Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Overhead Valve shopping experience:
1. Compare - without doubt the biggest advantage that the Overhead Valve offers shoppers today is the ability to compare thousands of Overhead Valve at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.
2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about
3. Testimonials - don't know anybody that has bought a Overhead Valve? Wrong! If the Overhead Valve is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.
4. Questions - Got a question about Overhead Valve then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....
5. Reputation - Never heard of the company selling Overhead Valve? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Overhead Valve and build up a picture of their reputation for sales, returns, customer service, delivery etc.
6. Returns - still worried that even after all of the above your Overhead Valve wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.
7. Feedback - happy with your Overhead Valve then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.
8. Security - check for the yellow padlock on the Overhead Valve site before you buy, and the s after http:/ /i.e. https:// = a secure site
9. Contact - got a question about Overhead Valve, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.
10. Payment - ready to pay for your Overhead Valve, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.
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
poppet valve. Lifters or
tappets reside in the engine block between the camshaft and pushrods.
This contrasts with an overhead cam (OHC) design which places the camshafts 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 I-head engine the camshaft (rarely more than one) is part of the main engine block assembly.
In 1949, Oldsmobile introduced the Oldsmobile V8 engine. 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 Supercharged V6 (260 hp, 280 lbf·ft torque), GM LS engine#LS7 Chevrolet Corvette 7.0 L V8 Engine (505 hp, 475 lbf·ft torque) and GM LS engine#LS4 5.3 L
Active Fuel Management V8 (303 hp, 323 lbf·ft torque). Few pushrod type engines remain in production. This is a result of few manufacturers wanting to design both OHV and OHC engines, and competitively OHC racing engines have an advantage in power due to rpm limits. However, in 2002, Chrysler introduced a new pushrod engine: a 5.7L 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.
History
In
automotive engineering, an
overhead valve internal combustion engine is one in which the intake and exhaust
poppet valve 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 (geometry) to the
pistons and this is still in use today. This contrasts with previous designs which made use of Cam-in-block#L-head 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 Overhead camshaft#Single overhead camshaft or Overhead camshaft#Dual overhead camshaft).
Pushrod engines have become less common in recent years, serving primarily as either truck engines or as budget V6 models for General Motors, though Chrysler's HEMI engines and GM's LS series are a notable exception. Pushrod engines are nearly extinct among other automakers.
Advantages
In contrast, pushrod engines have specific advantages:
- Smaller overall packaging - Because of the Cam-in-block, pushrods are 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 5.0 L I-head Ford Windsor engine V8 it replaced. 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 I-head GM LS engine 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 when compared with OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a Roller chain or multiple Roller chains. These systems require the use of tensioners which add some complexity to the engine. In addition, failure of the timing belt or chain can sometimes result in the pistons colliding with the open valves, resulting in severe damage to the engine.
- Reduced major servicing times - Operations which require the cylinder head to be removed (such as head gasket replacement) can be performed without removal of the camshaft and therefore the camshaft drive does not need to be replaced or re-timed, saving both time and expense. If the head and block are cast iron, this can be achieved very quickly as full cooling of the parts may not be necessary.
Limitations
Two specific problems remain with pushrod engines:
- Limited engine speeds or revolutions per minute - Pushrod engines have more valvetrain moving parts (the pushrod itself) thus more valvetrain inertia, suffer more easily from valve "float" due to the innate valve actuation rocker design, and exhibit a tendency for the pushrods, if too long, to flex or snap at very high engine speeds. Therefore, a pushrod engine cannot revolve ("rev") at engine speeds as near as high as an OHC design. Modern pushrod engines are usually limited to 6,000 rpm. Compare this to modern OHC engines that have rev limits from 6,000 rpm to 9,000 rpm, while rev limits near 20,000 rpm are for Formula One racing engines. High-revving pushrod engines have also been developed, albeit solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette, Camaro Z28, and other models with a solid lifter cam pushrod V8, the ZL1, that could rev to 8,000 rpm. The Volvo B18 engine 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. Various pushrod racing engines are capable of varying from 9,000 to 10,500 rpm.
- Limited cylinder head design flexibility - The biggest benefit that an OHC design has is the ease of using multi-valve and variable valve timing. 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 efficiency and power. Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' Cummins B Series engine#ISB is a 4-valve pushrod straight-6. The GM 3900 was the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between only two settings, it can not vary the intake and exhaust timing independently. Presently there is even a company called Arao Engineering, formerly Dominion Performance, that has developed, patented, and sold a 4-valve per cylinder aluminum cylinder head for various pushrod engines like the small/big block Chevrolet engines, Ford small/big block engines and others.
