OHC vs PUSHROD

[Muscle Master]

So can someone explain the differences between SOHC, DOHC, and Pushrod engines, what are each of their strength and weaknesses, and why do people say the American pushrod is a ancient inefficient peace of junk considering we crank out huge amounts of torque and the sound is just...... haaaaaaaaaa it's beautiful

[stovt001]

On Edmunds Straightline blog, they just posted a Most Underrated List, and pushrod engines are on there. Some people, even some writers, still understand the benefits.

[Mr. Wyndham]

Over head cam designs are largely considered more "refined" than pushrod.

The benefit to the design is increased airflow due to the ability to poke more valves in there...resulting in equal power from a smaller displacement engine (which means increased efficiency)...see: Ford 4.6L vs LS4.....

BUT -- when you do it right, like GM and Chrysler(only recently)...it doesn't really matter. We're getting 400hp for the same mpg as their 300hp V8. So.

I don't buy it. When someone brings it up...I laugh at them.....like this:

[TFord]

Originally Posted by Dragoneye

Over head cam designs are largely considered more "refined" than pushrod.

The benefit to the design is increased airflow due to the ability to poke more valves in there...resulting in equal power from a smaller displacement engine (which means increased efficiency)...see: Ford 4.6L vs LS4.....

BUT -- when you do it right, like GM and Chrysler(only recently)...it doesn't really matter. We're getting 400hp for the same mpg as their 300hp V8. So.

I don't buy it. When someone brings it up...I laugh at them.....like this:

thats about as much as I know about OHC/DOHC. As far as Im concerned the less moving parts the better, I never jumped on the overhead cam bandwagon. Part of the reason I only buy GM. I think of pushrods like an AK-47 Simplified, and dependable, and on the rare occasion easy to fix.

[gunslinger]

One advantage that a 2-valve "Pushrod" or "OHV" engine has is the ability to induce more mixture swirl of the incoming combustion charge, resulting in a more homogeneous mix. This allows for more low end torque, when air is not moving quite as fast.

The OHC does allow for more airflow with the same size bore, but this is generally accomplished at a higher RPM because the larger runner size does not keep the velocity of the incoming air at a high enough speed, where as the single, smaller runner on the OHV does.

Also, less moving parts and a lower center of gravity. The above mentioned shortcomings of the OHV can be overcome with extensive port testing as is found in the GenIII and Gen IV General Motors V8's.

There have been whole scientific articles devoted to this topic, and trust me GM would not be using the Pushrod design if it was an "old" (actually OHV is newer than OHC) outdated design. When the LS6 (GenIII V8) was put in a 3100 LBS Corvette ZO6, 30 MPG was attainable while still having 405 HP and world class acceleration and top speed...

Can you tell I love GM's motors?

[DGthe3]

Originally Posted by Muscle Master

So can someone explain the differences between SOHC, DOHC, and Pushrod engines, what are each of their strength and weaknesses, and why do people say the American pushrod is a ancient inefficient peace of junk considering we crank out huge amounts of torque and the sound is just...... haaaaaaaaaa it's beautiful

Why are pushrods considered anchient while OHC's are new? I don't know. They are only about 20 years older than OHC designs.

Pushrod engines have a single cam mounted in the block which activates pushrods which open the valves.
+ inexpensive
+ simple
+ compact
+ lighter than OHC
+ fatter torque band
- exceeding 2 valves/cylinder is difficult (more valves=better breathing)
- difficult to achieve high revs (7500+)

SOHC engines have 1 cam per cylinder bank (inlines have 1 cam, V's have 2). These cams are located above the cylinders and are linked with a long timing chain or belt to keep them in sync. The cams activate the valves directly.
+ improved fuel efficiency
+ It is relatively easy to have a 3 or 4 valve design.
+ freer (sp?) to rev
+ better control over valve timing
- larger
- heavier
- more expensive

DOHC engines are very similar to SOHC but they have 2 cams per cylinder bank. As such, their problems and benefits are the same as those from SOHC but more pronounced.


Also, a OHV can be made to be high reving (302cu Z28's, Nascar) and an OHC can be made to produce lots of torque (5.7L Toyota). Likewise, an OHV powered vehicles are not always less efficient than equivalent OHC's (Ram vs Tundra). These mostly exceptions though.

[Grape Ape]

Originally Posted by DGthe3

Why are pushrods considered anchient while OHC's are new? I don't know. They are only about 20 years older than OHC designs.

Pushrod engines have a single cam mounted in the block which activates pushrods which open the valves.
+ inexpensive
+ simple
+ compact
+ lighter than OHC
+ fatter torque band
- exceeding 2 valves/cylinder is difficult (more valves=better breathing)
- difficult to achieve high revs (7500+)

SOHC engines have 1 cam per cylinder bank (inlines have 1 cam, V's have 2). These cams are located above the cylinders and are linked with a long timing chain or belt to keep them in sync. The cams activate the valves directly.
+ improved fuel efficiency
+ It is relatively easy to have a 3 or 4 valve design.
+ freer (sp?) to rev
+ better control over valve timing
- larger
- heavier
- more expensive

DOHC engines are very similar to SOHC but they have 2 cams per cylinder bank. As such, their problems and benefits are the same as those from SOHC but more pronounced.


