Why DOHC and not SOHC ??

[Enchanter]

I know twin cam engines are better than single camers, but why ?
I cant see logicaly why having an extra cam will benefit the engine (apart from being able to adjust them independantly which is beyond the scope of this post).
A single cam has just as many lobes as a pair of twin cams (Iam talking about number of cams too, not number of valves).
I just need to know WHY they are better and not simply accept that they are.

Cheers

[ed]

valve and port angle, and consequently conbustion chamber design and combustion dynamics

[MR22ZZ]

independant timing on the inlet and exhaust valves

[Nim]

valve and port angle, and consequently conbustion chamber design and combustion dynamics

Actualy, I think you'll find it's not. A closer angle head actualy makes for a better combustion chamber.

However, in a way that's right. A wider angle on the head means you can have larger valves and ports, and allow for less curve of intake runners (which reduces turbulence).

The main reason though, is to reduce weight, stress and friction of moving parts.
This means the engine can have more revs with less friction in the head.

More info:
http://www.billzilla.org/2v4v.htm

Bill Sherwoods site is awesome. I suggest you read the whole thing.

[Enchanter]

Actualy, I think you'll find it's not. A closer angle head actualy makes for a better combustion chamber.

However, in a way that's right. A wider angle on the head means you can have larger valves and ports, and allow for less curve of intake runners (which reduces turbulence).

The main reason though, is to reduce weight, stress and friction of moving parts.
This means the engine can have more revs with less friction in the head.

More info:
http://www.billzilla.org/2v4v.htm

Bill Sherwoods site is awesome. I suggest you read the whole thing.

I have read chunks of bill's site before, dam good reading.
But he really only talks about why 4v per cyl is better than 2v per cyl, pushrods vs ohc etc, not the advantages of a second camshaft. My thinking is a second camshaft is creating MORE weight and friction than the same setup with a single camshaft. There are after all plenty of sohc 16v engines about, but they are never the performance versons and they generally don't rev as well as the dohc variant.
I have heard the BMW M3 is sohc but I may be wrong. This car may be an exeption to the rule, but again I don't know why .....

[Nim]

Naw, the BMW M3 is DOHC.

It does go more into 4 vs 2 vales, but it has some stuff at the start about rockers. Right at the start. Read it.

[o_man_ra23]

Righto, well, its been stated in this thread, but ill reiterate, the lower valve to port angle means theres less curve on the port, and as such, less turbulance... also you can run MUCH bigger valves... Just look at the 18RG compared with the 18RC or 22RE even(which is crossflow too). The 18RG's valves are the size of 351 cleveland valves (ive compared 88230 valves with the clevo valves... pretty close to identical face size)... Where as the 18RC and 22RE both have pissy little valves, and not too much room for bigger ones (the 18RG can get stupidly big valves). On top of this, a twincam head allows you to remove the rockers, pushrods, lifters, tappets, and anything else that may move independantly and create more problems... you just run a bucket and adjusting shim on top of the valve to actuate the valve from the camshaft. THIS reduces the valve train weight significantly, and reduces friction more than the additional friction of the second shaft. From here, because there is less room for slop, you can run tighter clearances, and harder valve springs, again allowing for higher RPM.

There are many complex reasons for running TWIN CAM... or DOHC, but all of them are logical, and all of them lead to a higher revving motor. And generally SOHC, and pushrod suck the big one.

Cheers, Owen

[Nim]

SOHC isn't as bad as pushrod! That's for DAMNED sure.

Think of this. An extra cam is allways spinning in the one direction. The force required to keep it spinning is pretty low, and the friction it creates is also low.
A rocker must slap back and foward pretty fast. The force required must first speed it up, then it has to stop, then it has to move back again. This creates a lot more friction and takes up more power etc.

Now, if I may side track for a moment... one interesting thing about the Honda VTEC and Toyota VVTLi engines is that they have lash adjusters in the head. I hear these aren't as bad as rockers, as a rocker creates lateral movement, where a lash adjuster doesn't (or something like that). They're also not as big and heavy as rockers. The reason they have lash adjusters is to run the variable lobes which far outweighs the loss of having lash adjusters. The Nissan SR20 engine, however, runs lash adjusters for... seeming no reason.

Just think about it for a moment though. Think about how a SOHC engine works... visualise it. Then visualise a DOHC engine... it just... FEELS better. The two cams working directly on the buckets, rather than using this big old lumpy hunk of rocker...LOL. Perhaps I'm going crazy.

[tricky]

Lash adjusters for seemingly no reason? Not really. There are HEAPS of different DOHC engines with hydraulic lash adjustment. The reason? To... Adjust the lash. It seems some designs aren't to keen on the limited adjustable lifetime of shims. Also, this design doesn't require the removal of the cam cover in a thorough service (less labour, no gasket cost).

[Nim]

I think Nissan was out to make the SR20DET as cheap as possible. So yeah, that makes sense. I kinda meant performance reasons though.

[oldcorollas]

yes you are going crazy.
main benefit (apart from adjustability, as eccluded at top of thread) is lack of rockers = less valve train weight/complexity.
the SR20 is an abortion of a top end...

[takai]

And yet it still flows like a bitch. The SR20 in NA form is actually a really nice engine.
EDIT: Pity its a Nissan.

[Rinmax]

If you guys take a look in under 2 ltr sports sedans at the moment there are heaps of guys running the SR20 but sod all using the 3SGE or other toyota engines, so lets not flame the SR20 too much its a fine engine. Take a look at the new Alloytech V6 from Holden, DOHC, but its still got rockers. Personally i think it is easier and quicker to set up than the bucket and shim job on the Toyotas, so I belive its to help out in servicing.

As for the why use DOHC instead of SOHC, I look at the rally Escorts from the 70's and early 80's. Ford hade both Lotus (early) and Cosworth (late) have a go at what was their SOHC Pinto engine. Biggest advantage was the angle of the valve giving a better flow while allowing larger valves. These engines were what dominated the sport before the onslaught of turbo 4WD.

[Hen]

I think I am reiterating the above people here, but I think there is no huge golden reason why twin cam is better. You could set up a great twin cam engine (duration, overlap, lift etc), and then theoretically grind a single cam to match the setup and install it with the necessary rockers (and other junk) so that it can actuate the valves. Performance should be near on identical.

It is just that you have the extra hassle, weight and losses of rockers and the associated hardware. And you miss out on the independant adjustability (short of grinding a new cam).

Plus I think most people naturally believe twin cam is better because the twincam versions of most engines was the "sporty" version and so had changes other than the second cam to make it more powerful/revvier (eg 2T vs 2T-G)

In any case, single cam engines can make damn good power (eg Datsun L20s and Peugot 205 GTI motors).

Hen

[wagonist]

One of the really big advantages to a twin cam is the ability to stick the spark plug right in the middle top of the combustion chamber instead of off to one side because there's a camshaft in the way.
This gives more even combustion, less chance of detonation, etc.
That is why the twin cam escorts, celicas etc of the 70s were started. The plug may not be in middle, but at least its at the top & not in the side.