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.
May 10, 2018 - Unlike DOHC engines, pushrod engines are not complex in construction. In the engineering world, if a solution can be achieved with a simpler. 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 Continue reading In-Block Vs.
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. OHCIn 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.OHVIn 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 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. 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. 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. 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. If an engine has been sufficiently over-designed with strong enough components then it can exceed the expectations of any consumer level product. I could change the springs to stiffer ones, buy lighter, stronger valves, stiffer pushrods, forged pistons and connecting rods and a forged crank and push well above the stock HP and RPM levels.
However in this article we're comparing apples to apples, which is to say, the type of engines that your average consumer can afford. For a given price point, OHV and OHC have distinct pros and cons, as mentioned. Great article, my 3.3L OHV is no powerhouse but i prefer it's reliability.
It has a timing chain instead of my previous vehicles that had a belt that I have to worry about breaking and destroying my valves. Also the cam is better lubricated at the bottom as you mentioned and engine is quiet as can be at 150k(have owned DOHC's and lifter ticks would come and go due to cams being on top with high mileage engines) drived me crazy trying different weight oils to quiet it as I didn't feel fixing it and shelling out tons of money to be quiet was worth it as it still got me from A to B. Respect to our elders, newer isn't always better.
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Components of a pushrod valve actuation systemAn overhead valve engine ( OHV engine), or pushrod engine, is a whose are situated in the. An OHV engine's operates its valves via a camshaft within the, cam followers (or '), and.The OHV engine was an advance over the older, whose valves were situated in the. Some early 'OHV' engines known as ' used both side-valves and overhead valves. A variation over the OHV design is the, or 'OHC', engine, whose camshaft lies in the cylinder head itself, above the valves. To avoid confusion, OHC engines are not referred to as OHV despite also having their valves in the head.
Valve-In-Head engine, illustration from 1904 patent, Buick Manufacturing CompanyIn early 1894, Rudolf Diesel's second Diesel engine prototype was built with a cylinder head featuring push rods, rocker arms, and poppet valves. Diesel had published this design in 1893. In 1896, U.S. Patent 563,140, awarded to William F. Davis, illustrated a gasoline engine with the same head configuration, patenting his solution to the problem of how to cool the head, which problem had made the overhead valve engine difficult before then. Henry Ford's of 1896 had valves in the head, with push rods for exhaust valves only, the intake using suction valves.
In 1898, bicycle manufacturer built a motor-trike with a one-cylinder OHV engine with push rods for both exhaust and intake. In 1900, hired Marr as chief engineer at the Auto-Vim and Power Company in Detroit, where he worked until 1902.
Marr's engine employed pushrod-actuated rocker arms, which in turn pushed valves to the. Marr left Buick briefly to start his own automobile company in 1902, the, and made a handful of cars with overhead valve engines, before coming back to Buick in 1904. The OHV engine was patented in 1902 (awarded 1904) by Buick's second chief engineer Eugene Richard, at the Buick Manufacturing Company, precursor to the. The world's first production overhead valve internal combustion engine was put into the first production Buick automobile, the 1904 Model B, which used a 2-cylinder, with 2 valves in each head. The engine was designed by Marr and David Buick.Eugene Richard of the Buick Manufacturing Company was awarded US Patent #771,095 in 1904 for the valve in head engine. It included rocker arms and push rods, a water jacket for the head which communicated with the one in the cylinder block, and lifters pushed by a camshaft with a 2-to-1 gearing ratio to the crankshaft.
Was awarded US Patent #1,744,526 for an adapter that could be applied to an existing engine, thus transforming it into an Overhead Valve Engine. Picture of a (with removed), showing the, and rockers.The built their own airplane engines, and starting in 1906, they used overhead valves for both exhaust and intake, with push rods and rocker arms for the exhaust valves only, the intake valves being 'automatic suction' valves. They even built a V-8 engine with this valve configuration in 1910. In 1949, introduced the, the first V-8 engine with OHV's to be produced on a wide scale.is the world's largest pushrod engine producer, producing I4, V6 and V8 pushrod engines.
