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Pushrod motors vs DOHC

23K views 60 replies 25 participants last post by  TheZman 
#1 ·
Can someone give me the benefits of a DOHC motor vs a typical pushrod engine? Why the the mustang use the 5.0?

From reading online, I've found that typically a DOHC makes much more hp per liter of displacement but:

They're typically much larger in dimensions
They are heavier
They are more expensive to produce
They make power by revving to higher RPM's
They produce less torque

So why does Ford use it? Is there a benefit?
 
#2 · (Edited)
Can someone give me the benefits of a DOHC motor vs a typical pushrod engine? Why the the mustang use the 5.0?

From reading online, I've found that typically a DOHC makes much more hp per liter of displacement but:

They're typically much larger in dimensions
They are heavier
They are more expensive to produce
They make power by revving to higher RPM's
They produce less torque

So why does Ford use it? Is there a benefit?
The coyote 5.0L is the exception to the size issue. It does make a good bit more power per liter than the 6.4L. I personally prefer a push rod design over a DOHC engine. Push rod engines are less complicated to work on and have a advantage concerning torque.

However, an all aluminum DOHC 7.0L could change my tune.
 
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#5 ·
You're also comparing a 4-valve/cyl vs. 2-valve/cyl.

It would be very costly and complicated to have a pushrod 4-valve setup (lots of moving parts).

The main reason is lower valvetrain mass / inertia (higher revving capability) there's fewer parts in a OHC layout - the pushrod layouts tend to have more low-end torque and of course the cost is lower with a simpler cam drive (cam in block vs. OHC layout).

Many OHC design peak torque tend to be further up in the RPM band than their pushrod counterparts.

Pushrod layouts tend to be viewed as somewhat lower tech, but from a cost standpoint and performance you still see a lot of domestic makes running pushrod setups.
 
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#6 ·
Besides the lighter valvetrain enabling higher rpms, there is also the ability in recent times to adjust the lobe separation angle between the exhaust and intake peak lift. This allows moving peak torque around...or constantly adjusting to achieve the highest/flattest torque curve/ most cylinder pressure at X rpm.
 
#8 ·
In theory at least, DOHC engines, especially with 4 or more valves per cylinder, are better. If you think of an engine as a big air pump, all else being equal, a DOHC 4 valve engine, allows for more air in an out of the engine, with better airflow since you don't have pushrods in the way. They are also, however, usually bigger and more complicated, as well as more costly to make.

The amount of torque that an engine produces has nothing whatsoever to do with having pushrods or DOHC's.
 
#12 ·
In theory at least, DOHC engines, especially with 4 or more valves per cylinder, are better. If you think of an engine as a big air pump, all else being equal, a DOHC 4 valve engine, allows for more air in an out of the engine, with better airflow since you don't have pushrods in the way. They are also, however, usually bigger and more complicated, as well as more costly to make.



The amount of torque that an engine produces has nothing whatsoever to do with having pushrods or DOHC's.

That's fair. I was kind of getting at that DOHC engines typically are smaller displacements in order to fit into the engine bay. Displacement and torque are proportionally related, yes? Or am I oversimplifying this?


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#13 ·
As far as power, torque curve, and all that goes, whether the valves get opened by pushrods or by overhead cams really doesn't matter, that's all the cam profile, the head breathing ability (flow), and so on.

Since overhead cams actuate the valves directly (or by a finger-follower), the amount of mass being moved by the cam is very small- essentially just the mass of the valve, the spring, and the finger follower. With a pushrod engine, the cam moves the mass of a lifter, a pushrod, a rocker arm, and also the valve.

In the old days, that meant that OHC could spin much, much faster before the valves started to 'float,' with the springs unable to move all that mass back the other way fast enough.

Today, modern valve spring designs and modern pushrod/rocker arm materials, and so on have made it so that OHC really only becomes an advantage at 8000+ RPM. They spent a lot of money doing this, but even before NASCAR engines went to roller lifters this year, they proved that they could operate reliably at 10,000 RPM using pushrods. 7000 RPM for a street pushrod engine is no big thing, so obviously going OHC only becomes a real advantage at higher RPM than most street engines ever need to spin.

