Thread: SBC Heads

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Thanks for your input Max, But I would still like to know what advantages 202/160 valve would have over 194/150 valve on a 64 cc head ?

The advantage is this:
A bigger valve has a bigger circumference and so the cylindrical area through which air can flow is bigger at the same valve lift (about 4% larger for the 2.02 valves than on the 1.94 valves). So more air can flow through the opening the valve creates in the same time. As a result of which you could use a lower lift cam for the same performance (less stress on the valve train) or you automatically get higher performance because of a bigger amount of mixture being drawn into the cylinder with the same lift cam.
Sounds good, BUT:
This is only theoretical: when valve sizes are bigger the edge of the valve is nearer the chamber wall, so the area between chamber wall and valve edge decreases (shrouding). Depending on the exact chamber shape this can even cause performance to drop with a bigger valve. One move is to deshroud the chamber somewhat, but then your compression will go down slightly. A rule of thumb is that walls which are more than a quarter of the diameter of the valve away from it aren't an impediment to airflow. This isn't possible in any two valve engine I know of, you would have to cut more material out of the chamber wall than is actually there.

So what does that mean?
If you're going to go aftermarket anyway you'll alomst certainly have 2.02 valves installed, because most performance heads have them anyway. I guess the Techs there at Dart or World did their flow testing and found 2.02 valves to be better for airflow reasons, so they put them in. I'd trust them in those things.
Also: the resale rate is higher because people think bigger is better

The bigger exhaust valves are actually more of a benefit, shrouding isn't so much of an issue here because pressures are so much higher. A bigger exhaust valve causes less backpressure. The pistons has to press the burnt charge out through the exhaust valve opening, this robs power. The faster and easier it is for the piston to push the charge out of the cylinder the more torque will be available at this engine speed. The valve being bigger it has a larger seat area and heat gets transfered from the valve to the head faster. Exhaust valves get damn hot and they don't like it
I don't know of a downside to bigger exhaust valves except for cracking issues because the amount of cylinder head material between the valves is less, but I guess this isn't much of an issue.

Originally Posted by MadMax

The advantage is this:
A bigger valve has a bigger circumference and so the cylindrical area through which air can flow is bigger at the same valve lift (about 4% larger for the 2.02 valves than on the 1.94 valves). So more air can flow through the opening the valve creates in the same time. As a result of which you could use a lower lift cam for the same performance (less stress on the valve train) or you automatically get higher performance because of a bigger amount of mixture being drawn into the cylinder with the same lift cam.
Sounds good, BUT:
This is only theoretical: when valve sizes are bigger the edge of the valve is nearer the chamber wall, so the area between chamber wall and valve edge decreases (shrouding). Depending on the exact chamber shape this can even cause performance to drop with a bigger valve. One move is to deshroud the chamber somewhat, but then your compression will go down slightly. A rule of thumb is that walls which are more than a quarter of the diameter of the valve away from it aren't an impediment to airflow. This isn't possible in any two valve engine I know of, you would have to cut more material out of the chamber wall than is actually there.

So what does that mean?
If you're going to go aftermarket anyway you'll alomst certainly have 2.02 valves installed, because most performance heads have them anyway. I guess the Techs there at Dart or World did their flow testing and found 2.02 valves to be better for airflow reasons, so they put them in. I'd trust them in those things.
Also: the resale rate is higher because people think bigger is better

Max,is this very well written.
I am sure there are people who have done dyno testing to see at what point and time you start to benefit from the larger valves.
Never forget that the port shape is also very important to flow numbers along with the intake being an extension of the port.
One area I have noticed a lot better flow numbers is going to an 8mm valve stem.
The 8mm is just a little smaller than the 11/32 which tends to help the flow a lot around the guide area.

To be exact, there is of course more to it, than I wrote in my previous posts. If you want the more or less full story behind the valve size and flow characteristics of a head, here goes:

You have to make a compromise between flowing speed and flowing volume.
The reason is this:
Flowing fluids and gases have certain physical properties:
(1) flowing speed and internal pressure are indirectly proportionate
(2) speed and mass are directly proportionate
(3) high pressure enhances condensation, low enhances atomization
(4) port area perpendicular to the flowing direction is indirectly proportionate to the flowing speed (i.e. it is directly proportionate to the pressure, see (1))
(5) a high pressure difference from one end of a tube to the other causes high flowing speed
(6) a large perpendicular area lets more gas pass at the same pressure difference

Directly proportionate means that when one size is increased, the other is increased by the same amount, so if you double the flowing speed you automatically double the flowing mass per second.
Indirectly proportionate means that when one size is increased, the other is decreased by the same factor, so if you double the speed, the pressure will be only half of what you had before.
Perpendicular area is the area, through which the flowing takes place at a ninety degree angle
I think the other things are clear. These simple physical relationships are almost all you need to calculate the flowing numbers if you know the mathematical function of the port shape. As this is unknown I will not go into it any further, but will try to stay "unmathematical"

Now I would like to join a few of these statements together to make sense in a cylinder head:
The ultimate goal is to get a certain amount of evenly distributed mixture into the cylinder in a short time. The way an engine does this is by moving the piston down the bore on the intake stroke, thus causing a pressure drop, and the "missing" air wants to be replaced by something else. The pressure drop is always the same because the same amount of air is missing on every stroke (an eightth of the engines displacement). But at WOT the "vacuum goes down", because the throttle blades are open and a lot of air can pass through them (6) (high speed => low pressure).

