Whats a "cam" (DOHC, single cam, that stuff...)

Yeah so whats a cam in a car, cos after watching videos on youtube i see comments talking about single cam cars and DOHC, i learned a while ago that DOHC stands for dual overhead cam, but what is that supposed to mean? Thanks for the replies.

Excellent, simple description of the principles and configurations:
http://auto.howstuffworks.com/camshaft1.htm

First person to claim one is better than the other gets banned for a day.

reed valves are best *neener*

reed valves are best *neener*


Thats a mighty big pair of brass balls your carrying tonight Uncle. :shakehead

:lol: :lol: :lol:

First person to claim one is better than the other gets banned for a day.

What if we say OHV is better? :biggrin:

What if we say OHV is better? :biggrin:


You can get away with it, only because I know you know what your talking about :evillol: :evillol:

First person to claim one is better than the other gets banned for a day.

Can I vote for pushrods?:grinyes:
Simply so you can fit a bigger engine in a smaller place.

Two stroke FTW.

What will your gods do now?

Ok im still completely confused. But ill explain what i grasped and you tell me if im correct.

There are valves that take in air/fuel mixture, from previous knowledge i remembered that there are at least 2 valves per cylinder, intake and exhaust valve. So a cam is what makes the valves take in mixture and exhaust it. So a single cam is only 1 cam working to take in air/fuel and exhaust it. And the DOHC has 2 instead of one, and the reason 2 is makes it faster is because the faster the car goes the more air/fuel it needs, and DOHC does the job faster allowing air/fuel to be taken in a exhausted out.

Am i right?

Ok im still completely confused. But ill explain what i grasped and you tell me if im correct.

There are valves that take in air/fuel mixture, from previous knowledge i remembered that there are at least 2 valves per cylinder, intake and exhaust valve. So a cam is what makes the valves take in mixture and exhaust it. So a single cam is only 1 cam working to take in air/fuel and exhaust it. And the DOHC has 2 instead of one, and the reason 2 is makes it faster is because the faster the car goes the more air/fuel it needs, and DOHC does the job faster allowing air/fuel to be taken in a exhausted out.

Am i right?

The advantage of twin cams (DOHC) is that you can put the valves on vastly different angles, it gives a lot more freedom in how you want to shape the combustion chamber.
It can also make it easier to change intake timing without affecting exhaust or vice-versa.

two over head cams also allows an easier and cheaper packaging route for variable valve timing and lift mechanisms. basically your statement is based on good information, but you kind of got the wrong conclusion from it. every part in a car is a comprimise. the most powerful engines also are the most complicated and expensive. thats why people still modify stone age two valve pushrod engines. because you cant get a 350+ hp DOHC 4 valve variable valve timing engine without spending easily twice as much if not much more. and sacrificing reliability to a degree. (more parts always = more broken parts at some point).

so yes as a purely theoretical measure, DOHC is better. but in reality, each engine is better for some people.

two over head cams also allows an easier and cheaper packaging route for variable valve timing and lift mechanisms.


Honda would disagree.
The vast majority of their VTEC engines are single cam.



so yes as a purely theoretical measure, DOHC is better.


Bob, Curtis, what do you reckon, 1 day ban or not? :evillol:

oh cmon. you know im right.

oh but i didnt mean to say dohc is better for VVT. i meant over head cam engines are better for VVT, as opposed to under head cam engines.

oh cmon. you know im right.


Only if you can justify it.



oh but i didnt mean to say dohc is better for VVT. i meant over head cam engines are better for VVT, as opposed to under head cam engines.

I would disagree with that.
I think it is simply that more and more modern engines are overhead cam, and it is the same modern engines that also have VVT systems fitted.
But they are not built as over head cam engines just so they can be fitted with VVT systems.

Only if you can justify it.




I would disagree with that.
I think it is simply that more and more modern engines are overhead cam, and it is the same modern engines that also have VVT systems fitted.
But they are not built as over head cam engines just so they can be fitted with VVT systems.


#1. im talking purely theoretical, with efficiency and flexible output being the only goals. in other words, not in the real world.

#2. how would you incorporate VVT into a pushrod engine? why arent more engines using it? i mean the 3800 series III is still pushrod and they dont have any sort of VVT system. the viper is the only pushrod VVT system i know, and its only variable timing on the exhaust, with no lift control.

