One interesting change Bieneman...
One interesting change Bieneman makes to the 9 Degree head that is unique to MBE Cylinder Heads is the intake is moved 0.050 closer to the center of the cylinder. This not only unshrouds the valve from the side of the cylinder wall, it also moves the valve pocket closer to the center of the piston top, which allows engine builders to move the top ring land closer to the top of the piston for greater efficiency.
Cfm numbers may not mean much to everybody, but when you consider that at this level of race engine every cfm you gain equals approximately one horsepower, that's really eye opening. And it's even greater if you are looking at torque, which is very important in dirt track racing. There, every count of cfm you gain is about 1.2 lb-ft of torque."
Of course, there's more to it than simply raising the port height. Raising the port also increases its length (if you keep everything straight) and also raises the carburetor. But runner length, which includes both the runner in the intake manifold and the port in the cylinder head, is also a critical factor for determining where in the rpm range the engine will make peak power. A shorter runner moves peak power higher in the rpm range, while a longer runner moves it lower. If you raise the port too high you run the risk of lengthening the port so much that peak power is moved outside the useable rpm band.
MBE also holds its tolerances...
MBE also holds its tolerances to within 0.0001 when honing the valve guides. The valve stem-to-guide clearance is the same, but Bieneman says holding the tolerances this close allows him a better fit to the pilot when cutting the valve seats. Less movement of the pilot equals a more concentric seat, which, in turn, equals more power.
Runner length is also one of the main reasons Bieneman says that the Chevy SB2 hasn't surpassed the venerable small-block as the preferred race engine for GM-branded cars. While expense is also a factor, it turns out the cylinder head design is a product of a very focused design to make the best NASCAR Sprint Cup race engine possible.
If you look at the bottom of the head, you will instantly notice that, unlike the small-block head design, the intake and exhaust valves in the SB2 aren't in line. This does have some advantages. Primarily, it makes for a very efficient combustion chamber. But by moving the intake valve closer to the edge of the head, it also makes the port shorter. This is no accident. Remember, a shorter port helps move an engine's peak power higher in the rpm range.
While we were in the MBE shops,...
While we were in the MBE shops, we also spied its latest Sprint Car racing cylinder head. Check out the lightening program. It's is a Brodix 13-degree casting.
At the time the SB2 was being developed, there were no rpm or gear limits in Cup racing, and engineers were pushing rpm limits ever closer to the mythical 10,000-rpm barrier. They even exceeded it at some tracks before NASCAR put rules in places to bring the rpms back down. But that short port is part of the package for that reason. In fact, Bieneman says the intake runner and port for an SB2 head is right at 1.750-inch shorter than the Dart 9 Degree head (0.650 for the intake port alone). This difference in length is a primary reason why the peak power in a well-built SB2 engine is in the range of 7,000 to 9,500 rpm, while an engine built around the 9 Degree head is between 3,800 and 8,000.
You will notice the peak rpm range isn't just lower, it's also much broader.
While working with Bieneman...
While working with Bieneman on this story, he kept stressing how every port design must be done specifically for the type of engine it will be run on. For example, this is the combustion chamber of the Sprint head we just showed you. Notice how the spark plug hole has a "wall" between the plug hole and the intake valve. This is to protect the tip of the plug from all the alcohol that enters the port during every intake stroke to keep from drowning the spark plug in fuel.
"On a 11/2-mile asphalt oval, the rpm range can stay in a very narrow band," Bieneman explains. "And the higher you can push that band the more gear you can put to it for more speed. This makes the SB2 great for Cup racing, and we're also having some good success building SB2 heads for drag racers because their rpm requirements are approximately the same.
"But on a 1/2-mile dirt oval that's not going to do you much good. You just can't keep the rpms that high, especially coming out of the turns or on restarts," he continues. "The engine will bog down and you just won't get anywhere. Instead, our porting package combined with the 9 Degree head is designed to broaden the useable rpm band and provide a lot more torque down low. This really pays off on the restarts because every racer will tell you that the easiest place to pass is the restart when you can just out accelerate the other guy to the next turn.
"After all, it doesn't matter where you pass the guy in front of you as long as you get around him before the checkered flag flies."