Pushrod engines may not be high-tech by today’s standards, but they soldier on and have benefits of their own. One downfall, however, is their ability to rev. Pushrod engines tend to have rather low redlines.
You won’t find a GM small-block V8 revving to 9,000 rpm, but there’s a reason for it all. Jason Fenske of Engineering Explained is here to help us understand why we can’t push a pushrod engine to higher rpm limits.
It comes down to valve float, which is when the pushrod assembly begins to lose contact with the camshaft profile. As the pushrod forces the rocker arm down to activate the valve, high revs can cause things to fall out of sync.
It’s basically due to mass within the entire system. The spring, which pushes the valve back to keep in contact with the camshaft profile, will become unable to keep up at high rpm, which will lead to valve float.
This requires the spring to do the heavy lifting and keep the valve in contact with the camshaft via a tappet. When the tappet doesn’t make contact with the cam profile, the valve is suddenly no longer related to the cam profile.
What happens when valve float occurs? Jason says the best case scenario is an efficiency loss, but it could get worse. The valve could begin to contact the piston that’s on its way up as things fall out of concert with one another.
Airflow is another big reason pushrod engines don’t boast sky-high redlines. Most pushrod designs feature two valves per cylinder. Any more than that becomes a complex design, which means the vast majority of mass-market engines feature just two valves. Without extra valves, the engine can’t take in enough air at higher rpm and it becomes starved for air. Thus, it can’t rev as high.
The number of camshafts and valve timing are the final reasons pushrod engines don’t rev very high. A single-camshaft pushrod engine doesn’t allow for as many changes in cam profiles to as you get in a dual-overhead cam engine. That means valve lift and duration can’t be adjusted for higher rpm needs.
A pushrod engine also generally doesn’t allow for the timing of the intake and exhaust valves to be optimized for the airflow needed for high rpm.