There was something in motortrend about GMC offering VVT on the small block v8's now.

Honda VTEC system would be a little complicated in a pushrod engine, but variable valve timeing simply needs a solenoid fitted between the cam and its drive mechanism, usually as part of the cam gear.
That is very easy to do on a pushrod motor.

If the Viper engine is a single cam, push rod engine, then the variable timing will effect both exhaust and inlet events :)


In purely theoretical applications how far the valves open, and how long, is more important than what is used to open them, and I have yet to see the performance of a pushrod engine limited by its pushrods.

I believe the biggest reason multi over head cam engines only become better when you start to use more than 2 valves per cylinder.
The more valves you have the more cam lobes you need and the more connections you need between the cam lobes and valves.
Could you imagine the cam and block design you would need to run a 32 valve V8 on a single, in block camshaft?

Honda VTEC system would be a little complicated in a pushrod engine, but variable valve timeing simply needs a solenoid fitted between the cam and its drive mechanism, usually as part of the cam gear.
That is very easy to do on a pushrod motor.

If the Viper engine is a single cam, push rod engine, then the variable timing will effect both exhaust and inlet events :)


In purely theoretical applications how far the valves open, and how long, is more important than what is used to open them, and I have yet to see the performance of a pushrod engine limited by its pushrods.

I believe the biggest reason multi over head cam engines only become better when you start to use more than 2 valves per cylinder.
The more valves you have the more cam lobes you need and the more connections you need between the cam lobes and valves.
Could you imagine the cam and block design you would need to run a 32 valve V8 on a single, in block camshaft?
Thats been done often I believe. It's really quite simple, you just make the rockers split into two on the valve side.

The biggest reason for OHC and DOHC that I know of is to reduce the weight in the valve train, if you intend to rev a motor up very high. If the engine can do those revs safely with OHV though, there is no reason to just move the cams for kicks. Alternately, if you have two exhaust or two intake valves, and want the pair of them to run on different profiles, generally multiple cams is required.

Honda VTEC system would be a little complicated in a pushrod engine, but variable valve timeing simply needs a solenoid fitted between the cam and its drive mechanism, usually as part of the cam gear.
That is very easy to do on a pushrod motor.

apparently not. this is the only system in use right now. mechadyne makes it for dodge for the viper. this website is very informative on the topic.
http://www.mechadyne-int.com/vva-media/display-media/SCP-camshaft.swf/400/292/sCP-cam-assembly-animation


If the Viper engine is a single cam, push rod engine, then the variable timing will effect both exhaust and inlet events :)

look at the animation. seems to me only the one set of lobes are variable.



In purely theoretical applications how far the valves open, and how long, is more important than what is used to open them, and I have yet to see the performance of a pushrod engine limited by its pushrods.

I believe the biggest reason multi over head cam engines only become better when you start to use more than 2 valves per cylinder.
The more valves you have the more cam lobes you need and the more connections you need between the cam lobes and valves.
Could you imagine the cam and block design you would need to run a 32 valve V8 on a single, in block camshaft?


what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.

apparently not. this is the only system in use right now. mechadyne makes it for dodge for the viper. this website is very informative on the topic.
http://www.mechadyne-int.com/vva-media/display-media/SCP-camshaft.swf/400/292/sCP-cam-assembly-animation




look at the animation. seems to me only the one set of lobes are variable.



That is pretty clever, I wonder how easy it would be to retrofit to other engines.




what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.


In theory yes, but there are plenty of real world examples that prove otherwise.

what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.
Have you heard of an Australian Institution called the Taxi Race (aka Bathurst )?
The rules tie them down to australian sedan bodies, rwd and pushrod V8's of I think 5 litre capacity. They spin to 7,500rpm and produce a little under 500kw.

I know these figures don't compare with motorbike engines, but they show what's possible with comparitively ancient technology.
They're racing in New Zealand this weekend. It doesn't really interest me though.

http://en.wikipedia.org/wiki/V8_Supercar

The major benefit IMO of pushrods is space. You can fit a bigger capacity engine in a smaller hole.
Even many slow revving diesels (limited to 2800rpm) have gone to OHC recently.

yes. i agree with the above statements. like i said from a purely theoretical performance standpoint DOHC is the best one out there right now. but in the real world and for other goals besides pure performance, there is no definitive answer as to whats best.