused by exhaust pulse waves and waves generated by closing intake valves, or the effects of valve opening all of which which cause all kinds of pressure pulses going up and down the intake tract. Generally low lift flows are also high pressure low speed flows dictated by the ram effect as flow velocity is converts to inertial mass. However this switches quickly where mid and upper cam lift flows tend to become high speed low pressure events. These variable flow characteristics prefer different shapes within the port and arond the valve. Of course we're subject to finding the compromise shape that offers the best solution obtainable for either case. At low lifts, upon valve opening the piston speed is slow as it’s going around TDC. The previous valve open cycle ended with a high speed low pressure flow that transitioned to a low speed high pressure flow as the valve closed. This happens because inertia tends to keep the flow pouring into the valve pocket where velocity falls off and pressure builds. This also sends a shock wave back up the port. Clever designs catch this wave as it reflects back into the port just as the valve reopens for the next cycle, this adds some over boost or natural supercharge to the mixture. However, when the wave is out of time with engine events it will conspire to reduce cylinder filling. When the valve reopens, this high pressure mixture starts to enter the cylinder even though the piston is hardly moving. Being a low speed high pressure flow the shape of the port and the backside of the valve don’t have too much influence yet except that a smooth backside to the valve or of the port wall causes the fuel and air molecules to stick and form a sluggish boundary layer. A good example of this is the slow speed that rainwater has as it crosses your windshield, this shows the airstream adjacent to the surface is moving very slowly compared to the speed a few inches above that surface. This surface air is moving much more slowly because of molecular adhesion between it the adjacent surface. Inside a pipe (port) this slow moving air or fuel mixture makes the pipe seem smaller to the passing gas flow. In the case of an airplane it adds weight and increases drag. That’s why if you look closely at a jet airliner you will see flush riveting everywhere except at the tail, here you will see headed rivets to roughen the surface. This is used break up the adhesive qualities of the passing air so it will peel off the skin. Otherwise a 747 would be dragging a layer of sluggish air 10 feet deep all around the tail. You need the same thing inside a port especially where the sections are divergent (opening away from each other).  Getting back to piston movement and air flow, by the time the valve is at mid lift the piston is moving very quickly down the bore and gas velocities are becoming quite high. At this point port pressure is falling and flow is very port shape dependent as inertia forces everything toward the outside of turns. Here clever shaping to induce small but extremely low pressure eddies can be used to force the mainstream to make turns it otherwise wouldn’t. Some of these features such as an abrupt “Kamm” cut on the short side turn or a lip on the backside of the valve help make the apparent size of the port seem larger to the flow than the more intuitive design of smooth streamline shapes would. Once the piston passes half way down its stroke it begins to quickly slow, but the gas velocity it imparted to the incoming mixture is so high that the flow keeps coming even though the piston has slowed and even reversed direction past BDC all while the valve is closing. In a competition engine there is significant flow well into the compression phase before the valve must be shut to trap the charge. Which emphasizes the need to shape for low speed flow as the result of some compromise on the high speed flow. This is a way of saying you need a compromise that makes the best of both low and high speed flows but will not be perfect for either. Again the use of the engine will dictate which way the compromise favors. A full out competition engine with a very high lift cam will lean toward making optimum use of the extreme lift. Of course pressure continues to build behind the closed valve because of the inertia effect from the intake flow and the process repeats as the valve reopens. At low speed, of course, these inertia effects are not present and the engine 9especially with a long duration, high  lift cam) reverse pumps the intake charge back out the carb. This results in the ragged idle of a hot cam. Now appriciate that this is a simplification as I haven’t gotten seriously into wave effects which can be significant for or against flow. Lingenfelter, Morrison, Holley, TPIS and others have good books out there on the subject of porting. Type "Engine Porting" into your search engine and stand back, whip out your credit card and start buying books.
Now general theory suggests that opening the valve beyond a fourth of its diameter doesn't improve flow to any great extent. If you review the published data on head flow you will see an indication of truth to this in that at lifts higher than .25 the valve diameter show a very marked decrease in the rate of flow improvement. However, if your building a competition engine, these small improvements at .6 to .8 inch lift for a valve around 2 inches in diameter can provide the difference between winning and loosing. So while theory correctly predicts the slowing the increase in the rate flow, it by no means suggests that a competitive racer should not go there.
Some engines like 30 degree intake seats and some don't care. Again Vizard discusses this in his book, or aims you at another one of his books that does. Way back in my FoMoCo days, the Ford FE (332-428) liked 30 degree seats, though the Tunnel Port 427 seemed immune to them vis-à-vis a 45 degree seat. The Chevy Rat and the MoPar Hemi didn't seem to care either way. The AMC engine has a pretty tight turn into the pocket, similar to the FE Ford and probably would like a 30 degree seat.
