Overhead Cam Vs. Pushrod

Pushrods: as timeless as black suits and malt milkshakes, combined.

Computers vs carburetors, CVT vs DIY and overhead-cam vs pushrod: these are the conflicts that both divide us and serve to spark the fires of innovation and understanding. Both OHC and pushrod engines have been around since the dawn of the internal combustion engine, and more than a century later the debate is still going strong.

Configuration Basics

Call them "pushrod" or "cam-in-block," but this engine family remains one of the most popular of all time. Cam-in-block engines are so-called because the camshaft sits in the engine block just above the crankshaft. It actuates the intake and exhaust valves via a set of pushrods and rocker arm levers. An overhead-cam engine uses a camshaft mounted on top of the cylinder head to actuate the valves directly via a set of camshaft followers. While the overhead-cam configuration has been getting more popular on V-configured engines for the last 20 years or so, the design really lends itself more toward in-line four- and six-cylinder engines. On an in-line engine, it's actually cheaper to mount the cam atop the engine than it is to configure the block for a cam.

Airflow and Horsepower

At some point, every engine hits horsepower limits via the deficit or availability of oxygen. The more oxygen goes into the cylinders, the more fuel the engine can burn and the more power it can make. There are two basic ways to get more air into and exhaust gases out of the cylinder: you can either open the valves further and keep them open longer with a bigger camshaft, or you can change the shape and size of the intake and exhaust passages. Larger passages allow more air in and exhaust gases out, while straighter passages help them to get in and out faster and more efficiently.

Intake Ports and Airflow

The intake ports in a pushrod engine are compromised by shape and size because the passage has to bend to go around the pushrod "pinch" in the cylinder head. Short of using rocker arms with one side offset toward the pushrod -- as some engines do -- there's no way to get around the pinch without getting rid of the pushrods themselves. Enter the overhead-cam configuration. With the cam safely above the intake passages, there's no reason the intake passages can't be perfectly straight and optimized for maximum flow and velocity. This allows for much higher rpm and horsepower potential while preserving proportionately more of the engine's low-rpm torque.

Single vs. Dual Overhead Cam

A single overhead cam -- or SOHC -- engine uses a single camshaft over each cylinder head, and a dual overhead cam motor uses two. While the SOHC configuration is an improvement over the cam-in-block, a DOHC engine offers a few advantages. First, using one cam for the intake valves and another for the exhaust gives engineers a bit more freedom in setting the valve angle, which determines the combustion chamber and port shape. Even more important, using two cams allows for real-time adjustment of the cams relative to each other. This allows engineers to decrease valve overlap -- how long the intake and exhaust valves are open at the same time -- for low-rpm torque, fuel efficiency and a smooth idle, and to increase overlap for increased flow and more top-end horsepower.

A Case for Pushrods

You may be wondering why, if OHC engines are so clearly superior and have been around for so long, pushrod engines still exist. The first reason has to do with packaging; the V-configuration naturally lends itself to the cam-in-block architecture just as the in-line configuration works best with overhead cams. Slapping a pair of OHC heads onto a V-6, V-8 or V-10 adds a significant amount of weight, complexity, cost and physical size to the engine; this last is the main reason we don't see more OHC V-8s, and why you'll almost never see one more than 350 cubic inches. The sheer width of a long-stroke engine saddled with OHC heads means the engine bay has to be designed around it. From a manufacturer's standpoint, the V-configured OHC engine's physical size limits design options for a particular chassis and inter-chassis compatibility for that engine.

References

• The Ultimate American V-8 Data Book; Peter Sessler
• How to Port and Flow Test Cylinder Heads; David Vizard
• Building 4.6/5.4L Ford Horsepower; Richard Holdener
• Engine Airflow: A Practical Guide to Airflow Theory, Parts Testing, Flow Bench Testing and Analyzing Data; Harold Bettes