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Elsee
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 7, 2003 - 8:15 am: |       |
Still waiting on Part 3. Thanks |
Bogie
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 8, 2003 - 2:54 pm: | |
Elsee, I working on it, had to travel a lot and my laptop croaked all conspire to slow me down.
I'm home again and the "DEll form Hell" is reliably functional again, that's always good for a couple weeks. |
Bogie
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 8, 2003 - 2:55 pm: | |
Elsee, I working on it, had to travel a lot and my laptop croaked all conspire to slow me down.
I'm home again and the "Dell form Hell" is reliably functional again, that's always good for a couple weeks. |
Bogie
Rating: 
Votes: 3 (Vote!) | | Posted on Monday, May 19, 2003 - 1:55 pm: | |
Well I'm the road again, this time to Dallas. As I was whipping up a story on how to grind the Swirl Port heads, I got to looking at some flow data in the books I recommended. What became apparent to me was how often low lift flow was sacraficed to enhance flow at high lift. So I thought I throw in a Preamble that discusses some of the engineering behind why an engine gets designed the way it does. Hopefully this bit of knowledge will help you understand why low lift flow is important to maintain, other than the fact the valve sees these low lifts twice a cycle. Where low lift occurs in the cycle is important.
Part 3; Porting, Section 1 Background
Back when I promised to write about porting Swirl Port heads, I didn’t realize I’d start the Great American Novel, but I did. For practical concerns there clearly isn’t space in Sallee’s Discuss Board for this much writing so I’m chopping this up in to more bite size pieces.
In my previous installment I recommended that you get a hold of a couple books; the first was “How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads” by David Vizard. It’s a very good read with lots of pictures and charts that help put the words into a physical perspective. I also recommended that you get a copy of “Small Block Chevy Engine Buildups” by Chevy High Performance magazine. This book is a compellation of articles they published monthly. These are also viewable at the following web site http://www.chevyhiperformance.com/toc/ but they are not organized into a comprehensive subject so you’ll need to tease them out. Look for How to Build Your First Engine; The Goodwrench Quest Parts 1 thru 7; Street Fighter 377; Agent 87 Parts 1 thru 3; Hot, Hot, Hot Cam; Son of Muscle Mouse; Gladiator vs. Muscle Mouse; Battle of the Small-Block Strokers; Flow to Go; Flow Power; Bolting on Vortec Heads; A Tale of Torque; Porting for Power; Compression Lessons; New Wave TPI; It’s a Spring Thing; Angling for Power; Cam Basics; Cam Overlap; Roller Cam Basics. I need to refer you to these books because for as much as I’ve tried I can’t get the Discuss Board to let me insert drawings and photos.
Lets talk economics versus results for a minute. Porting a set of swirl ports will never be as effective as just buying a set of Vortecs, bolting them on and driving away. The reason is in the combustion chamber more so than the ports. The Vortec uses a very effective high squish/quench combustion chamber and properly shrouds or un-shrouds the valves for very effective flow between the ports and combustion chamber. The swirl port head is a kin to the L-98 in terms of combustion chamber design. This design is less effective in establishing late compression cycle turbulence from the effects of the squish/quench zone when compared to the Vortec or Fastburn designs. This chamber, also, does not as effectively shroud the exhaust valve to reduce flows from competing for space within the port. We’ll tear into that subject later. Because of this, like the L-98 head, the swirl port will always trail the Vortec head by 10 to 15 horsepower. There is also the issue of the “ramp”. Its low and moderate speed effect is to enhance swirl, this is a good thing in terms of building torque while enhancing fuel mileage and minimizing emissions. The down side of the ramp is at higher rpm where it interferes with high-speed flow. This is because as mixture velocity increases with engine speed the swirl from a properly designed port becomes the dominant flow characteristic without need for a gadget to force the event. At those engine speeds the “ramp” interferes with flow more than helps swirl to the detriment of top end horsepower.
