The Twist on Swirl
Since the Cleveland flow data we gathered for this article included thought-provoking swirl data and because "swirl" is a term you'll see tossed around internet forums and magazine pages, we thought we'd research the mysterious "twist". In doing so, FordMuscle opened the proverbial "can-of-worms" in trying to define the advantages and disadvantages of swirl by consulting various industry professionals like Rick Roberts from Edelbrock, Tony Mamo from Air Flow Research, Joe Sherman of Joe Sherman Racing Engines, and John Yelich from CHW. We also scoured our own reference materials and even referred to a UC Irvine study on the subject from their Department of Mechanical and Aerospace Engineering (see second sidebar below). Here's what we found.

By definition, swirl is the term for the corkscrew movement of an air-fuel mixture down the last few inches of the intake runner and into the cylinder, similar to the vortex you see when the last few gallons of water drain out of your bath tub. For those cylinder heads that do exhibit a swirl effect, the point at which a swirl is initiated based on valve lift can show great variation as you'll see with our subject Cleveland heads.

The illustration above showing an air-fuel path around the intake valve and into the combustion chamber is really the best way to visualize the phenomenon of swirl. Albeit, this image is greatly simplified for the purpose of gaining a quick understanding. The movement seems clear, looks powerful, and at first glance you might be inclined to say... Why that's gotta be the optimum intake charge for any head!

Unfortunately, for the enthusiast, there are a few lines of thought in the professional realm as it relates to swirl and whether it should be chased after, chased away, or politely ignored. These attitudes prevail within original cylinder head design as well as with engine builders and head porters. With that aside, the common opinion among professionals is that swirl is an important factor in atomization and combustion efficiency, although research on swirl specifically (as opposed to broader subject of turbulent flow) has not provided a foundation for which to draw any hard and fast conclusions.

Regardless, we've simplified two common positions on swirl that you might come across. Let us warn you that after reading the positions you'll still be forced to draw your own conclusions regarding what remains a quiet controversy. And while our three Cleveland head's flow data show interesting variations in swirl behavior, the only real test would be to throw each head on one motor designed for a specific application and dyno test the different combos.

Common Argument for Promoting Swirl
Swirl increases the mixing of fuel and air by diffusing the mixture to the
swirl path's centerline by forming a recirculation zone. Swirl increases
combustion efficiency through increased atomization. (See sidebar "UC Irvine Mechanical and Aerospace Engineering Studies Combustion Enhancement Using Induced Swirl").

Common Argument for Eliminating Swirl
Swirl decreases the mixing of fuel and air by "flinging" fuel particles
out of suspension in a centrifugal effect. Too create swirl an intake
runner must be shaped to promote the effect. Swirl promoting "shapes" in the form of unique intake contours or turns will impede an intake charge's rate of flow.

Keep the two positions above in mind next time you see the term "swirl" used by a peer, a manufacturer, or even included on your own cylinder head's flow chart. According to Rick Roberts, Edelbrock's Director of Engineering, a more accurate way of rating the performance of an intake or exhaust runner is to measure the head's flow coefficiency. Flow Coefficient or the relationship between the pressure drop across an orifice and the corresponding flow rate (Static Pressure and Velocity) provides us with measurable, dependable, and relevant scientific results. Swirl is just one aspect that can affect the Flow Coefficient..

Just show me the Cleveland flow numbers please!

	Turbulent Flow and Swirl The term "swirl" fits into the broader and more comprehensive category of turbulent flow. To express the swirling action of the pesticide stream coming off the tip of this crop duster in RPMs alone may be a bit short-sighted.

A Look Beyond Swirl
While a swirl meter integrated into a flowbench can provide a reference point by which to rank an intake runner's ability to set a charge into motion, a swirl meter does not fully express turbulent flow (see "Rendering 2" below and "Turbulent Flow and Swirl" at right). To simplify the comparison of swirl to turbulent flow, cylinder heads with identical swirl readings from a flow bench's swirl meter can have different behavior within the runner. By behavior we are referring to an array of high and low static pressure areas and the related high and low velocity areas. Therefore, to only address swirl is to only look at a small part of a more complex picture of turbulent flow.

What this all really means to the enthusiast staring at cylinder head swirl data is that swirl defies the assignment of a set of hard and fast target numbers for optimum performance. Edelbrock's, Rick Roberts supplied these 3D renderings as an indication of just how much action is really going on within a cylinder head. Clearly it would take an entire education in Fluid Dynamics to intelligently study these renderings, however it doesn't take much see that static pressure and velocity take precedent over swirl at this level of intake runner development.

Rendering 1: Static Pressure Within an Intake Runner
There is an inverse relationship between static pressure and airflow. As static pressure increases, airflow drops. For example, note the red "hotspot" around the valve guide boss below. That area represents a high static pressure point which correlates with low velocity.



Rendering 2: Static Pressure and Velocity Within an Intake Runner
Note arrows that indicate airflow velocity and direction. 3D renderings like this one are use by Edelbrock to evaluate turbulent flow.

(Flow and Swirl Testing)