Valves in different engines and with differently ported heads respond differently to back cutting. Some like the back side to be smooth and others don't. This gets back into the discussion of energizing the boundry layer flow to get to let go of the port wall and the backside of the valve. The same can be said about the shape of the port as it blends toward the valve seat or as the bottom of the runner comes around the tight turn into the pocket. Some ports like this area to be smooth and diverging to the seat such as those of Small Block Chevy and MoPar engines. Others like a smooth venturi shape just ahead of the seat. Some respond well to what could be thought of as a sharp edged venturi ahead of the seat. The smooth flow around the valve that is portrayed in Smoky Yunick's book while very pretty, doesn't exist because in the real world the flow is not symmetrical around the valve and in the case of a 2 valve wedge would be fatal to power output as it would foster a tumble port which 2 valve wedge chambers do not like.
You can use various shapes in the port to pull air around or put it where you want it. Cutting off the bottom of the port as it joins the pocket on the short side radius making a "Kamm" style termination can be effective in pulling the airflow around this corner. Some people also dimple this area like a Golf Ball’s surface for the same effect without causing fuel to puddle ahead of the Kamm spoiler shape. These create a very tight low pressure local swirl that pulls the passing mixture tightly around the turn. An airplane example from back in the early days of the F-111 test flight program was that the aircraft was incapable of hitting its top dash speed requirement. An engineer at Convair got the idea of actually cutting the end of the fuselage off and putting a kick up spoiler on it as Dr. Kamm's experiments suggested was good to terminate the flow and recover forward momentum from the passing airflow. While this created more drag than a smoothly blended shape between the engines, it provided more “thrust” than drag and helped push the airplane forward. The rest, as is said; "Is History".
So the lesson in all this is that this is less an area of uniformity where one size fits all and more an area of continued experimentation. That said, his statement of his heads generating 80 more horsepower than yours because he has better max lift flow but less low lift flow would be true for the right cam and RPM leading to a “more” useful port velocity combination. Obviously, if you used a cam with .45 inch lift with his heads the superior ultra high lift flow would be of no consequence. The reverse would also be true, in that your porting would peter out in these high lifts so a cam of .5 inch or more would not be effective. All this assumes that the duration remained the same for the lower or higher lift cam. At some point the port or the valve reaches all the flow it can pass in a given moment. Holding everything else steady, the only improvement is more time with the valve open and that's controlled by cam duration in this case. Certainly lobe shape also plays in this but for argument I'm considering them to be the same.
He's right about the valve flow on the cylinder wall side for a two valve wedge. For the most part there is little to no flow on that side of the valve adjacent to the cylinder wall and you don't want to encourage flow from there as it will oppose the swirl pattern and reduce overall flow. Now 2 valve hemi chambers and 4 valve pent chambers, or engines with blowers exhibit different flow patterns largely grouped into what's called "Tumble Ports" However, if you force this to occur on a 2 valve naturally aspirated wedge power goes out the window and fuel consumption goes up indicating a loss of efficiency. The suggested larger valve will help top end power by increasing the area of flow which also tends to reduce top end port Mach numbers. You don't want to drive the flow into local supersonic as the shock wave is as solid to flow as a piece of metal obstructing the passage. How you use the engine should be your guide. If you can keep the revs high, then big high lift flow is beneficial to you, if the engine has to grunt its way up from low RPMs, then you need to design around more torque which usually means smaller ports and less cam timing and lift. My own approach to ports is to get the most flow with the smallest volume necessary to get the job done. So I compute the theoretical max volume needed to fill the cylinder. Then work to achieve that flow with the smallest port that keeps max velocity at or below .5 or .6 Mach. This usually leads to unusual shapes, raising the port and sometimes filling the lower sections. But all this is variable as to what it takes to get the engine to perform for the task at hand. Sometimes big flows just kill the engine even at high revs, you just have to cut and try if you're on the competitive edge. If you want a hot street engine that occasionally races then a set of machine ported heads are the most bang for the buck you're likely to find. If you’re a competitive racer, then you stand up against the line of known knowledge and experiment on the other side of the line.
The subject of 11/32 valve stems against 3/8s in my experience is highly overrated. The 1/32 less diameter just doesn’t buy much flow and reduces available stem strength. This is especially true of hollow stem valves. Having had valves fail, I fall on the side of greater reliability against a couple CFMs of flow as the cost of a failed valve is high, not only do you become a DNF but your engine becomes several hundred pounds of junk and those expensive custom heads are almost always a total loss.
I can't really comment on his ideas and theories other than to say that "Mainstream Ideas" as you read in books and articles tend constitute the status quo. The guys who step over the edge are the guys that discover things that make the new status quo or find themselves up a blind alley. Either way experimentation is what's required to separate what works from what doesn't. I say that 2 grand for a set of heads is an expensive way to discover things, but to a large extent it's part of the admission price to being a competitive racer. That being said it won't take you many heads to pay for a decent flow bench whether homemade of a Superflow. You'd choke over the pile of junk heads I've made over the years, which is why I started making silicon patterns of stock ports then casing them in plastic and modifying the plastic ports for the flow bench, or making acrylic models of port's I'd like to see, it's a lot less expensive than wrecking heads with ideas that don't work. And in the case of long out of production engines such as the AMC, you’re not ruining things that are increasingly rare.