Now as a designer, your always trading features to achieve functional goals/requirements. In the beginning SBC combustion chamber design was a tight chamber with moderately good squish/quench. I don’t know if the phenomenon of swirl was actually addressed as a design parameter back in the 1950s, or whether it was something that just happened if you got the design correct. I know the first set of Ford FE heads I ported back in 1958 had a peculiar bulge in the port wall on the side that favors the center of the cylinder. Since straight and shiny was what we learned on flat heads the FE got the same treatment. The instant that engine fired it wheezed to life rather than barked. I knew,,,,, I knew I did something wrong. The Ford guys put that bulge there to force flow back on the port wall adjacent to the cylinder wall, thus producing a swirl past the valve. So one must assume the engineers at GM also knew about swirl at this time as well as Ford or anyone else. But gas was cheap and emissions not a concern and generally engine design reflected this.
When the emissions age began to affect engine design, the impact was quite severe as requirements were imposed with little thought of collateral engineering impacts. The “knee-jerk” reaction was to lower compression, retard timing, lean the mixtures, increase operating temperatures, toss a little exhaust into the fresh mixture, and pump the exhaust manifold full of air. Horsepower, torque, fuel mileage, reliability, and longevity simply collapsed. That’s the way things were through the 1970s and into the mid 1980s. There was no improvement in this situation until the application of computerized engine management systems and electronic fuel injection. From the introduction of CEM and EFI, you begin to see the application of cylinder head science here-to-for unknown in production automotive engines. The L-98 head is a classic representative of initial ideas leading to better spark plug placement, control and utilization of shrouding to enhance port and valve flow, a return to a squish/quench zone that enhances turbulence and builds mechanical octane into the engine, and the beginnings of designs that purposely encourage swirl characteristics into the mixture entering the cylinder. The swirl port, as stated earlier, is a variation on this theme and is designed specifically to enhance swirl in low and moderate RPM engines. It uses a ramp or vane cast into the port to mechanically enhance the swirl characteristic. Finally, we arrive at heads like the Fast Burns and Vortecs (notice I’m not getting into SB 2 and 3s where these ideas are further enhanced) where careful attention is placed on the relationship of combustion chamber shrouding or lack there-of with enhanced port flow swirl characteristics without the need of ramps, or vanes. The spark plug location becomes more idealized as it moves toward the top of the chamber and tends to be more centered in relation to the bore diameter. Often it is biased toward the exhaust valve. Raising the plug toward the top of the chamber provides for a more consistent burn. If you’re running domed pistons, this higher plug location really speeds the burn and lessens the need for excessive spark advance. The more centralized plug location tending to be within the swirl flow does several things; it stabilizes the spark plug’s operating temperature and allows the incoming charge to provide a clean “prime” between the electrodes. Moving the plug close to the bore center reduces burn time. The burn now is downward and outward from the center of the combustion chamber as compared to low at one side and needing to burn up, down and across. The reduced burn duration allows the engine to use less spark advance. Reduced advance does a couple things; 1) it reduces the amount of energy trying to turn the engine backwards against normal rotation, that makes more power available; 2) it softens the pressure/temperature spikes that cause pre-ignition and detonation and the production of nitrogen oxides. The net effect is to allow for more compression that in turn generates even more torque, horsepower and improves fuel economy. The last improvement has been with foundry processes. These allow greater casting precision resulting in ports, guides, and combustion chambers that are much closer to the design ideal than was possible just a few years ago. This has the effect of greatly enhancing total flow and swirl, where previously many hours with a die grinder were required to achieve similar results. So lets hear it for precision casting. This of course also tells you that getting more flow out of Fast Burns and Vortecs is increasingly difficult compared to older heads, simply because they’re so much better.
Now for several times you heard me say that valve shrouding can be a useful thing. All “Hot Rodders” no that to be heresy. There isn’t a porting book; tape or CD out there where the author doesn’t exclaim that valve shrouding is the root of all evil. But here’s Bogie saying it’s not necessarily a bad thing; stone him or is he already stoned, oh-well whatever we’ll get into the details of my apparent insanity a bit later.
Now I’m going to repeat some of what I said about combustion chambers and their relationship to flow from Part 1 of this epistle, only because it’s important that you see what’s going on. An engine is a complex system, now there isn’t space to discuss all the complexities, but understanding the relationship between the combustion chamber and the ports with valves as they relate to piston and valve movement is really important to your successful porting efforts.
