Discussion
a little confused:
does a sohc engine mean single cam controlling/changing the inlet and exhaust valve
and
a dohc engine mean two cams, each seperately control the inlet and exhaust valves
or
does a dohc simply mean that there are 4 valves above the piston? two intaking and two outaking (so 2 cams needed).
whichever is right, what the benefits of each?
does a sohc engine mean single cam controlling/changing the inlet and exhaust valve
and
a dohc engine mean two cams, each seperately control the inlet and exhaust valves
or
does a dohc simply mean that there are 4 valves above the piston? two intaking and two outaking (so 2 cams needed).
whichever is right, what the benefits of each?
SOHC = Single Over Head Cam
This means there is one cam above the valve that opens both inlet and exhaust valve's (usualy 8 valve on a 4 cylender engine)
DOHC = Double Over Head Cam
This means there are two cams usualy one for inlet and the other for exhaust (there may be exeptions) can be 8v or 16v in a 4 cylender engine.
IIRC some sierra's had 2l DOHC 8 valve's
>> Edited by seaton on Monday 29th September 20:44
This means there is one cam above the valve that opens both inlet and exhaust valve's (usualy 8 valve on a 4 cylender engine)
DOHC = Double Over Head Cam
This means there are two cams usualy one for inlet and the other for exhaust (there may be exeptions) can be 8v or 16v in a 4 cylender engine.
IIRC some sierra's had 2l DOHC 8 valve's
>> Edited by seaton on Monday 29th September 20:44
goin2fast52 said:
you can also have a 16v sohc engine, with one cam controlling all the valves like some hondas
me also getting slightly confused!?!
what would be the point of a 4v per cyl single cam??? surely you're just making work for yourself and it would be easier to have a twin cam head!?!
littlegearl said:
you can also have a 16v sohc engine, with one cam controlling all the valves like some hondas
me also getting slightly confused!?!
what would be the point of a 4v per cyl single cam??? surely you're just making work for yourself and it would be easier to have a twin cam head!?!
SOHC installations are generaly more compact, giving a less deep cylinder head (although sometimes taller). They are also cheaper to manufacture than a twin cam design.
The disadvantages of SOHC is mainly in the extra mass of the rockers, possibly less than optimal valve angles and spark-plug placement.
littlegearl said:Wrong, you can get single cams with 4 valves per cylinder and double cams with 2 valves per cylinder
i'm pretty sure you can only get dohc engines with 4 valves per cylinder...
2 valves cheaper, 4 valves more efficient
single cam more compact but uses rockers, double cam needs more camshaft bearings etc etc
Then you can add VVT.....
4 valves per cylinder give more flow at low lift (better for high revs) for equivalent port area, they are useful in increasing flow in small capacity engines as they require a smaller diameter valve and so more port area for the available head area.
for high revving engine they are more useful, but for example a large capacity v8 there is much less to be gained.
sohc is lighter and less difficult to configure, but if running 4 valves, then there is generally a requirement for rockers, overlap can be a problem and there can be a lot of play in the valve train, leading to increased wear and tear, i.e. the rocker gear becomes heavier losing the weight advantage.
dohc has the main advantage of being able to increase the valve angle which generally increases flow. Plus the two seperate cam shafts allows on the car changing of the inlet/exhaust phasing. the real advantage of 2 comes with 4valves.
so although 2valve twin cam systems and single cam 4 valve systems exist. generally a 2 valve single cam suits a large capacity engine or low revving torquey engine and 4 valve twin cam suits a small capacity engine or high peak power high revving engine.
for high revving engine they are more useful, but for example a large capacity v8 there is much less to be gained.
sohc is lighter and less difficult to configure, but if running 4 valves, then there is generally a requirement for rockers, overlap can be a problem and there can be a lot of play in the valve train, leading to increased wear and tear, i.e. the rocker gear becomes heavier losing the weight advantage.
dohc has the main advantage of being able to increase the valve angle which generally increases flow. Plus the two seperate cam shafts allows on the car changing of the inlet/exhaust phasing. the real advantage of 2 comes with 4valves.
so although 2valve twin cam systems and single cam 4 valve systems exist. generally a 2 valve single cam suits a large capacity engine or low revving torquey engine and 4 valve twin cam suits a small capacity engine or high peak power high revving engine.
I'm sure there was a production Ferrari engine with 5 valves / cylinder and dohc on both sides of the block. Can't seem to find a ref though.
Z
Ah! The F50.....
Marketed as a 'Formula 1 car for the road', the F50 proved that it was just that. Launched at the Geneva Motorshow, two years short of the company's 50th birthday, it was the successor of the F40. F1-technology and materials were found throughout the F50.
A one piece carbon fibre monocoque type chassis is used. Directly bolted on the monocoque, the V12 engine carries the rear suspension, in true F1 style. This V12 engine is directly derived from the contemporary 3.5 litre V12 engines and used the same 5 valve per cylinder technology. The 520 horses produced are sent through a six speed manual gearbox to the 355 mm wide rear wheels.
>> Edited by z_chromozone on Tuesday 7th October 15:46
Z
Ah! The F50.....
Marketed as a 'Formula 1 car for the road', the F50 proved that it was just that. Launched at the Geneva Motorshow, two years short of the company's 50th birthday, it was the successor of the F40. F1-technology and materials were found throughout the F50.
A one piece carbon fibre monocoque type chassis is used. Directly bolted on the monocoque, the V12 engine carries the rear suspension, in true F1 style. This V12 engine is directly derived from the contemporary 3.5 litre V12 engines and used the same 5 valve per cylinder technology. The 520 horses produced are sent through a six speed manual gearbox to the 355 mm wide rear wheels.
>> Edited by z_chromozone on Tuesday 7th October 15:46
There is a photo of the 5 valve cylinder head at the bottom of the page. http://autozine.kyul.net/technical_school/engine/tech_engine_2.htm#3,%204,%205-valve
And to quote from the page:
4-valve engines
....
4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduce more friction and hurt high speed power. Therefore the sportiest Honda and Mitsubishi still use dohc.
And to quote from the page:
4-valve engines
....
4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduce more friction and hurt high speed power. Therefore the sportiest Honda and Mitsubishi still use dohc.
The valve number and orientation will also dictate the flow patterns within the combustion chamber.
e.g a 2 valve cylinder will have a SWIRL flow- usually promoting more low rev torque (low end power).
and a 4 valve head promotes a combustion chamber flow characteristic known as TUMBLE which produces a better burn at high revs and more top end power.
Whilst not even considering valve angles, combustion chamber shape, etc etc....
e.g a 2 valve cylinder will have a SWIRL flow- usually promoting more low rev torque (low end power).
and a 4 valve head promotes a combustion chamber flow characteristic known as TUMBLE which produces a better burn at high revs and more top end power.
Whilst not even considering valve angles, combustion chamber shape, etc etc....
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