Pushrod vs. DOHC

[marleyskater420]

Today my friend and I were discussing this.. What is better for all out hp and performance? What are the advantages of both? How the heck does DOHC work besides cam on top?

We were stuck on "if DOHC is so good why doesnt chevy go DOHC LS1's" and "ferrari uses DOHC" crap like that.. I just argued that fact of how much room you have for each valve and if you can get your 1 valve to open as much as 2 valves,whats the point of 2.

Also we talked about flat pistons vs. wedged ones. Why are there different types...care to explain anyone?

 

[jerrad]

DOHC engine's can rev higher. Also you have more valves for the air/fuel to get in through and then the spent gas to get out of. Besides that I don't know.

 

[marleyskater420]

Originally posted by jerrad
DOHC engine's can rev higher. Also you have more valves for the air/fuel to get in through and then the spent gas to get out of. Besides that I don't know.

I know for a fact it isnt because DOHC's can rev higher. Im working w/ a alchohol funnycar driver and he has a pushrod v8 that revs to 10k. So that isnt the case.

I know you have more valves, but cant you just make the one pushrod valve bigger? Can you a 3 valve pushrod?

 

[Sleeper-6]

There have been many chapters written on the questions asked here. I'll try to give the quick and dirty version.

DOHC vs Cam in block : DOHC eliminates the some of the valvetrain components. Therefore you have less stuff in motion and changing directions or force, things become more efficent. With less weight in motion because there are no pushrods filled with oil, rocker arms, and conventional lifters, lighter valve springs can also be used. Again another gain in efficency and reduced wear and tear on parts. Cam in block style engines are still so popular I believe because of the simplicity of having just one cam and 1 timing chain/belt. It's much more cost effective to machine 1 cam instead of 2 or 4. GM has been very successful with optimizing this setup in their LS1/2/6/7 engines.

2 valve vs. 4 valve : If you were to draw 2 circles (representing cylinders) of the same size on a sheet of paper and the draw 2 of the biggest circles (2 valve head) possable without overlapping in one and draw 4 of the biggest circles (4 valve head) possable without overlaping in the other, the one with 2 circles would have less surface area in the two circles, than the one with 4. if those circles were valves in a cylinder, the greater the area that is opened by the valve the more flow is possable.

 

[marleyskater420]

Originally posted by Sleeper-6
There have been many chapters written on the questions asked here. I'll try to give the quick and dirty version.

DOHC vs Cam in block : DOHC eliminates the some of the valvetrain components. Therefore you have less stuff in motion and changing directions or force, things become more efficent. With less weight in motion because there are no pushrods filled with oil, rocker arms, and conventional lifters, lighter valve springs can also be used. Again another gain in efficency and reduced wear and tear on parts. Cam in block style engines are still so popular I believe because of the simplicity of having just one cam and 1 timing chain/belt. It's much more cost effective to machine 1 cam instead of 2 or 4. GM has been very successful with optimizing this setup in their LS1/2/6/7 engines.

2 valve vs. 4 valve : If you were to draw 2 circles (representing cylinders) of the same size on a sheet of paper and the draw 2 of the biggest circles (2 valve head) possable without overlapping in one and draw 4 of the biggest circles (4 valve head) possable without overlaping in the other, the one with 2 circles would have less surface area in the two circles, than the one with 4. if those circles were valves in a cylinder, the greater the area that is opened by the valve the more flow is possable.

Thanks for the info. Keep more info coming!

How does the DOHC head design work then?

 

[Sleeper-6]

DOHC cam engines have two cams in each head, one for the intake valves one for the exhaust valves. Sometomes each cam has a sprocket and they are both driven off of the timing belt. In others the belt drives one cam and the other is gear or chain driven off of the first cam.
Exactly how they work depends on the manufacturer, In general the valve stems and springs sit as the would on any other head, but instead of being pushed down by a rocker arm there is a "lifter" (for lack of a better word) that sits directly above the valve stem. On top of that would be the cam. As the cam rotates, the lobes push down on the "lifter" which pushes the valve open. Everything travels inline, instead of haveing to change directions like a cam in block setup.
Sometimes the cam will push on a rocker arm in the head, which in turn will move the valves. Still eliminating the pushrods.

