It seems that only american manufacturers still build and use OHV(pushrod) engine(am I right?) while other manufacturers have given up and build SOHC or DOHC engines instead. What's the advantage and disadvantage of OHV to OHC engine?
What's the advantage of OHV(pushrod) to OHC engine?
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Part of it has to do with reciprocating mass of the engine parts, the less, the better. There are other factors as well.
One advantage of OHC is that the valves can work much faster therefore you can get higher RPM's.
Think harley, that has a redline of 5,000 rpms, vs a ducati V-twin that revs over 10,000 RPMS.
Both are v-twins but the limiting factor is how fast the valves can open and close.
BTW I think racing harleys can redline a little higher maybe 7500 or so.
Think harley, that has a redline of 5,000 rpms, vs a ducati V-twin that revs over 10,000 RPMS.
Both are v-twins but the limiting factor is how fast the valves can open and close.
BTW I think racing harleys can redline a little higher maybe 7500 or so.
The pushrod design has some advantages. First it is cheaper to produce. It is probably easier to "time" and keep in "time".. The chain or gear to drive the cam would be closer and more direct, compared to an overhead design with the cam on top of the head(s). With the cam being directly attached, no tensioners are needed to keep the belt or chain tight. The chain or gears on a pushrod design are always lubricated with the oil, whereas overhead designs usually are exposed and not lubricated. The chain or gears for the cam rarely wear out in an overhead design, as opposed to an over head cam which needs the belts or chains replaced or retensioned from time to time. A pushrod design is also easier to hop-up cheaply in the aftermarket, by changing different cam designs, or different rocker arm ratios to produce more lift which could require differing springs and or valves. Also the overall width and height of a comparable pushrod design will be more compact, helping packaging. Hopes this answers most of your questions.
Why would an OHV be cheaper to produce? (unless you have an extreemly long production run) it has more parts.
I don't see valve timing as being a problem with OHC's, and timing chains are normally lubricated, Belts are often used these days to make the engine quieter, and can be an issue if they break.
For 'Hop up's' I can change the cam in my OHC in about about 10-15 min, in an OHV the cam is deep in the engine and normally will require the head to be pulled.
OHC's do tend to be a bit taller in IN line engines, but no wider.
I think the OHV designe does have some advantages in the 'V' engine, where one cam can sit in the V and serve both banks, but in a smaller, in line engine, esp one that needs to rev, the OHV is an anachronism.
Ahem! The Cadillac Northstar V8 is quad cam, four valves per cylinder with VVT. Check the GM Powertrain site if you'd like proof. Scroll down a litte farther and read about the Global V6, also a quad cam, VVT motor.
That same page refers to the old, pushrod 3800 Series III. It's 200 hp, 230 lb-ft, gets 29 mpg on the highway and meets SULEV requirements for 2005. Now tell me that technology is oudated and inefficient. There's a reason they still make pushrod motors.
Besides, lawnmowers have very little correlation to cars. They haven't been subject to anything close to the same amount of government pressure that the auto companies have and don't have any pollution controls or fuel consumption requirements. If they did, you'd see the old flathead designs disappear very quickly. Those motors also don't have to work all that hard in order to perform as expected. The old expression "If it ain't broke..." definitely applies. Not so for car engines.
There was an interesting discussion of this issue in Car and Driver a few months ago. There are advantages and disadvantages to each. The article also notes that OHC designs are really no newer than pushrod designs, each has been used at different times for different reasons.
One of the big advantages of a pushrod design that is often overlooked by those looking down on the American industry for its tendency toward pushrods is weight and packaging (keep in mind that our two family vehicles are a Toyota and an Infiniti). For any given displacement and block material, the pushrod engine is almost always smaller and lighter. It has one camshaft vs. two or four (in V-engines, of course) and the hardware atop the heads is usually smaller and lighter than with an OHC design. One of the reasons that the much-maligned GM 60* V-6 family has hung on for so long is that it fits so well in so many tight places. This engine has even done duty in the Cavalier and Fiero. At the inception of the platform that has been the Pontiac Aztec and Buick Rendezvous, it was the only engine that GM had that would fit (the 90* 3.8L was too big).
