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OHV vs OHC

20K views 36 replies 13 participants last post by  Nightrider 
#1 ·
Question....
OHV engine has any serious advantages compared with OHC?

(answer "it's cheaper" consider as incorrect)
 
#3 ·
just for the record, ohc does not mean absence of rockers and associated parts. for example, the mitsubishi eclipse is an overhead cam engine, yet it still uses cam followers (rocker arms of sort), and "lifters", or more accurately hydraulic plungers that keep the followers running at zero lash against the cam. definately no pushrods though.............some engines do however use "shim buckets" that ride directly on the valve, or a hydraulic version of this. older volvos use shim buckets with replaceable shims for adjusting valve clearance, whereas older saabs used something very similar, except that it was hydraulic, and worked in the same way a hydraulic lifter does. these are just a couple of examples though.......
 
#4 ·
The main advantage of OHC is more precise timing control. There is a little clearance in all the parts between the cam and valve with a push-rod design. All that has to be taken into consideration, and varies with oil pressure and wear. Some of those parts, such as the pushrods, have a little flex under load that has to be considered also. Since a standard passenger car engine doesn't need real precise control, pushrod technology has persisted. Small high power engines, on the other hand, need to squeeze every little bit of power out of each cubic inch (or centimeter). More precise timing helps. As emission regulations and fuel economy needs get tighter, precision in control becomes more important.

Assuming a single OHC in-line, cost is probably less than a comparable push-rod engine due to assembly cost as well as fewer parts. But that's from a manufacturers point of view. I believe that's the main reason GM came out with the new Vortec in-line six a few years ago -- cost of the variable timing configuation (and control of it) would be much greater with two cams in a V configuration.
 
#5 ·
As Nightrider didn't make the ohc/ohv connection I just answered on the kiss principle. You're right about there being ohc engines with lifters, rockers and in one instance that I know of even rods. This was an experimental aircraft engine I saw at Dayton a ton of years ago.
If you want to see a neat motor, they have one there that has no cam or crank. It is an 8 cylinder opposed motor. The pistons run on a swashplate with short rigid conrods. Intake and exhaust duties are carried out by a perforated rotating flat plate that is fixed to the same shaft as the swashplate. Total moving parts, about 16. 300+ hp in a 350lb package using 40s technology. Todays tech would make this a scandalous motor. :)
 
#7 ·
One of the British engine manufacturers (Bristol?) also came up with a pushrod engine with pushrods in series. Instead of a long rocker arm to pick up the exhaust valve with a hemi chamber, they kept the primary pushrods all in a row and used short, horizontal, secondary pushrods and additional rockers to actuate the exhaust valves. Yes, it would be possible to provide an aftermarket hemi cylinder head, using this little trick, for any conventional pushrod wedge engine. The flow could even be reversed and the second rocker used to actuate two intake valves. Ah, the joy of bench racing!
 
#8 ·
BillyShope said:
One of the British engine manufacturers (Bristol?) also came up with a pushrod engine with pushrods in series. Instead of a long rocker arm to pick up the exhaust valve with a hemi chamber, they kept the primary pushrods all in a row and used short, horizontal, secondary pushrods and additional rockers to actuate the exhaust valves. Yes, it would be possible to provide an aftermarket hemi cylinder head, using this little trick, for any conventional pushrod wedge engine. The flow could even be reversed and the second rocker used to actuate two intake valves. Ah, the joy of bench racing!
Sorry, not quite understand, WHAT YOU SAY??? :eek:
 
#9 ·
both pushrod and OHC are good. Look at the power the Chevy Gen III engines are producing with pushrods. Most Over head cammers need a huffer to make 400 let alone 500 horse. Then there's Honda's S2000 engine, 120 HP per litre in a production engine!

I guess it's preferance. I like how the DOHC's can alter their LSA without buying a new cam (cams)
 
#10 ·
Most OHC engines need huffers because most OHC engines are smaller than big American V-8s. Even most American OHC engines are smaller than most of their pushrod counterparts.

But you are correct on packaging. If OHC were as compact as pushrod designs the pushrods would have been thrown out long ago. For in-line engines it's not to bad -- the engine can be leaned over ala Chrysler Slant Six and still have plenty accessory room -- just put them all on the "open" side. A V engine is different because of width. Accessories have to be tucked in the V as with the Caddy NorthStar (starter is in the V in back!) or hidden under each bank. Space gets precious fast!
 
#12 ·
KulTulz

Ahhh.. . . . . . . . I think this is the photo that graced the cover of hot rod magazine back when Ford's SOHC motor was introduced. Mid 60's I think. 600 HP on one carburetor, with gasoline. And a streetable idle. Got my vote for the best V8 motor ever!!

Pat
 
#13 ·
PatM said:
KulTulz

Ahhh.. . . . . . . . I think this is the photo that graced the cover of hot rod magazine back when Ford's SOHC motor was introduced. Mid 60's I think. 600 HP on one carburetor, with gasoline. And a streetable idle. Got my vote for the best V8 motor ever!!

Pat
Introduced and banned! As I know, banned, because it was unstreetable...although I can mistake.
 