 

[turbo buicks]

Originally posted by marleyskater420
Thanks for the info. Keep more info coming!

How does the DOHC head design work then?

like this

http://static.howstuffworks.com/gif/dohc.jpg

http://auto.howstuffworks.com/question562.htm

 

[WE4]

hrmmmm

So what about Formula1 DOHC engines? They only rev ahhh...18,000 rpm or so and that is with 50 lbs of boost. So how does this fit into the equation?
Somehow I think Sleeper-6 has the answers or at least the concept down, now if we could only get him to keep typing we may all learn something.
I am interested in this also. I do not have much info on the subject, but I could come up with a ton of devils advocate sides to this.
Instead of the , how did you put it, "I'll try to give the quick and dirty version" What is the long and clean version of what you know about this?
Sounds like an very interesting topic. I would love to learn more.

Thanks man, help us to understand.....

Anybody else know anything about this subject?

I always thought the DOHC engines seemeed smoother and revved quicker, but I really do not know.


Bruce
WE4
Tb.com

 

[marleyskater420]

Re: hrmmmm

Originally posted by WE4
So what about Formula1 DOHC engines? They only rev ahhh...18,000 rpm or so and that is with 50 lbs of boost. So how does this fit into the equation?
Somehow I think Sleeper-6 has the answers or at least the concept down, now if we could only get him to keep typing we may all learn something.
I am interested in this also. I do not have much info on the subject, but I could come up with a ton of devils advocate sides to this.
Instead of the , how did you put it, "I'll try to give the quick and dirty version" What is the long and clean version of what you know about this?
Sounds like an very interesting topic. I would love to learn more.

Thanks man, help us to understand.....

Anybody else know anything about this subject?

I always thought the DOHC engines seemeed smoother and revved quicker, but I really do not know.


Bruce
WE4
Tb.com

I want the long and clean version!!!

I hope this can bring some REALLY good tech info.

 

[bruce]

Originally posted by marleyskater420
Today my friend and I were discussing this.. What is better for all out hp and performance? What are the advantages of both? How the heck does DOHC work besides cam on top?

We were stuck on "if DOHC is so good why doesnt chevy go DOHC LS1's" and "ferrari uses DOHC" crap like that.. I just argued that fact of how much room you have for each valve and if you can get your 1 valve to open as much as 2 valves,whats the point of 2.

Also we talked about flat pistons vs. wedged ones. Why are there different types...care to explain anyone?

Air flow is about valve curtain area, ie the amount of area that gets exposed by the valve opening. With DOHC you can get more air flowing at lower valve lifts, with adds alot to the possible intake tract tuning. Also, with that design, you can eliminate the need for using valve springs, and use compressed gases to close the valves. BTW, it's the cam that opens them, and the valve springs are just needed to close them. That may help understand how they use gases instead of springs. Also with DOHC, you can reduce the valve train component weights, and that reduces the loads on the springs.

DOHC, has been around for about forever. It's nothing new or magicial. Seems like it appeared in the 1920s or so, as I recall.

Early one most engine parts were iron, or steel, and heavy, so the pushrod engines were popular. Easy to machine, and they did the job. The early problem with DOHC, was that the heads were so large, that if iron, the engine would literally weight a ton. With the advent of *cheap* AL, bigger castings, were acceptible. BTW, at one time, AL, was called *Metal Electricity* since it took so much electricity to refine it. It wasn't until the TVA, and WWII that the US had enough to generate meaningful amounts of AL.

Piston design is about getting the gases to swirl and mix. That gives the most even *reaction* across the dome of the piston, which then can be recovered thur the movement of the piston. With the mania for cheap, using offset wrist pins has lost favor, but at one time was a viable way of adding a few ponies.

