Why DOHC and not SOHC ??

[mic*]

Twin cam is better than single for the same reason as twin turbo is better than single - if you tell people there are TWO - more is better and they're sold.

There is no inherant reason why twin is more powerful than single. Its about the design of the combustion chamber & the inlet manifol & ports. Usually improving one thing comes at the expense of another. Twin cam is more flexible in terms of being made to work effciently and dynamically within fairly strict physical design limitations.

I beleive that the reason we are saturated with SR20's is not coz they are supeior (or inferior) but coz some clever bastard brought a few thousand s13's to OZ in the mid to late ninties, then Australia caught this Nissan bug that spread like a disease till every bugger has an R33 Gts in their yard. I think the use of RB30's by holden has had a major influence over Australia's adoption of Nissan. Only now are they all realising they could have spent half as much to build twice as reliable a motor from a 1JZ rather than an RB25.

[Nim]

Rinmax, there are plenty of reasons why the SR20 may be used so much, and they may not be that they're the best design performance wise. Having a narrow angle head is good for the combustion chamber and the whole setup is cheap. But that's what Nissan is about. Cheap performance. Atleast they HAVE DOHC, and aren't pushrod. And atleast it's a lash adjuster, not a rocker.

[Nim]

Twin cam is better than single for the same reason as twin turbo is better than single - if you tell people there are TWO - more is better and they're sold.

There is no inherant reason why twin is more powerful than single. Its about the design of the combustion chamber & the inlet manifol & ports. Usually improving one thing comes at the expense of another. Twin cam is more flexible in terms of being made to work effciently and dynamically within fairly strict physical design limitations.

'cept if you read Bill Shirwoods site, he does actualy go into why DOHC is better than SOHC.
It's not as big an improvement as 4 valves over 2, but it is an improvement.

[ed]

remember - bill sherwood isnt an absolute authority on these things, he's just a guy like the rest of us on here. an avid enthusiast. i suggest reading his stuff, yes, but read furher also

[Nim]

I have done so, but Bill is the one who explains it the best... and explains almost everything. Unlike some sites that just give bits an peices, that's why I keep referencing his site.

[mic*]

'cept if you read Bill Shirwoods site, he does actualy go into why DOHC is better than SOHC.
It's not as big an improvement as 4 valves over 2, but it is an improvement.

The only real reason Bill gives are; reduced friction in valve running gear, and thus efficient operation of four valves per cylinder, which we do why? To get more flow for less loss, and because it can be made to operate more dynamically.

These things are not limited to twin cam, just generally easier to acheive therewith. OK.

For example, look at a SOHC with roller rockers and 3 valves per cylinder (12V), as can be found in many late model four cylinder egnines. Now read Bill's page and tell me where the significant advantage is over a DOHC engine of the same capacity, bearing in mind that combustion chamber design has probably suffered slightly too by adding the twin cam...

I stick to what i said;

Twin cam is more flexible in terms of being made to work effciently and dynamically within fairly strict physical design limitations.

[3sgte]

Better is a very subjective term.
Having said that, as mentioned, the ability to use VVT to separately control the timing of one or both cams is a crucial (if I may say) need for modern engines that have to meet any kind of emission standards.

You can see some of the emission benefits in a comparison between two 1MZ engines, one with VVT and one without. The timing and overlap changes on the VVT motor result in lower HC output, and no need to equip the motor with EGR.

The ability to dynamically adjust the cam timing and overlap, according to engine RPM and load, allows DOHC engines to run cams that are more agressive than a SOHC.

Single cam VTEC is a partial workaround, but it would be a compromise having only one "stage" of change (Toyota's VVT-i is continiously variable across its adjustment range). More compromising needs to be done on production engines that are SOHC.

As HEN mentioned, a SOHC cam can be ground agressively, and it can perform well. Unfortunately, depending on the severity of the grind, the usability of the engine may suffer in some road use conditions.

