Ford’s been full-send on overhead-cam V8s since the mod motor launched way back in 1990. Now, all of a sudden, they’re going back to pushrods in their latest great big 7.3L engine. So what’s the difference, and why pick pushrods over ocks and docks? And what does that even mean?
Every engine needs valves of some sort that open and close. Well, almost every engine, there are some oddballs out there like two-stroke diesels, but they’ve long gone the way of the dinosaur. But there are many different ways of opening and closing those valves. Some cars, like Formula 1 racers, use pneumatic actuators. Others use a mechanical link like Ducati’s Desmodromic system. But most use a camshaft that pushes against the valve. Either through a rod, a rocker arm, or directly, with a spring to close the valve when the opening part is done.
The overhead valve engine’s name is a bit misleading these days. It’s been a long time since flatheads were at all common, and both types of overhead cam engines also have valves located over (at the top of) the cylinder head. So you’ll also hear them called “pushrod engines” though there are DOHC engines that can have pushrods (looking at you, Roger Penske!). Some people call them cam-in-block engines, which might be the most appropriate. Since the camshaft is located firmly inside the engine block, and not inside the cylinder head like the overhead cam types. We’ll call them pushrod engines for the purposes of this piece.
Pushrod engines have the cam in the vee of the engine (we’re also sticking with vee engines here). From that camshaft, a lifter, well, lifts the pushrod, which is really just a rod of metal with a hole in it to allow oil to flow through, makes the long journey to the cylinder head. In the head, it pushes one side of a rocker arm. The other side of that arm pushes the valve open. Then a spring pushes the valve closed when the rod isn’t trying to open it.
The benefits here are simplicity. Adjusting the rocker arm and adjustment in the oil-filled hydraulic lifer means that the arms don’t even have to be exactly the right length. Though it’s better when they are. There are fewer complex parts since you need just one cam instead of two or four, and the camshaft sits in the otherwise useless space in the middle of the vee. That proximity also means that the chain (or in rare cases belt) to drive the cam is shorter. That makes it cheaper, and more accurate. It also makes sure you don’t end up with that nightmare of a chain that certain Audi engines had a few years back.
The disadvantage of those lifters and long rods is weight. And inertia. The parts might weigh just a few ounces each, but it ads up. And they’re moving fast. Really fast. Back and forth about 50 times a second at 6,000 rpm. Spin them too fast, and the valve springs are no longer strong enough to close everything in time. That’s called valve float, since they float, or stay open when you don’t want. That could see them hit the piston or push the intake charge back out the intake. So pushrod engines, at least those built for drivability, have a lower redline than overhead cam engines.
Single overhead cam engines, on a vee engine, put one camshaft in each cylinder head. Suddenly you don’t need pushrods. And if you’re ok with two valves per cylinder, you don’t need rocker arms either. Now, instead of all those parts moving, there’s a cam spinning and a valve moving. You can spin everything faster allowing higher rpm. Which allows for more power since power is torque x rpm divided by a constant.
Want more than two valves? Add some rocker arms. Or add a second cam in each head. Four or even five valves per cylinder with the ability to flow more air and rev higher. The engines tend to be smoother, too, since there are fewer parts rattling around in there.
The downsides here are cost. Camshafts are expensive. So are the fancy heads to hold them and bearings to support them. And the long chains to drive the cams generally need a tensioner. Something that over time will need to be replaced, and it’s just one more component that could fail.
There’s one other big downside to overhead cams. Instead of the cam hiding in that wasted block space, the cams stick out at the edges of the engine. A Ford Coyote 5.0L V8? It’s nearly a foot wider and significantly taller than a Chevrolet LS motor. That means it needs some fancy packaging to fit underhood. And is why the LS is far more popular as a swap option. It’s positively tiny in comparison.
So why is Ford’s latest big block a pushrod engine? Four valves in a cylinder can flow more air. That means more power at high rpm. But having just two valves improves air velocity. That means they can get more air (and so fuel) into the cylinder at low rpm. It also means better mixing and better combustion at low rpm. That means more torque at (you guessed it) low rpm.
If you’re only going to have two valves per cylinder, and keep rpm low, like you would in a truck engine that’s designed for low-end torque, there’s no point in adding the complexity of an overhead cam. So the engine is torquier, which truck buyers want, cheaper, which truck buyers and accountants want, simpler, which mechanics want, and smaller, which engineers and hot rodders want.
Which is why GM keeps its pushrod V8s around, and why Ford has made its latest Super Duty engine of the pushrod variety.