MBE never tests a cylinder...
MBE never tests a cylinder head design bolted directly to the flow bench. Instead, it uses specially designed plates that mimic the deck of an engine block and the way in which the first inch of the cylinder wall affects valve shrouding. This also allows the traits of a specific head gasket to be worked into the picture, as well.
Flow: Intake versus Exhaust One of the most important relationships in a cylinder head is the amount an intake port is capable of flowing versus the exhaust port. According to many sources, the best ratio is an exhaust port that flows approximately 75 percent of what the intake is capable of flowing. Bieneman has seen the same results in his work and says this ratio holds true no matter what type of engine you are building. A good example is the big-block Chevy head program he has begun for DIRT Modifieds.
"We don't believe in putting a big exhaust valve in anything," he says. "In the big-block racing head, the casting was designed for a 1.88-inch exhaust valve, and we ended up putting a 1.8 valve in. We even had to change the seats to get it to accept a valve that small, but it just makes the headwork better. Even with the smaller valve, the port flows what the 1.88 valve did. That brings up the velocity and makes the head more efficient-not to mention what a smaller valve means in terms of valvetrain stability.
"Even if we have room to create a bigger exhaust port, we believe the relationship between the intake and exhaust is so important that we won't. If that relationship is off, you may not see it on the dyno, but you will see the difference on the track. A good 75-percent head seems to always accelerate better. We've all heard stories about engine builders that can build a race engine that looks great on the dyno but is a dog on the track, and I believe this has a lot to do with it."
A good head designer will...
A good head designer will take the height of the spring pads into account. If the spring height stays the same, a tall pad means a longer valve stem must be used, increasing the weight that the springs must control at high rpm.
Once Bieneman settles on a design for the ports and combustion chamber, he turns his attention to the requirements that his design places on the valves. MBE has a working relationship with Stealth, which allows him to design titanium intake and exhaust valves exactly to his specifications. Using specialized software, Bieneman can design a valve and determine exactly how much it will weigh. If it is too much, he can tweak either the valve or head design until it meets his criteria. His reasoning for going to such great lengths is simple: Cylinder head design has a big impact on valvetrain stability.
When it comes to the cylinder head, many factors that affect the valvetrain are in the geometry it dictates. MBE typically designs heads to accept the biggest pushrods possible. The company works to drop the spring pads as close to the combustion chamber as possible. For a given spring height, a thick spring pad means a longer valve stem is required. Long valve stems mean unnecessary weight in the valvetrain, and we don't have to tell you what that means.
This shot shows the optimum...
This shot shows the optimum relationship between the intake manifold and the cylinder head intake port. There are no obstructions, the angle of flow is continuous, and you can almost see the bottom of the intake valve. The result is fantastic flow and plenty of signal at the carburetor.
Include the Intake Manifold
Intake manifold matching has been going on for years. Typically, it means taking parts of either the intake runner exit or the cylinder intake entrance to make them match up without any ridges to impede airflow. Smart engine builders and cylinder head specialists have begun taking that a step further. An engine does not recognize where the intake runner ends and the cylinder port begins; it only recognizes if there is an obstruction that disrupts the flow of the incoming air/fuel charge or something that causes the fuel to drop out of suspension.
This can be caused not only by a ridge that results from incorrect alignment of the intake manifold runner and the cylinder head intake port, but also from a change in the direction of flow, which is much harder to correct. An engine is able to pull in the largest, most homogeneously mixed air/fuel charge when it can create a strong signal at the carburetor venturis. The best way to do this is to create a straight shot between the carburetor and the seat of the intake valve. This isn't always possible in a pushrod V-8, but generally speaking, the more you can do, the better.