If your head hurts now.,. there is more to come.

Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

pepsi1 pepsi1
Guru | Posts: 1718 | Joined: 09/11
Posted: 09/22/13
06:48 PM

I have been flowing heads for nearly 40 years now. I made my first flow bench. The truth is as you say Tuff what works for one engine may not work for another. Valve angles are a big part or your read. I have flowed different heads from different companies, and it's makes me wonder how they say they're heads do this and that when they really don't. I did a set of XYZ heads for a friend that did not even come close to the flow sheet company XYZ supplied.
  Thanks Tuff it's along read, but well worth it.

Bob  

76Skylark 76Skylark
Guru | Posts: 853 | Joined: 12/11
Posted: 09/23/13
08:02 AM

also the folks doing the testing have only one thing in mind. So they use a bore size that is often larger than what you yourself may have and valve lifts you will never use to Prop up their numbers.  Peter and Bob are very good at what they do.and Offer great insite to this Voodoo. So Hey guys, I am in the middle of my First Flathead, Its going in a 1953 Mainline, this car was the least expensive car Ford Offered that year and came Equipped with the First Overhead valve engine a 6 cylinder they made. doing a LOW budget build for a man and instaling a V-8 as this was also the last year for the flathead V-8. Kinda Rat Rod, should be Fun!  

Dave632 Dave632
Addict | Posts: 2217 | Joined: 07/08
Posted: 09/23/13
05:33 PM

That's some interesting stuff Tuff. I have never gotten into porting that much as I left that to those who knew better. The Pro Stock big chief heads on my Beretta were a study in experimentation, not by me however. Someone spent a lot of cash and time on them, moving intake ports around, filling areas with epoxy and grinding other areas. The result was a pretty good set of heads which were able to push the car into the very low 5s on motor alone in the eighth mile.  
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tuffnuff tuffnuff
Moderator | Posts: 7827 | Joined: 12/09
Posted: 09/23/13
06:40 PM

Ported BB Chevy head and Eddy manifold

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Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder