Discussion
I suppose that you can have valves in the form 8 8 8 8 instead of 00 00 00 00 which allows a better shaped casting and port design to be employed. (For both OHC designs). With a TC, you would be looking at having dyno variable intake and exhaust timings without looking at cam changing.
That is pretty much it.
Directly actuated instead of semi. I suppose that would take some weight/clearance issues out.
Must be missing something. I wonder what the price difference between single pattern and dual pattern cam grinds are????
That is pretty much it.
Directly actuated instead of semi. I suppose that would take some weight/clearance issues out.
Must be missing something. I wonder what the price difference between single pattern and dual pattern cam grinds are????
I'm a 8V fan ! GM actually made some fine SOHC engines before they made the best 16V Engines in the XE and the LET before recently screwing it all up with The EcoshiiTEC.
I would also point out that the Mk1 Astra GTE and Mk2 Cavalier SRI share the same 1.8 8V and lets face it they should have stopped there. You don't get much better really.
I would also point out that the Mk1 Astra GTE and Mk2 Cavalier SRI share the same 1.8 8V and lets face it they should have stopped there. You don't get much better really.
5USA said:
DOHC. far better for emissions as well as performance.
I commend to you the General Motors LS1 engine in the Corvette. 350 bhp, 350lb/ft, fully emissions compliant....
....and push-rod.
yes, but it is far easier to control one cam for intake then one for exhaust than one cam for both. not saying that a single cam won't do both, just that it's more difficult. all credit GM
I would say that almost all modern engines are DOHC (and four or five valves per cylinder). I suppose an SOHC would be cheaper (fewer parts, less compex) but DOHCs offer clear performance increases, so presumably market forces are doing their thing. Dunno about emissions. Presumably the performance gains can be had from greater valve areas and lower valve mass/inertia for a given valve area permitting higher RPMs.
Interestingly the Honda VTEC system can be configured to use just one of the pair of valves at lower RPMs/throttle openings to increase air speed through the valve promoting fuel/air mixing and improving fuel consumtion and emissions.
Kingr
Interestingly the Honda VTEC system can be configured to use just one of the pair of valves at lower RPMs/throttle openings to increase air speed through the valve promoting fuel/air mixing and improving fuel consumtion and emissions.
Kingr
At the risk of sounding excessively geekish:
DOHC enables an engine to have hemispherical combustion chambers, which gives the best design compromise for combustion efficiency, thus leading to greater power. It is possible to use pushrods to operate a valve arrangement leading to a similar combustion chamber design, but this has gone out of favour.
DOHC v. SOHC is of course quite a different argument than that alluded to above regarding single versus multivalve designs.
DOHC enables an engine to have hemispherical combustion chambers, which gives the best design compromise for combustion efficiency, thus leading to greater power. It is possible to use pushrods to operate a valve arrangement leading to a similar combustion chamber design, but this has gone out of favour.
DOHC v. SOHC is of course quite a different argument than that alluded to above regarding single versus multivalve designs.
SOHC - fewer moving parts, fewer parts for the engine to drive = less valvetrain noise, less flywheel effect, less expensive if your cambelt goes!
DOHC - what the bloke above said about combustion chambers I guess
The Lotus twin-cam is DOHC but with only 8 valves (ie 2 valves per cylinder), and this in-between stage by my experience behaves somewhat more like a DOHC 16v (4-valves per cylinder) than an SOHC 8v.
Most of Mercedes' current line-up is SOHC but with 3-valves per cylinder (= 12 valve four-cylinders, 18 valve V6s), another sort of crossover. I'm not sure why, but since they already had developed and used 4-valve per cyl engines all through the '90s, maybe they reverted to this because of cost, or because you get less noise and drivetrain stress combined with still good combustion with the 3 valves rather than 2.
?
