The basic 2.05/1.60 valve seat: Compare the 60-degree cut's width to that of the 1.94 seat
Fully porting the 1.50 (small-valve) exhaust runner produced no discernable real-world advantage. Overall, it's a wash compared to just bowl and short-side work. Compare that to the 1.60 runner, which flowed a whopping 217.4 cfm at 0.700 lift, up 58 percent over stock and 20 percent over the fully ported 1.50 runner configuration. The shape and overall contour of the exhaust side is critical; to maintain real-world driveability, the overall area (in square inches) of the exit should be equal to the area (in square inches) of the valve head. This means that the 1.50 port exit should not be enlarged to equal the 1.60 port's cross-sectional area. The new small-valve-runner critical area is the limitation of the exhaust valve size itself.
These tests showed that stock-size valves work great in a stock-size port (as they were intended to), so long as you get a professional valve job. A good three-angle valve job in conjunction with bowl-porting is the most cost-effective mod; the small valves still perform adequately at this level of modification. For no-compromise fully ported heads, big valves are definitely the answer. If because of rules restrictions you're valve size-limited, fully porting the intake side while only doing the short-side and bowls on the exhaust makes sense. In any case, match-porting isn't required until after you've done the good valve job and bowl work. Naturally, this data is valid only for traditional small-block Chevy heads. Other makes and models (as well as aftermarket race heads) may respond differently.
Does More Flow Always Mean More Power?
Not necessarily. The big flow numbers must occur at a lift that's commensurate with your camshaft. For example, if the flow gains occur primarily over 0.450-inch lift but your cam has only 0.447-inch lift (like the old 327/350hp L79 hydraulic grind did), they won't do you much good. Overall top-end power might increase slightly, but the car might be a dog under part-throttle, daily driving conditions.
Huff points out that it's important to consider the engine as a total combination. How much air and fuel the intake side of the heads suck in determines the engine's peak horsepower rpm point. Decide what rpm you want to achieve peak power at, then have your porter optimize intake flow accordingly. Below is the basic equation relating the max theoretical horsepower rpm point with intake port flow, in cfm. The factor "1.68" converts common 28-inch-water-pressure-flow bench readings to 10 inches. Naturally, it can be algebraically reshuffled to solve for any desired missing variables. With the cylinder head flow established, it's a relatively simple matter to select a complimentary cam, intake manifold, and carb.
You must also pay attention to maintaining the proper intake/exhaust flow ratio. According to Huff, the exhaust side determines the width of the powerband. The greater the exhaust flow as a percentage of intake flow, the narrower the powerband. He offers these guidelines that equate exhaust flow to the engine's intended maximum operating rpm:
The 2.05/1.60 seat after porting the bowl areas: Dial calipers illustrate how much the bow
The cylinder head flow table includes both the theoretical peak horsepower rpm points for each stage of the porting mods described in this article (based on a 350ci engine displacement), as well as the exhaust versus intake flow percentages.
|Flow in cfm @ 28in
||= Peak hp rpm
|Engine displacement (ci)
No. of cylinders