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 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 in order to reduce the amount of
Turbocharger#Boost 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.
Comparison of engine configurations and types
Comparing engines is not an exact science. This table shows the comparison of some of the most important features when looking at an engine{| class="wikitable"|-! Engine name! Capacity! Geometry! Type! Car! Engine weight! Power! RPM power! Torque! RPM torque! Power/Weight|-|| (liters)||| (application)| (lb)| (HP,SAE)| (rpm)| (lbf·ft)| (rpm)| (hp/lb)|-| F140| 6.0| V12| DOHC| 2002
Enzo Ferrari (car)| 496| 660| 7,800| 485| 5,500| 1.33|-| 13B-MSP (
Renesis#13B-MSP RENESIS)| 1.3| 2-Rotor|
Wankel engine| 2003
Mazda RX-8| 472| 605| 8,000| 435| 5,750| 1.28|-| F130| 4.7| V12| DOHC| 1995 [Ferrari F50| 458| 505| 6,300| 470| 4,800| 1.10|-| AMG 6.3| 6.2| V8| DOHC| 2007 [Mercedes-Benz CLK-Class| 439| 475| 6,800| 465| 5,000| 1.08|-| LS3| 6.2| V8| pushrod| 2008 Chevrolet Corvette C6 & [BMW M6| 650| 510| 5,600| 535| 4,600| 0.79|-| S62| 5.0| V8| DOHC| 2003 [BMW M5s for race and road legal track day cars
{| class="wikitable"|-! Engine name! Capacity! Geometry! Type! Car! Engine weight! Power! RPM power! Torque! RPM torque! Power/Weight! Reference|-|| (liters)||| (application)| (lb)| (HP)| (rpm)| (lbf·ft)| (rpm)| (hp/lb)||-| [BMW P84/5]
Formula One| 203| 925| 19,000| NA| NA| 4.56| |-|
Ferrari Tipo 052]
Formula One| 203| 920| 19,500| NA| NA| 4.53| |-| Powertec RPB V8| 2.8| V8| DOHC| Radical Motorsport SR9| 194| 450| NA| 250| NA| 2.32| |-| Motopower RST-V8| 2.0| V8| DOHC| Various| 163| 340| 10,250| 190| 7,000- 7,800| 2.09| |-| Powertec RPA| 2.6| V8| DOHC|
Radical Motorsport SR8| 194| 380| NA| 215| NA| 1.96| |-|}
See also
External links
- Pushrod (OHV), SOHC and DOHC engine animated diagrams
- LS7 torque and power by rpm chart
- Ferrari Enzo: The Engine
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
poppet valve. Lifters or
tappets reside in the engine block between the camshaft and pushrods.
This contrasts with an
overhead cam (OHC) design which places the camshafts 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 I-head engine the camshaft (rarely more than one) is part of the main engine block assembly.
In 1949, Oldsmobile introduced the
Oldsmobile V8 engine. 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 Supercharged V6 (260 hp, 280
lbf·ft torque), GM LS engine#LS7
Chevrolet Corvette 7.0 L V8 Engine (505 hp, 475 lbf·ft torque) and GM LS engine#LS4 5.3 L Active Fuel Management V8 (303 hp, 323 lbf·ft torque). Few pushrod type engines remain in production. This is a result of few manufacturers wanting to design both OHV and OHC engines, and competitively OHC racing engines have an advantage in power due to rpm limits. However, in 2002, Chrysler introduced a new pushrod engine: a 5.7L 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.
History
In
automotive engineering, an
overhead valve internal combustion engine is one in which the intake and exhaust poppet valve 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 (geometry) to the pistons and this is still in use today. This contrasts with previous designs which made use of
Cam-in-block#L-head 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 Overhead camshaft#Single overhead camshaft or Overhead camshaft#Dual overhead camshaft).
Pushrod engines have become less common in recent years, serving primarily as either truck engines or as budget V6 models for General Motors, though Chrysler's HEMI engines and GM's LS series are a notable exception. Pushrod engines are nearly extinct among other automakers.
Advantages
In contrast, pushrod engines have specific advantages:
- Smaller overall packaging - Because of the Cam-in-block, pushrods are 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 5.0 L I-head Ford Windsor engine V8 it replaced. 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 I-head GM LS engine 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 when compared with OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a Roller chain or multiple Roller chains. These systems require the use of tensioners which add some complexity to the engine. In addition, failure of the timing belt or chain can sometimes result in the pistons colliding with the open valves, resulting in severe damage to the engine.