Also, a OHV can be made to be high reving (302cu Z28's, Nascar) and an OHC can be made to produce lots of torque (5.7L Toyota). Likewise, an OHV powered vehicles are not always less efficient than equivalent OHC's (Ram vs Tundra). These mostly exceptions though.

More DOHC: Adding a second cam lets you change intake and exhaust cam timing independently so as to allow higher overlap at high RPM without destroying low RPM power or emissions. The high end DOHC rigs also offer variable lift for the same race cam when you want it, little cam when you don’t.

A big cam sounds great, but it can make parallel parking a real pain.

[syr74]

A lot of great things have been covered here already. A important one which is missing? Four smaller valves covering a similar amount of territory as two larger valves have an easier time maintaining decent port velocity, which nis important to both driveability and performance.

[Oracle]

OHC engines have the ability to use multiple valves. this gives greater high rpm flow velocity, meaning it can cram more air into the cylinder at high rpms, meaning similar HP with less displacement. also, a OHC has less rotating mass than an OHV motor so it can rev higher and faster. because of these traits the technology was conceived for racing applications where displacement was implimented to limit performance. OHC engines allowed engineers to squeeze more out of the limits that were imposed. the reason many manufacturers use this is because they have racing teams that they sponser, and already get a good deal of R&D done on the track. also the ability to make decent hp at smaller displacement means the engines are generally smaller resulting in less parasitic losses that become apparent at times where the vehicle idles often, like at city stoplights.

OHV engines have a trend of 2 valves per cylinder, and those two valves are very big. this large size alows high flow volume, meaning it can cram more air into a cylinder at low RPM. this gives the motor a greater amount of tourque in a range where fuel has time to burn, at low rpms. this is superior for applications which require that extra force down low. OHV motors also have greater rotating mass but less overall mass meaning they are lighter. an OHV engine also has a trend of being large displacement. this large displacement allows for more air in the cylinders at any rpm, meaning once again it has more force at low RPMs. this combination makes this type of motor ideal for providing low end power to move a lot of mass such as towing or heavy vehicles. the reason we see these in corvettes and ford GTs is because of the tradition way back when, sticking the largest truck motor they had into a lightweight sports car. the amount of torque is what made the cars so successful. not the power. the reason a modern OHV engine is so powerful is because of advancements in valve design that increases flow velocity. however these will still find it difficult to produce as much hp as an OHC engine. that is why you will see professional racing series producing 600hp out of a 5.0L NA OHV engine (Chevy Aurora engine out of a V8 supercar) while producing 780hp out of a 2.4L NA OHC engine (F1 car motor). on the plus side of all that torque from a OHV motor it can cruise the highway all day content at 1500 RPMs, which is where its fuel economy numbers will shine.

[rayhawk]

SOHC seems like such a waste to me. I think it's only reason for existing is as a less expensive alternative to a manufacturer who relies heavily on DOHC motors, and where their engine architecture does not lend itself well to pushrod design. The only slight benefit is not having the pushrod weight to move up and down, but the additional cost, complexity, size, weight for a moderately better ability to turn higher rpm don't seem worth it to me.

DOHC vs. pushrod is a better debate, and the DOHC setup lends itself to better flexibility due to smaller valves and the ability to independently vary intake and exhaust timing (although certainly at the cost of complexity). You can also achieve loads of flow with small valves and low spring pressures-easier to reach high rpm's.

With that said, in my opinion, a pushrod motor is the way to go as the best compromise. You are usually limited where engine mods are concerned in an OHC setup, if nothing else than for historical purposes. I am more concerned with an engine's displacement than the cam configuration, because in my experience displacement is what makes a street car fun to drive, more than anything else. To me an engine that needs 5000 rpm to get moving is not as fun to drive as one that pulls hard at 3-4k rpm.

[Muscle Master]

Originally Posted by rayhawk

SOHC seems like such a waste to me. I think it's only reason for existing is as a less expensive alternative to a manufacturer who relies heavily on DOHC motors, and where their engine architecture does not lend itself well to pushrod design. The only slight benefit is not having the pushrod weight to move up and down, but the additional cost, complexity, size, weight for a moderately better ability to turn higher rpm don't seem worth it to me.

DOHC vs. pushrod is a better debate, and the DOHC setup lends itself to better flexibility due to smaller valves and the ability to independently vary intake and exhaust timing (although certainly at the cost of complexity). You can also achieve loads of flow with small valves and low spring pressures-easier to reach high rpm's.

With that said, in my opinion, a pushrod motor is the way to go as the best compromise. You are usually limited where engine mods are concerned in an OHC setup, if nothing else than for historical purposes. I am more concerned with an engine's displacement than the cam configuration, because in my experience displacement is what makes a street car fun to drive, more than anything else. To me an engine that needs 5000 rpm to get moving is not as fun to drive as one that pulls hard at 3-4k rpm.