In a race engine you want to cram as much mass of this mixture into the cylinder as you possibly can, so the perpendicular area of the whole port has to be large (reason (6)). There is then a lot of mixture in the engine, the correct ratio of which is what is important. You don't always get the same amount of mixture into the cylinder per stroke, at WOT the cylinder draws in much more than at small throttle openings (reason (2) also applies to the throttle opening, which is where your intake runner starts: Small opening keeps the flowing speed down, so less mixture per time unit is sucked through it).
In a truck engine you want maximum efficiency, so you don't always want a lot of mixture in the cylinder, you just want it to burn as well as possible, so a fine distribution of the fuel molecules in the air is required. The fine distribution is enhanced by the low pressure (3), which is to be achieved through high flowing speed (known as port velocity, which you have probably heard of, this would be reason (1)), and this high flowing speed is achieved by making the runner area smaller.

This would also explain why bigger valves generally flow more air: The perpendicular area is larger. High lift also achieves this, but the physical reason behind this is also (6). Shrouding, too, is a direct effect of there being too little area between the valve and the chamber wall, through which the flow is slowed down (smaller area => slower flow => less mass per time).

An interesting effect with bigger valves is: the valve throat is then bigger than the nominal area of the rest of the port: So right before the valve the pressure goes up and the mixture forces itself into the cylinder (4). In an optimal engine this effect would be timed when the piston is actually already moving upwards again. As you know, air has a mass, so it has momentum, and the flow in a certain direction doesn't stop immediatley the suction from the cylinder is gone. It continues for a short while to travel in the same direction of its own accord. So if the pressure rise is experienced in this time you can close the intake valve a good deal later than otherwise, thus having a higher mixture amount in the cylinder as would normally be possible.

This immediately shows one common misconception: A large port will actually keep power down at low RPMS, because the flowing speed isn't high enough (because of the bigger area the speed goes down) to keep the mixture finely atomized (because the pressure is high). So the mixture won't burn smoothly (only the outside of the droplets of fuel in the mixture is burnt, so the smaller the droplets the better). And there wont be much charge in the engine at low speeds, because the pressure difference occurs slower, so less air is drawn into the cylinder. Only at high RPMs these big heads come into their own: When the engine turns faster the pressure drop becomes more abrupt and pulls on the mixture harder, so it will flow through the big port at high speed, so it also gets well distributed. But only at high engine speeds. A small port is better for low RPMs because the mixture is drawn into the cylinder fast, so it's finely atomized and will burn smoothly. But at high RPMs this small-port-engine will "starve", because of (6) - the flow area simply isn't big enough to get a lot of air through the port at the pressure drop we have. Because you don't need a lot of mixture in the cylinder in a low-RPM engine you can save the chapter about the bigger valves pressing mixtur into the cylinder after the pistons has passed BDC.

That about sums it up I think. It's still quite physical I'm afraid and rather long, but I think it gets you in the ballpark when choosing cylinder heads, even if I have added no exact numbers: I specially didn't want to do this, as the only numbers air is really concerned about are the flow numbers, a port with a different shape might flow worse at a bigger volume than a small one, depending on the exact shape funtion it has. So the question is not : do I want 190cc runners or do I need 230cc ones? The question should be: do I need 230cfm at .200 lift or more like 260cfm?

BTW, "swirl" and "tumble" are fine words, but they're all included in the port shape, generally at high flowing speeds you will get more tumble and swirl (imagine running through a tight tunnel with a million other people. It's much more difficult to go in a straight line then as if you were only walking slowly. In fact, this is pretty exactly what happens in "port-reality"). This also keeps the abolute speed of the mixture up: The molecules don't only move straight towards the cylinder, they also move in all sorts of other directions, all adding to the overall speed, keeping pressure down and atomization up, so you get a nice, efficient burn even at low engine speeds, which is what you want in a daily driver. (btw, "swirl polish" doesn't get you anywhere at all, as the air doesn't know what finish you put on your valves, hardly any of it ever touches the valve...).

Right, this is the end.

OK, not quite: I like the following quite precise analogy:
Imagine you have a 20ft rope lying in front of you. Give it a slow pull of 2 ft (one slow swing of your arm). What happens? The rope has travelled 2 ft in your direction, it is still straight. Now give the rope a sharp tug of the same length, but much harder. With luck the whole rope will be lying tangled up at your feet. This is what race engines rely on: At high engine speeds the pistons pulls so hard on the air, that more than a cylinder load is pulled into the cylinder and this is well disordered.
But when you combine these fast engine speeds with a lot of "rope" being in your cylinder you can easily understand that the amount of rope you need goes up. So does the amount of fuel you need.

This is it now, finally

Max, that's allot to absorb. Thanks for the time you put put into writing it. I'll print it out so I can go back and read again when I have a laps in memory. Thanks for the information.