#14 ·
In an earlier post in this thread, I mentioned a six cylinder engine with 18 pushrods. I wasn't certain of the manufacturer, so I did a little "googling" and found the following:


http://www.boc.net/buyers_guide.html

The above will give the full history, for those who are interested. Seems I was right on the Bristol origins, but it was BMW that actually developed the aluminum hemi head with the "extra" pushrods. Bristol re-acquired the engine after the Second World War and ended up churning out 170 horses from two liters for their LeMans version. This would be equivalent to 488 horses from a Chevy 350 on low octane French pump gas! And, this was 50 years ago.
 
#15 ·
smallblocker2 said:
both pushrod and OHC are good. Look at the power the Chevy Gen III engines are producing with pushrods. Most Over head cammers need a huffer to make 400 let alone 500 horse. Then there's Honda's S2000 engine, 120 HP per litre in a production engine!

I guess it's preferance. I like how the DOHC's can alter their LSA without buying a new cam (cams)
You are referring to 346 cubes versus 281 cubes ??????
 
#18 ·
Another advantage is less reciprocal motion. A bit of power is to be gained by eliminating the pushrods and rockers. Pushrods must be accelerated up repeatedly. Likewise the rockers pushed up. (The valve springs return them to position). With a mechanical OHC the cam acts directly on a shim and a cup over the valve, so the valve, cup, and shim are the only things bouncing up and down. The rest of the valve train just needs to be spun. Spinning stuff continously, all else being equal, takes less power than repeatedly bouncing them up against spring pressure. OHC is no perfect solution but does pretty well minimize the reciprocal stuff. And the less reciprocal stuff going on, the more potential for higher engine RPM's too.
 
#19 ·
Full electronic valve control is the next big step for internal combustion engines. I can see a pair of solenoids for every cylinder now! Then timing will be controlled totally by computer. Some european auto makers have been experimenting with this idea, I don't know about US makers. I know some of you will dread that day, but the aftermarket would soon have a controller where you literally upload whatever cam timing you wanted. Change the cam by plugging in a laptop or controller and uploading new specs!
 
#20 ·
farna said:
Full electronic valve control is the next big step for internal combustion engines. I can see a pair of solenoids for every cylinder now! Then timing will be controlled totally by computer. Some european auto makers have been experimenting with this idea, I don't know about US makers. I know some of you will dread that day, but the aftermarket would soon have a controller where you literally upload whatever cam timing you wanted. Change the cam by plugging in a laptop or controller and uploading new specs!
Sounds like funeral music for our old OHV V8... :smash:
Or NOT? :evil:
 
#21 ·
Flashbacks to the good old days

KULTULZ, ya brought tears to my eyes with that pic of the 427 cammer. Even had the pleasure to see and hear one run onetime.
BTW, the 4 bbl version was rated at 615 HP if I remember correctly, and that was a weeeeeeeeeeee bit on the concervative side of things.
 
#22 ·
Full electronic valve control is a reality, in a way. Some guy designed and built a smallish twin cylinder engine with fully electric valve control. He even designed the circuitry to control it. He installed the engine in a radio-controlled model power boat and it certainly works. He said something about the limited speed and strength of the available solenoids being the limit of the engine size. Bigger solenoids were needed to control larger valves and as the solenoids got larger they got slower. Thus solenoids past a certain size couldn't respond fast enough above a certain RPM. Anyways, the guy is one smart SOB with some hellacious design and machining abilities. Details of how to build a duplicate engine with plans are in back issues of "Home Shop Machinist". I believe he was selling plans, or even completed circuit boards and software, for the controls. I have a copy of the issue with the engine on the cover somewhere if somebody wants to know more.
IIRC, his idea of duplicating a cam profile was to just build the computer into the control circuit. "Cam profiles" would change on the fly according to use, even throttle position, humidity, engine load, whatever you cared to have be an input variable. And of course you could have optimum presets in place for simplicity, "cruise", 1/4 mile. 1/8 mile, roadcourse, etc. If you found it more convenient to use a laptop, you could do that too. Really cool stuff.
 
#25 ·
selonoid operated valves

One problem is the fixed reaction time or limited flexibility of opening and closing selonoids. At idle a valve will open and close 300 times a minute at 600 RPM but will have to open and close 3000 times a minute at 6000 RPM. Im not an electronics or electrical engineer, but Im not familiar with any selonoids that operate that fast. There is also the issue that electric selonoids have been snown to not stand up very well to engine heat, or the heat they produce opening and closing at high rates of repitition, as many experiments with selonoid controlled valves has proven, at least in years past.
The biggest problems seem to have been the lack of flexibility in opening and closing speeds, as well as the heat problem. Maybe someday, someone will cure both problems.
In the meantime, since I can honestly call myself Brokeassed White trash, Ill stick with the mechanical setups for the time being.
 
#26 ·
selonoid operated valves

One problem is the fixed reaction time or limited flexibility of opening and closing selonoids. At idle a valve will open and close 300 times a minute at 600 RPM but will have to open and close 3000 times a minute at 6000 RPM. Im not an electronics or electrical engineer, but Im not familiar with any selonoids that operate that fast. There is also the issue that electric selonoids have been snown to not stand up very well to engine heat, or the heat they produce opening and closing at high rates of repitition, as many experiments with selonoid controlled valves has proven, at least in years past.
That being the case, maybe someday, someone will cure both problems.
In the meantime, since I can honestly call myself Brokeassed White trash, Ill stick with the mechanical setups for the time being, and leave the experimentation of the selonoid thing to the likes of IBM, ect.
 
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