Here’s the reality in Spec Miata. The days of finding a cheap donor car with 100,000 miles on the clock, adding the required safety equipment and going racing in hopes of running with the pack are gone. Engine modifications permitted in the rulebook, specifically tuning with the fuel pressure regulator and performing cylinder head machine work, have been made by 90-plus percent of the field no matter where you race.
Your driving ability notwithstanding, if you expect to run with the pack in your region, you will need to make these modifications so you can make enough horsepower just to keep up.
According to the rulebook, you can resurface the head, plunge cut the throats to correct for core shift and make radial cuts on the combustion chamber wall around the intake in exhaust valves. Finally, a three-angle valve job is legal, provided all the valves and guides are factory OEM Mazda parts, all of which are available through Mazdaspeed Motorsports Development.
Resurfacing the head is simple and understandable, and practically a necessity on aluminum heads. If you’re going to go to the trouble to remove the head and do a valve job, you should resurface it to ensure a true surface.
A three-angle valve job is customary, with seat angles of 45 degrees, with a bottom cut angle of 70 degrees and a top cut at 30 degrees. The cylinder head used for this story had been removed and sitting for a while, so the guides and valve stems and valve faces were corroded and pitted, which meant everything had to be replaced. That is not uncommon, according to Todd McKenzie of McKenzie Cylinder Heads in Oxnard, Calif.
“On most imports with high miles, it’s best to replace it rather than grind it because when you grind it you take away the margin of the valve, which is the thickness underneath the face, and then it has a tendency to burn,” he said.
Once the valves were out, he could proceed with the permitted machine work. To correct for core shift, the throats on 1.8-liter Miata engines can be cut 9 millimeters from the bottom of the ferrous metal valve seat. What is core shift?
“Core shift is when the sand mold moves when the aluminum is poured, so you’ll have a bowl on one side that’s bigger than on the other side,” McKenzie explained. “Plunge cutting tends to even that up as well as opening the throat diameter to allow more air in past the valve.”
Next come the radial cuts in the combustion chamber walls. Like the plunge cuts to the throats, the radial cuts were done on McKenzie’s Serdi grinding machine. The tooling inserts into the valve guides to ensure exact placement and uniform cuts. The radial cuts are a bit tough to see in the photos, but that’s because they are slight, just .760” on the intake and .675” on the exhaust side for the 1.8. The cuts are even smaller for the 1.6.
“It unshrouds the valve so nothing is obstructing the flow past the valve in that one section where the valve is adjacent to the cylinder wall,” said McKenzie, who did the head work on the turbocharged Honda V6 in the Muscle Milk LMP2 car. “The rest of the valve, which is probably 85 percent of it, is free flowing, but it’s just that one section. Every little bit helps.”
McKenzie has been machining cylinder heads since 1983 and in his current shop for the last 17 years. He’s done everything import and domestic, for street and racing applications, including a twin turbo 996 Porsche, which ran 229 mph at the Texas Mile.
This video takes you through the whole process and shows how the machine work is done.
Special Thanks To:
McKenzie Cylinder Heads, Oxnard, Calif.