Sure, there are plenty of disadvantages to the pushrod design, but it's not near as bad as some would have you believe AND it's not an "old" design vs. a "new" (OHC) that Detroit refuses to adopt. Rumor has it that Toyota is even working on a racing engine that is a ... pushrod V-8!
One of the big advantages of a pushrod design that is often overlooked by those looking down on the American industry for its tendency toward pushrods is weight and packaging (keep in mind that our two family vehicles are a Toyota and an Infiniti). For any given displacement and block material, the pushrod engine is almost always smaller and lighter. It has one camshaft vs. two or four (in V-engines, of course) and the hardware atop the heads is usually smaller and lighter than with an OHC design. One of the reasons that the much-maligned GM 60* V-6 family has hung on for so long is that it fits so well in so many tight places. This engine has even done duty in the Cavalier and Fiero. At the inception of the platform that has been the Pontiac Aztec and Buick Rendezvous, it was the only engine that GM had that would fit (the 90* 3.8L was too big).
Sure, there are plenty of disadvantages to the pushrod design, but it's not near as bad as some would have you believe AND it's not an "old" design vs. a "new" (OHC) that Detroit refuses to adopt. Rumor has it that Toyota is even working on a racing engine that is a ... pushrod V-8!
IIRC, the Toyota design is to be able to compete in some racing class that only allows pushrod engines.
Pushrod engines need to make room for the pushrods in the heads. That puts some limitation of port arrangement.
OHC valve gear between the cam lobe and valve has much less inertia and is much less flexible than the pushrod equivilants.
Some of the later pushrod engines like the LT-4 dinasaur in my Chevy, LS-1, LS-2 and the new semi-Hemi from Chrysler have outstanding combinations of performance and economy, but you won't see many new "clean sheet of paper" performance engines with pushrods unless there is some compelling reason that has nothing to do with engine design.
Even cheap little s-boxes have given up on pushrods.
I think a lot of it has to do with ''The way it has always been done.'' Camshafts once were cozily next to the crank where they could be driven by a pair of gears, and later chains. Later pushrods were run up through the valve guides and a rocker arm added to push the OHV down. I once saw an engine from the 20's where the pushrods were external to the block.
OHV may still make some sense for a Vee engine.
I fail to see how a simple SOHC like in my old truck can be more complex or expensive than a straight OHV. Most of the OHV engines already have a timing chain, chain tensioner, and cover. Does it add that much to make the chain a little longer, and the cover a little higher? My truck not only doesn't have push rods, it doesn't even have lifters or buckets or anything except call them what you want levers. The levers do have a mechanical adjustment on them, but I have never messed with them much and remained free from clatter and burned valves.
OHV may still make some sense for a Vee engine.
I fail to see how a simple SOHC like in my old truck can be more complex or expensive than a straight OHV. Most of the OHV engines already have a timing chain, chain tensioner, and cover. Does it add that much to make the chain a little longer, and the cover a little higher? My truck not only doesn't have push rods, it doesn't even have lifters or buckets or anything except call them what you want levers. The levers do have a mechanical adjustment on them, but I have never messed with them much and remained free from clatter and burned valves.
Another advantage of pushrod engines, which has been alluded to, but I didn't see mentioned specifically, is the lower center of gravity. This can make a real difference in sports and race cars and is one reason the Corvette still has a pushrod motor. Did I mention that the 'Vette motor gets 28 mpg highway and packs 400 hp?
No, they don't make mostly flatheads. The only flatheads they still make are the very low end, low HP disposable lawnmower engines. The only reason they're still flathead is that they need to stay dirt cheap, that's their market. Without them there would be no $99 walmart lawnmower. The vast majority of their engines are OHV.
In a small engine, I'd much rather have OHV over OHC. The simpler, the better.
An OHC engine has the ability to rev higher than a comparable OHV motor due to less valvetrain weight and parts. The OHC heads especially the DOHC ones flow large amounts of air and love boost. An OHC motor will always be larger and heavier than an OHV motor. I dislike the NA OHC motors due to them having a loss of lowend torque compared to an OHV motor. My Grandpa's 03 Trailblazer for ex has an 4.2L I6 DOHC motor. The thing is simply a dog until 3500 rpms where the DOHC breathing takes effect. OHC motors are noted for peaky torque curves whereas OHV motors are noted for flat broad curves.
