Difference between revisions of "Pushrod"

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{{ redirect-acronym|OHV|[[Off highway vehicle]] }}
  
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A '''pushrod engine''', '''overhead valve (OHV) engine''' or '''[[Cam in  block]]''' is a type of [[piston engine]] that places the [[camshaft]] below the pistons (usually beside and slightly above the [[crankshaft]] in a [[straight engine]] or directly above the crankshaft in the V of a [[V engine]]) and uses '''pushrods''' or ''rods'' to actuate rocker arms above the [[cylinder head]] to actuate the [[poppet valve|valves]]. [[Lifter]]s or ''tappets'' reside in the engine block between the camshaft and pushrods.
 
  
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This contrasts with an [[overhead cam]] ([[OHC]]) design which places the [[camshaft]]s above the [[cylinder head]] and drives the valves directly or through short rocker arms. In an OHC engine, the camshafts are normally part of the cylinder head assembly, while in an OHV engine the camshaft (rarely more than one) is part of the main engine block assembly.
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[[Image:Pushrod2.PNG|thumb|Components of a pushrod valve actuation system]]
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[[Image:pushrod engine.jpg|thumb|Picture of a [[V8]] [[engine block]] (with [[intake manifold]] removed), showing the camshaft, pushrods, and rockers.]]
  
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Pushrod engines are maligned as "old fashioned" by the modern automotive press. The cause is historical:  The OHV engine came first while OHC engines were developed as more expensive high-performance engines and have largely replaced the pushrod design, particularly in countries where cars are taxed based on [[engine displacement]]. In 1949, Oldsmobile introduced the Oldsmobile V8 engine. It was the first [[high-compression]] OHV design, and is the archetype for most modern pushrod engines.
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An '''overhead valve (OHV) engine''', also called '''pushrod engine''' or '''I-head engine''' is a type of [[piston engine]] that places the [[camshaft]] in the [[cylinder block]] (usually beside and slightly above the [[crankshaft]] in a [[straight engine]] or directly above the crankshaft in the V of a [[V engine]]) and uses '''pushrods''' or ''rods'' to actuate [[rocker arm|rocker arms]] above the [[cylinder head]] to actuate the [[poppet valve|valves]]. Lifters or ''[[tappet]]s'' reside in the engine block between the camshaft and pushrods. The later [[overhead cam|overhead cam (OHC)]] design avoids the use of pushrods by putting the camshaft in the cylinder head.
  
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==Limitations==
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In 1949, [[Oldsmobile]] introduced the [[Oldsmobile V8 engine|Rocket V8]].  It was the first high-compression I-head design, and is the archetype for most modern pushrod engines. [[General Motors]] is the world's largest pushrod engine producer with engines such as the [[Buick V6 engine#L32 Supercharged|3800 Series III]] [[Supercharged]] V6 260 hp, 280 lbft, [[GM LS engine#LS7|LS7]] [[Chevrolet Corvette]] 7.0 L V8 Engine 505 hp, 475 lbft and [[GM LS engine#LS4|LS4]] 5.3 L [[Active Fuel Management|DOD]] V8 303 hp, 323 lbft.
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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 due to the fact that 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.
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However, in 2002, Chrysler introduced a new pushrod engine: a 5.7 L Hemi engine.  The new [[Chrysler Hemi engine]] presents advanced features such as [[variable displacement]] technology and has been a popular option with buyers.  The Hemi was on the [[Ward's 10 Best Engines]] 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 600hp SRT-10 engine for the 2008 [[Dodge Viper]].
  
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Three specific problems remain with pushrod engines:
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==History==
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In [[automotive engineering]], an '''overhead valve''' [[internal combustion engine]] is one in which the intake and exhaust [[poppet valve|valves]] and ports are contained in the [[cylinder head]].
  
