In today’s performance world of seemingly limitless selection in aftermarket cylinder heads, there are still enthusiasts steadfastly retaining ancient OEM production castings. If the name of the game is all-out power and performance, the improved flow and bolt-on appeal of new aftermarket heads leave little room for argument. While one might think the bulk of these factory-headed engines involves budget-minded builders using the parts they have on hand, there is another perhaps larger group within these ranks—the nostalgia/resto crowd. For many, authenticity is every bit as important as performance, and high-performance aftermarket cylinder heads just don’t meet their standards of originality.

We were faced with just such a situation in building a Chevy big-block 396 for a resto Chevelle project. The “numbers” block was the starting point, and in keeping with an OEM-style restoration of the vehicle, the engine had to play the part. Externally, the engine had to have the factory look and parts to fit the theme of original 1960s muscle. Internally, however, we were given a free hand to modify for increased power output, driveability, and reliability.

Naturally, when the subject turns to improved performance, the centerpiece is the cylinder heads. Our examples were small-chamber oval-port heads—casting number 3931063—factory-equipped with 2.07/1.72-inch valves and featuring 101cc chambers. With these cylinder heads, the OEM 396 carried a rating of 325 or 350 hp, depending on the version. Our goal was to substantially up the ante on power output from our 396, including the addition of a 4.00-inch stroke crank. To find the extra power, cylinder head modifications were a key consideration. Now, there is virtually no limit on how far you can go with a porting and modification program, but we were looking for practical increases with maximum results for the time and effort spent. This meant we would not pursue a max effort full porting and polishing. We would look to improve the flow substantially with a few key mods.

Practical Head Modifications

Any cylinder head rehabilitation program on old, used castings is going to include the basics, such as a valve job, new valves, and refurbished valveguides. Here was the cornerstone of the upgrades, moving up to new Manley valves in the 2.19/1.88-inch specifications of the OEM rectangular port heads. The larger valves open the door for more throat area in the bowl just beneath the valves, along with more curtain area for improved low- and mid-lift flow. Since our engine would be built to mimic the idle quality of an OEM muscle car, the engine would be necessarily limited on camshaft specifications. Under these constraints, a flow curve that is fat in the midrange and up to peak lift is what will count, and the valve curtain area of the larger valves provides the potential for significant improvement.

While we were spec-ing the valve upgrade, we also stepped down from the OEM valve stem diameter of 3⁄8 inch to the smaller 11⁄32-inch dimension, reducing unneeded valve mass in the process. The lighter valves would assist in maintaining valvetrain stability with a fast-acting hydraulic roller cam, which was also part of our engine-build game plan. This was easily accomplished by installing K-line bronze valveguide inserts, refurbishing the guides in the process.

Simply installing bigger valves is not a sure path to flow improvement. Without corresponding work to the valve seat, throat, chamber, and bowls, it can be a wasted effort. Modifications are required here to take advantage of the potential offered by the bigger valves, starting with the valve job machining. Here, we turned to the experts at Dr J’s Performance for the necessary work. The machining included a chamber cut to reduce valve shrouding, a competition valve job, and machining in the port throat just below the valve to open this area to match the larger valve diameter. The high flow seat form and material removal above and below the valve is what unlocks the flow potential of the larger valve diameter.

Dimensional Comparison
Stock 2.06/1.72-inch vs. 2.19/1.88-inch
Measured Throat Diameter, Inches
Valve Radial Clearance to Chamber, Inches
Stock Modified
Intake Throat Diameter1.8201.970
Exhaust Throat Diameter1.5201.640

With the machine work enlarging the throat area of the bowl below the valve, there will be a considerable mismatch with the untouched portion of the port below. This remaining step or ridge requires hand blending with a carbide burr to complete the job. Many call this minimal level of hand porting a “bowl blend,” but we extended the operation to include a moderate blending of the important short-turn radius leading from the bowl around to the port floor.

While this may seem like a modest level of modification, the flow bench proves that the effort yields substantial gains. Looking at the accompanying flow chart we see gains just where we expected, with significant increases beginning in the low-lift range, carrying through with a big boost in the mid-lift and top-end flow numbers. A peculiarity of the Chevy big-block architecture is that there are actually two distinct intake port configurations, with two ports per head biased to the cylinder wall, which normally show a flow disadvantage compared to the other two ports biased to the cylinder centerline.

We can see that even the poorer ports of the modified heads ended up with much more flow than even the “good” ports on the stock cylinder heads. Flow in the 0.200-0.500-inch lift range gained dramatically, adding far more area under the flow curve for a moderate street cam to take advantage of. Even at the top of our test range of 0.600-inch lift, the advantage gained held firm.

Even though we were limited in this particular engine project to the stock cylinder head castings, a well-planned and executed modification program greatly improves the power potential of the build. Based on the flow numbers, we could conservatively expect a gain of 50 to 60 hp compared to a stock set of cylinder heads. That is a healthy amount of power ready for the taking, for just a little more effort than a stock rebuild.

Cylinder Head Flow Results
Big-Block Chevy No. 3931063 Oval Ports
2.06/1.72-inch valves vs. 2.19/1.88-inch
SuperFlow 600 Flowbench
Tested at 28 inches Water Depression
Intake Stock (1) Mod (1) Stock (2) Mod (2) Stock Exhaust Mod
LIFT
0.100 64 75 62 77.6 54 78
0.200 115 152 115 153 96 138
0.300 179 220 180 216 121 164
0.400 226 263 225 251 143 193
0.500 244 288 238 270 162 207
0.600 264 292 251 274 172 217
Legend:
Stock (1) -- C/L intake port; 2.06-inch valve
Stock (2) -- Wall intake port; 2.06-inch valve
Mod (1) -- C/L intake port; 2.19-inch valve
Mod (2) -- Wall intake port; 2.19-inch valve
Stock -- Exhaust port; 1.72-inch valve
Mod -- Exhaust port; 1.88-inch valve

SOURCE
Comp Cams
3406 Democrat Road
Memphis
TN  38118
800-999-0853
http://www.compcams.com
Dr. J's Performance
436 S Montgomery Street
Orange
CA  92868
714-808-9780
http://www.j-performance.com
Manley Performance Products
1960 Swarthmore Avenue
Lakewood
NJ  08701
732-905-3366
http://www.manleyperformance.com