Also, its easier to actuate 4 valves per cylinder with OHC than it is with cam-in-block, so small-bore engines that can't fit big valves benefit some from that. There are 4-valve pushrod engines out there- the Cummins ISB diesel for example (aka the "24 Valve" diesel in the Ram).

Short story: it *looked* for a while like OHC was going to be the future, and that's about the time Ford designed their Modular engine family in the early 90s. Then materials and cylinder head design sorta made it moot by the time Chrysler designed the Hemi in the early 2000s. Plus pushrod engines are much more compact. Ford stuck with OHC because there's not enough "down-side" to go back to pushrods now that they have the basic architecture.

Chrysler went DOHC with the Pentastar because its a small-bore engine that needs the 4 valves.
 
#16 · (Edited)
As far as power, torque curve, and all that goes, whether the valves get opened by pushrods or by overhead cams really doesn't matter, that's all the cam profile, the head breathing ability (flow), and so on.

Since overhead cams actuate the valves directly (or by a finger-follower), the amount of mass being moved by the cam is very small- essentially just the mass of the valve, the spring, and the finger follower. With a pushrod engine, the cam moves the mass of a lifter, a pushrod, a rocker arm, and also the valve.

In the old days, that meant that OHC could spin much, much faster before the valves started to 'float,' with the springs unable to move all that mass back the other way fast enough.

Today, modern valve spring designs and modern pushrod/rocker arm materials, and so on have made it so that OHC really only becomes an advantage at 8000+ RPM. They spent a lot of money doing this, but even before NASCAR engines went to roller lifters this year, they proved that they could operate reliably at 10,000 RPM using pushrods. 7000 RPM for a street pushrod engine is no big thing, so obviously going OHC only becomes a real advantage at higher RPM than most street engines ever need to spin.

Also, its easier to actuate 4 valves per cylinder with OHC than it is with cam-in-block, so small-bore engines that can't fit big valves benefit some from that. There are 4-valve pushrod engines out there- the Cummins ISB diesel for example (aka the "24 Valve" diesel in the Ram).

Short story: it *looked* for a while like OHC was going to be the future, and that's about the time Ford designed their Modular engine family in the early 90s. Then materials and cylinder head design sorta made it moot by the time Chrysler designed the Hemi in the early 2000s. Plus pushrod engines are much more compact. Ford stuck with OHC because there's not enough "down-side" to go back to pushrods now that they have the basic architecture.

Chrysler went DOHC with the Pentastar because its a small-bore engine that needs the 4 valves.
What are some examples of high-revving, push-rod production cars made today? There is a plethora of high-revving DOHC designs. We can be here all day listing them.

Ford's modular engine design duds of the 90's (and early 2000's) were SOHC. The OP is about pushrod vs DOHC, and many of the Ford DOHC cars are still sought after to this day.
 
#15 ·
Don't let anyone fool you, the 5.0L Coyote is a huge engine. Just the block itself is smaller than the competition (Chevy's 6.2L, Dodge's 5.7L & 6.4L), but once you put those heads on the engine it is huge. Having owned both the 5.0L Coyote (took that engine out twice) and the 6.4L Hemi, there definitely is a difference between the two (4 valve vs. 2, etc). The coyote block is a great engine, more advanced than our pushrod but it is also more complicated to work on.
 
#25 ·
I prefer not to say "more advanced"...

The coyote block is a great engine, more advanced than our pushrod but it is also more complicated to work on.



I used to think that OHC technology is a more modern design than OHV. It is not. OHC technology actually dates back to the beginning of the 20th Century and it is actually older than OHV. Think about these two technologies more as different ways to accomplish the same task. OHC has some clear technical advantages but as usual the Devil is in the details of the implementation. The two engines mentioned are both great engines and I would consider them both technologically equals. I believe the reason why you see so many DOHC's is because the Europeans have a displacement tax. This means they MUST squeeze more power from smaller displacement motors because the displacement tax is HUGE. About $5000 for a V8 when I lived there (annually). 4 valves per cylinder solves some of the problem.


 
#24 ·
I could not imagine taking the heads off that thing. That's why I like OHV, smaller and easier to work on.

