Here's a comparison between...
Here's a comparison between a Chevy SB2 head (bottom) and a cylinder head for a 500ci drag racing engine. Although drag racing heads like these will burn alcohol or nitromethane instead of gasoline and produce three-to-five times more horsepower, Bieneman says they are actually quite similar. "The sizes are different," he says, "but the relationships from the port, to the seat, to the chamber, to the exhaust are a lot more alike than you might think."
A little work is required to make the flow patterns in the intake manifold even. Just like on a small-block, the offset cylinder banks indicate slight variations between the two interior runners on each side. It's more pronounced on a big-block because of the greater distance between bore centers. On the stock manifold design, the intent is to make the angle almost identical on all runners at the transition where the manifold meets the cylinder head. However, this means one of the runners must make a very sharp turn before the entrance into the cylinder head intake port. Bieneman says it is better for the engine if that runner is shortened to match its mate. This is done by grinding the wall on the inside of the turn to straighten the path a little.
The details may be slightly different, but these are the same tricks that small-block porters have been taking advantage of for years. Although it is often impossible to achieve for many different reasons, the more you can equalize the volume and quality of the air/fuel mixture entering all eight combustion chambers, the more power you can make. This allows you to achieve the best possible tune for every cylinder. If, for example, you have a runner that is restricting flow or causing the fuel to fall out of suspension, the air/fuel mixture entering one combustion chamber will be of a different quality from the air/fuel mixture entering the other seven. Since there is no way to have the carburetor pump more fuel to this one cylinder or set the timing differently for that cylinder, it won't produce as much power as the other seven.
Relocating the ports also...
Relocating the ports also entails moving the pushrod holes. Bieneman handles this by bushing the existing pushrod holes and then cutting new openings exactly where he needs them.
Believe it or not, Bieneman says one of the tricks he has found to make more power with these heads is to shrink the exhaust port and use a smaller valve. He says that when he has finished porting a set of heads, the exhaust port will actually be about 5 cc's smaller, and the usual 1.900-inch exhaust valve will be replaced by one that's only 1.800 inches in diameter.
This achieves two significant results. First, Bieneman says the smaller port produces more velocity, which improves that port's volumetric efficiency. We often hear that better velocity numbers in the intake ports is important because it means better cylinder filling at high rpm levels, but it is also important in the exhaust. At high rpms, higher exhaust port velocity translates into better evacuation of combustion remnants from the combustion chamber and also helps with scavenging.
Veteran cylinder head porter...
Veteran cylinder head porter Dennis Warner puts the finishing touches on a new combustion chamber. Although CNC machinery has taken over the vast majority of all finish porting work, the best models are still done by hand. After they are perfected, the ports and chamber will be digitized so that the CNC machinery can mimic the original.
Second, the smaller port and valve have an interesting advantage that many of us wouldn't think of. Bieneman actually uses the port and valve design to limit the effects of valve overlap. Overlap is helpful to use the exhaust's scavenging effect to begin filling the cylinder with fresh air and fuel before the piston has begun its downward stroke. However, large valves can sometimes block the air and fuel from actually getting into the cylinder. Instead, the tops of the intake and exhaust valves form a floor and guide the incoming air/fuel charge out of the exhaust port until the exhaust valve closes. In order to get enough duration, cam designers are forced to put in excessive amounts of overlap.
Bieneman says his exhaust port design actually reduces flow at lower levels of valve lift. For example, the exhaust flow at 0.100 and 0.200 inch of valve lift is actually worse than the traditional port. But at 0.300 and up the flow is much greater. As a result, the incoming air/fuel charge that enters the chamber during the overlap period is less likely to flow out of the exhaust port and be wasted. On the dyno, the effect is more low-end torque and greater fuel efficiency.
An example of the raw port...
An example of the raw port Warner begins with and his finished work.
Bieneman points to a similar phenomenon that many small-block cylinder head porters-himself included-discovered when they began putting radical 55-degree valve seats into high-lift heads. The steep seat angles are very harmful to flow levels at lower valve lifts, but in race engines these seats somehow resulted in better low-end torque. Bieneman says it confused many engine builders until they realized that with the shallower seats and current cam lobe designs, they were sending too much of the incoming air/fuel charge right out the exhaust ports. The assumption was that you wanted to maximize flow at every point of valve lift to get as much fuel and air into the cylinder as possible. But too much of that air and fuel was being lost. Killing off a portion of low-lift flow actually helped performance because more air and fuel was being burned inside the combustion chamber. The lesson to be learned here? Bieneman stresses that good numbers on the flow bench are nice, but unless those numbers relate directly to success on the racetrack, they don't mean a thing.