The 85 Percent Rule
Talk to most cylinder-head experts and you will run across something called the 85 Percent Rule. This refers to the diameter of the throat area directly underneath the valve job. Most production heads make this area significantly smaller, sometimes around 70 percent of the valve-seat diameter. This means that if you have a 2.00-inch intake valve, the throat diameter may be as small as 1.5 inches in diameter. The temptation for backyard porters is to immediately grind this throat area out to the base of the valve job. When that happens, this eliminates the shortside radius (port floor) into the intake seat and disrupts flow, especially at the higher valve lifts.
The 85 Percent Rule merely limits the throat diameter to no more than 85 percent of the valve-seat diameter. So again, if you have a 2.00-inch intake valve, the maximum size of the area just below the valve seat must not exceed 1.70 inches. Some cylinder head porters will tell you the number should be 90 percent (which is another 0.100 inch for our 2.00-inch valve example) but our sources tell us you must be extremely careful with this high percentage so we'll stick with 85 percent.
The distance shown by the line across the exhaust-port throat should be 85 percent of the diameter of the valve. At 88 percent, the stock throat is already larger than we would prefer. | 
This is the stock 460 exhaust port. Note the large wedge downstream that must be narrowed. |
This is the exhaust port once we had narrowed that wedge (A) and blended a slight bulge in the port roof (B). | |
Valve Angles
While accurate porting will certainly improve airflow, a good valve job can also make a huge difference. If this is your first attempt at porting, the usual suggestion is to experiment on a junk head to get some experience. The idea is to remove metal around the valve-seat area without hitting it with the carbide cutter. Once most of the port work is complete, take your heads to the machine shop where they can add the new guides and machine the seats to your specs.
JGM has years of experience with production heads and valve angles that work, even with heads like the iron 460. For the intake, JGM cuts both the intake and exhaust seats with three angles of 37-45-60 degrees. For a performance street-engine application, Grubbs prefers to run an 0.080-inch-wide seat for the intake and 0.100-inch wide for the exhaust for durability.
Narrowing the intake-seat width will improve flow, but for street use this will soon pound out and the flow will probably suffer. Grubbs also places the seat as far out toward the edge of the intake valve as possible to gain valve diameter and improve flow, but for street use, this distance is generally around 0.020 in from the outside edge of the valve.
JGM has also found that placing a 32-degree back-cut on both the intake and exhaust valves will improve flow. Generally, this helps low-lift flow but hurts higher valve lifts of 0.400 inch and higher. In this case, the back-cuts on both the stock and aftermarket valves improved flow throughout the entire lift curve. Every head, port, and seat-angle combination will react differently to these types of modifications.
JGM uses a Sunnen valve-seat machine that simultaneously cuts all three seat angles. The cutting tool is custom-made to his specs. Older valve-grinding tools also do a good job of establishing seat angles, especially in the hands of a qualified operator. | 
JGM machined a 32-degree back-cut (arrow) to improve flow in this Manley 1.71-inch stainless steel valve. Our testing shows that both intake and exhaust flow improved with this procedure. |