One of the problems with current ICE, internal combustion engine, is getting the gases trapped between the piston, block, and above the top ring to burn, correctly.

Gm has done a marvelous job of *Turd Polishing* with the SBC, and that is a compliment, nothing negative meant. It's really amazing that the latest Vette does what it does, with a 100K mile emission warranty. Makes one wonder what they have going on in the labs.......

HTH

PS, really long story made really short........

 

[wlaukaitis]

My monkey wrench into the system

So, lets talk about DOHC vs SOHC! In theory you could have a 4 valve head operating with 1 cam. Make a peace symbol, your 2 fingers are the rockers that operate the valves. Whay have another cam/sprocket/longer timing chain/etc. Wouldn't that be more efficient?

But, you cannot have VVT with 1 cam, because I think that the cams aret timed differently. I also think this is done by engine oil pressure. You can have the cams timed at lets say 0* at normal cruise and then at higher rpm's have the intake advanced 1 or 2*

I have seen an F1 engine where there are no valves at all rather a rotating shaft that has passages milled into it similar to things like this

http://www.dself.dsl.pipex.com/MUSEUM/POWER/RotaryValveIC/RotaryValveIC.htm

http://www.coatesengine.com/eGallery/pages/RotaryValveSystem.htm

So I think that DOHC is just another method of selling cars. I do think it has its advantages, but none come to mind.

 

[cool 84]

Also for street engines, you can flow a ton of air with a 4V and still have decent drivability due to a little extra velocity in the intake ports. Some even operate just one intake or only partially open one intake at low rpms for drivability/gas mileage/torque.

 

[CTX-SLPR]

Actually most DOHC engines I've seen are quite Peaky, even with some of the fancy manifold control schemes. If you look at the LT5 and the Ford 4.6/5.4 DOHC motors they all use what Ford calls IIRC or Internal Intake Runner Control. This uses a set of butterfly valves to completely block off the air path to the second valve at low throttle and high engine vacuum. The LT5 even has 16 injectors and the second set don't engage till higher throttles, if you've ever been in a ZR-1 Corvette there is a little "Valet Mode" key that disables the secondary runners and injectors effectively castrating the car so your valet can't beat the snot out of it while you're away. The problem with a DOHC motor is that the available port area for the air to flow through is huge compared to most pushrod engines. As such the same slug of air moves slower through the ports which gives you your mushy low end throttle response and generally poorer low end torque due to the poor flow velocity. Now that all changes as your accellerate and the air needs rise and the port is now capable of supporting massive amounts of air flow. If you look at the flow numbers a stock Eagle Talon/ Eclipse 16V head is capable of flowing near a NASCAR SB2 head yet you'd be hard pressed to run an SB2 motor on the street without some serious sacrifices in drivablity, while a Talon is perfectly drivable on the street with its fancy engine controls keeping the torque curve up at lower speeds.
As far as VVT or VTEC or Zetec, they use different means of controlling the cam phasing, but oil pressure is the most common while some use electronics. You can adjust the valve timing on an SOHC motor but you can not adjust the lobe seperation angle since all the camshaft lobes are on the same stick. With DOHC you can adjust the timing of each cam independently so you can decrease overlap at low RPM to build cylinder pressure, ie torque, then increase the overlap at higher rpms to increase the effeciency of cylinder filling. For a race motor on pump gas you can do the reverse to bleed off load speed cylinder pressure to prevent detonation in the same way you can crutch compression by running more overlap in an NA motor without having to sacrifice the topend needs or having a different tune for race gas with less overlap.

Just my mad musing on the subject, I actually scared some of my friends by having a 2 hour discussion about this at a party with one of my fellow gearheads,

 

[buickpower]

The problem with a DOHC motor is that the available port area for the air to flow through is huge compared to most pushrod engines. As such the same slug of air moves slower through the ports which gives you your mushy low end throttle response and generally poorer low end torque due to the poor flow velocity.

damn, you beat me to it!

