Project Focus SVT - 2003 Ford Focus

The stock cast surface inside the exhaust port can be significantly improved with polishing, which will transfer more exhaust energy to the turbo.

The stock cast surface inside the exhaust port can be significantly improved with polishin

Cutting the Valves
We decided to stick with just basic head work, which means all the stock components are going back into our Zetec head. The stock valves are refaced to remove irregularities from mass production machining and wear on the 45-degree seat angle. Cosworth also back-cut the intake valves, which removes material from the back side of the valve to smooth the transition from the 11.5-degree back surface to the machined 45-degree seating surface. The average of these two angles, 28 degrees, was cut into the back as a transition face. Less abrupt changes in flow direction-several stepped angles vs. one sharp transition-helps minimize flow separations and turbulence, which increases efficiency. The smooth cut surface is also an advantage over the stock valve surface. Cosworth typically back-cuts both intake and exhaust valves, but the mild work on our head only warrants the cutting of the intake valves.

Three-angle Valve Job
The steel valve seats are remachined to match the refaced valves. A special mill that sits on a hemispherical mount, allowing the cutting tool to be swiveled to align with the valve guides is used for cutting valve seats. The valve guides were left in this head for ease of machining and because it was hand ported, which requires less tool clearance.

Luckily, there is little geometry and calculation involved in cutting valve seats. A generic three-angle cutting tool is used to cut all three valve angles, 30, 45 and 60 degrees at once. The stock valve seat is first colored with a Sharpie so that machinists can tell if the seats have been completely cut by the tool. Cut depth is set once the Sharpie marks are machined off. After the first seat is cut, a valve is inserted and lapped to mark the diameter of the seat on the back of the valve.

Lapping is a process in which an abrasive compound is applied to a valve as it is spun in its closed position. The resulting ring from the abrasion indicates how well the valve seals against the seat. If the lapped ring is more than five thousandths of an inch from the outer edge of the valve, the seat will be cut deeper. The deeper cut increases the seating diameter, which increases the flow area past the valve at low lift.

The overall profile of the exhaust port was not changed in the port and polishing process.

The area around the seat is deshrouded with a radius cutter to knock down casting features and spark plug bosses inside the combustion chamber. This removes flow obstructions to the seat and further smoothes the flow path. A final 75-degree cut is performed at the base of the seat leading into the intake ports. This further eases the transitions between the cut angles. The depth of this cut also dictates the width of the 60-degree cut relative to the width of the 45-degree cut that the valve seals against. Cosworth claims the ratio of the widths between these two cuts is critical in achieving maximum flow.

Porting and Polishing
The porting performed on the Zetec head was fairly minimal. Even though our turbocharged engine will be moving a lot more air, we left the port profile alone instead of removing as much material as possible because proper port work takes a lot of back-to-back flow testing.