Russ
'86 190E SOHC 8v, '62 Elan DOHC 8v
IMHO
DOHC - what the bloke above said about combustion chambers I guess
The Lotus twin-cam is DOHC but with only 8 valves (ie 2 valves per cylinder), and this in-between stage by my experience behaves somewhat more like a DOHC 16v (4-valves per cylinder) than an SOHC 8v.
Most of Mercedes' current line-up is SOHC but with 3-valves per cylinder (= 12 valve four-cylinders, 18 valve V6s), another sort of crossover. I'm not sure why, but since they already had developed and used 4-valve per cyl engines all through the '90s, maybe they reverted to this because of cost, or because you get less noise and drivetrain stress combined with still good combustion with the 3 valves rather than 2.
?
Russ
'86 190E SOHC 8v, '62 Elan DOHC 8v
IMHO
Any valve configuration for an 8v can be achieved with 1 or 2 cams, you just need rocker arms. Obviously this becomes more of an issus with BIG engines like a Jag 4.2.
With TC's, you can drive 2 valves per cam as with a SC, you would be looking at having a weird configuration which would either have loads of lobes or wear problems.
The "Pent roof" chamber is best achieved with a 4V/C and that comes from a TC. Otherwise you can get close with a SC but no cigar.
With TC's, you can drive 2 valves per cam as with a SC, you would be looking at having a weird configuration which would either have loads of lobes or wear problems.
The "Pent roof" chamber is best achieved with a 4V/C and that comes from a TC. Otherwise you can get close with a SC but no cigar.
admiral said:
At the risk of sounding excessively geekish:
DOHC enables an engine to have hemispherical combustion chambers, which gives the best design compromise for combustion efficiency, thus leading to greater power. It is possible to use pushrods to operate a valve arrangement leading to a similar combustion chamber design, but this has gone out of favour.
DOHC v. SOHC is of course quite a different argument than that alluded to above regarding single versus multivalve designs.
Interesting point, but was the Ford CVH (the "h" in this case standing for hemispherical) a SOHC?
Fair point about DOHC not meaning multivalve. It was something that had occured to me, but I should have been clearer.
Kingr
danhay said:Overlap does give the nice burble as un-burnt exhaust gases explode in the pipe, but overlap can be achieved with single (pushrod or OHC) and double cams. As has been said DOHC allows for a free design of combustion chamber, usually hemispherical. Multi valves (as opposed to SOC or DOHC) allow a larger area of valve opening to be achieved within the same size of head. E.g. by increasing the inlet valve size to increase performance, eventually the valve would be bigger than area available within the combustion chamber, two (or three) inlet vales allows smaller valves but gives a bigger opening. Rich...
I understand that 2 valve per cylinder motors sound nicer...something to do with valve timing overlap?
tuscan_thunder said:
think most are DOHC. far better for emissions as well as performance. can't imagine cost would be significantly different
love machine said:
Must be missing something. I wonder what the price difference between single pattern and dual pattern cam grinds are????
Generalisations like DOHC being far better for emissions are quite dangerous. One could argue that there is more valve-margin area potential unburned HCs to sit on with a 4 valve engine. On the other hand one could also argue that Variable cam phasing is also not possible with a SOHC so this is a real hitter for emissions. Advancing the cam timing in many cases allows lower Nox because exhaust residuals are recycled back from the exhaust to the intake- lowering combustion temperatures- while HCs can go down as some of the unburned HCs are also recycled and reburned. It’s a difficult one- but I would say the emissions benefit in terms of a SOHC 2 valve engine versus a DOHC 2 valve engine is minimal- as there are bigger fish to fry in the scheme of things.
