Sohc/dohc

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Is there any performance difference?
Would a 250hp DOHC perform the same as a 500hp SOHC? Or is there no ratio?

posted by  Kardon

first of all do you know the difference between dohc and sohc?

posted by  Import-tuner

Not really, I thought DOHC was the standard, then I was browsing and saw that the Mustang is a SOHC. So I'm not sure of the difference. :oops:

posted by  Kardon

SOHC = single overhead cam (1 cam)
DOHC = Dual Overhead Cam (2 cams)

sohc = less valves = get air in/out semi-fast
dohc = more valves = get air in/out faster,

car companies generally use SOHC to make cars cheaper.

posted by  Godlaus

Sooo... A SOHC engine with a four valve head has less valves than a DOHC engine with a two or three valve head? How exactly does that work?

Think, type, submit.

posted by  vwhobo

DOHC and SOHC are generally just for timing... my DX civic is SOHC, and it has just as many valves as an SI DOHC civic... you just get a little more controll over timing with DOHC than you do with SOHC

posted by  mazda6man

And how exactly is it that you "get a little more controll over timing with DOHC than you do with SOHC"? I think a factual explanation of the alleged principle involved is in order here.

posted by  vwhobo

you can turn one cam without it affecting the others position... with a single cam, you turn it, and both intake and exhaust is affected... so say your off a feww degrees with say the intake timing cycle, but youre alright with the exhaust timing, you can mess with the intake to fix it and not bother with the exhaust

posted by  mazda6man

I was generalising, my bad. A Ford gt uses somehting like 32 valves off a pushrod, but was speaking in general terms.

Welcome back, by the way.

posted by  Godlaus

That's a stretch at best. You're assuming that he's talking about a modified engine... As an engine would have to be to have adjustable timing gears/camshaft sprocket. Why can't we simply modify it by using a different cam? Same results, different approach and you "control the timing" in exactly the same manner.

Bottom line is you have no more "control of the timing" with a DOHC engine, you simply make the adjustmants in a different way.

posted by  vwhobo

Hahahaha, you don't have more control over timing with a DOHC, why would you want to change the timing from one cam to another anyway... that would be pointless.

Every post in this thread has been WRONG except vwhobo's posts.

The difference between the two obviously is one has two cams when the other has one, both have same amount of valves.

Mustang only has 3 valves by the way so that's a bad example.

Okay, the reason that DOHC is better is the head design, SOHC is limited to the design of the combustion chamber, and is not able to create a very efficient chamber, because the cam sits between the exhaust and intake valves so it's pushing away. DOHC has a cam for intake and exhaust and is able to push the valves the other way (towards the inside of the combustion chamber) making it more efficient.

Hope that was explained well enough, I ain't no teacher.

posted by  88GrandPrixSE

Now including yours as well. There are several reasons you might "want to change the timing from one cam to another". The most obvious being adjusting your powerband by varying the LSA.

BTW, your reasoning for why DOHC is better than SOHC is, in a word, wrong. While it has some merit in a theoretical kind of way, in the real world there are many ways manufacturers have found to overcome this minor nuisance

posted by  vwhobo

a correction here. the mustang is not really an SOHC.

if it was an SOHC, then it would have the camshaft on top of the valve head. the mustang is an OHV type of engine, aka Pushrod.

OHV = OverHead Valves

in this case, the camshaft is located right on top of the crankshaft case, and it moves the valves by pushing these "sticks" that are called pushrods. quite simple.

also, not all mustangs have just 3 valves. also, the number of valves really doesnt have anything to do with the timing of the cams. they just affect the performance delivered by affecting how well the engine breaths. engines with fewer valves have even bigger sized valves in place.

also, an SOHC engine does NOT have a limit to the size of the cam or anything. in fact, i have never seen a camshaft thick enough to be compared to the width of a hotdog inside of a bun and napkin (not counting the displacement of the lobes, just the center shaft). in the newer VR6 engines, the intake camshaft controls the intake valves on the opposite side of the engine via some extended shafts that come out of the rocker arms, and vice versa with the exhaust camshaft. this can be done with a simple SOHC engine, in fact, it would be even easier. how? simple, cuz this is how pushrod engines literally work (differently, but they "push sticks to open the valves", in simple terms).

posted by  Inygknok

A litle correction here... The Mustang hasn't had a pushrod engine since '95. The standard V8 Mustang since then has been the 4.6 liter SOHC (yes, overhead cam) engine, with the later Cobras getting a DOHC version.

Man, why is it that people can't even get the basics right?

Oh, and while technically, pushrod engines are OHV, so are SOHC and DOHC engines! In all cases, the valves are "above" the piston, i.e. overhead.