As I previously said, in spite of being a student of the internal combustion engine, I don’t know where the current heart shaped wedge chamber design comes from. I first saw such a configuration back in the early 1970s on a Vega head designed by Doug Roe. The stock Vega head was an open chamber with no squish/quench at all. This engine was extremely ping prone and became the poster-child of how not to design an engine. But the emissions were low. Anyway, Mr. Roe came up with a modified head in an attempt to make a SCCA competitive car out of the Vega. I acquired a couple of the heads to top off the Vega engines I was trying on a midget, and they were quite successful.
Another interesting variation on the theme of heart shaped high squish/quench chambers can be found on the Yamaha Virago (cum V-Star) engine. (Although in the latter 1100s Yamaha partially abandoned the heart shaped Quench/Squish and had to reduce compression ratio as a result.) This engine was originally designed in the late 1970s. Here the chamber is rotated 90 degrees such that the intake valve is on the inside (carburetor side) of the engine and the exhaust is opposite. This forms a natural hemi chamber, but that’s not quite what Yamaha did. They set the valves into a half-hemi tub with the spark plug quite centered adjacent to the tangential approach of intake and exhaust valves. Opposite the spark plug location, the combustion chamber forms a heart shaped squish/quench zone. The intake port is off set to establish a swirl pattern tangent to the perimeter of the cylinder wall, as is the major flow characteristic of contemporary SBC wedge chamber design. Anyone who has ridden many bikes well knows the 1100 Virago spots a 1340 Harley or 1360 Suzuki Intruder 300+ cc but will easily eat their lunch. We’ll it’s all in the heads. Anyway, the point here is that the heart shaped high quench/squish combustion chamber is very effective, it’s nothing new, it just got lost for a while.
Of course the BBC semi hemi engine, 396-454, and the Ford 429/460 (a cheap and obvious copy of the BBC) can also be viewed as a pre-existing variation on somewhat the same theme as the Virago; by designing a combustion chamber and valve layout that takes advantage of the high turbulence squish/quench wedge with some hemi-ish valve un-shrouding at high lifts, thus you get good bore circumference swirl characteristics of the inlet charge with utilization of more of the valve diameter for increased flow area and some wedge shrouding to protect the flow at lower lifts as the rising piston begins to press the vortex in on itself while the valve is still open late in the compression cycle.
Shrouding you say, it keeps coming up, well this is something you don’t see on a flow bench, yet is a necessary characteristic to maximize inlet and exhaust flow. Smokey Yunick, undoubtedly, the most insane automotive engineer that ever lived, suggested that the only real flow bench would include a cylinder and piston that could be motored at speeds seen within an engine rather than tube with a fan on the end of it. If you had such a bench, you’d see some different things concerning flow and its interaction within the cylinder and the mechanical parts therein.
Let's contrast flow events in a simple 2-valve wedge to a similar hemi, such as the Chrysler. In here is also the answer as to why the oversize ported engines like the Ford Tunnel Port this and Boss that never worked up to expectation. You can put the humongous square ported BBC on that pile too.
Intake airflow in a wedge headed engine passes about 80% of the flow through 1/3 to 2/5s of the valve’s diameter facing the spark plug, forming a strong swirl into the combustion chamber as it does so. The remaining 20% of the flow finds it’s way out along the shrouded sides and its direction is often contrary to the prime flow, so not maximizing the minor flow is a real good idea. (Refer to page 74, figure 7-4 of “How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads” for a drawing of this). The port needs to be designed to enhance the 80% flow to 90% or better without encouraging more backside flow. As it passes into the cylinder, the natural flow forms a vortex tangent to the bore diameter that spirals down the cylinder as the piston descends toward BDC. (Refer to page 112, figure 11-1 of “How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads” for a cartoon of this). Now here’s where I get heretical; a major advantage of the wedge chamber is that the intake valve is shrouded by the quench step and on the cylinder wall side. As the piston ascends toward TDC, while the valve is still slightly open, the close in quench step and the cylinder wall side of the combustion chamber helps to prevent the swirling flow arriving at the backside of the valve from “throttling” the mixture entering from the spark plug side of the valve. It may, in-fact, actually help flow by inducing a siphon effect as the swirl flows over the valve face and away from the open side. (Reference a Vortec chamber compared to a open chamber on page 25 of “Small-Block Chevy Engine Buildups”.)