The irony about all the talk of rockers, and the advantages of direct cam on bucket previously favoured by Toyota, is found in the new engines from the #GR series.
They employ hydraulic lash adjusters, and roller rocker arms. Based on an SAE article I read, the reason for the roller rocker arms is that a more agressive cam profile (different ramp) can be used.

Apparently, cam on bucket as used in production vehicles has a poorer "pratical" limit for the valve opening rate.
I initially was very surprised at the change of direction taken in valvetrain design, but I am guessing that there must be a legitimate reason, as the engine (2GR) surely must be more expensive to build than the old 1MZ.

Sorry for the tangent, the question is at face value very simple, but in fact, there are many overlapping facets tangents and different priorities that can be considered.

Edit:
To clarify, the term "rocker" that I used was not meant to imply that there is a rocker shaft.

[ViPeR_NiPPleX]

hmm, maybe i should look into modifying some 5mge 'rockers' to rollers if toyota thinks its the go

[wagonist]

Why is a 3S-GE or 4A-GE more powerful than a 3S-FE- or 4A-FE?
They're all twin cam 16 valve engines.

GE heads have cams which are individually driven.
Therefore:
The cams can be further apart meaning you can fit larger valves
The cams can be built smaller, especially as one on the FE head has to transfer drive load through to the other cam
There is less friction from the belt driving a cam than from the metal cogs on the scissor gear arrangment.

Now both of these are more powerful than a SOHC because they both have more valve opening area (the single biggest reason) & the spark plug position produces more efficient combustion.

There are SOHC 4v/cyl motors.
eg the early Subaru Liberty came with a SOHC 16v 2.2. The top spec Jap Legacy had a 2.0 DOHC 16v.
The 2.0 DOHC has, in factory form (& even when run in Oz conditions) about 15% more power than the 2.2
Why?
The cam/valve friction loss is the same, the DOHC has more friction from driving 2 extra cams, but the positioning of the spark plug in the "top" middle of the combustion chamber, instead fo sticking at an angle through the side means its can be tuned for more efficient combusition.

http://auto.howstuffworks.com/camshaft.htm

[CrUZida]

Why is a 3S-GE or 4A-GE more powerful than a 3S-FE- or 4A-FE?
They're all twin cam 16 valve engines.

For the simple reason they are tuned differently, and have different profiles on the cams.

[mic*]

And FE valve angle supports mid range torque & "economy style" driving whereas the greater angle on the GE heads supports high flow & top end power - "performance style".

[CrUZida]

And FE valve angle supports mid range torque & "economy style" driving whereas the greater angle on the GE heads supports high flow & top end power - "performance style".

Proof of this?

[Nim]

GE head allows for larger ports. It also allows the port to have less of a curve to the valve, making for less turbulence in the head. Also, they allow for less crap in the head too which means less weight to move and less friction.

However, a head doesn't HAVE to be wide angle to be performancy. Having a wide angle just makes it easier. As stated previously, the SR20DET is a narrow angle head, similar to a Toyota FE head.

ViPeR_NiPPleX, you have lash adjusters don't you? Not rockers?

[YelloRolla]

I think that the reasons behind why twin cam is preferred is well established in this thread.
If you are running agressive cams with a SOHC arrangement (IE copy the cam profile as suggested in an earlier post), then you will need to run killer valve spring pressures to control the valve train. This takes power to do so and further reduces the reliability of the rocker assembly.

SR20 valve trains are not very reliable at high RPM.

[roadsailing]

SR20s are DOHC, so are 4AFEs

DOHC doesn't always mean bucket and shim, and plenty of high performance engines use rockers of some form (M3 BMW for example)

my thought is that DOHC is just a simpler way to do it.

also, with SOHC it is impossible to adjust inlet and exhaust cam timing independantly, which is a real problem when trying to tune your powerband where you want it.

I can't think of any 4 valve engines that use SOHC, so i'd say it boils down to "why 4 valve and not 3 or 2 valve heads"