And Kardon, as to your original question,



Think about it... In the same car, would a 250 hp engine of ANY sort perform like a 500 hp version? If they did why would having more power in the same car be any better?

With the same gearing and same powerband, a 250 hp engine would perform like a 500 hp engine only if the car it was in was HALF the weight of the car that the 500 hp engine was in. Basic math. Has nothing to do with how the valves are actuated.

posted by  ChrisV

Actually, no they haven't. A DOHC engine will always make a bit more power than a SOHC equivalent. They're close, yes, but DOHC still does make more power. Might only be another 10hp on a 200hp engine, but still an improvement no matter how you look at it. You're much more limited to how you can design the heads of a SOHC over a DOHC.

posted by  88GrandPrixSE

No. How the valves are actuated hasn't got any bearing on it. Only the actual cam lobe profile, how many valves are actuated, compression ratio, porting, timing, displacement, etc. Hmm. In fact everything BUT how many cams it has.

Think about it for a minute: teh lift, duration, and overlap of teh valves is teh key. Separating the intake lobes from the exhaust lobes (so they are on two cams instead of one) won't let the engine make more power, only the actual SHAPE of those lobes. ANY engines you want to use as an example will have differnt lobe shapes, too, so that what is making the power is teh lobe shape, not how many cams it has.

The primary benefit of having the lobes on separate cams is teh ability to tune them separate. But at the point where they are optimized on separate cams, you made a single cam that duplicated that timing, the engine would make identical power with that single cam as it did with the dual cams.

posted by  ChrisV

i believe ppl here know that the valves are located on top of the piston area. wat i stated about the OHV ordeal was something a bit formal in the way that pushrod engines are classified as OHV in sites and stuff, as not to cause confusion, but im quite aware of the simple facts, and i was hoping other ppl are aware of it. simply put, actual "Pushrod" engines, not just any motor that pushes rods, i mean specifically PUSHROD engines (just in case somebody around here doesnt understand im just talking about engines classified as pushrod engines n not just anything that pushes a rod, or else males would be pushrods too) have their cams on top of their crankshaft case while the valve head is all alone up there.

PS: my bad bout the newer stangs.

posted by  Inygknok

Damnit!!I can't find a picture online because I don't remmber what exactly it is called, but:

Not all engines are overhead valve desings. I was reading a recent edition of Bike magazine (or maybe it was two wheels only...), and in it they had a list of different engines and the way they worked. Of course they had the classic piston and rotary engines, then they had a few other engines like the Veselsovy rotary/piston engine:http://www.machaon.ru/tetra/veselo.htmAnd an engine driven only by compressed air:http://www.abc.net.au/newinventors/txt/s1072065.htm

Then they had an enine which was set up much like a conventional piston engine but the combustion chamber was underneath the piston, so that the piston was pushed upward to spin the crank shaft. Since the combustion chamber was underneath the piston, so were the valves. Thus the engine wasn't an OHV design. The valves were opened and closed by special lobes on the crankshaft. Since there was no need to have a head, this engine was very small, which was the point of inventing it.

They also had another rotary pisto engine in which the piston was placed on the same centerline as the crankshaft, and instaed of having the combustion simply push the pisto down the cylinder, it used a set of grooves in the cylinder walls to make the piston spin. OK I know how I said that was confusing...basically the force of the explosion pushes the piston down, but since the grooves that the piston are set in wont allow it to go down it spins, and in turn spins the crankshaft...Get it? I suck at explaining stuff.

Sorry for making you read all that nonsensical dribal, but I found it very interesting.

posted by  Zalight

Of course not all engines are overhead valve engines. Whoever thinks that that is true must be stupid. But you guys sure love bitching huh? Like women... I have a real arguement for you guys; who'll win Batman or Spiderman? And please dont have the comeback of me saying you all bitch like "We dont bitch, we are merely intent on correcting others mistakes something u obviously don't have the capabilities of you nimrod. So please think before you type" because that is really gay and lame.

posted by  VMJYogi

It's called a 2-stroke, T head, flat head, L head...

Chris V:

SOHC limits the design of the head, and as I'm positive you know, the head plays a great role in the power an engine is able to produce. If it was just lift that limited the power, they could easily change that, they could make the lobes for the intake more aggressive and leave the exhaust however they wanted it. Or they could just increase lift all the way around. That's not the limiting factor of SOHC. If it was, why don't they just put a more aggresive cam in them?

posted by  88GrandPrixSE

Did you read my whole post?



Thats not how a 2 stroke works, not even close.

posted by  Zalight

No, no i didn't. I highly doubt there was ever an engine invented that ignited fuel under the piston, for, that just wouldn't work. It raises questions:

1. How was it ignited? Piston would hit spark plug if it was in the wall
2. What about the oil and crank over-heating?
3. How does it suck fuel in/push it out? It's not trying to push it at all.
4. How does it compress it to ignite it?