 

[buickpower]

also, pushrod engines are sometimes chosen over DOHC because of space constraints. as someone mentioned the latter heads are larger and this can be a proble with V-configuration engines. basically with the pushrod setup you can often have more cubic inches within the same engine bay. you guys did such a good job covering all the points i wanted to make while reading down this thread that I actually feel gypped . btw, from what I understand SOHC has a torque curve that is closer to that of a pushrod motor, at least in 2 valve form.

 

[jerrad]

Originally posted by marleyskater420
I know for a fact it isnt because DOHC's can rev higher. Im working w/ a alchohol funnycar driver and he has a pushrod v8 that revs to 10k. So that isnt the case.

I know you have more valves, but cant you just make the one pushrod valve bigger? Can you a 3 valve pushrod?

Ok so a fully built push rod motor can rev to 10K a stock dohc vtec engine can redline 8000.

 

[buickpower]

Ok so a fully built push rod motor can rev to 10K a stock dohc vtec engine can redline 8000.

alot of that is due to the rather large reduction in valvetrain mass, and fewer potential places for "slop". but you are right in that they rev inherently higher because of this.

 

[marleyskater420]

Originally posted by CTX-SLPR
Actually most DOHC engines I've seen are quite Peaky, even with some of the fancy manifold control schemes. If you look at the LT5 and the Ford 4.6/5.4 DOHC motors they all use what Ford calls IIRC or Internal Intake Runner Control. This uses a set of butterfly valves to completely block off the air path to the second valve at low throttle and high engine vacuum. The LT5 even has 16 injectors and the second set don't engage till higher throttles, if you've ever been in a ZR-1 Corvette there is a little "Valet Mode" key that disables the secondary runners and injectors effectively castrating the car so your valet can't beat the snot out of it while you're away. The problem with a DOHC motor is that the available port area for the air to flow through is huge compared to most pushrod engines. As such the same slug of air moves slower through the ports which gives you your mushy low end throttle response and generally poorer low end torque due to the poor flow velocity. Now that all changes as your accellerate and the air needs rise and the port is now capable of supporting massive amounts of air flow. If you look at the flow numbers a stock Eagle Talon/ Eclipse 16V head is capable of flowing near a NASCAR SB2 head yet you'd be hard pressed to run an SB2 motor on the street without some serious sacrifices in drivablity, while a Talon is perfectly drivable on the street with its fancy engine controls keeping the torque curve up at lower speeds.
As far as VVT or VTEC or Zetec, they use different means of controlling the cam phasing, but oil pressure is the most common while some use electronics. You can adjust the valve timing on an SOHC motor but you can not adjust the lobe seperation angle since all the camshaft lobes are on the same stick. With DOHC you can adjust the timing of each cam independently so you can decrease overlap at low RPM to build cylinder pressure, ie torque, then increase the overlap at higher rpms to increase the effeciency of cylinder filling. For a race motor on pump gas you can do the reverse to bleed off load speed cylinder pressure to prevent detonation in the same way you can crutch compression by running more overlap in an NA motor without having to sacrifice the topend needs or having a different tune for race gas with less overlap.

Just my mad musing on the subject, I actually scared some of my friends by having a 2 hour discussion about this at a party with one of my fellow gearheads,

So on DOHC basically the heads flow so good,that at low rpm, the runners or whatever are too big to push the air through it fast enough...basically its too big in there to give the air a lot of velocity?

How do you adjust the cam timing while driving?I dont get how the oil pressure part works..Please explain GREATLY

So with DOHC you can almost change the LSA while driving???Please explain HOW you can change the cam timing w/ the oil pressure..this is so cool!!!!!!

Expand further why the talon can flow the same as the SB2,but the SB2 sucks on the street..is it because of the electronic controls in the DOHC motor?

