Overhead Valve (OHV) vs Overhead Cam (OHC): Which Engine Design is Better?
Why Does it Matter?
Whether you're buying a car or doing an engine swap in your existing car, if you're interested in performance you will have asked yourself whether overhead cam or overhead valve is better for you. There are a wide range of engines available in both designs although the overhead valve design is older and has more of a historical significance while overhead cam engines are becoming more pervasive in today's automotive world.
Because of the fact that both designs have not always been on level playing fields throughout history, one factor that influences people is style. Do they want the retro feel of a pushrod engine or do they want the more modern overtone of a DOHC for example? With the OHC engines, that modern feeling is less profound than the retro feeling you'd get from the OHV engines because they are still in use today and some companies like Chevy and Dodge are sticking to them for their muscle cars.
The other main difference between the 2 engine types is performance. Not that one engine necessarily has MORE performance than the other. Rather the performance characteristics are simply different, offering benefits in certain areas at the expense of others.
Then there's manufacturability and cost, which is of primary interest to the auto manufacturers themselves and to some extent the end user. The complexity of the engine, the weight, how easy it is to fix and numerous other practical considerations will weigh in when it comes to which design you want for your car.
What OHC Looks Like
First, What Are OHC and OHV?
In overhead cam engines, whether it's a V configuration or a straight configuration, the cam which actuates the valves is located directly on top of said valves. The cam rotates and the lobes push down on the valve stems, causing the valves to open and then close when the lobe rotates away. The valve springs of course provide the return force. A chain or belt is used to couple the overhead cams to the main shaft and quite often there are multiple intake and exhaust valves per cylinder.
In overhead valve engines, there is only 1 cam, nestled in between the V of the opposing cylinder banks. The lobes on this single cam actually push on... "pushrods". Pushrods are long rods that transmit the linear displacement of the cams up to rockers when then redirect the motion down so that the valves can be pushed open in the downward direction. OHV engines almost always have only 2 valves per cylinder but this isn't always the case.
What OHV Looks Like
Practical Comparison of the OHC and OHV Designs
Let's start with the overhead cam design. Since its cams are placed on top of the cylinders they actuate, the rotating mass of the system is lower but packaging space is increased on the top end because there has to be space for the cams obviously. In an OHV design, the little nook in the V shape between cylinder banks uses the engine space more efficiently so the engine can be smaller for a given displacement.
OHV valvetrain components tend to be cheaper to make than their OHC counterparts but there are also more moving parts. Furthermore, as the cams on an OHC engine are up top, lubrication becomes a design concern, whereas the lower placement of an overhead valve cam keeps it lubricated at all times. Then there's the timing chain/belt needed to maintain tension and synchronization of the overhead cams. A longer chain/belt is necessary and tensioners required to engage over a longer distance leading to increased maintenance and reliability concerns with OHC engines. So while OHV engines have more valvetrain components, OHC engines have more components related to the cams and cam timing.
OHC Dyno Example: Notice the High End Torque
Performance Comparison of OHC and OHV
Perhaps the most notable differences in these 2 engine designs which affect performance are the number of valves and the RPM limitations.
On an overhead valve engine, the pushrods contribute to a larger rotating mass as well as the pushrods being long slender members which can flex under abrupt transient loads. You would see such abrupt transient loads when the engine is spinning really fast and this tendency for the rods to flex due to their inertia and inability to remain rigid while communicating rapid loads presents a danger to the engine. In order to operate such an engine safely, the engine speed has to be limited to around the neighborhood of 6000 RPM. Sometimes improved metallurgy and lighter weight components can permit some leeway, as in the case of the LS3, where hollow stem valves enable that engine to operate up to 6600 RPM because the lighter valves offer less resistance to the rods and rockers but generally speaking, OHC will always outperform OHV in engine speed, which is critical for top end tuning and maximizing horsepower.
Another feature of the OHC design is that the overhead placement of the cams allows for more valves per cylinder. The pushrods and rockers get in the way and make placing more than 2 valves an engineering challenge. With only 2 valves, the valve diameter is limited on how large the cylinder is. Being able to reduce the size of the valves and have more of them actually increases the effective area covered by the valves and this translates to more airflow, which is why OHC engines tend to produce more high-end torque and horsepower. However, it's not a clear win since the extra flow offered by multiple valves comes at the expense of more laminar flow. What I mean by this is, the smaller valves require less severe changes in flow direction around the valve so the air is less turbulent. In an OHV engine, the air has to flow around a very wide diameter valve and recirculate on the other side, making it turbulent. Turbulence encourages air and fuel to mix more completely prior to combustion so the OHV design offers gains in low-end torque naturally while it suffers at the top end, where absolute flow is more important than mixing efficiency.
This makes picking one over the other a matter of deciding what you want your torque curve to look like and what sort of gearing and power you expect to have. If you have longer gears lots of power, as you might in a muscle car, you might opt for the pushrod engine because it can make better use of its low end torque. If you have many shorter gears and/or a weaker motor that's tuned to produce most of its power at the top end, the OHC design would help you since it can rev higher and will flow better where your RPMs will tend to linger most of the time.
One other minor difference between engine types is that the overhead valve engines have a lower center of gravity, owing to the cam, timing gear/chain and pushrods being low in the engine. Performance cars like the Corvette have a very low center of gravity, giving them excellent handling, grip and safety, which they owe in part to the engine.
OHV Dyno Example (Supercharged 3800): Notice the Low End Torque
I don't know if it counts for much but OHV is the way it's always been done in the past, including the 60s and 70s when all those beautiful muscle cars dominated the roads. It has a long track record of reliability so it's no surprise that the generation who grew up in that era look back at these engines with fond memories and nostalgia. The simpler design and "tendency" not to include displacement on demand and variable valve crap gives the engine a retro, time-tested and "won't die" personality that you just gotta love.
So Who's the Winner? OHC or OHV?
Well, you can probably tell I'm an OHV kind of guy. I bought my 2013 Camaro in part because the manual was coupled to the LS3, which is a pushrod. Sure if it had been a DOHC like the Mustang Coyote engine I might be able to get more out of it per liter of displacement but that's why I always go big or go home with pushrods. The best way to make up for a highly tuned DOHC is with a huge mother of an OHV :D What can I say OHC... you lose!