sohc/dohc

Author
Discussion

dragstar

Original Poster:

3,924 posts

218 months

Monday 29th September 2003
quotequote all
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?

littlegearl

3,139 posts

225 months

Monday 29th September 2003
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i'm pretty sure you can only get dohc engines with 4 valves per cylinder...

so there are two inlet and two outlet...

seaton

400 posts

222 months

Monday 29th September 2003
quotequote all
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

goin2fast52

57 posts

228 months

Monday 29th September 2003
quotequote all
you can also have a 16v sohc engine, with one cam controlling all the valves like some hondas

littlegearl

3,139 posts

225 months

Monday 29th September 2003
quotequote all
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!?!

deltaf

6,806 posts

221 months

Monday 29th September 2003
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Ford Granadas have a twin cam 8 valve engine...it has a plastic cam cover, but i cant remember its desigantion number..

mr2mike

20,143 posts

223 months

Tuesday 30th September 2003
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littlegearl said:

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!?!


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.

liszt

4,329 posts

238 months

Tuesday 30th September 2003
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Also 4 smaller valves (2 inlet and 2 exhaust) can give a greater flow than 2 larger valves (1 inlet 1 exhaust)

Incorrigible

13,668 posts

229 months

Tuesday 30th September 2003
quotequote all
littlegearl said:
i'm pretty sure you can only get dohc engines with 4 valves per cylinder...
Wrong, you can get single cams with 4 valves per cylinder and double cams with 2 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.....

annodomini2

6,329 posts

219 months

Friday 3rd October 2003
quotequote all
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.

z_chromozone

1,436 posts

217 months

Tuesday 7th October 2003
quotequote all
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

Incorrigible

13,668 posts

229 months

Tuesday 7th October 2003
quotequote all
Don't forget desmodronic valve action

Has anyone got a good cutaway pic of a 5 valve head

forrestgump

62 posts

219 months

Tuesday 7th October 2003
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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.

Mr2Mike

20,143 posts

223 months

Tuesday 7th October 2003
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I hope no one is forgetting Honda's NR500 with 8 valves per cylinder and oval pistons and two conrods per piston! Although Honda seems to have forgotten them...

kevinday

8,985 posts

248 months

Wednesday 8th October 2003
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z_chromozone said:
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.


I think the 355 and 360 are also 5 valves per cylinder.

gt5s_1985

703 posts

224 months

Wednesday 8th October 2003
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Audi makes a bunch of 5-valves per cylinder. My A6, for example, has a 2.8L 30-valve V-6...

annodomini2

6,329 posts

219 months

Wednesday 8th October 2003
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vw 1.8 is 20valve, audi 2.4/2.8/30 30 valve engines and the new s8 will have a 4.2 40 valve v8

jv_as

129 posts

223 months

Thursday 9th October 2003
quotequote all
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....

z_chromozone

1,436 posts

217 months

Tuesday 14th October 2003
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All I can say is that I am glad my 16V VW daily driver is only 16V. It proved to be a right pain in the arse to do the head (for the second time in 200,000 miles). If it had 8 valves / cylinder I would have no hair left by now.

Z

wedg1e

26,736 posts

233 months

Wednesday 15th October 2003
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z_chromozone said:
If it had 8 valves / cylinder I would have no hair left by now.

Z


EIGHT VALVES PER CYLINDER???!!! You been on the happy baccy?