Ohv

[USA Racer]

Originally posted by Moppie
[BThe puch rod does not open the valve, its the Rocker which is actuated by the push rod which has direct contact with the valve.
And very few modern OHC engines use cam lobe to valve contact to open and close the valves, most use some kind of rocker assembly between the two.

[/b]

I know. I just firgured we all knew this. Didn't I talk about the rockers in my first post?

I agree with what your saying mr. shoping cart I was just bringing out that OHV and OHC are different. That's it. One has the cam over head. Tiz you know.

Like in my examples. An LS1 and S54 are different. OHV is one, thy other is bestowed with over head cams. I know the nearly all engines are OHV dooood.

EDIT: ^^ ...nearly all engines HAVE OHValves....

[fieroturbo]

Okay, no one has given a straight answer to our good friend "AEstud," so I will.

All (modern) automotive engines have the valves directly above the pistons in what's called the "cylinder head," which is attached to the top of the lower half of the engine, the "block," and the headgasket is in between them.

Now, OHV (Over Head Valve) is a system in which there is one valve for exhaust and one for intake per cylinder. The camshaft, which controls the action of the valves, is usually close to the crankshaft, which moves the pistons up and down. The crank is sometimes attached to the cam with two or four gears, or more commonly, a small timing chain on two sprockets. From the camshaft (which is mounted in the block) comes the lifters, (which is what wears down instead of the pushrods) which guides the pushrods, which goes from the block to the head, and connects to one rocker arm per valve, and then the rocker arm attaches to the valve and spring. Typically, OHV systems are best for trucks, because there's fewer valves, but they're large valves, which means higher torque with less horsepower is possible, more than any other valvetrain.

OHC (Over Head Cam), or SOHC (Single Over Head Cam), means that the cam is in the cylinder head, not the block, This results is a slightly thinner, and less complicated block, with a more complex, and taller cylinder head, which isn't a bad thing. But still, one exhaust and one intake valve per cylinder. The system is from top to bottom; cam, lifter, valve and spring. The cam attaches to the crank with a sproket and chain deal, or slotted belt , but longer than the OHV system.

DOHC (Dual Overhead Cam) results in the same thing as OHC, but it can support more, smaller, valves. This results in higher horsepower per cubic inch, because more air and fuel can get in. Given that a cylinder, is, well, ROUND, what's better? Two large circular valves, or four or more smaller valves? The more, the merrier. Usually though, the heads are larger, heavier, and more complex, but in the hands of a good builder, you can get much more horsepower per cubic inch than any other valvetrain. The DOHC, 32 Valve Dominion heads for Chevy Small Blocks create 120 more HP than the best OHV head available for that block. WOW. And I think Ferrari has Quad Cams in their 360 Modena.

So, with all of that said, CAN I GET A HELL YEAH?!?!?:frog:

Oh, and with lawnmower engines, typically, the valves are next to the cylinder (I don't know why), but instead of the fuel/air going down, it goes sideways :licker:. The cam is attached to the crank like OHV, so I guess it can be OHV, but it's really PWPV (Parallel With Piston Valve).:silly2: I think Honda lawnmower engines may be OHC, I'll ask my neighbour. He has an 18HP Ride on.

Anymore valvetrain questions? E-mail [email protected]

If you ask nicely, I send pictures of my PWPV Briggs & Stratton Snowblower Engine with the head taken off. I'll even send pics of the blue flames that are shooting out the back since I rebuilt it (I rebuilt it too well ). I should throw the engine in a go-kart or something.

Happy Heading! (That really didn't sound right )

[AEstud]

haha, I like your sense of humor fieroturbo.

[enginerd]

Oh no. Two pages and still... . OHV means the valves are in the head. That's it. It really is that simple. OHC means the cam is over the head. You could, theoretically make a OHC engine with side valves, but that would be retarded. Lawnmowers use side valves because manufacturing costs are much lower (besides, how much power does a lawnmower need?).

[fieroturbo]

Well OHC engines aren't labeled as OHV/OHC, just OHC. It's how the manufacturers label it. If it bugs you so much, write a letter to Bob Lutz, car guru and head of GM product design, production engineering, and I think CEO as well. Heck, he may relabel GM engines as we speak if you write to him. I can see it now.....

"In automotive news today, Bob Lutz, GM product designer, relabels all car engines as OHV according to "enginerd's" request on automotiveforums.com."

Best of luck to you! :sun:

[enginerd]

Oh, come on. Like 90% of car buyers know what the hell a cam does anyways. Most people couldn't tell a valvespring if it bit them on the ass. It's not terribly important.

[ivymike1031]

Originally posted by fieroturbo
Now, OHV (Over Head Valve) is a system in which there is one valve for exhaust and one for intake per cylinder.

I've never heard that definition before... where that come from?

Typically, OHV systems are best for trucks, because there's fewer valves, but they're large valves, which means higher torque with less horsepower is possible, more than any other valvetrain.