Cost is much easier- DOHC is MUCH more expensive then a SOHC system- even when you ARE just comparing 2 valve versus 2 valve. When the DOHC engine in question is a 4 valver- then prices are driven even higher.
kingr said:
Interestingly the Honda VTEC system can be configured to use just one of the pair of valves at lower RPMs/throttle openings to increase air speed through the valve promoting fuel/air mixing and improving fuel consumtion and emissions
In tests I have seen with a pent roof engine- there is very little difference in terms of WOT performance when one of the valves are blanked off completely even up to 3000 rpm. If done properly- using the two valve mode allows more of a swirl motion within the combustion chamber- which could allow use of more ignition retard in the warm up phase- to get heat out of the exhuast and warm up the catalysts. During part load operation- this set up could also tolerate high amounts of Exhaust Gas redirected back INTO the combustion chamber- something that is good for both fuel economy ( on modern catalysed cars) and Nox emissions.
pentoman said:
The Lotus twin-cam is DOHC but with only 8 valves (ie 2 valves per cylinder), and this in-between stage by my experience behaves somewhat more like a DOHC 16v (4-valves per cylinder) than an SOHC 8v.
Most of Mercedes' current line-up is SOHC but with 3-valves per cylinder (= 12 valve four-cylinders, 18 valve V6s), another sort of crossover. I'm not sure why, but since they already had developed and used 4-valve per cyl engines all through the '90s, maybe they reverted to this because of cost, or because you get less noise and drivetrain stress combined with still good combustion with the 3 valves rather than 2.
Actually I will contradict this- a two valve twin cam engine versus a two valve SOHC engine with good port design and well optimised rocker arms will behave similarly. An example being the twin cam Alfa engine versus the classic M10 BMW “2002” engine. Both cylinder heads flowed very similarly. But here we’re comparing an excellent engine (Alfa) with an exceptional one (BMW)(in their time). BMW didn't resort to the added weight and friction and complexity of the twin cam layout , however the Alfa has more potential for tuning, more flexibility - as you can muck about with the cam timing independently (just like YOU can with your Lotus) and they went to the expense of an Alloy block- so for the enthusiast tuner- it's obvious which engine looks more attractive! Four valve raises the plane totally, partly because of valve area and partly due to favourable plug postion- which has a HUGE effect on combustion burn rates.
I think Daimler Benzs driver for the 3 valve lay out was emissions- or so they quoted publicly. They said that the single exhaust valve allowed less surface area and flow speed – thus not dissipating the heat for the vitally important catalyst warm up stage. It is CERTAINLY true from work I’ve looked at that flow speed is a HUGE variable on cooling. So a bigger low velocity exhaust port would conduct less heat to the port walls/cyl head even though the surface area was larger. [ This is why air cooled Porsche 911s have unusually large exhaust ports for a given exhaust valve size]. The other thing is that the twin plug arrangement allows a lot of ignition retard –again during the critical catalyst warm up phase. So all you people brainwashed into buying new cars which are “better” can be safe in the knowledge that you cars are being optimised to run very very inefficiently JUST to get the catalysts warmed up
As for NVH? Have you ever put a microphone up to a Mercedes engine and recorded it- while comparing it’s NVH waterfall plot to its contemporaries? (Ok…well I don’t suppose you would have
) Well lets just say the words excellent NVH optimisation aren’t what spring to my mind! RichB said:
danhay said:I understand that 2 valve per cylinder motors sound nicer...something to do with valve timing overlap?
Overlap does give the nice burble as un-burnt exhaust gases explode in the pipe, but overlap can be achieved with single (pushrod or OHC) and double cams. As has been said DOHC allows for a free design of combustion chamber, usually hemispherical. Multi valves (as opposed to SOC or DOHC) allow a larger area of valve opening to be achieved within the same size of head. E.g. by increasing the inlet valve size to increase performance, eventually the valve would be bigger than area available within the combustion chamber, two (or three) inlet vales allows smaller valves but gives a bigger opening. Rich...
Actually the valve overlap is often responsible for popping and banging on the over run from unbunt fuel NOT burbling. This popping on over run- although “fun” is really due to bad calibration and control of fueling. Burbling is a different phenomenon and usually comes about from uneven interactions from close firing cylinders. A classic example is the archytypical cruciform crank V8. The valve event that often gives a RICHER exhaust note is exhaust valve Opening. In this regard a 2 valve DOES often sound richer because it IS more to do with the TIMING of this event rather then the flow area.