Another member on this forum posted the pic below comparing the size of the coyote to a 454 lsx. Gives you an idea of just how big the 5.0 really is.
 

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#22 ·
Interesting! I had heard of experimentation by GM on the same but did not know it was in production. Are there any other single cam in block engines using this development? Thanks

I take it the V10 is the only Dodge engine with this?

That's why I asked the Mopar vendor here about the 392. New developments happen in 5 years.
 
#23 · (Edited)
Well, I downloaded the pdf from Mahle on this Viper cam. Yes, LSA is adjustable but only the exhaust side moves relative to TDC. They felt this was good enough because advancing intake center line would only increase torque down low which the V10 did not need. It did need the ability to cut overlap to get better idle and lower the amount of exhaust dilution of the intake charge.
 
#26 ·
over your head:

my experience is this. iv'e had both over head and push rod bikes and cars. like most of you iv'e found that my 07 cobra mustang at 305H.P. with the double over head cam set up, compaired to my push rod 275 H.P. 95 Z28, i found red line on the mustang was if i remember right, 6800 rpm. the camaro was at 5200. the chevy pulled stronger all thru the rpm range. the stang woke up at 4000rpm.at 5500 it really pulled hard up to red line. reved quicker, after 4000. mustang a 5 speed, chev a 6. closer raito. raced them together, changed drivers, if the camaro stumbled for some reason, the stang was gone. otherwise the camaro with closer raito, better tourqe, had an edge. iv'e found simular comparisons in bikes. i curently own a 2012 srt challenger. which is a humbling experience in it's own right. 69 yrs. young, and not looking back!
 
#27 ·
my experience is this. iv'e had both over head and push rod bikes and cars. like most of you iv'e found that my 07 cobra mustang at 305H.P. with the double over head cam set up, compaired to my push rod 275 H.P. 95 Z28, i found red line on the mustang was if i remember right, 6800 rpm. the camaro was at 5200. the chevy pulled stronger all thru the rpm range. the stang woke up at 4000rpm.at 5500 it really pulled hard up to red line. reved quicker, after 4000. mustang a 5 speed, chev a 6. closer raito. raced them together, changed drivers, if the camaro stumbled for some reason, the stang was gone. otherwise the camaro with closer raito, better tourqe, had an edge. iv'e found simular comparisons in bikes. i curently own a 2012 srt challenger. which is a humbling experience in it's own right. 69 yrs. young, and not looking back!
An 07 Cobra makes a hell of a lot more than 305 horsepower.
 
#30 ·
The big advantage of DOHC is it breathes great and will rev higher than a pushrod engine, allowing the DOHC engine to make more power for a given amount of displacement. However, that advantage is offset by all the engine bay space a DOHC requires (4 cams take up a lot more space than a single cam!).

From a practical standpoint, this means a comparable sized DOHC should make more power per liter but delivers fewer liters in the same sized package as a pushrod engine. The pushrod engines won't rev as high, but you can fit a lot more displacement under the hood using only 1 cam instead of 4. This is why pushrod engines tend to have more torque---because they usually have more displacement, and displacement is a great way to generate low and mid range torque.

If you are looking strictly at performance, it is a close race but the pushrod engines have the advantage in power density (how much power they can produce in the limited space available under the hood).

But the DOHC has another big advantage that does not involve performance, and that's why they are so popular these days. By making the same power with less displacement the DOHC engine can be tuned to get better fuel economy while producing lower emissions. Displacement is your enemy when chasing CAFE numbers, and that is a big reason Ford has switched to DOHC.
 
#39 ·
Question

Since the DOHC makes more power with 4 valves and high RPM...and the pushrod design gets it's power primarily from increasing displacement.....


and if we accept that the DOHC is a larger motor (if displacement were equal between the two)...


Could you not create a pushrod motor equal in overall size/weight, but with even LARGER displacement....to the point where the pushrod motor makes more power than an equally sized/weight DOHC (offsetting the advantages of 4 valves/cylinder and high revving capacity)?
 
#41 ·
Since the DOHC makes more power with 4 valves and high RPM...and the pushrod design gets it's power primarily from increasing displacement.....


and if we accept that the DOHC is a larger motor (if displacement were equal between the two)...


