Why Don’t Pushrod Engines Rev As High As Overhead Cam Designs?

Being an EngineLabs reader, you are probably more familiar than the average bear with how and why engines work. You probably also know most of the generalities associated with different engine types. One of those generalities is that pushrod engines aren’t meant to be RPM monsters. While the statement is mostly accurate, first, it’s relative, and second, it’s a generalization.

However, Jason Fenske of Engineering Explained fired up his trusty video camera and dove deeper into the reasoning behind the statement that “Pushods can’t rev high.” Fenske starts with a cool overview of a disassembled single-cylinder engine, and breaks down how the rotating assembly and valvetrain work in unison, Barney-style. He then translates that into automotive V8 pushrod engines.

Uncontrolled Valve Motion

As Fenske explains it, there are three major drawbacks when trying to get a pushrod engine to perform at high-RPM. The first reason is uncontrolled valve events – also known as valve float.

“The entire [valvetrain] assembly has to reciprocate back and forth very fast. You have a lot of mass in the system that is trying to change direction very quickly as you get into higher RPM,” Fenske explains.

“The reciprocating mass can start to outrun the spring. There comes a point where parts of the system lose direct contact with one another, and the valve movement is no longer directly related to the camshaft’s movement.”

Using a disassembled single-cylinder engine as a demonstration tool, Fenske illustrates a typical in-block-cam, OHV valvetrain setup, with the lifter, pushrod, rocker arm, valve, spring, retainer, and lock. All those parts moving in unison make for a lot of reciprocating mass that have to be controlled at high-RPM.

Once you encounter valve float, it can be as minor as simply a loss of power, as the valves are no longer doing what they are supposed to do, when they are supposed to do it, or it can be so severely out of sync, that things start impacting one another.

“Stiffer springs will help you keep everything in contact in the higher RPM ranges,” says Fenske. “However, [the stiffer valvesprings] result in an efficiency loss, as it takes more system effort to compress that stiffer spring.” That increased parasitic loss can negate any benefit you might see by spinning the engine higher, unless you have a system that is specifically designed to operate in the higher RPM range.

Another factor in the equation is pushrod deflection in high-RPM applications. “The length does play somewhat of a role. If the pushrods are not strong enough, they can bend, meaning you won’t have proper valve opening and closing,” says Fenske.  “I think this is an easier problem to address, by simply developing stronger pushrods.”

The main component eliminated in an overhead cam engine is the pushrod, and removing that variable is one of the key factors in allowing more control of the valve at higher rotational speeds for a variety of reasons.

“DOHC engines have four valves per cylinder rather than two, so this means the valves are smaller, each with their own spring,” says Fenske. “There’s also no pushrod, so you’ve eliminated a good amount of reciprocating mass.”

This is where the “generally-speaking” and “relatively” statements come into play. Afterall, we can list off several forms of racing where pushrod engines spin to the moon. Some of them are designed to live for 500 or so miles without being torn apart, while some are lucky to make it 10 miles before needing new components, but they do perform quite well. And as Fenske points out, “Then you see Formula 1 engines spinning to 20,000 rpm.”

To accomplish that feat, the dual overhead cam engines used in F1 have done away with coil valve springs altogether, instead using pneumatic springs.

“By using a pneumatic spring with air pressure instead of a steel coil spring, the valves are easier to control, and you don’t run into harmonic vibration issues,” says Fenske. “Generally speaking, dual overhead camshaft designs, especially using a pneumatic valve, will be able to rev higher than a traditional pushrod setup in the same application.”

Due to packaging constraints. most OHV engines are two-valve designs. While those typically work well in lower-RPM applications, a multi-valve-per-port design will inherently outflow a two-valve design at elevated RPM.

Airflow Restrictions

“At high RPM you need lots of air,” says Fenske. “While it is possible for a pushrod engine to use more than two valves per cylinder, it’s quite a complex design.” That  design not only adds cost and complexity to the system, we’d have to imagine it would also add significantly more mass to the valvetrain, further exacerbating the previously discussed issue. “If you were to go to a 4-valve setup, you’d reduce the size of the valves so you would be partly improving the situation, but you’d be eliminating one of the key advantages of a pushrod engine which is its beautiful simplicity.”

“[A two-valve-per-cylinder] design generally limits airflow at high-RPM,” Fenske says. “At lower RPM, you can actually benefit from the two larger valves, as it will increase velocity of the intake charge and promote better fuel mixing. However, an engine revving at 10,000 rpm vs 5,000 rpm, ideally, will be pulling in about twice as much air.”

Here’s a model of a four-valve pushrod design. While you improve airflow potential, you also increase complexity and valvetrain weight by adding double the pushrods and rocker arms.

By increasing the number of valves in the chamber, even though they are smaller valves, a simple area calculation shows that your total valve area is increased. While there are chamber and camshaft designs out there to maximize the two-valve high-RPM flow, they are still outmatched in the upper-RPM range by a multiple-valve-per-port design. “Ultimately, a four-valve system will be able to flow more air through it at higher RPM,” says Fenske.

Valve Timing

Another interesting subject Fenske broaches in his video is adjustable valve timing and how it affects engine performance. While most domestic V8 race engines lock out any variable timing features, he raises the point that not being able to control each side of the equation (intake and exhaust timing) independently and dynamically is a hindrance on a pushrod engine.

“Having a single camshaft controlling everything limits your control. All that can be changed is when the valves are opening and closing in relation to camshaft rotation,” Fenske says. “While Variable Cam Timing is present in some OHV V8s, that only changes the timing of the system as a whole. You can’t change duration of the intake or exhaust independently of one another, adjust overlap, or lift.”

Using this awesome 3D-printed LS engine model, Fenske explains the variable camshaft timing limitations of an OHV cam-in-block engine design.

All of those variables can be adjusted independently on a dual overhead camshaft engine, and with modern controls and designs, adjusting them on the fly is reality, not fantasy. “You’re able to control the intake and exhaust valves independently of one another, as well as have multiple lobe profiles for lift and duration on each camshaft,” says Fenske. “That, along with the ability to adjust valve overlap allows for optimal airflow efficiency in any RPM range.”

So while it’s obvious that an overhead camshaft arrangement is advantageous in a high-RPM application, it’s bit of an oversimplification to simply say that pushrod engines can’t rev high. Sure they have some inherent challenges to overcome, but like anything in this world, with enough effort, knowledge, and money, anything is possible.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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