Expand more EXPAND MORE!!!!

Originally posted by wlaukaitis
So, lets talk about DOHC vs SOHC! In theory you could have a 4 valve head operating with 1 cam. Make a peace symbol, your 2 fingers are the rockers that operate the valves. Whay have another cam/sprocket/longer timing chain/etc. Wouldn't that be more efficient?

But, you cannot have VVT with 1 cam, because I think that the cams aret timed differently. I also think this is done by engine oil pressure. You can have the cams timed at lets say 0* at normal cruise and then at higher rpm's have the intake advanced 1 or 2*

I have seen an F1 engine where there are no valves at all rather a rotating shaft that has passages milled into it similar to things like this

http://www.dself.dsl.pipex.com/MUSEUM/POWER/RotaryValveIC/RotaryValveIC.htm

http://www.coatesengine.com/eGallery/pages/RotaryValveSystem.htm

So I think that DOHC is just another method of selling cars. I do think it has its advantages, but none come to mind.

I dont get the first thing you said..Are you saying that it is better to have 2 cams, or 1? Wouldnt it be better for 2..?

How would you use the VVT? How does oil pressure speed up the cam? Does the oil pressure speed up what drives the cam gear? Please go into great detail about this... I wanna know why people say they need a certain cam to get VTEC and crap like that.

That picture you showed...it just has swirling circles and they just let air in and out w/ no valves?

Originally posted by bruce
Air flow is about valve curtain area, ie the amount of area that gets exposed by the valve opening. With DOHC you can get more air flowing at lower valve lifts, with adds alot to the possible intake tract tuning. Also, with that design, you can eliminate the need for using valve springs, and use compressed gases to close the valves. BTW, it's the cam that opens them, and the valve springs are just needed to close them. That may help understand how they use gases instead of springs. Also with DOHC, you can reduce the valve train component weights, and that reduces the loads on the springs.

DOHC, has been around for about forever. It's nothing new or magicial. Seems like it appeared in the 1920s or so, as I recall.

Early one most engine parts were iron, or steel, and heavy, so the pushrod engines were popular. Easy to machine, and they did the job. The early problem with DOHC, was that the heads were so large, that if iron, the engine would literally weight a ton. With the advent of *cheap* AL, bigger castings, were acceptible. BTW, at one time, AL, was called *Metal Electricity* since it took so much electricity to refine it. It wasn't until the TVA, and WWII that the US had enough to generate meaningful amounts of AL.

Piston design is about getting the gases to swirl and mix. That gives the most even *reaction* across the dome of the piston, which then can be recovered thur the movement of the piston. With the mania for cheap, using offset wrist pins has lost favor, but at one time was a viable way of adding a few ponies.

One of the problems with current ICE, internal combustion engine, is getting the gases trapped between the piston, block, and above the top ring to burn, correctly.

Gm has done a marvelous job of *Turd Polishing* with the SBC, and that is a compliment, nothing negative meant. It's really amazing that the latest Vette does what it does, with a 100K mile emission warranty. Makes one wonder what they have going on in the labs.......

HTH

PS, really long story made really short........

What is offset wring pistons?

Make the story longer! I want to learn more!


THANK YOU SO MUCH EVERYONE FOR THE INFO!!!

 

[BoostKillsStres]

Marley, don't forget google is your friend, just punch in variable valve timing and have at it.

 

[Drac0nic]

Originally posted by marleyskater420
How would you use the VVT? How does oil pressure speed up the cam? Does the oil pressure speed up what drives the cam gear? Please go into great detail about this... I wanna know why people say they need a certain cam to get VTEC and crap like that.

If I'm not mistaken the VVT motors have a cam that has a tapered lobe (someone club me over the head with a clue by four if I"m butchering this) and the cam is hydraulicly pushed over. Ideally you'd have the big lumpy chunk for optimum power when you're kickin along at higher RPM and then the smaller lobes so you didn't castrate your motor's low end.