Contrast this to a hemi headed engine. On paper or on the flow bench, the hemi appears to have it all over a wedge combustion chamber. The hemi utilizes 1/2 to 2/3s the circumference of the valve compared to 1/3 to 2/5s utilized by a wedge head. So why doesn’t the hemi just run away from a wedge? The answer lies in the swirl pattern. A hemi with a straight on port (Chrysler) forms a flow pattern is vertical, that’s to say the principle flow pours around the top of the intake valve, passing over the sparkplug and exhaust valve. It then floods down the far side of the cylinder wall chasing the piston as it descends toward BDC. All’s good so long as the piston continues on the down stroke, but as the piston rises on the compression stroke, the vertical orientation of the flow crosses the piston crown and rises up the intake valve side of the cylinder wall and rushes back over the still open valve. Since the valve isn’t shrouded, the rising flow essentially “throttles” the incoming charge, reducing low lift flow. This is because the flow can get into the still slightly open valve from its, low flow, back side rather than being forced over the valve face by the chamber shrouding found in Vortec type chambers. Therefore, the cylinder never fills as well as theory and flow benches would indicate. Yet, typically, it fills better than a wedge but not so much more that a well-executed wedge can’t be competitive. Again the Yamaha Virago design gets around this problem by skewing the intake port relative to the valve centerline and providing a conventional quench/squish step, which should have been done by Chrysler.
Flow on the exhaust side is greatly controlled by the angle the port makes to the valve and the radius of the short side floor. Sharper angles and radiuses flow less well than more open designs. This is where Buick got into “big time” trouble with their early 50s OHV V-8. This engine has a very nice two valve pent roof chamber. Both valves are on the intake side of the chamber, this forces a very tight turn on the exhaust port. Back then “Blow-Down” theorists believed this didn’t matter much as the rising piston would force the spent gasses out. This is true at moderate engine speeds, but as RPM climbs the resistance raises to a power factor and cost of energy lost to the exhaust effort becomes quite large. Tommy Ivo with four blown Buicks couldn’t make them competitive against a single engined Chrysler or Lincoln.
Exhaust flow on the short side radius (port floor) must be carefully controlled. Emphasizing flow here will act as a valve across the port substantially reducing overall flow. On the other hand too little flow promotes excessive reversion as the lower pressure, compared to the upper portion of the port, will tend to draw the major flow from the top of the runner down and back toward the valve. This, at its extreme reduces the apparent size of the port limiting total flow as a result. (This is a place where Ford got into real trouble with the FE block, the Cleveland, and the 429. On the FE this is solved by the porter with careful shaping of the guide; on the Cleveland and the 429 you milled the outside of the port boss right off the head and bolted a thick plate that raised the ports onto the head. If you didn’t do this, you were never competitive against anything GM or Chrysler threw at you.) At the other end of the scale, when done right, the reversion can make the apparent radius larger which helps flow in the upper part of the port. Quite a balancing act ain’t it. (Please reference page 81, figure 8-1 of “How to build and modify Chevrolet Small-Block Cylinder Heads” for a cartoon of what reversion looks like).
Once again the shrouding of a wedge chamber can be used to your advantage in establishing a measured exhaust flow on the short side, port floor. Whereas the Hemi combustion chamber is simply open to the port from all sides, if the port radius is not or cannot be made such that it controls short side flow, your stuck with what you’ve got because there’s no shrouding in the chamber that you can play with.
Let me get into a little design history here which is important for understanding the need to maintain good low lift flow. If you read my reading list, one thing should be apparent to you, that is many reworked ports show lower flow at low lift compared to the flow before the cutting. Now if your building whomper-stomper race engine that sees little to no street action, feel free to pass this by, but, if your going to drive on the street, tow and do things the rest of us ordinary mortals do read this to understand the importance of low lift flow. Race guys get great low lift flow as a result of high engine speeds, it’s kind of like natural supercharging, this is simply taking advantage of inertia in the gas flow an certainly some wave tuning. If you drive a speeds where inertia effects are minimal because of speed than you need to design the porting around that constraint.
