by Jon Mikelonis

Introduction
Are 4V Cleveland heads OK for my application, or should I run the 2V Cleveland head? The question can spur disputes as fierce as rival religious sects jockeying for land rights. In fact, the debate has been hashed out so many times within the FordMuscle forums that we thought we'd add some fuel to the fire in an pursuit for clarity on the subject. In this article we'll gather flow and swirl test data, and take a three-dimensional look at the design of Cleveland intake and exhaust runners for Ford's factory 2V iron head, the factory 4V iron head, and Edelbrock's Performer RPM 351C head. Once compared with basic cylinder head theory and the often misunderstood topic of "swirl", the results will provide you with a better understanding of which head is best for your next Cleveland buildup whether it's a 351C, Clevor, or the suddenly hip 351M/400.

	351C Intake Ports. Is What You See What You Get? Pictured from top to bottom: factory iron 351C 4V, Edelbrock Performer RPM 351C, and the factory iron 351C 2V cylinder head. While the size of an intake port can provide an indication of a cylinder head's flow potential, you've got to get inside to see what's really going on. Page two or this article will take you there.

Quick Cleveland Head Facts
Complete lists, varieties, and other facts about the factory and aftermarket canted valve "Cleveland" head are available all over the web. With that said, this article is not intended to be a reference to identify every Cleveland casting produced nor is it intended to be a historical facts and figures resource. The intention is to highlight new and exclusive FordMuscle material on one of the most critical and diverse aspects of all Cleveland heads designed for stock manifolds; the intake and exhaust runners.

We'll venture forward assuming you know the following basic facts about
the Cleveland cylinder head.

Factory Cleveland heads were produced from 1970-1974
Factory Cleveland heads can be found in both 2V and 4V castings
2V and 4V castings are available with and without a quench chambers
All Cleveland heads can be used on 351C, 351M, and 400 blocks
Edelbrock, CHI, and AFD all produce aluminum alloy Cleveland heads
Factory quench chambered 351C 4V Cleveland heads are identical to Boss 302 Heads other than water jacket locations

For complete historical data on the Cleveland see The Ford 335-Series Engine at Wikipedia.

Basic Cylinder Head Theory
While combustion chamber design and volume, valve position, and many other design aspects of a cylinder head have an effect on useable power gains, the design characteristics of any cylinder head's intake and exhaust port have the greatest influence on airflow. This is why so much time and effort is spent by DIY head porters and aftermarket engineering departments on optimizing these areas of a cylinder head for the specific applications in which the head will be used.

Again, while this article is not intended to be an absolute reference on the Cleveland head or cylinder head theory, the following excerpts from the SpeedPro Series book "How To Build , Modify, and Tune Cylinder Heads" by Peter Burgess and David Gollan", will provide you with a basic foundation for which to evaluate the information presented on the following pages.

"A small intake port feeding a large cylinder will have a high gas speed at low RPM, it will be unable to supply sufficient air at higher rpm. Conversely, a very intake large port feeding the same cylinder (think 351C 4V) will only achieve high gas speeds at high RPM and will have very low gas speed at low rpm. Standard port dimensions are the result of designers aiming to achieve the best compromise in terms of filling across a wide range of engine operating speeds."

"Airflow is more sensitive to shape than size, so big ports are not necessarily better than small ports at flowing air. Airflow also hates experiencing sudden changes in direction, volume, and shape. These concepts are supported by the fact that the areas of the port that are easy to get at (when porting) normally have small to moderate effect on airflow; it is more often those bits that are really difficult to get at that usually have the greatest influence on the head's airflow capability"

Both excerpts are applicable to the obvious differences seen between a 351C 2V head and a 351C 4V head, where the 4V's large intake port design is known for better high RPM operation and the 2V's relatively smaller intake port design is known for better low RPM or street performance. Keep in mind however, that port volume alone is just one aspect of an intake runner that contributes to performance and drivability. Port shape and contour are also critical factors, the following pages will take you inside three different Cleveland cylinder heads to demonstrate the variations in runner shape that affect flow performance.

(Port Mold Analysis, The Twist on Swirl, Flow and Swirl Testing)