CAD/CAM Design, Cylinder Head, Lead To Faster Motorcycle

Major corporations had been trying to top eight seconds for the quarter-mile on a Harley-Davidson motorcycle. Precision Developments (Corona, California), a seven-employee custom shop owned by George Natzic and managed by his brother Rob, who also programs machining, had a hand in the production of the Stage I street model cylinder head for aftermarket customization of Harley-Davidson motorcycles, which led to the Stage II prototype head on the first Pro Stock Harley, owned by Bill and Theresa Hannon. This head, they thought, might help the Harley break the seven-second barrier. There had to be a lot of work done by a lot of people before getting to this stage, however.

Dan Baisley, owner of Baisley Hi-Performance (Portland, Oregon), had a plan to market powerboosting cylinder heads for HarleyDavidson Sportsters and Big Twin EVOs for street or racing use. Also involved were design consultant Paul Paine (Buckmaster CAD/CAM Programming and Manufacturing, Portland, Oregon), the Natzics' Precision Developments, Crown Pattern and Foundry (Alhambra, California) for casting, STD Development (Chatsworth, California) for preliminary finishing, and then Mr. Baisley would do custom finishing and delivery.

Mr. Baisley and Mr. Paine, working together using Mastercam CAD/CAM software (CNC Software, Tolland, Connecticut), designed and redesigned the cylinder head. They had to modify the entire head, originally feeding both cylinders from a single rightside Ymanifold to the left side to permit single carburetion to each head. Other changes were also made.

Then it was time for Precision Developments to make the master part and sand cast tooling for the Stage I cylinder head. By choice, Precision Development acquired its first seat of Mastercam. Rob Natzic set to work creating the tool path for the tooling board master. Though he had never used Mastercam, he was able to catch on quickly.

Mr. Paine explains that "so many of the 1,300 surfaces in this part had very complex trimming and blending requirements. Because Mastercam has a tremendous amount of surfacing capabilities," he says, "I was able to combine ruled, blended and NURBS surfaces, constant and variableradius fillets, and more, to blend these multiple surfaces smoothly. Each area had some hitches."

"Since the part was well designed using good CAD tools," says Mr. Natzic, "it saved me a lot of time that usually goes into part surgery on many imported files."

The head was intricate, according to Don Wear at Baisley. However, he was able to break the part down into different layers. "This made it easy to create tool paths for specific areas and machine them separately," says Mr. Natzic.

He chose ½-inch and ¼-inch ball end mills, finishing with a 1/16-inch ball mill in places such as the area around the nameplate. In addition, he used the new constant scallop feature. While a standard stepover tool path works well for many parts, the linear stepover does not compensate for sloping surfaces. "It produced variable scallop depths on the leading and trailing edges of sloped surfaces," he says. "But using Mastercam's constant scallop machining, I can expect a consistent finish everywhere on the piece."

The decision was made to use a rotary table and index the Mastercam file to work around the part in 90-degree increments, simulating a 4axis approach. Mr. Natzic had Mr. Paine extract the specific geometry for the ports and vents. Then he exported them from Mastercam into STL (stereolithography) files and Precision Developments created these details on its Stratasys FDM 8000.

The accuracy of this step was critical for the success of Precision Developments' part in the project. "It's the placement of the ports that gives the cylinder head its edge," says Mr. Baisley. "Not only were we able to channel the intake mixture more efficiently, but the redesign allowed the use of a separate carburetor for each cylinder."

"We isolated the powerrobbing pulses you'd get from a single carb splitting off a "Y" manifold to feed two cylinders, and now fed the fuel mixture to the cylinders with fewer velocity reducing angles," Mr. Baisley explains. "Performance picks up drastically, the engine runs smoother, builds more torque and horsepower, and spreads it across a wider power band." Typically, 74 cubic-inch HarleyDavidson Sportster engines, modified to 88 cubic inches, deliver about 100 hp at 8595 foot-pounds of torque. Adding the Baisley modification jumps it to 153+ hp and 120 footpounds of torque.

The machining yielded a tooling board master head, fins and all. Next, the Natzics made the multiple mold parts from the master. Beginning with the drag, or base level mold section, they built five different core plugs in sections around the master, making the general outline on the bench, fitting it up against the master, and pouring plastic between to capture the details as an impression of that section.

Additional internal features were added by taking dimensions from the Mastercam CAD file. The STLproduced intake/exhaust channels and other parts were "hung" inside the assembled core boxes, the master replaced within the boxes and the top, or cope, set on and plastic poured in to capture the top details. From each of eight core plugs, Precision Developments cast iron filled epoxy core boxes and shipped them to the foundry, where they are used to make the cores into which the casting metal is poured. When the casting is poured, the sand is broken off to reveal the metal part.

Once extracted from the molds at the foundry, the aluminum cylinder head is separated from the riser systems of the molding process, the riser attachment points are ground down and the entire head is sand blasted. The foundry forwards the parts to the machine shop for preliminary finishing of the top and bottom surfaces, valve guide holes, spring seats, chamber, valve seats, and all bolt holes and basic roughing in the ports.

Then, it's back to Baisley's for custom intake/exhaust port and valve seat configuration to customers' cylinder bore and valve specifications, aesthetic finishing—and out onto the street or track.

And, after all this, the Hannon's Pro Stock Harley clocked 7.931 seconds at 166.13 mph for the quarter mile at the U.S. Nationals in Richmond, Virginia, in June, 1998, successfully breaking the seven-second barrier.