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* ''Limited revolutions per minute (rpm)'' - Pushrod engines have more rotational mass, suffer more easily from valve "float", and exhibit a tendency for the pushrods themselves to flex or snap at high rpm. Therefore, conventional wisdom says that a pushrod engine cannot rev as high as an OHC design. Modern pushrod engines generally rev to 6,000 rpm. Compare this to modern OHC engines that rev to 7,000 rpm in the case of ordinary engines, to 9,000 rpm in the case of high-performance engines like the one used in the Honda S2000, and even past 10,000 rpm in specialty engines. However, high-rpm pushrod engines have also been developed — in 1969, Chevrolet offered a Chevrolet Camaro Z28 with a pushrod V8 that revs to 8,000 rpm. Volvo B18 and B20 engines can rev to more than 7000 rpm, and still last for hundreds of thousands of miles. The 2006 Chevrolet Corvette#Z06 2 features a 7.0 L GM LS engine#LS7 engine capable of revving to 7000 rpm. Higher engine RPMs results in more power overall.  
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The original overhead valve or '''OHV''' [[piston engine]] was developed by the [[Scottish-American]] [[David Dunbar Buick]]. It employs [[pushrod]]-actuated valves [[parallel (geometry)|parallel]] to the [[piston]]s and this is still in use today. This contrasts with previous designs which made use of [[Flathead engine|side valves]] and [[sleeve valve]]s.
  
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* ''Difficulty in using [[crossflow cylinder head]]s'' in [[straight engine]] configurations - A few straight pushrod engines have been manufactured with crossflow heads, such as the six cylinder [[Humber Super Snipe]]. These engines combined much of the performance of the overhead camshaft with the ease of service of the pushrod, but were more expensive to manufacture than either competing design.  
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Nowadays, side-valves have virtually disappeared (except perhaps in lawn-mower engines) and valves are almost all "overhead". However most are now driven more directly by the [[overhead camshaft]] system and these are designated OHC instead (either [[Overhead camshaft#Single overhead camshaft|SOHC]] or [[Overhead camshaft#Dual overhead camshaft|DOHC]]).
  
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* ''Limited valve flexibility'' - The biggest benefit of an OHC design is the use of multi-valve (multiple intake and exhaust valves) and [[variable valve timing]]. Most modern pushrod engines have two valves per cylinder, while many OHC engines use three, four or even five valves per cylinder to achieve greater efficiency and power.  Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' Cummins B Series engine#ISB is a 3-valve [[pushrod]] [[straight-6]].  For the 2006 model year, General Motors will introduce the ''GM Vortec engine#6200''.  This is the first mass-produced pushrod engine to feature variable valve timing.  The system adjusts both intake and exhaust timing between two settings.
 
  
 
==Advantages==
 
==Advantages==
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Pushrod engines have specific advantages:
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* ''Smaller overall packaging'' — Because of the [[Cam-in-block|camshaft's location inside the engine block]], pushrods are more compact than an overhead cam engine of comparable displacement.  For example, [[Ford]]'s 4.6 L OHC [[Ford Modular engine|modular]] V8 is larger than the 5.0 L I-head [[Ford Windsor engine|Windsor]] V8 it replaced. GM's 4.6 L OHC [[Cadillac Northstar engine|Northstar]] V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head [[GM LS engine|LS]] V8. The [[Ford Ka]] uses the venerable [[Ford Kent engine#Endura-E|Kent Crossflow]] pushrod engine to fit under its low bonnet line.
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* ''Less complex drive system'' — Pushrod engines have a less complex drive system to time the camshaft when compared with [[OHC]] engines. Most [[OHC]] engines drive the [[camshaft]] or [[camshaft]]s using a [[timing belt]], a [[Roller chain|chain]] or multiple [[Roller chain|chain]]s. These systems require the use of tensioners which add some complexity to the engine. In contrast a pushrod engine has the camshaft positioned just above crankshaft and can be run with a much smaller chain or even direct gear connection.
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In contrast, pushrod engines have specific advantages:
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==Limitations==
  