Take the old 5.7L LS1 OHV vs Ford's 4.6L DOHC motor.. The LS1 is vastly lighter and smaller than the 4.6L, gets better gas mileage and produces more power in stock form. Jon Colleti of SVT said that they could not get the performance they wanted to beat the LS1 out of the NA 4.6L DOHC motor so for 2003 they strapped on a Eaton Roots Blower at 8lbs boost to help the lil 4.6L out. Boost plus DOHC = Orgasim. This happens because the instant boost of the supercharger makes alot of lowend torque whereas the NA 4.6L does not. You will find the 96-01 Cobra guys running very steep rear end ratios...4.10s, 4.30s @ 4.56 just to make up for the fact of having no low end torque. The lower gearing allows the DOHC motor to rev and get into its high rpm power range faster and out of the dead zone low end powerband.
The Corvette @ Viper retain the OHV design as it's **** well perfected in the LS2 and upcoming LS7 a 7.0L 427CI 500HP in the new hi po C6. They get great gas mileage, are light and compact and relatively easy to work on. The new LS2 revs to 6,500 rpms whereas the Ford 4.6L DOHC maxes out at 6,800rms. Not much difference. Ford screwed the pooch with it's modular line trying to put sports car with truck together. The bore size of the 4.6L is too small negating the fact of the high flow DOHC heads. If Ford redesigned the 4.6L to a 4" Bore, 3" stroke with DOHC high flow heads and a stout valvetrain capable of high rpms they would have a motor Ferrari would be jealous of!
Many ask the why did the LT5 ZR1 C4 have such a successful DOHC V8 with lotsa lowend punch and the ability to rev to 7,500rpms? This was due to a $6,000 package of sixteen seperate runner intake manifold with sixteen seperate injectors. Solenoid operated throttle plates blocked every other runner along with a seperate three stage throttle at the mouth of the plenum for better low end torque. This setup was awesome for performance and the LT5 had awesome lowend @ midrange torque along with the awesome DOHC breathing up top but it was freaking $$$$$$ and not reasonable for anyhting other than a high priced niche car like the ZR1 was.
I hope this helps..
Take the old 5.7L LS1 OHV vs Ford's 4.6L DOHC motor.. The LS1 is vastly lighter and smaller than the 4.6L, gets better gas mileage and produces more power in stock form. Jon Colleti of SVT said that they could not get the performance they wanted to beat the LS1 out of the NA 4.6L DOHC motor so for 2003 they strapped on a Eaton Roots Blower at 8lbs boost to help the lil 4.6L out. Boost plus DOHC = Orgasim. This happens because the instant boost of the supercharger makes alot of lowend torque whereas the NA 4.6L does not. You will find the 96-01 Cobra guys running very steep rear end ratios...4.10s, 4.30s @ 4.56 just to make up for the fact of having no low end torque. The lower gearing allows the DOHC motor to rev and get into its high rpm power range faster and out of the dead zone low end powerband.
The Corvette @ Viper retain the OHV design as it's **** well perfected in the LS2 and upcoming LS7 a 7.0L 427CI 500HP in the new hi po C6. They get great gas mileage, are light and compact and relatively easy to work on. The new LS2 revs to 6,500 rpms whereas the Ford 4.6L DOHC maxes out at 6,800rms. Not much difference. Ford screwed the pooch with it's modular line trying to put sports car with truck together. The bore size of the 4.6L is too small negating the fact of the high flow DOHC heads. If Ford redesigned the 4.6L to a 4" Bore, 3" stroke with DOHC high flow heads and a stout valvetrain capable of high rpms they would have a motor Ferrari would be jealous of!