I think that you're mistaken about that. My best guess for why a pickup truck (I assume that's what you meant) would use 2 valves per cyl (or 3) instead of 4 is that at the lower rpms where the truck engine operates, breathing isn't a big enough problem to justify the extra valves. Having less valves means lower engine cost, so why put them in if you don't absolutely need them? There is probably a trend that exists in oem engines, that 2-valve engines have higher low-end torque, and 4-valve engines rev higher, but I think that the effect is more likely to be a result of other (more fundamental) differences in the design than the number of valves. I am aware that there are some racing applications where 4-valve/cyl heads are an option, but are rarely chosen, and I'm not sure exactly what the reasoning is there. In case you're wondering, many semi truck engines use 4 valves per cylinder.

OHC (Over Head Cam), or SOHC (Single Over Head Cam), means that the cam is in the cylinder head, not the block, This results is a slightly thinner, and less complicated block, with a more complex, and taller cylinder head, which isn't a bad thing. But still, one exhaust and one intake valve per cylinder.

There are also SOHC engines with 3 valves per cyl, and SOHC engines with 4 valves per cylinder...

DOHC (Dual Overhead Cam) results in the same thing as OHC, but it can support more, smaller, valves.

Fair enough, but perhaps it would be worthwhile to point out that you can get larger cam lobes on a DOHC than on a SOHC, regardless of the number of valves.

So, with all of that said, CAN I GET A HELL YEAH?!?!?:frog:

Sorry, not from me.

[Steel]

just get a coates rotary valveset and we can all stop bickering.

[fieroturbo]

Larger, and fewer valves result in more torque and less horsepower in the sub 4000RPM range. More, smaller valves result in more horsepower with less torque in all ranges. IN UNLEADED ENGINES!!!

quote: In case you're wondering, many semi truck engines use 4 valves per cylinder.

Diesel is a whole other world, with compression ratios in excess of 20:1, different fuel, and much larger pistons. Why do you think it's a separate course at technical schools?

And, yeah there are 3 valve SOHC (2 in, 1 ex. VW 2.8 V6), but where have you seen a 4 valve SOHC? Seriously, I'm not being cocky. I'd like to know what company (companies?) makes a 4V SOHC.

This is a learning environment, not a web word fighting arena.

[ivymike1031]

More, smaller valves result in more horsepower with less torque in all ranges.

Perhaps you didn't mean to say what you seem to be saying? If you have two engines, "A" and "B", running at the same speed (let's say 3000 rpm), and engine A is putting out more torque than engine B, then engine A is also (necessarily) putting out more horsepower than engine B. The only way to increase the power output of an engine at a given speed is to increase the torque output at that speed, right?

Assuming that you didn't mean what you seemed to be saying, is there any particular reason why you would expect that more valves will result in less torque, or is that just something that someone told you? Given an engine operating speed, and the goal of producing as much torque (and thus horsepower) as you can, I can't seem to think of a single advantage that a 2-valve system would have over a 4-valve system... perhaps you can enlighten me?

IN UNLEADED ENGINES!!!
Diesel is a whole other world, with compression ratios in excess of 20:1, different fuel, and much larger pistons. Why do you think it's a separate course at technical schools?

Wouldn't want to confuse the students?

And, yeah there are 3 valve SOHC (2 in, 1 ex. VW 2.8 V6), but where have you seen a 4 valve SOHC? Seriously, I'm not being cocky. I'd like to know what company (companies?) makes a 4V SOHC.

Nissan also makes a 3-valve SOHC, and uses it in pickup trucks (or at least they used to). If I remember correctly, that engine was quite a bit torquier than the competing, similar displacement, Toyota engine (which I think had 2 valves/cyl).

Honda makes several varieties of SOHC engines w/ 4 valves per cylinder(their 96-00 SOHC/non-vtec and SOHC/Vtec engines are both 4valve/cyl).

Even though you don't want to hear about diesels, I feel compelled to mention that many diesel engines w/ pushrods have 4 valves per cylinder (but usually only 2 pushrods per cylinder)...

[FYRHWK1]

I'm going to have to agree with USA Racer, it makes much more sense. ohv is the valves are over, or above the cam, OHC is the cam is over or above the valves, at least thats how i've always put it, i've never heard of an OHC engine with the cams below the valves or vice versa in an OHV motor so i dont see any problemwith it, but its all semantics anyway. i had always put more VPC as a high RPM valve arrangement because the charge air moves slower at low speeds, but i'm sure thats got alot to do with valve size, cam choice and intake valving, i'm sure theres great 2V topend motors and same for lowend 4 VPC engines.

[ivymike1031]

hmm... I editted my previous post, but for some reason it didn't stick. I was just going to add to the list of OEMs with SOHC, 4valve/cyl engines:

In addition to Honda/Acura, Mazda, Mitsubishi, Suzuki, Dodge, Ford, and Chevy all have 16V/cyl SOHC engines.

[SaabJohan]

Originally posted by ivymike1031
16V/cyl

16 Valves per cylinder, that's a lot of valves : )

[454Casull]

And they're run by a single camshaft. =)

[ivymike1031]

hehehe... yeah, I meant 16V/4cyl SOHC.... oh well.