Holler: Cylinder Head Porting, Part 2
Last time we performed some rather conventional modifications to Bob O’Neill’s 2.2 Turbo head as far as porting goes. As promised, here comes the good stuff. In the Allpar archives you can find an article entitled “Performance and Economy” that outlines my passion to maximize whatever air and fuel the engine is able to ingest. So above and beyond improvements in the breathability of the engine, we also want to maximize the power potential of what volumetric efficiency we are able to get.
It is a well known fact that liquid fuel doesn’t burn. So, in an attempt to convert more of the liquid droplets into a dry vapor, I have come up with what I call Powre Lynz. I had some hardened steel tools made up at my brother’s machine shop (LL Brown in Enola, PA) that enable me to put screw threads in the intake ports. There are 3 different pitches I can scratch: 12, 16, and 20 pitch. Unlike a tap that has spiraling threads, these Powre Lynz tools have concentric threads. The shank is ¼” to fit my grinder.
With patience and time and I can score very well laid-out screw threads into the ports. These Powre Lynz will create somewhat of a variable boundary layer. At low flow demands, the boundary layer constricts, keeping velocity high. This is really good for low end torque and off-boost power. It also keeps the fuel actively suspended in the incoming air stream. Larger droplets will bounce off the sharp edges and scatter away in a dozen or more smaller (easier to vaporize) droplets. Some of the fuel will even stick, radiating out laterally and vaporizing into the air stream. As flow increases, the boundary layer collapses allowing improved breathability.
There is a very ugly patch of 12 pitch threads under the valve seat on the short side radius of the intake ports. This really excites the fuel in the vicinity to turn a spray into a light fog. The low velocity area around the manifold gets a 20 pitch thread, while the higher velocity areas get a 16 pitch. Because Bob will be driving his Daytona mostly on the highways, I didn’t use much of the coarse 12 pitch Lynz.
The combustion chamber is another area of dramatic combustion efficiency enhancements. You’ll notice that the normally sharp edges around the heart are now very rounded. This accomplishes many things, and I owe a debt of gratitude to Allpar’s dana44 for teaching us about this incredible trick. For illustration, try this experiment. Place a square quart milk container on the table. Grab a hand-held mirror and a birthday candle. Place the lit candle behind the milk container, and try to blow it out from the other side. Use the mirror to gauge your success. Next, get something cylindrical about the same diameter as the milk carton was wide. Try blowing out the lit candle again. See how much easier it is to blow out a candle placed behind a round object?
The radiused squish pad allows incoming air to flow up and over with greater ease, improving intake flow numbers. It allows exhaust gasses to pass toward the valve with greater ease, improving exhaust flow numbers. As the charge ignites and begins to propagate, it is able to work its way back into the quench zone easier. More of the air and fuel are able to release their chemical energy in the form of thermal energy sooner in the combustion cycle. The piston sees a more even force across more of the top than in the stock configuration. This means better performance, fuel economy, and lower exhaust emissions. With no sharp edges, there are virtually no hot spots to cause pre-ignition or detonation. The engine is able to withstand higher compression, higher boost, and do it on lower octane fuel!
The first step is to mark the cylinder with a bare block preferably. Sit the head on the block, and reach up from the crank area and mark the cylinder. An alternative is to overlay a head gasket on the head and mark the chambers. This gives parameters that cannot be violated.
Deshrouding the valves and improving combustion efficiency; laying back the shroud.
With the markings in place, the restrictive shrouding around the valves gets laid back. A set of trimmed valves are placed in the seats to protect them while a hungry carbide bit does its work in the neighborhood. The area around the exhaust valve is taken out to the edge of the cylinder. If mismatched ports between the manifolds and head are bad, then a ledge around the exhaust valve is equally bad. By taking this area out to the edge of the cylinder, there is no turbulence on the exhaust stroke. The exhaust gasses flow smoothly around the valve and exit the engine.
Squish pad is then gently radiused while the rest of the chamber is smoothed.
The laid back area is then radiused to create a seamlessly smooth transition to the squish pad. The point of the heart is also laid back and radiused. This radius gets blended into the sides around the valves. The head was then milled about 0.063” to yield an 8.6:1 compression ratio.
Most of you have heard of the Somender Singh Grooves by now. I am a believer. I have experimented with 1, 2, 3, and 4, grooves with several angles and configurations. With the radiusing, I find that a single groove angled so that it splits the area between the seats and aims just to the exhaust valve side of the plug works best. The Groove is cut at a 6 degree angle so that it is shallower near the cylinder wall. A 1/8” round nose bit is used for a rounded floor in the groove. Since I have one, I use a mill for this process. After milling the groove, the sharp edges are removed to reduce hot spots and improve flow around the groove.
Even Grooves get smoothed over.
The head gets a thorough cleaning before final assembly. We used a typical ’87 TII Turbo cam with roller followers. New cam seal and plug were installed. The exhaust manifold studs are homemade 316 stainless steel studs with black anodized nuts. Temp sender and heater hose fitting were cleaned up and installed with Permatex Super 300 sealant.
I for one am excited to see how well this head does for Bob. He has rebuilt the bottom end with everything machined and trued. The foundation is solid. The car is getting the early log TI ditched in favor of the more attractive and more efficient later style intake manifold. Again, he is aspiring for 200 HP at a mere 12 psi boost, while getting at least 30-40 mpg on the highway (staying out of boost, of course).
|Mike Holler, known on Allpar forums as mpgmike, also contributes to mpgResearch.com. He has contributed many columns to Allpar:|
|Interiors||Budget interior restoration: making the inside of your car look like new again
Red carpet treatment: Installing new carpet in an old car
|Porting||Porting heads for performance: step by step
Head porting example, part 1 | part 2
Intake manifold porting
Exhaust manifold porting (turbo)
Poly Quad heads: porting revisited
|Turbochargers||Turbochargers - all you need to know (interview)
Turbocharging the slant six for power and economy • Revisiting the turbocharged slant six
Budget turbocharger rebuilding (and Turbo Rebirth)
Installing a boost gauge
|Other||Powder-coating for a brilliant, durable finish
Custom pistons: roll your own!
Prepping valves for performance: grinding and polishing
Old cars: an opinion
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