Many ask the why did the LT5 ZR1 C4 have such a successful DOHC V8 with lotsa lowend punch and the ability to rev to 7,500rpms? This was due to a $6,000 package of sixteen seperate runner intake manifold with sixteen seperate injectors. Solenoid operated throttle plates blocked every other runner along with a seperate three stage throttle at the mouth of the plenum for better low end torque. This setup was awesome for performance and the LT5 had awesome lowend @ midrange torque along with the awesome DOHC breathing up top but it was freaking $$$$$$ and not reasonable for anyhting other than a high priced niche car like the ZR1 was.
I hope this helps..
The Car & Driver article also said that OHV engines make their torque down low in the rpm band. Not that OHC engines don't or can't, but their advantage doesn't come into play until higher rpm, where the average American driver doesn't go because of traffic, those pesky speed limits, etc.
i think american companys are generally cheap and also slow to adapt compared to the europeans and asians.
heres an example:
look at briggs and stratton, they still make flathead lawnmower engines (and ohv too, but mostly flatheads) . where as honda makes belt driven SOHC lawnmower engines. honda hasnt made a flathead mower engine is 20 years. their lowest economy model engine still features overhead valves, which is only somthing that high end briggs motors have.
briggs and stratton could of made a ohc engines decades ago if they wanted to, but they are cheap and wanted to milk the flathead profits.
i think this is a similar problem with american automotive companys. they like milking the old designs to get maximum $$$ out of them before switching onto somthing new. you can see this "milking the design" in everyting from engines to suspensions.
almost any american company could produce a quad cam v8 with 5 valves per cylinder but they never will. its too advanced and american companys like to progress is small steps so as to milk current designs for all they are worth.
heres an example:
look at briggs and stratton, they still make flathead lawnmower engines (and ohv too, but mostly flatheads) . where as honda makes belt driven SOHC lawnmower engines. honda hasnt made a flathead mower engine is 20 years. their lowest economy model engine still features overhead valves, which is only somthing that high end briggs motors have.
briggs and stratton could of made a ohc engines decades ago if they wanted to, but they are cheap and wanted to milk the flathead profits.
i think this is a similar problem with american automotive companys. they like milking the old designs to get maximum $$$ out of them before switching onto somthing new. you can see this "milking the design" in everyting from engines to suspensions.
almost any american company could produce a quad cam v8 with 5 valves per cylinder but they never will. its too advanced and american companys like to progress is small steps so as to milk current designs for all they are worth.
So he can cut the grass really really fast.
Hey, I run Mobil 1 in my snowblower.
"What's the advantage of OHV(pushrod) to OHC engine?"
Smaller overall size, lower weight and center of gravity, good reliability and low cost.
It's only a way to open and close the valves. Biggest disadvantage is pushrod flexing and usually can not achieve as high rpm's as OHV.
My wifes DC minivan has the 3.3L V6 pushrod engine. It is short enough to fit under the low hood, and has a very good reputation for reliability (unlike the mediocre 4 speed auto transmission). It's one of the best engines made and I intend to get 250K miles out of it with nothing more than oil changes, spark plug and belt replacement.
Smaller overall size, lower weight and center of gravity, good reliability and low cost.
It's only a way to open and close the valves. Biggest disadvantage is pushrod flexing and usually can not achieve as high rpm's as OHV.
My wifes DC minivan has the 3.3L V6 pushrod engine. It is short enough to fit under the low hood, and has a very good reputation for reliability (unlike the mediocre 4 speed auto transmission). It's one of the best engines made and I intend to get 250K miles out of it with nothing more than oil changes, spark plug and belt replacement.
So does it mean a larger displacement pushrod engine can possibly be smaller than a smaller displacement OHC engine in size(weight and dimension)?
Assuming you are talking about equal performance from pushrod vs OHC with weight held constant and displacement allowed to vary....That's a good question, because there are no displacemnt classes on the street.
Potentially. It's difficult top make the comparison because with the exception of Chevy, there aren't many high performance, modern techology, production, pushrod engines.
The pushrod advantage, if it really exists, would be limited to engines with relatively large cylinders. The reason for that is that longer strokes require lower rev limits, all else being equal. Smaller engines with the potential to be durable at high rpm benefit more from OHC because they need faster acting valve trains that you can do with pushrods.
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