Overhead-Valve Engines: Their Advantages and Disadvantages

By: Jacob Hampton

 

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If there ever was a perfectly versatile valvetrain for achieving as much power and fuel economy as possible whilst cutting down on harmful emissions, the dual-overhead-camshaft (DOHC), four-valve setup is the best. It provides a massive power band for performance cars, lower frictional losses for reduced emissions, and incredible airflow for the best fuel economy. But DOHC is also a very complex valvetrain type, requiring a long and complicated drive from the crankshaft (the large central “squiggly stick” of an engine’s bottom end, which is connected to its pistons) to each smaller camshaft (which look like tiny, bumpy sticks) in the engine’s cylinder head. Before this valvetrain was forced into mass-production after the fuel crises of the 1970s and the increasing fleet-economy standards of the 1980s and ‘90s, much simpler valvetrains like pushrod overhead-valve (OHV) systems were the norm, especially in America. Despite its problems, OHV’s specific advantages over DOHC allow it to still be used today in pickup trucks, muscle cars, and some sports/supercars. 

For example, OHV is so simple to engineer that the time it takes to improve it to muscle-car levels of performance will take roughly the same amount of time as a standard DOHC setup. In “Automation: The Car Company Tycoon Game,” a carbureted OHV 90-degree cast-iron V8 with the top end improved to +5 or even +8 quality “takes roughly 30-40 months to engineer, if even that” (Camshaft Software). This is about the same time it will take to engineer a +0-quality DOHC four-valve V8 of otherwise comparable specs. This does not account for the heaps of familiarity that manufacturers (particularly the American ones) had/have with OHV, which would allow this simple valvetrain to be engineered even quicker. Pushrod engines’ typically lower rev limits means that they are less strained than an equivalent DOHC for comparable power, allowing them to be much more reliable. 

Because OHV engines have their camshaft in the block (the large metal chunk that makes up most of the engine and determines its shape and cylinder count), not in the head(s) (the smaller block(s) of metal that house the valves), they often are much easier to service and modify than DOHC engines. There is no need to mess around trying to readjust valve timing for when the heads need to be serviced, and even if there is, there will only be one camshaft instead of two or four to mess around with. The timing chain or belt (the device that drives the camshaft from the crankshaft) is much shorter on an OHV engine, especially in vee engines, where the camshaft is in the valley between the heads. This means that the replacement intervals of an OHV engine’s timing belt or chain will take much longer than an equivalent DOHC engine, and it might not be necessary to change a classic American car’s timing device before that vehicle’s entire engine needs an overhaul.  

These pushrod wonders also are far more compact than any other engine type, especially DOHC vee engines, which typically require much more width and height to clear their enormous heads. OHV engines allowed American cars to gradually become longer, lower, and wider after the war, even as their V8s grew to over seven liters in displacement, as basically all the valvetrain was placed far below where a DOHC’s valvetrain would be. This allows a car’s hood to be much shorter with a pushrod engine than a typical DOHC equivalent, which improves aerodynamics and often makes a car look much better in the eyes of the buying public. 

However, OHV engines have their issues, as well. For instance, they are typically much less efficient, as their airflow (how well the air/fuel mixture flows in and out of the cylinders) is severely hindered by only being able to have two valves per cylinder. This means that they cannot clean out the pollutants from the burnt fuel as well as a DOHC engine could, giving them higher emissions. These engines also experience valve float (a point where the valvetrain can no longer keep up with the revs of the pistons and crankshaft) much earlier than any other engine type, as their heavy valvetrain components often cannot move fast enough to keep in time with the rest of the engine. 

In conclusion, while OHV engines might be best for when a massive 8-liter V8 needs to be as small on the outside as possible while being easy to service and reliable, stringent fuel economy and emissions regulations, as well as a pursuit of greater power per liter, has relegated these engines to muscle cars, pickup trucks, and the odd supercar/hypercar. They simply are no longer feasible for a regular econobox or sports car, which both need to squeeze out every ounce of power from their tiny engines. They might be better for the tinkerer or hot-rodder who wants to fiddle around with their first car, but for everyone else, twin cams and four valves per cylinder is the better way to go.  

 

 

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