Flow and Swirl Testing
After that heavy section on the meaning of swirl, you're probably ready for some good old-fashioned flow numbers for our three different Cleveland heads discussed. As FordMuscle has indicated in other articles involving flow data, we must be careful not rank the performance value of a cylinder head entirely on flow numbers since flow benches fall short in simulating reality in two primary areas:

Flow benches do not show the effect of heat on airflow

Flow benches do not mimic the unsteady flow produced by an engine

With that said, the following flow and swirl data was collected from John Yelich at Cylinder Head Works (CHW) in Edmonton, Alberta. The data will provide you with a relative flow "personality" of each head and offer more clarity as to which head is right for your application.

Note: We felt it was OK to use CHW's Boss 302 flow and swirl data in place of data acquired from an actual quench chambered 351C 4V. Other than water jacket locations, both are essentially the same head.

In addition to being one of the first independent cylinder head shops to flow test the Edelbrock 351C performer head back in 2005, CHW has also experimented with reshaping the intake and exhaust runners of the factory Boss 302 head to maximize flow throughout low to high lifts. We've included that data for you here in the gray columns of each chart.



Intake Runner Flow Remarks
Large volume Stock Boss 302 head shows a relative sacrifice of flow performance at low valve lifts for increased flow at high valve lifts.

Edelbrock's focus on street performance and drivability are apparent in the Edelbrock 351C head. Great flow for lifts of .550" and below.

Although the Edelbrock 351C runner is not much larger than the Stock 351C 2V runner, its ability to flow is vastly improved, proving that contour and shape are critical for performance.

CHW's Modified Boss 302 head shows encouraging numbers that match Edelbrock's flow performance at low valve lift while offering increased flow performance over the Stock Boss 302 at high valve lifts.

Swirl Remarks
CHW's Modified Boss 302 head shows the elimination of a swirl effect at lifts below .600 inches. For those who consider swirl a detriment to
producing power this is a positive thing.

The Edelbrock 351C head shows a controlled "swirl ramp" that comes on lightly at 100 RPMs starting at .450" lift and increases steadily to 2000 RPMs at .600" lift. Past .600" lift, the Edelbrock 351C swirl value flattens out. This swirl plateau coincides flow numbers beginning to drop.

The Stock 351C 2V, which showed the lowest flow performance of the group, had an early onset of swirl as well as the highest swirl RPM.

Exhaust Runner Flow Remarks
Despite the large variations in exhaust runner volume and shape, the Stock 351C 2V, Stock Boss 302, and Edelbrock 351C all show fairly similar flow performance on the bench.

Exhaust plate added to the Modified Boss 302 head shows improved exhaust flow performance over the Stock 351C 2V, Stock Boss 302 and Edelbrock 351C head at .400" lift and beyond.

Article Outakes
Here are a bunch of pictures taken during the production of this article. We didn't use them in the main portion of the story but thought they might be interesting support material.

Factory Boss 302 head with quench chamber. Prepped by CHW. Note guide bosses removed.		Note the radiused "Yates Inspired" chamber design of the Edelbrock 351C head compared to the stepped design of the stock castings.		A look down the Edelbrock 351C intake port. Note CNC'd port entry.
Close-up of valve guide, valve stem seal, and spring cup on Edelrbock 351C head.		Edelbrock heads include stainless swirl polished valves, guide plates, and ARP rocker arm studs.