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* ''Smaller overall packaging'' - Because of the camshaft's location inside the engine block, pushrods are generally more compact than an overhead cam engine of comparable displacement.  For example, Ford's 4.6 L OHC Ford Modular engine V8 is larger than the 4.9 L OHV Ford Windsor engine V8 it replaced and GM's 4.6 L OHC Cadillac Northstar engine V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L OHV GM LS engine V8.
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Some specific problems that remain with pushrod engines:
  
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* ''Peak [[torque]]'' - Pushrod engines develop maximum power at a lower rpm than overhead cam engines and also tend to be torquier.  Combine this with higher gearing and that can add up to superior fuel economy. The GM LS engine#LS6 V8, for example, makes significantly more torque and power than the Northstar while exhibiting similar fuel economy.  Though some would consider this a questionable comparison since the Northstar engine is typically used in applications where engine output is sacrificed to improve NVH (Noise, Vibration and Harshness) characteristics. While it is true that pushrods produce more torque at lower RPMs, this also has a lot to do with the tuning of the specific engine. Many engines used by Mercedes-Benz utilize OHC and have lots of torque at very low RPMs.
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* ''Limited engine speeds or [[revolutions per minute|RPM]]'' — Pushrod engines have more valvetrain moving parts thus more valvetrain 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, pushrod engine designs cannot revolve ("rev") at engine speeds as high as OHC designs.
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Modern pushrod engines are usually limited to about 6,000 RPM 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 legal car engines and up to 18,000 RPM in current [[Formula One]] race engines using [[pneumatic valve springs]]. High-revving pushrod engines are normally solid (mechanical) lifter designs, flat and roller. In 1969, [[Chevrolet]] offered a Corvette and a [[Camaro]] model with a solid lifter cam pushrod V8 (the ZL-1) that could rev to 8,000 RPM. The [[Volvo B18 engine|Volvo B18 and B20]] 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 7100 RPM.
  
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* ''Complexity'' - By their nature OHC engines tend to be more complex than pushrod engines, particularly considering that an OHC V6 or V8 can have four camshafts. This adds cost, mass and increases the number of moving parts.
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* ''Limited cylinder head design flexibility'' — [[OHC]] engines benefit substantially from the use of [[Multi-valve|multiple valves]] as well as much greater freedom of component placement and intake and exhaust port geometry. Most modern pushrod 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 pushrod engines exist their use is somewhat limited due to their complexity and is mostly restricted to low and medium speed diesel engines. In pushrod engines the size and shape of the intake ports as well as the position of the valves are limited by the pushrods.
  
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==1994 Mercedes Indianapolis 500 engine==
 
  
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The [[Indy 500]] race in [[Indianapolis]] each year 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 from racing engines was the use of pushrods, rather than the [[overhead cams]] used on most modern racing engines; Mercedes-Benz realized before the 1994 race 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 state of the racing art in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and, as expected, dominated the race. After the race, the rules were changed to prevent a recurrence, and the engine became obsolete after just the one race, as Mercedes-Benz knew it would when deciding a victory at Indy was worth it.
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==1994 Mercedes/Ilmor Indianapolis 500 engine==
  
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The [[Indy 500]] race in [[Indianapolis]] each year 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 cams used on most modern racing engines; Mercedes-Benz realized before the [[1994 Indianapolis 500|1994]] race 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, as expected, dominated the race. After the race,  the rules were changed in order to reduce the amount of [[Turbocharger#Boost|boost pressure]] allowed to be supplied by the [[turbocharger]]. 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.
  
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==External links:==
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===See also===
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* [[Valvetrain]]
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* [[Overhead camshaft]]
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* [[Cam-in-block]]
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* [[Flathead engine]]
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==External links==
 
*[http://www.samarins.com/glossary/dohc.html Pushrod (OHV), SOHC and DOHC engine animated diagrams]
 
*[http://www.samarins.com/glossary/dohc.html Pushrod (OHV), SOHC and DOHC engine animated diagrams]
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*[http://media.gm.com/us/powertrain/en/product_services/2007/HPT%20Library/Gen%20IV/Gen%20IV%20Car/2007_LS7.pdf LS7 torque and power by rpm chart]
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*[http://www.web-cars.com/enzo/motor.html Ferrari Enzo: The Engine]
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*[http://www.caranddriver.com/features/columns/c_d_staff/larry_webster/the_pushrod_engine_finally_gets_its_due_column The Pushrod Engine Finally Gets its Due]
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{{Automobile configurations}}
 
{{Automobile configurations}}
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[[Category:Motorcycle engines]]
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[[Category:Engine valves]]

Revision as of 08:11, 14 June 2009

Template:Redirect-acronym


Components of a pushrod valve actuation system
Picture of a V8 engine block (with intake manifold removed), showing the camshaft, pushrods, and rockers.

