Fully ported heads are one of a high-performance engine's most expensive components. Are they really worth the bucks, or can you get by with just a valve job and bowl work?
To find out, we worked with Charlie Hupp at GT Alley Performance, a unique facility where you'll find 600ci Merlin big-blocks going together alongside 750hp Honda DOHC V-6 engines. For our portwork comparo, Hupp selected casting No. 3947041, a typical factory small-block Chevy head with 1.94-/1.50-inch valves and 64cc combustion chambers. Originally installed on many '69-'70 350ci/300hp engines and (with larger 2.02-/1.60-inch valves) on 350ci/350-360-370hp engines, the 041 heads' general port layout is similar to millions of its '60s and '70s small-block siblings, so our findings will be applicable to most other factory small-block Chevy heads from this period.
Baseline: Dead Stock
Hupp began by flowing the head in bone-stock configuration. The intake port peaked at 208 cfm at 0.650-inch lift, while the exhausts became saturated at 0.550-inch lift, and 137.4 cfm were recorded on GT's Super Flow flow bench. However, as shown in the Cylinder Head Flow Data table, the heads are essentially done by 0.450-inch lift, with little flow increase beyond that point on either the intake or the exhaust side. From the original GM designer's viewpoint, this is perfectly acceptable because production hydraulic cams all have under 0.450-inch lift.
Tim Jackson at GT Alley does the valve job the old-fashioned way using seat grinders; in t
Three-Angle Valve Job
The first improvement was a basic NHRA-legal, three-angle Pro valve job. Overall, the Pro valve job resulted in gains of 2.8 percent on the intake side, and 4.4 percent on the exhaust. Below 0.450-inch lift the intake improved by about 5 percent, while the exhaust was up 6 percent; above 0.450-inch lift, the improvement was only 1 to 2 percent.
The next step was to install larger 2.05-inch intake and 1.60-inch exhaust valves. On the intake side, the 2.05 valve flowed less than the 1.94 valve, except at 0.700-inch lift. Average flow numbers for the big-valve intake were less than those achieved with the 1.94 valve. Average flow numbers were also down on the exhaust side. Looking at actual numbers, the 1.60 valve flowed less than the 1.50 valve from 0.350 lift on up.
With an otherwise stock port, these results show that on small-block Chevy factory cylinder heads, big valves aren't effective-the question is, why? The answer involves the concept of Critical Area (CA). Think of CA as the minimum restriction or cross-sectional area in the port. The CA establishes the port's flow potential. GM factory engineers designed the head to flow down low to complement mild production cams, establishing this flow coefficient by the cutter diameter used with the stock-size valves. With factory ports, the smallest part of the bowl is the main flow-limiting factor.
With 1.94/1.50 valves, the three-angle valve job improves on the '69 factory valve job-it helps blend the CA of the port to the valve, and opens it up at the valve-but the throat-to-valve seat transition remains relatively smooth, maintaining the factory's original design intent. Big valves upset this relationship. Flow won't improve until you make the CA bigger to match the larger valves.
Port-matching the head's intake entry and exhaust exits to their respective gaskets is an enduringly popular modification, especially because it's one mod the average car crafter can do at home. Hupp uses enlarged dimensions based on his intended performance window instead of any specific gasket outline, transitioning down to the original port diameter about 1 1/2 inches in from the entrance. The exhaust ports are merely "squared-off" rather than enlarged, with the final shape based on Hupp's years of experience.