Also it would be so inefficient it's not even funny. It would be building up pressure in the the hole block.

I could be wrong but it just doesn't make logical sense to me.

Besides, you can build a two stroke smaller than anything. Hell, R/C cars generally have a .12ci (3.5cc) motor.

Also there is no piston in a rotary motor, it's called a rotor. The groves don't make it spin either. It's the way the fuel is ignited. They have two spark plugs, one at each end of the combustion chamber, they ignite the fuel just closer to the end of the compression stroke, just before the power stroke, so the far plug is more compressed than the ealier one, which creates a circular motion because of the length of the combustion chamber. It's a very simple motor, quite a bit simpler than the conventional piston engine, just they're very in-efficient due to low compression. Though I do believe if they put the money into the wankel that they did the piston, they wankel would be the better engine.

posted by  88GrandPrixSE

I had no idea so many people had so little knowledge on DOHC :mrgreen: I really liked the pushrod DOHC humour.

Here's one reason F=ma, there are another two very compelling reasons and a few other minor ones.

posted by  Wally

argh, didnt know ppl were gonna be such a bitch about things. ok, if we are going to include engines that even work on weed whackers (which are usually 2-stroke.... though ive never seen a 4 stroke one in my life) then no, not all engines have their valves over the head. old steam engines dont even have valves (as far as i remember at least). a train's engine doesnt have valves either (maybe new ones do since im not a train expert). in fact, many other engines actually operate via actuators that are not referred to as valves to handle the whole process of intake and exhaust.


geez.... thats the problem in this forum, everyone gets so bitchy about technicalities and specifics in general when the subject originated from something completely different.

as for igniting in engines, diesel engines dont really ignite the fuel, the compression just makes the air/fuel mix ignite due to the high compression.

as for the 2-strokes in mopeds and such, they do get ignited with a spark (im betting a great percentage of them, if not all of them), but part of the mix actually goes around the engine itself while some of it stays on top of the piston, n then goes out. they have small levers that act as valves (they are called read valves, or reed valves, or something), n if i remember correctly, they dont have a valve at the exhaust port, most of the time i believe, since the piston itself handles when the gases will exit since it performs everything in 2 strokes (goes up once to take in everything, then goes down and everything goes out while allowing more to come in).


as for the rotary piston engine thing zalight explained..... wat the hell?


lets see, another technicality since ppl seem to wanna bitch this week about changing the categories, from low category of 4-stroke engines with valves from the modern days (or pre-modern) to any engine ever created.

electrical engines..... whoever says something like that needs any sort of fuel or anything by themselves.... well u know the rest.

hydrogen powered engines.... i actually still havent bothered reading how they worked, i did once with the BMW's that were being produced, by i didnt pay any attention. so, someone else can take care of that.


everyone happy now?

posted by  Inygknok

In street engines, NONE of them are optimized, otherwise no one would be able to make more power from their stock vehicle.

But, the point we are discussing is now head design. SOHC limiting the head design? With the cam above the valves OR in the center higher than the valves, the port size and shape can easily be identical, whether comparing 2 valve per cyl or 4 (and yes, there are 4 valve per cyl SOHC engines, jut like there are 2 valve per cyl DOHC engines).

But since you brought it up, please show how the placement of a single cam above the valves vs two cams above the valves "limits" the port size or shape in any way. What is it about the CAM number that limits the head? I'm really interested to see you work your way out of this one.

posted by  ChrisV

Haha thats funny. :driving:

Note: Yeah there are four-stroke leaf blowers, snow blowers, weed whackers. For the really tough bush!

posted by  VMJYogi

88GrandPrixSE the number of valves and roof design in a SOHC can be pretty much the same as a DOHC.

If say you have 4 valves per pot on a SOHC the room for cam lobes is pretty tight and generally do not line up with the valve stems (the intake and exhaust stems are usually opposite each other). Therefore the rocker arms are offset angled. Have you ever noticed how much more effort and less stability you get when you try to tighten a nut with the spanner lifted at an angle to the the nut's flat? At the same time have you noticed how your grip deforms because the effort is concentrated on a smaller part of you palm?

posted by  Wally

God damnit, dude, READ my post before you criticize it.

I wasn't talking about the wankel rotary, I was tralking about a completly different engine....I know how a wankel works (hmm, notice the sig?) This one was invented by some Italian guy.

posted by  Zalight

rofl really??? kinda makes sense though... even though that would be one mighty strong weed whacker :D




i dont think he even bothered reading wat i said about the 2nd gen VR6 engines n how they operated the valves despite the location of the valves vs location of the cams.

posted by  Inygknok

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