I change the title

This is great I love you guys, I learning alot

..... keep it coming, more info

I wanna hear from you spike!

[MrIcky]

It seems to me that there are great engines in all 3 categories.

There's something to be said for gobs of low end torque and then again there's something magical about the wail of an engine over 8000 rpm. I'm really starting to get hooked on the whole direct injection plus turbocharged powerplant thing too.

If my money was infinite, I'd want a boulevard cruiser with a huge V8 and a <3000 lb canyon carver that revs to the sky. I also love a good diesel truck.

[DGthe3]

Originally Posted by Muscle Master

I change the title

This is great I love you guys, I learning alot

..... keep it coming, more info

I wanna hear from you spike!

Now the title is changed, I generalized it down to overhead cam vs pushrod

[CamaroSpike23]

Originally Posted by Muscle Master

I change the title

This is great I love you guys, I learning alot

..... keep it coming, more info

I wanna hear from you spike!

lol I looked into this thread earlier... a lot of good info out there already.



one thing to keep in mind is rotating mass. the reason we can make gobs of torque with our "prehistoric" pushrod motors is that we have larger and more cyls/pistons which allow us to make more power down lower and per rpm. its not a necessity for a pushrod v-8 to have to spin to the moon just to get any power. now if you've got a dyno queen car, then spin to whatever the hell you want to to get your peak numbers. I like power under the curve. I like useable power. I LOVE TORQUE.

as ive said before, its nice to rev, its nicer to move.

that being said, there are v-10s, 12s, 14s, w-16s, etc out there making gobs of power (n/a or other wise) but take a v12 lamborghini making 500 hp.... compare that to a v-8 corvette making.... 500 hp. but look at the power bands and where their power is being made. v-12 is spinning to 7k before it starts making max power. v-8 is a little more than half that.


now, there's also v-8 OHC motors as well out there. like the ford 4.6 for example. as said before, with more cams, you've got more options for valve timing, but you also have more shit to break. I like the fact that the weakest part of my valvetrain is my pushrods. meaning that if something is going to break, a pushrod is going to be one of the first things to go. and thats fine, id rather break a pushrod, than snap one of my 30 foot timing chains (they arent really 30 ft long, just exaggerating my point) then ive got a valve sticking down into the chamber when the piston rises..... and boom. there goes my motor. pushrod bends/breaks..... car runs like ass till I get it shut down, then im out 40 buck to replace them.


one big bonus for pushrod motors (all things being equal, v-8 to v-8) is that it is more compact. If OHC were as compact as pushrod designs the pushrods would have been thrown out long ago. For in-line engines it's nice, cus you have plenty of room on either side and if it gets too tall, lay the engine over like the slant 6 from chrysler years ago. For most inline engines (either four-bangers or straight-sixes), OHC makes perfect sense. Smaller engines that make use of extremely high revs are also obvious applications. But if we look at passenger-car applications where the displacement is over 3 liters or so and revs rarely top 6K, then there is absolutely no reason to believe that a properly-designed pushrod engine cannot deliver power, refinement, and compactness comparable to an OHC engine. OHC engines allow higher rev operation, and thus are better for peak power output. They're found in many different exotic car and racing applications (Ferrari, IRL, etc) where high rpms are desirable. Using two cams instead of one (DOHC vs SOHC) can give you a bit more flexibility when designing lift profiles, as you'll have less contact stress, and you may be able to achieve higher speeds. DOHC is a logical choice if you have a variable cam phasing mechanism, as you can put your intake and exhaust lobes on separate shafts.

Both OHC and pushrod valvetrains can use timing chains or timing belts, but timing chains are more common on pushrod engines, and timing belts are more common on OHC engines. The advantage of a chain is that it lasts longer; belts are quieter, easier to package, and offer better high-rev performance if the drive load requirements aren't too high. OHC and pushrod configurations can both use hydraulic lash adjustment, which almost eliminates the need for periodic manual lash adjustments. Valvetrain friction can go either way. For reduced valvetrain friction, it is preferable to use a rolling follower, and it is preferable to use manual lash adjustment. Either of those can be done on a pushrod engine, but they seem to be more common on OHC engines. An OHC valvetrain will generally have less mass to control (hence the potential for higher operating speed), which translates to lower spring forces for a given operating speed, and lower friction.


they are both good for different things. however the old pushrod v-8 has beaten nearly every other type of motor in nearly every type of race out there. pushrod v-8s make (on average) 75% or more of their power from idle to redline where as most OHC engines dont make hardly any power till they are spinning high. for small econoboxes, for decent mpg (which also matters on weight of the vehicle), OHC is great. for power to move the whole family sedan loaded down with camping gear, for decent mpg at the same time you are towing a loaded down trailer, for MOVING when you hit the gas.... pushrod is FTW.