A common mis-conception for four valve engines is to say that they are inherently peaky because they are not. It is true though that 4 valve applications are often for sportier engines that rev so have been optimised to give better top end, but it doesn’t necessarily have to be that way. An example of an exceptionally torquey 4 valve engine is the outgoing M5 V8 which makes almost 12 bar BMEP from 1600 rpm onwards.
The thing that a pent roof 4 valve really allows is more valve area from a given compact combustion chamber. The air cooled Porsches could not go toward the 4 valve route due to cooling issues- which meant that had to go very oversquare- this lead to an adverse surface to volume ratio for combustion chamber and this was poor in terms of thermodynamics. A pent roof 4 valver could get similar flow for a much more compact combustion chamber- and there for be more thermodynamically efficient. Porsche mitigated this some what by going to twin plugs and raising the compression ratio. This is the other area where a 4 valve engine scored over a 2 conventional 2 valve engine- by being able to have a central plug,
As for which system is better direct acting DOHC or SOHC with rockers- well this is a difficult one.
The DOHC system allows a stiff valve train and rapid valve accelerations but a SOHC system allows a rocker with a good ratio. A well designed rocker has a number of benefits: You can apply a good rocker arm ratio- thus allowing you to package a compact diameter central camshaft- you can have a curved pad on the rocker arm- again allowing more valve lift for a given duration and all this for a little increase in valve system mass ( the rocker arm mass isn’t figured into it as a whole- more like a third of it- depending on the position of its centroid). The SOHC system will also tend to have lower losses in terms of friction. However- the DOHC will typically be able to rev much higher and there’s more flexibility with the cam timing.
>> Edited by Marquis_Rex on Monday 29th November 17:56
>> Edited by Marquis_Rex on Monday 29th November 20:18
I have a question about old Chrysler Hemi Engines thats been bugging me for ages.
They are Hemispherical combustion chambers (hence the name) but I'm assuming (as I've never seen one) that its a single cam push rod engine. So how the hell is it laid out.
Anyone got a photo with the rocker arms in it? Is it a cross flow engine?
And as a side note what's the modern Chrysler Hemi. Is it a SOHC, one per bank or is it a rodder too?
Andy
They are Hemispherical combustion chambers (hence the name) but I'm assuming (as I've never seen one) that its a single cam push rod engine. So how the hell is it laid out.
Anyone got a photo with the rocker arms in it? Is it a cross flow engine?
And as a side note what's the modern Chrysler Hemi. Is it a SOHC, one per bank or is it a rodder too?
Andy
pentoman said:
Most of Mercedes' current line-up is SOHC but with 3-valves per cylinder (= 12 valve four-cylinders, 18 valve V6s), another sort of crossover. I'm not sure why, but since they already had developed and used 4-valve per cyl engines all through the '90s, maybe they reverted to this because of cost, or because you get less noise and drivetrain stress combined with still good combustion with the 3 valves rather than 2.
?
Russ
'86 190E SOHC 8v, '62 Elan DOHC 8v
IMHO
I read somewhere that it was a cost issue.
I got the gist of most of it, and this kind of explanation highlights why some of us are mechanics and some of you are engineers. 
andytk said:
I have a question about old Chrysler Hemi Engines thats been bugging me for ages.
They are Hemispherical combustion chambers (hence the name) but I'm assuming (as I've never seen one) that its a single cam push rod engine. So how the hell is it laid out.
Anyone got a photo with the rocker arms in it? Is it a cross flow engine?
It is. The pushrods come up sort of between the cylinders, and there are two sets of rockers, one set going "forwards" to operate the exhaust valve and the other set going "backwards" to operate the inlets.
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