A century ago 4 cycle engines only used the cam to activate the exhaust valve. Engineers thought that since the descending piston created a vacuum all that was needed to control the intake valve was a light spring. Thus when vacuum was sufficient to overcome the spring and lift the valve off its seat, the mixture would begin to flow into the cylinder, this obviously happened somewhere well after Top Dead Center (TDC). Since the piston slows pretty rapidly after passing the half stroke mark, the spring quickly overcame suction and closed the valve well before Bottom Dead Center. Therefore, these engines had no overlap or late closing intake; as a result they neither turned RPMs above a few hundred nor developed much power. As experimenters and scientists such as Sir Harry Ricardo turned their attention to the low efficiency problem of the 4-stroke engine, it was discovered that there were substantial gas dynamic effects occurring in the ports that a cam operated intake valve could take advantage of. Thus was born, the high-speed internal combustion engine with its quantum improvements in power and efficiency. This engine is the result of taking advantage of early intake opening and late closing in an area of the operating curve where the valve is in a low lift regime and the piston is on one hand hardly moving during overlap and is moving in opposition to the incoming flow at the other end of the induction cycle as it begins the compression cycle. So a very substantial amount of your engine’s power is generated at moderate and low valve lifts and you can’t ignore the need to maintain good flow here, especially for a street driven engine, racing can be a different solution as high RPMs and the gas inertia that comes with those speeds will tend to hide low lift intake flow deficiencies.
Just to rub everybody’s face in this subject some more, visualize what’s happening with the piston and intake valve. Remember your old high school algebra book. Somewhere in there was a graph of a parabolic curve. That’s where a line representing a moving object like a piston connected to a crankshaft at dead stop on TDC starts slowly, accelerates faster and faster till a maximum is reached, then it begins to slow doing so very rapidly at first then with less and less urgency till it once again stops at BDC. The valve follows a similar curve, due to overlap it would start to open some number of degrees before the piston starts its downward movement. The valve slowly begins to lift off its seat, then at an increasing rate opens further and further till reaching full open. Then it hangs at full open for a period defined by the cam profile. This point of maximum lift coincides closely the pistons point of maximum velocity. Then coming off the cam, the valve rapidly moves to closure at some point after the piston begins it’s upstroke. It’s pretty simple to overlap these two cartoons to see the relationship of piston velocity against degrees of rotation and valve opening to degrees of crankshaft rotation.
What’s happening here? As the piston completes the exhaust stroke but is not yet at TDC and the exhaust valve is still open, the intake valve begins to open in the what’s called the overlap period. At this point, the velocity of the exiting exhaust gases have, (hopefully) reduced cylinder pressure below that of the intake. By cracking the intake valve open fresh mixture begins to flow into the cylinder. While both valves are open some fresh intake will flow out the exhaust. The down side is that with a carburetor or constant flow injection unburnt fuel passes out the exhaust which ends up as smog unless something is done. Obviously sequential injection gets you around this problem. At the other end of the intake cycle, you find that the valve closes at some point after Bottom Dead Center (BDC) because the incoming charge has inertial pressure greater than the cylinder pressure generated by the piston rising on the compression stroke. So the valve is heal open till these pressure equal then its closed to trap the charge. This inertia of the incoming mixture is relative to the speed of airflow in the port, which is related to the speed of the engine among other factors. Thus low speed engines use small ports, BBC Peanut ports as an example, and a high-speed engine can use large ports. The problem is that we typically drive at variable speeds form idle to freeway or faster and one port must satisfy all these requirements. This is where infinitely variable port size would be nice. The reason why a long cammed engine sputters at idle occurs here, at low speeds there isn’t enough inertia in the intake flow to overcome the reverse pumping of the rising piston, so some amount of fresh charge gets pushed back into the intake. The resulting thin mixture lowers the effective compression ratio forcing you to idle the engine faster and this combined with exhaust backflow on overlap causes that staggering idle we so love to hear.
Had enough yet? I need to go to Texas next week as my employer expects some work out of me from time to time. I hope this reads OK I’m cutting out reviewing it so I can get other things done. I’ll be working Section 2 of this part where we’ll actually take grinder in hand and do some carving. |
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