Most other companies use overhead cams. Nowadays, automotive use of side-valves has virtually disappeared, and valves are almost all 'overhead'. However, most are now driven more directly by the system. Few pushrod-type engines remain in production outside of the United States market. This is in part a result of some countries passing laws to tax engines based on displacement, because displacement is somewhat related to the emissions and fuel efficiency of an automobile. This has given OHC engines a regulatory advantage in those countries, which resulted in few manufacturers wanting to design both OHV and OHC engines.However, in 2002, Chrysler introduced a new pushrod engine: a 5.7-litre Hemi engine.
The new presents advanced features such as technology and has been a popular option with buyers. The Hemi was on the list for 2003 through 2007. Chrysler also produced the world's first production variable-valve OHV engine with independent intake and exhaust phasing. The system is called CamInCam, and was first used in the 600 horsepower (447 kW) SRT-10 engine for the 2008.Early air-cooled OHV BMW motorcycle engines had long pushrods and a single centrally-mounted camshaft; but the post-1992 BMW R259 'Oilhead' boxer engines had a camshaft in each cylinder head, located between the combustion chamber and the rocker arms.
The pushrods were very short, allowing higher rpm and more power. For instance, the (which had a R259 engine) could achieve an output of 98 hp (73 kW) at 8,400 rpm, with no risk of valve bounce. Since, BMW engines have had OHC valve actuation.Advantages OHV engines have some advantages over OHC engines:. Smaller overall packaging: because of the, OHV engines are more compact than an overhead cam engine of comparable displacement. For example, 's 4.6 L OHC V8 is larger than the 5.0 L I-head V8 it replaced. GM's 4.6 L OHC V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head V8.
The uses the OHV engine to fit under its low bonnet line. Because of the generally more compact size of an engine of a given displacement, a pushrod engine of given external dimensions can have significantly greater displacement than an OHC engine of the same external size. As a result, the pushrod engine can sometimes produce just as much power as the OHC engine, but with greater torque (contrary to popular belief, this is simply due to the greater displacement of the pushrod engine versus the OHC engine rather than any inherent advantage of the pushrod design for torque production). Only a single head casting is needed on vee and flat pushrod engines: Because of the need to drive both cams on one side of the engine, the camshaft orientation on OHC heads must be the same for both heads of a flat and vee engine. This means that the heads must be (more or less) mirror images of each other.
And this requires that two different head castings be produced. Pushrod heads can simply be flipped around, which allows a single casting to be used on both cylinder banks. Simpler drive system: OHV engines have a less complex drive system for the camshaft when compared with engines.
Most OHC engines drive the or using a, a, or multiple chains. These systems require the use of tensioners which add complexity. In contrast, an OHV engine has the camshaft positioned close to the crankshaft which may be driven by a much shorter chain or even direct gear connection.
However, this is somewhat negated by a more complex valvetrain requiring pushrods. Hydraulic lifters: Although RPM capability is limited by the use of hydraulic lifters, the valve lash is self-adjusting for the lifetime of the engine, reducing a significant maintenance requirement. Some OHC engines also use hydraulic lifters/lash adjusters, but the implementation is more complex in OHC designs. Simpler lubrication system: Because OHV engines have no camshafts in the heads, the heads have much more modest lubrication requirements than the heads in OHC engines.
Therefore, there is no need for oil galleys to supply the heads with oil or oil galleys in the head to provide lubrication for the cam bearings. OHV heads only need lubrication for the rocker arms at the pushrod end, trunnion, and rocker tip. This lubrication to is typically provided through the pushrods themselves rather than a dedicated lubrication system in the head.
And lubrication for the camshaft is provided through the same block galleys that provide oil for the main bearings. The more modest lubrication needs of an OHV engine also mean that a smaller, lower capacity oil pump can be used.Limitations Some specific problems that remain with overhead valve (OHV) engines:. Limited engine speeds or: OHV engines have more moving parts. OHV engines also typically use only a single intake and exhaust valve, which results in large (and heavy) valves, valve springs, and retainers.
Thus, the valvetrain in an OHV engine has greater inertia and mass. As a result, they suffer more easily from valve 'float', and may exhibit a tendency for the pushrods, if improperly designed, to flex or snap at high engine speeds.