Could you not create a pushrod motor equal in overall size/weight, but with even LARGER displacement....to the point where the pushrod motor makes more power than an equally sized/weight DOHC (offsetting the advantages of 4 valves/cylinder and high revving capacity)?
Isn't that basically what dodge did with the 392(6.4L) Hemi vs Ford's latest 5.0 Coyote?
 
#43 · (Edited)
@crater. Not sure. I think the 392 is bigger dimension and weight wise. I ask because I believe the real motivation for DOHC is to get as much power as possible from a finite displacement. I think it's economy driven. We were debating the performance advantages. If we can establish that the pushrod design gets more HP for a given weight/size, we will know which one is the better performance motor.


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#45 · (Edited)
I believe Torque is a function of HP.

If all things are equal, engine dimensions/weight, but displacement is not a limiting factor. Or emissions. Or displacement taxes.....whichever one gets more power would be the better performance motor.
I'd like to hear any evidence or facts that addresses this.

We see comparisons between motors all the time. But the motors are not the same weight or dimension.

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#49 ·
so On the 5.0 coyote they only need 1 spark plug for emissions because it is more efficient. On our modern hemi engines we have 2 plugs per cylinder for emissions because they don't burn as well.
I think Ford uses the overhead cam engines because of emissions and they are building "world" vehicles. In europe ford has a 4 cylinder turbo-charged 1 liter engine that is well over 100 horsepower, it blows away a lot of other cars.
 
#54 ·
The only advantage of ohc is to exploit an extended redline. If you don't hit the extended rpm, then you don't get the benefit at pk hp. Given that context, a properly leveraged ohc engine seeks to maximize rpm to reach target hp. Then when you decrease displacement, you give up low end output. So the net result of most ohc engines is trading off low end output for high end output. The Coyote engine can be analyzed along these lines, as well...if you (for some reason) limited the range to 5500 rpm, it would only be a 380 hp engine. So it's similarly powerful to the Eagle Hemi, right?...not exactly. The Coyote also gives up a significant amount of low rpm torque due to the lower displacement. There is also the effect of 12:1 compression ratio vs 10.5:1 compression ratio between the 2 engines, so even then, it is not exactly apples to oranges when normalizing by rpm.

As far as potential flow capability, the Hemi design (canted valves) comes the closest to the typical 4-valve design. "Potential" is in italics because just having 4-valves doesn't automatically mean more breathing. In typical real 4-valve designs, they back-off the lift, so in net valve window terms, it is very close to a wash. The valve window area isn't something you just go as wide-open as you can in a given combustion chamber size. Since the window size affects resonance tuning, there is a "zone" of optimal size.

The wedge head design is the farthest approach as far as optimal valve flow (but it makes up a lot of that with heavier optimization via quench and swirl effects).
 
#57 ·
The major advantage a flat crank has is that it's much lighter, without all the counterweights a cross-plane uses. So it revs a lot quicker, and can rev higher. This doesn't require an OHC head, but an OHC will exploit the flat-crank's rev capabilities better.

Neither design has an advantage when it comes to torque, assuming the rest of the engine is equal. But equal torque plus superior RPM capability gives the flat crank a nice HP advantage.

Flat-crank engines also have better exhaust scavenging due to the alternating-banks firing order. The downside is vibration. With a flat crank you lose the inherent balance a crossplane crank enjoys.
 
#58 · (Edited)
I can tell you two things:

A: A simple tune will bring the peak torque ratings down in the Coyote quite a bit even with the FRPP (very conservative tune).

B: The engine revs so freely, I just can't describe it properly.

Is it better? I guess that all depends on what you want.

I believe Porsche also uses flat-plane crank engines in their cars. Could be wrong about this. The downside to flat-plane crank is they are noisy and create a lot of vibration that has to be dealt with. Or maybe it is Ferrari, or both. I can't remember.
 
#59 ·
I really like the simple design of the push rod engines.. Easier to work on and in alot of cases have a lower profile on the top of the engine. Look at the new Corvette with its push rod engine its so compact and flat on top even my 392 in my Scat Pack is alot more compact on top then the new 5.0 Mustang coyote engines.. Less to go wrong down the road as well when you get up into the higher mileage.. Just my preference..
 
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