"Heads are everything." That's what Air Flow Research's Tony Mamo said to us in a recent phone conversation. "What to do with the heads is the most critical decision to make in any engine build."
To see firsthand what goes into building a set of heads, we spent an afternoon with JMS' Pete Hillemeyer, a guy who's been building cylinder heads for more than 20 years. We also sought the counsel of the School of Automotive Machinists' Judson Massingill and Advanced Induction's Phillip Odom, in addition to the advice Mamo gave us. We wanted their take on where to spend your money when assembling a pair of heads.
The baseline for any cylinder head buildup begins with a thorough reconditioning of the heads, and that typically involves disassembly and cleaning, surfacing (if needed), regrinding the valve angles and seats, knurling or replacing the guides, installing new valve seals, and checking the springs. We'll show you what can be done during this process to tune up the heads to flow more air.
Guides And Seals
The first step to performance cylinder head work involves checking the valveguides for wear. Worn guides and seals cause oil consumption by allowing oil to leak past the valve stem into the combustion chamber. A telltale sign of worn seals and guides is a puff of blue smoke on a cold start-up or when they're really bad, a smoke screen when you pull away from a stop after idling for a while.
Performance (left) versus a stock valve.
Hillemeyer uses Dykem Machinists Blue when machining the valves and seats. The new cut wil
Guides are usually made of bronze (or cast iron in an iron head); seals are Viton or anoth
More detrimental than bad oil control, worn guides don't control the movement of the valve properly. Floppy, erratic movement makes it difficult for the valve to seat fully during each event cycle. If the valves don't seat properly, they won't seal the combustion chamber-and there goes your horsepower out the exhaust port.
Guides can be knurled if they are badly worn, but if they've been fixed more than twice, they should be replaced with press-in bronze inserts called false guides. Once the guides are in spec and properly located in the head, the valve seats can be cut in their proper locations. After the guides are repaired, always replace the valve seals.
Valve Jobs And Airflow
Mamo says the best thing you can do to a stock pair of heads is improve the low- and mid-lift airflow levels, and there are a couple of relatively inexpensive ways to do that. The first is to back-cut the valves. Most valves have a wide, 45-degree-face angle. By narrowing that angle with an appropriate back-cut, you increase low-lift flow because there is less material for the air to flow around and there is a smoother transition on the backside of the valve. Mamo says this adds 10 to 12 cfm of flow from the crank of the valve up to 0.300-inch lift. He says valves are generally back-cut at a 30-degree angle in the area between where the valve seats to the head and the valve stem.
The second flow improver is a three-angle valve job. Three-angle refers to three different cuts, 30, 45, and 60 degrees, machined into the seat in the cylinder head, not the valve. The three angles in the head are arranged from shallow to steep and form a funnel shape leading from the combustion chamber down into the runners. The middle angle, 45 degrees, must match the valve cut. That is where the valve seats or when it is closed. The shallow 30-degree cut is between it and the combustion chamber, and the 60-degree cut is between it and the bowl area of the runner.
Also, you should know the 30- and 60-degree numbers are generic figures that represent a range of possible angles your machinist prefers for your application. Massingill tells us small-block Chevys react well to 38-, 45-, and 58-degree angles, while Gen III Chevys and most Fords like 38-, 45-, and 64-degree valve jobs. Each head is different, and the valve job angles and widths are some of engine builders' most closely guarded secrets.