Pushrod vs Modular
One of the major divisions between engines comes down to modular vs pushrod, or, more accurately, overhead cam and overhead valve cylinder head configurations. Though overhead valve (OHV) engines have largely been replaced by overhead cam engines (OHC), there are still dedicated pushrod proponents out there, and certainly, both styles have their pros and cons.
Overhead Valve (Pushrod) Engines
In an overhead valve engine, the camshaft sits in-between the cylinder heads of the engine. Sometimes people refer to this as a cam-in-block style of setup. The crankshaft in an OHV engine is connected to the camshaft via a chain or gear system, so that when the crankshaft turns the camshaft follows. When the camshaft turns, a lifter pushes a pushrod that opens and closes the engine’s valves. You can already see how this engine style gets both of its names. The valves rest on top of the engine, and lifting them to bring in airflow and push out exhaust requires the use of a pushrod.
This engine design has been in use for a long time, since the early 1900s, but it wasn’t utilized in a V8 until 1948 when Oldsmobile introduced the first widescale pushrod V8. This popular setup was in wide use until the early 1990s and is still being used by GMC/Chevy, though most other major manufacturers have moved to overhead cam designs.
Overhead Cam Engines
When we’re talking about OHC engines there are two important style distinctions: Single and double overhead cam designs (SOHC and DOHC). SOHC units have one camshaft that is responsible for both intake and exhaust valves. In a DOHC, two camshafts exist in each cylinder head, one operating the intake and the other operating the exhaust valves.
There are some unique brand-specific variations on this idea, but they all operate similarly.
The camshafts in an overhead cam engine become the highest point on the individual cylinder heads instead of existing in-block.
Most modern engines are DOHCs.
Advantages of OHV Engines
OHV engines tend to be more compact than OHC engines. This is because the space in-between cylinders is essentially wasted in an OHC engine instead of being used for a camshaft. They also have the potential to be much lighter, since most OHC engines have several camshafts, which adds on quite a bit of weight.
If you’re looking for an authentic classic muscle car sound and feel, then OHV is the only way to go. Relocating the cams and valves definitely has an overall impact on some of the driving aspects of a car in ways that are hard to quantify.
This isn’t true or necessary on all engines, but it’s still worth mentioning. In older engine styles that rely on less than precise fuel injection techniques, ensure that the air and fuel mixes thoroughly can improve engine performance, and with the way the OHV engine opens and closes valves, there’s certainly a lot of turbulent air being introduced into the engine, creating a better mixture.
This contributes to improved low-end torque. Of course, with fuel injection techniques continually improving as drivers look for ways to improve their air/fuel ratio, an OHV head configuration won’t offer those engines any improvements to speak of.
Lower Center of Gravity
Moving the camshafts, timing chains, and other heavy engine components higher in the car definitely causes the center of gravity to be higher, which is never a good thing when it comes to stability and handling. The lower the center of gravity, the better, and OHV engines have the lowest possible center of gravity with regard to their engine’s weighty parts.
OHVs tend to be less expensive to produce. This makes a ton of sense. Only one camshaft that seems to just drop into a space designed just for it? That'll cut down on manufacturing costs. Of course, whether or not the savings will actually be passed onto you is an entirely different matter.
Disadvantages of OHV Engines
More Moving Parts
It might seem counterintuitive for an older engine type, but there are more moving components required in a pushrod engine. More tiny parts that are required to move in order to power the engine means more potential points of failure and more repairs. The worst engine in the world? That would be the one that’s in the shop instead of in your car.
This is tied to parts breaking but is its own beast as well. The pushrods that this engine is known for are thin pieces of metal with a lot of flex. Over time, it’s not uncommon for them to warp, leading for less accurate valve timing.
The only automotive companies that use pushrod-style engines anymore are Chevrolet and GMC. There are still aftermarket engine providers making OHV engines, and fortunately, parts tend to be plentiful.
Less Valve Timing Control
Alright. This is the big one. Variable cam timing is something most modern engines take for granted. Intake and exhaust valves need different timing, and that makes sense (most of us don’t inhale and exhale for the same length of time, why would our car’s lungs be any different?). This is a breeze with two camshafts and any number of valves. With an OHV engine? It’s a nightmare. Even installing two valves per a cylinder is really stretching this technology to the max.
Variable Valve Timing
It makes sense that the biggest con for OHV cylinder designs is the biggest advantage for OHC engines. Variable. Valve. Timing. It improves fuel efficiency. It improves power. It allows for the unexpected without negatively impacting your engine. It’s a big deal, which is why you see so many engines proudly stamped with VTEC or TiVCT or all of the other ways people have come up with to say that when your intake and exhaust are controlled by separate camshafts you have a lot more freedom.
Intake and Exhaust Valves are Separated
Being able to put the intake valves at a different angle than the exhaust valves allows for improved breathing and subsequently increased power. This isn’t really an option with OHV engines.
Higher Rev Ceiling
DOHC engines have a much larger range of revs, allowing them to hit redlines higher than any OHV configuration could dream of.
More breathing means more torque, eventually. Though OHV engines own the low torque range, overhead cam designs are able to catch up and surpass pretty quickly on a torque curve.
Multiple camshafts and timing chains? It adds on the pounds in a hurry. And it’s rotational mass, which is worse than static mass in terms of taxing your engine.
OHC failures will likely be catastrophic failures. Because the camshaft and pistons are proximally located and tied to the crankshaft with a literal chain, when one thing in that triad goes south it takes the other two with it, which results in the kind of failure you can’t just fix with an afternoon at the shop. OHVs may break more frequently, but OHCs break harder.
OHV or OHC Cylinder Configurations
Ultimately, these two cylinder configurations offer a very different set of pros and cons and which one is right for you is largely a matter of personal choice. If you want a high rev ceiling, good fuel economy, and a smooth ride, then you probably are looking for an overhead cam configuration.
On the other hand, overhead valve engines have been around for longer and are prized for their durability. It may also come down to practicalities. OHV engines are smaller and lend themselves more readily to classic car applications.
It’s for these reasons that we can pretty much rest assured that both engines have a place in the market and bring something valuable to the table.
Source: Axle addict | VW Vortex Image Credit: Car and Driver | Virginia Mustangs