pushrod vs. OHC/DOHC [Archive] - GrandAmGT.com Forum

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PontiacGT
01-25-2004, 10:24 AM
ok, i guess ive heard arguments and stuff about this topic, but what exactly is the difference, cuz i really dont know.....over head cam......does that mean the camshaft is on top and the pistons point down or something? i have no idea

Spoon
01-25-2004, 10:39 AM
Originally posted by fire_beast
ok, i guess ive heard arguments and stuff about this topic, but what exactly is the difference, cuz i really dont know.....over head cam......does that mean the camshaft is on top and the pistons point down or something? i have no idea OHC is overhead cam and of course DOHC is double........ There are a coule of advantages to the traditional overhead cam in that they offer larger effective valve area. The also with the latest technology offer the ability to phase the intake and exhaust valves, ( move them together and away from each other). This can effectively change the over lap of the intake and exhaust valve to give a broader efficient operating speed for the engine. The disadvantage is that it is complicated, expensive, and usually requires more packaging space than a traditional pushrod engine do to the rather wide cylinder heads to packge the camshafts. It also usually makes the engine longer to accomodate the cam drive.
Hope I don't confuse you, If so, ask again.

Filter
01-25-2004, 12:58 PM
Spoon is the man and he know his stuff. That's the answer fact for fact. I just wanna add my opinion.

I personally like pushrod because as Spoon stated they are simpler and cost efficient to tinker with. A shadetree mechanic can work on a pushrod engine alot easier than OHC. My opinion anyway.

Also, and I don't know if this is true, but I feel pushrods are capapble of making alot more torque than OHC. What this means is that a pushrod motor with less HP will actually pull just as hard and keep up with an OHC motor with more HP.

95-GT
01-25-2004, 01:50 PM
Another thing is....[D]OHC motors usually dont have heads avaiable for then, as they are just to expensive to make.

PontiacGT
01-25-2004, 03:24 PM
alright so what ive gathered is that DOHC engines are more expensive to fix due to complicated engineering, and pushrod seems to have more torque with same HP. I guess what im curious about is what's actually different inside? spoon when you said "larger effective valve area" what did that mean? define pushrod if you can, what makes a pushrod a pushrod? i dunno i guess i just dont know anything at all about DOHC so im just curious. does the camshaft push up the valves on a DOHC engine? if so, isnt that also pushing rods? when you said "phasing the intake valves" can you explain that process a little more to me? to help illustrate a picture in my mind.....thanks

Blackrider
01-25-2004, 03:54 PM
a pushrod engine usualy the cam is in a more : Centralized location, In V style engines is in the center, as the cam spins the lobs push up on the lifters witch Push up on the pushrods intern forcing the rockers to push the valves open, so you see there is a Hole lot of "Pushing" involved.

As were a OHC (DOHC or SOHC) engine has the cam ontop of the engine witch eliminates the need for Pushrods and and some cases rockers.

Dodge Neon DOHC's 2.0L still have Rockers as where the New Tiby DOHC 2.7L does not the Lobe of the cam pushes directly on the lifter

the GM 2.2L that comes in the base model Sinfire and Cav has the cam down in the block and off to the left if looking at it from the timing chain side. It is also Refurred to as a "Pushrod" its a good EG of a Pushrod 4 cyl.

PontiacGT
01-25-2004, 05:52 PM
thank you! that explains a lot to me. cool. is there any other places you guys can refer me to to learn more about OHC? like a website or article or something? thanks!

Blackrider
01-25-2004, 05:54 PM
Originally posted by fire_beast
thank you! that explains a lot to me. cool. is there any other places you guys can refer me to to learn more about OHC? like a website or article or something? thanks!

not one for soly OHC that i know of but just ask some questions I'm sure we can answer most/all of them

atc3434
01-25-2004, 05:58 PM
Here's a little picture so you can actually see the differences. Pushrod designs have much more valvetrain mass (Valvetrain is all the stuff needed to operate the valve) than OHC. This usually allows for a high rev capability in a OHC application. Also, pushrod designs do not offer any reason to generate more torque than a OHC design. Look at for V8's with OHC, they make plenty more torque than horsepower.
http://www.grandamgt.com/forum/attachment.php?s=&postid=334420

Blackrider
01-25-2004, 06:02 PM
Good Example!

PontiacGT
01-25-2004, 10:39 PM
ah sweet that's perfect. thats great! but im wondering how come it's harder to work on and more expinsive for OHV if there is less material than pushrod?
if the camshaft is on top, are the pistons still facing upward then, and the crankshaft is below? i would assume that the valves are on top but im just makin sure. hmmmm what else... not sure. just kinda tryin to learn a little about this topic so yeah anything else you wanna add that's cool....

can you think of any other advantages/disadvantages in comparrison?

Blackrider
01-26-2004, 12:15 AM
Originally posted by fire_beast
ah sweet that's perfect. thats great! but im wondering how come it's harder to work on and more expinsive for OHV if there is less material than pushrod?
if the camshaft is on top, are the pistons still facing upward then, and the crankshaft is below? i would assume that the valves are on top but im just makin sure. hmmmm what else... not sure. just kinda tryin to learn a little about this topic so yeah anything else you wanna add that's cool....

can you think of any other advantages/disadvantages in comparrison?

Wish OHC engies you have alot of times chain/belt parts they are really the hardest part to figure out.