Port Mold Analysis
Before reviewing any flow and swirl data on our Cleveland heads, we took port molds of the intake and exhaust runners on all three heads (2V, 4V, and Edelbrock). You may have see FordMuscle perform this process in other articles. Once cured and removed, the silicone molds are great supportive visuals for the corresponding flow data found on Page 4 of this article. Here's the process in case you ever want to try this at home. It's a good technique if you plan on home porting any cylinder head.

We popped an intake and exhaust valve from each head.		Mixed up the silicone RTV and catalyst.
Waited 6 hours for the exhaust port molds to cure.		Carefully coaxed them out of each head then repeated the process for the intake side.

Intake Runners
Intake runner design, shape, and contour is almost completely responsible for a cylinder head's intake flow velocity and ability to promote, prevent, or control the mixture's motion or swirl effect. The following visual comparison of our Cleveland intake runner molds offers information not seen by simply looking at an intake port's footprint or peering into the dark corridor we call the intake runner. Pictured from left to right is the factory 351C 4V intake runner, the factory 351C 2V intake runner, and Edelbrock's Performer RPM 351C intake runner. The average joe may not notice the subtleties between these three blue globs, but hardcore enthusiasts, cylinder head porters, and design engineers can quickly point to the improvements of modern head design over early 1970's designs. Let's take a closer look:

Factory Iron 351 Cleveland 2V Head Intake Port
Profile View The factory 2V intake runner shows a hard turn towards the valve forming an undesirable and abrupt "L" shape. Also, note the bullnose that essentially forces the intake charge into the runner wall right above the valve.		Top View From the top, the 2V intake runner has a fairly consistent cross-section which by itself is conducive to maintaining the velocity. However, based on the previous photo and the obtrusive valve guide boss (shown as an impression around the valve stem here) this runner can be improved with porting and machine work.
Factory Iron 351 Cleveland 4V Head Intake Port
Profile View The profile of the notorious 4V intake runner is like a bloated version of the 2V runner. The mold shows the same bullnosed "L" shape, unlike the 2V however, the 4V's port entry has a distinct "hogged-out" floor which creates an unwanted high static pressure area. This area is sometimes filled-in with epoxy or special port plates to give the intake charge a straighter shot at the valve, these modifications will help maintain flow velocity. (See sidebar)		Top View From the top, the 4V runner contradicts modern aftermarket runner designs with a "quick pinch" followed by a large jog outward as the runner approaches the valve. In theory, the sudden change in cross-section from small to large can only serve to slow flow velocity. While the "quick pinch" may act in a venturi effect to increase velocity, the effect is happening too soon slowing the charge well before the intake valve. However, the shear size of the port alone is what always contributes to high flow numbers for the factory 4V head. As with the 2V head, the obtrusive valve guide boss is also apparent.
Edelbrock 351 Cleveland Head Intake Port
Profile View The profile of the Edelbrock intake runner is a clear testament to modern computer-aided cylinder head design. Note the contoured shape that forms a "J" rather than an "L" like the factory iron head. There are no sharp turns or obstructions. The cross- section is a bit larger than the factory 2V but much smaller than the 4V. The cross-section gradually decreases in size as the charge approaches the valve, this helps increase flow velocity. Gone is the bullnosed shape that sends the mixture crashing into the runner wall.		Top View There's nothing too exciting about the Edelbrock intake runner when viewed from above, and that's a good thing! The lack of character indicates an unobstructed straight shot at the intake valve. As with most modern cylinder head designs, the valve guide boss does not protrude into the runner. Designs like these, by their nature, can create a controlled swirl effect. We'll discuss swirl on Page 3 of this article.

Exhaust Runners
Like intake runners, an exhaust runner's design, shape, and contour influence flow potential. While you may have been running Cleveland heads all your life, you probably have not seen the exhaust ports of a 2V and 4V displayed as we've done here. Let's take a look.

Factory Iron 351 Cleveland 2V Head Exhaust Port
Profile View The factory 351C 2V intake runner is remarkably contoured for its vintage era.		Top View Factory runner's still suffer from the large guide boss protruding into the exhaust stream. Otherwise the shape is rather uncharacteristic
Factory Iron 351 Cleveland 4V Head Exhaust Port
Profile View You can't help but think that the 351C 4V runner was simply an attempt to hog-out as much material as possible. The sharp turn towards the valve is accentuated by the oversized port entry.		Top View It's massive and resembles a glob of puddy but people still stand by 4V Clevelands. You'll see on page four that this oversized exhaust runner has no tangible advantage over the 2V runner.
Edelbrock 351 Cleveland Head Exhaust Port
Profile View The Edelbrock 351C exhaust runner is very similar to the factory 351C 2V runner from the profile view, leading us to believe that the 351C 2V exhaust runner was a good design for 1970.		Top View From the top, the Edelbrock 351C exhaust runner shows the obvious improvements like the lack of a protruding guide boss and a smoother shape. Other than that, it's very similar to the factory 351C 2V. The resulting flow data supports this.

(The Twist on Swirl, Flow and Swirl Testing)