That was great info. Who wrote that, you?
I want to add something about head porting that the average joe probably doesn't know, but it's importance is hinted to above. Two things greatly affect cylinder head airflow. Size and shape.
Size is the overall port volume of the runner, measured in cc's. This has a drastic effect on how much raw volume of air can go thru the ports. A larger size will feed a bigger inch motor much better, and is great for any type of forced induction usually. The forced induction cars have some type of blower thats pumping a ton of air into the motor, and the only thing that stops that air from actually filling the cylinder is the intake port path (including the intake manifold). When you're talking about an n/a car, too much volume is a killer of low end torque. On a forced induction car, we usually don't have that much trouble with low end torque anyway, unless we have a fairly large turbo for a compressor.
Shape is a different animal. The perfect shape would be a round tube with a valve at the end of it. This tube would be straight up and down, in the same plane as the cylinder bore. For obvious clearance reasons, this can't be done in a car. Besides, there would be no way to actuate the valve from inside the port. [Visualize a 2" plastic pvc pipe with a valve stuck in the end of it, and the valve tip being in the middle of the tube]. So the factory has to bend this tube in a 90 degree fashion towards the intake manifold to get all this to fit under the hood. This also allows the valve tip to stick up thru the top of the port (thru the guide), and be controlled from above by the valvetrain. (Stating the obvious here). The more they bend this intake port, the less straight of a shot the airflow has of going thru the tube. Remember, the air is going thru the tube rather fast, and suddenly it has to make a 90 degree bend to turn into the cylinder, past the valve. When the short turn (the bottom of the port, at the 90 degree bend) doesn't have the right shape, the fast-moving air simply "flies off the cliff", and hits the back side of the bend (the long turn). This makes all of the airflow go past only the long-turn side of the intake valve, which is what he was saying in the post above. That makes the port act like it only has half of the valve to flow thru. The correct shape for the short turn is a quadrant of the valve size. In other words, take the intake valve and carve it's shape into a piece of balsa wood. Now take one corner (25%
of that curved balsa wood, and that is a quadrant (1/4) of the valve shape. The short turn of the port should be exactly that same shape. The factory often shortens up that short turn by maybe 10% or so, because they've found that it increases low-lift flow, at the expense of top end flow. With any type of hydraulic cam, low-lift flow is important.
Picture the amount of air between the throttle body and the intake valve that's about to be open, as a set column of air. A train. You want the train not only to go fast, but to accelerate to that max speed fast. Good low lift flow will get the column of air moving faster, while total size and shape will dictate the total amount and speed of airflow once the valve is fully open. Get the train moving faster.
I was told by an expert that any airflow below .300 valve lift is totally decided by the size of the valve (bigger is better for low lift flow), and the angles on the valve job. The less angles, and cuts, such as a 2-angle 30 degree seat works best for low-lift flow. A multi-angle valve job (3 - 5 angles), and a 45 degree seat is better for higher lift flow, as it lets the large amounts of air past the valve faster. Of course, we all have to make that decision on the trade-off there, and that's where a flow-bench comes in handy.
Having the wrong shaped short turn radius will allow a fast-moving column of air to fly off the cliff, and only flow past the back side of the valve. This phenomenon is caused by having the incorrect shaped short turn. This is where a pro head porter has a big leg up on us home-hackers. He understands the shape of that short turn is critical. With a hydraulic cam, we can only pop the valve open so fast without collapsing the hydraulic lifter, so we can't "get to" the high lift flow numbers all that fast. A regular pushrod V8 engine with a solid roller can. So, we have to pay attention to low-lift flow numbers as much as peak airflow numbers, to get the column of air started moving faster. With a higher rpm motor, this column acceleration rate is less critical, because the engine already is turning faster. So what happens is the low-lift flow numbers become a little less relevant.
Now hopefully you can see why the B heads, and their large ports, are not so great for a n/a car at lower rpms. They're big. That's a good thing though, when it comes to racing at high rpms and especially with forced induction. Boost is already pushing that column of air from one side (the throttle body side), and as soon as the valve opens, that boost pushed that air into the cylinder. The air is under pressure, and the physics of a pressure drop take over at this point. So, essentially with a blower (more so than a turbo probably), we don't have as much to worry about by using big heads (large port volume).
Quickly, with respect to swirl vs tumble, think of it like this. There's a ballarina that's spinning her way thru a doorway, or there's a gymnist summersalting his way thru the same doorway. Which one is likely to get thru first, the forward moving one or the sideways moving one? That's swirl, vs tumbleport in a nutshell, although there's a lot more to it in the high tech explanation.
Another noteworthy point is that (and this is important to understand), the size of the bore dictates the size of the intake valve. Less so in a 4V motor, which is just great! The size of the valve dictates the choke point (smallest point) in the intake port. Usually the choke point on a perfectly ported head is about 94% of the size of the valve. The size of the choke point (cross section) determines the overall flow potential of the intake port. The flow potential dictates peak potential horsepower. Normally from the factory, the size of the choke point is much smaller than 94% of the size of the valve, so therefore we can port these heads (assuming there's enough meat left in them at these critical spots) to get to the 94%, and we normally don't have to change valve size. With a mod motor, the bore size is usually 3.55, so we can't fit much of an intake valve in there, and still have room for the exhaust valve. This gets better with a 4V motor, because we have 2 smaller valves to flow into instead of 1 large valve. Now you know why the 2 valve motors will never build much power in N/A trim no matter how much you spend. The bore is too small. Yet a 302 has a 4.00 inch bore, and you can slap some pretty big heads on that motor.
In summary, with a 4V motor and a blower, you don't exactly have to worry about lack of low end torque. The blower will create it soon enough anyway. And with traction at a premium already, a little loss of low end is usually just a benefit. If it's too weak down low, just come out harder.
The tumble port heads redesigned the B head for tumble and velocity at lower valve lift, at the expense of total port volume. This SHOULD mean that, assuming the port shape is correct, the B head will make for a better race head. That may be why you see so many racers running them. In 99, Ford redesigned that new tumble port head to fix the low end torque problem that the B head has. In 03, they had the bright idea to take this already torquey head and put a PD blower on it. The end result: Tire melting. They should have put the PD blower on the 96-98 head in the first place! That would have been a killer combo.
The number one reason why home porters screw up their work is that they enlarge the port in some areas, and leave other areas alone. And they screw up the short turn radius, causing the air to fly off the cliff. Often times, they just kill performance across the board. You would be better off just mildly enlarging the smallest parts of the port you can measure, without getting hog wild.
Take a look at the port volumes listed above, and you'll see that the B head is a rocker already, when it comes to total size. Airflow numbers might still be restrictive, based on choke points and short turn radius (which above was listed as being much better than the newer heads). However, if you port wisely on the B heads, you should be able to get the airflow numbers up quickly, without extensive redesigning of the port. I think they're a race head just waiting to be unleashed. But they'll always be plagued by the single injector, double port design, and also lack of aftermarket intake. I personally think that one reason there aren't many intakes out there aftermarket, is because they can't build one that's much better.
The big turbo cars basically take the runners out of the B manifold entirely, and fill the floor with epoxy to bring it up to the level of the port entrances. While this would kill all low end torque, a drag radial turbo car doesn't care about that anyway. Hope this helps some of you to understand the why's of cylinder heads. Remember too, that the factory usually does a great job of designing a head/intake for it's intended purpose. If they didn't, they probably redesign it within a few years. Case in point is the lack of low end torque on the naturally aspirated B head motors. But with a blower, it's a whole different story.