- Reduced major servicing times - Operations which require the cylinder head to be removed (such as head gasket replacement) can be performed without removal of the camshaft and therefore the camshaft drive does not need to be replaced or re-timed, saving both time and expense. If the head and block are cast iron, this can be achieved very quickly as full cooling of the parts may not be necessary.
Limitations
Two specific problems remain with pushrod engines:
- Limited engine speeds or revolutions per minute - Pushrod engines have more valvetrain moving parts (the pushrod itself) thus more valvetrain inertia, suffer more easily from valve "float" due to the innate valve actuation rocker design, and exhibit a tendency for the pushrods, if too long, to flex or snap at very high engine speeds. Therefore, a pushrod engine cannot revolve ("rev") at engine speeds as near as high as an OHC design. Modern pushrod engines are usually limited to 6,000 rpm. Compare this to modern OHC engines that have rev limits from 6,000 rpm to 9,000 rpm, while rev limits near 20,000 rpm are for Formula One racing engines. High-revving pushrod engines have also been developed, albeit solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette, Camaro Z28, and other models with a solid lifter cam pushrod V8, the ZL1, that could rev to 8,000 rpm. The Volvo B18 engine 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. Various pushrod racing engines are capable of varying from 9,000 to 10,500 rpm.
- Limited cylinder head design flexibility - The biggest benefit that an OHC design has is the ease of using multi-valve and variable valve timing. 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 efficiency and power. Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' Cummins B Series engine#ISB is a 4-valve pushrod straight-6. The GM 3900 was the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between only two settings, it can not vary the intake and exhaust timing independently. Presently there is even a company called Arao Engineering, formerly Dominion Performance, that has developed, patented, and sold a 4-valve per cylinder aluminum cylinder head for various pushrod engines like the small/big block Chevrolet engines, Ford small/big block engines and others.
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 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 in order to reduce the amount of
Turbocharger#Boost 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.
Comparison of engine configurations and types
Comparing engines is not an exact science. This table shows the comparison of some of the most important features when looking at an engine{| class="wikitable"|-! Engine name! Capacity! Geometry! Type! Car! Engine weight! Power! RPM power! Torque! RPM torque! Power/Weight|-|| (liters)||| (application)| (lb)| (HP,SAE)| (rpm)| (lbf·ft)| (rpm)| (hp/lb)|-| F140| 6.0| V12| DOHC| 2002
Enzo Ferrari (car)| 496| 660| 7,800| 485| 5,500| 1.33|-| 13B-MSP (Renesis#13B-MSP RENESIS)| 1.3| 2-Rotor| Wankel engine| 2003 Mazda RX-8| 472| 605| 8,000| 435| 5,750| 1.28|-| F130| 4.7| V12| DOHC| 1995 [Ferrari F50| 458| 505| 6,300| 470| 4,800| 1.10|-| AMG 6.3| 6.2| V8| DOHC| 2007 [Mercedes-Benz CLK-Class| 439| 475| 6,800| 465| 5,000| 1.08|-| LS3| 6.2| V8| pushrod| 2008
Chevrolet Corvette C6 & [BMW M6| 650| 510| 5,600| 535| 4,600| 0.79|-| S62| 5.0| V8| DOHC| 2003 [BMW M5s for race and road legal track day cars
{| class="wikitable"|-! Engine name! Capacity! Geometry! Type! Car! Engine weight! Power! RPM power! Torque! RPM torque! Power/Weight! Reference|-|| (liters)||| (application)| (lb)| (HP)| (rpm)| (lbf·ft)| (rpm)| (hp/lb)||-| [BMW P84/5] Formula One| 203| 925| 19,000| NA| NA| 4.56| |-| Ferrari Tipo 052]
Formula One| 203| 920| 19,500| NA| NA| 4.53| |-| Powertec RPB V8| 2.8| V8| DOHC|
Radical Motorsport SR9| 194| 450| NA| 250| NA| 2.32| |-| Motopower RST-V8| 2.0| V8| DOHC| Various| 163| 340| 10,250| 190| 7,000- 7,800| 2.09| |-|
Powertec RPA| 2.6| V8| DOHC| Radical Motorsport SR8| 194| 380| NA| 215| NA| 1.96| |-|}
See also
External links
- Pushrod (OHV), SOHC and DOHC engine animated diagrams
- LS7 torque and power by rpm chart
- Ferrari Enzo: The Engine