Therefore, OHV engine designs cannot spin at engine speeds as high as OHC Modern OHV engines are usually limited to about 6,000 to 8,000 (rpm) in production cars, and 9,000 rpm to 10,500 rpm in racing applications. In contrast, many modern DOHC engines may have rev limits from 6,000 rpm to 9,000 rpm in road car engines, and in excess of 20,000 rpm (though now limited to 15,000 rpm) in current using. High-revving pushrod engines are normally solid (mechanical) lifter designs, flat and roller. In 1969, offered a Corvette and a model with a solid lifter cam pushrod V8 (the ZL-1) that could rev to 8,000 rpm.
The engines can rev to more than 7,000 rpm with their solid lifter camshaft. However, the LS7 of the C6 Corvette Z06 is the first production hydraulic roller cam pushrod engine to have a redline of 7,100 rpm. The motorcycle engine has a 9650rpm redline, well above the usual limits for auto engines, due to the lighter weight of components.
Limited cylinder head design flexibility: (OHC) engines benefit substantially from the ability to use, as well as much greater freedom of component placement, and intake and exhaust port geometry. Most modern OHV engines have two valves per cylinder, while many OHC engines can have three, four or even five valves per cylinder to achieve greater power. Though multi-valve OHV engines exist, their use is somewhat limited due to their complexity and is mostly restricted to low- and medium-speed diesel engines, with a few notable exceptions such as the four valve per cylinder motorcycle, and the engine.
In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods and the need to accommodate them in the head casting. Spark plug placement is also less ideal in pushrod engines.
This is important, since a centrally located spark plug improves combustion efficiency and reduces both emissions and tendency to detonate by reducing flame travel distance (which also reduces combustion time). DOHC engines with four valves per cylinder can have a truly centrally located spark plug because this space is free from both valves in the combustion chamber and valvetrain above this central area. Even SOHC engines with four valves per cylinder can usually accommodate a central spark plug. But since pushrod engines almost always have only two valves per cylinder, it is impossible to have a central spark plug. Noise and refinement: OHV engines are generally noisier than their OHC counterparts owing to the increased complexity of the valvetrain and the adoption of chain or gear based camshaft drive. Maintenance: The location of the camshaft in the cylinder block often necessitates removal of the engine whenever camshaft work is required. This is particularly true for front wheel drive applications with a transversely mounted engine.
Longitudinally mounted OHV engines suffer less from this problem as the camshaft can be withdrawn from the front of the engine after removal of the. Additionally, replacement of lifters generally requires removal of the cylinder heads. And cam bearing replacement generally requires the removal and complete teardown of the engine. Increased valvetrain friction and wear: Because OHV engines generally use only a single intake and exhaust valve, the valves are larger and heavier than those used in multivalve OHC engines. Furthermore, when the valve is closed, the spring must accelerate not only the valve and rocker arm, but also the pushrod and lifter. Therefore, OHV engines must use heavier valve springs than OHC engines (multivalve or otherwise).
As a result, valvetrain friction and wear is increased. This is especially true with high performance OHV engines utilizing high lift/duration/ramp rate cams with heavier than stock valve springs. Limited ability to use variable valve timing: Because OHV engines use a single camshaft for all valves, the ability to independently vary intake and exhaust valve timing is limited, although it is possible to vary the phase of one set of valves while the other set's timing stays constant (as in the 'CamInCam' or 'DuoCam' system). Also, because all cam lobes are on a single camshaft, there is little to no room for the extra high RPM lobes required for two stage systems like Hondas’s VTEC. Because of these factors, variable valve timing on OHV engines is limited to cam phasing systems that change intake and/or exhaust valve timing with no ability to vary lift or duration.1994 Mercedes/Ilmor Indianapolis 500 engine. This section does not any. Unsourced material may be challenged.
( February 2015) Each year, the bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production engines from racing engines was the use of pushrods, rather than the overhead camshafts used on most modern racing engines; Mercedes-Benz realized before the that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole, but still design it to be a state of the art racing engine in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and because of the higher boost pressure and larger displacement that the 'loophole' allowed pushrod engines, dominated the race. After the race, the rules were changed in order to reduce the amount of supplied by the. This amount was still 13% higher than what was allowed for the OHC engines.
The engine was also allowed to retain its considerable displacement advantage. The inability of the engine to produce competitive power output after this change caused it to become obsolete after just the one race. Mercedes-Benz knew this beforehand, deciding that the cost of engine development was worth one win at Indianapolis.See also.References.