An overhead valve (OHV) engine, also called pushrod engine or I-head engine is a type of piston engine that places the camshaft in the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine) and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets reside in the engine block between the camshaft and pushrods. The later overhead cam (OHC) design avoids the use of pushrods by putting the camshaft in the cylinder head.

In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression I-head design, and is the archetype for most modern pushrod engines. General Motors is the world's largest pushrod engine producer with engines such as the 3800 Series III Supercharged V6 260 hp, 280 lbft, LS7 Chevrolet Corvette 7.0 L V8 Engine 505 hp, 475 lbft and LS4 5.3 L DOD V8 303 hp, 323 lbft.

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 due to the fact that 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 L Hemi engine. The new Chrysler Hemi engine presents advanced features such as variable displacement technology and has been a popular option with buyers. The Hemi was on the Ward's 10 Best Engines 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 600hp SRT-10 engine for the 2008 Dodge Viper.

History

In automotive engineering, an overhead valve internal combustion engine is one in which the intake and exhaust valves and ports are contained in the cylinder head.

The original overhead valve or OHV piston engine was developed by the Scottish-American David Dunbar Buick. It employs pushrod-actuated valves parallel to the pistons and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves.

Nowadays, side-valves have virtually disappeared (except perhaps in lawn-mower engines) and valves are almost all "overhead". However most are now driven more directly by the overhead camshaft system and these are designated OHC instead (either SOHC or DOHC).


Advantages

Pushrod engines have specific advantages:

  • Smaller overall packaging — Because of the camshaft's location inside the engine block, pushrods are more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the 5.0 L I-head Windsor V8 it replaced. GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head LS V8. The Ford Ka uses the venerable Kent Crossflow pushrod engine to fit under its low bonnet line.
  • Less complex drive system — Pushrod engines have a less complex drive system to time the camshaft when compared with OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a chain or multiple chains. These systems require the use of tensioners which add some complexity to the engine. In contrast a pushrod engine has the camshaft positioned just above crankshaft and can be run with a much smaller chain or even direct gear connection.


Limitations

Some specific problems that remain with pushrod engines:

  • Limited engine speeds or RPM — Pushrod engines have more valvetrain moving parts thus more valvetrain 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, pushrod engine designs cannot revolve ("rev") at engine speeds as high as OHC designs.

Modern pushrod engines are usually limited to about 6,000 RPM 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 legal car engines and up to 18,000 RPM in current Formula One race engines using pneumatic valve springs. High-revving pushrod engines are normally solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette and a Camaro model with a solid lifter cam pushrod V8 (the ZL-1) that could rev to 8,000 RPM. The Volvo B18 and B20 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 7100 RPM.

  • Limited cylinder head design flexibility — OHC engines benefit substantially from the use of multiple valves as well as much greater freedom of component placement and intake and exhaust port geometry. Most modern pushrod 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 pushrod engines exist their use is somewhat limited due to their complexity and is mostly restricted to low and medium speed diesel engines. In pushrod engines the size and shape of the intake ports as well as the position of the valves are limited by the pushrods.


1994 Mercedes/Ilmor Indianapolis 500 engine

The Indy 500 race in Indianapolis each year 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 cams used on most modern racing engines; Mercedes-Benz realized before the 1994 race 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, as expected, dominated the race. After the race, the rules were changed in order to reduce the amount of boost pressure allowed to be supplied by the turbocharger. 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


External links


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