OHV vs. OHC
Le Mans Master
Joined: Nov 2005
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Valves have not ALWAYS been where they are today (over the combustion chamber), some engines it still isn't (2 stroke engines for example). OHV means the valve is OVER the combustion chamber in the cylinder head. OHC means the cam is also in the head. DOHC means 2 cams in a single head.
Last edited by dougbfresh; Dec 3, 2007 at 10:10 PM.
Team Owner
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From: S.W. Ohio. . . . . . NRA Life Member
Racer
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From: Braselton GA
IIRC, the packaging of the LT5 was done quite well, with the dimensions essentially the same as the L98/LT1, but the ZR1 mill is 84 lbs heavier than an LS7, and with the majority of that extra weight in the 4 cam heads, has a CG probably several inches higher. While that may not seem like a lot in a vacuum (engine on a test stand in a lab) the engine has to move down the road with the car, lower weight and lower CG are both a + to anything trying to change direction, especially when it's sitting on the nose.
Last edited by drivinhard; Dec 3, 2007 at 11:54 PM.
Le Mans Master
Joined: Dec 2006
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From: Phoenix Arizona
You all are hitting on great ponts. I have heard the Internal combustion engine refered to as "An Elegant Balance", because you can make them so many different ways and with soo many different functions in mind. A Ship motor with a 3 meter bore has the same parts as the engine in your lawnmower, but they are completely diferent.
You will always be able to make more power from a DOHC engine than a OHV engine, all other things being equal, simply because you can spin it faster, but I agree that I don't want to spin my engine to 10k. I like driving down the highway at 1500 and getting 32 mpg
It is possible to use variable valve technology in OHV engines as well. GM uses quite a few cam phazers to advance and retard the timing of their valve trains.
You will always be able to make more power from a DOHC engine than a OHV engine, all other things being equal, simply because you can spin it faster, but I agree that I don't want to spin my engine to 10k. I like driving down the highway at 1500 and getting 32 mpg
It is possible to use variable valve technology in OHV engines as well. GM uses quite a few cam phazers to advance and retard the timing of their valve trains.
Burning Brakes
Joined: Nov 2002
Posts: 807
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From: Los Altos CA
One advantage of 4-valve DOHC engines is that at lower RPM, one intake valve is usually closed (actually the port is blocked) and this results in a flatter torque curve and a less peaky engine. However, I am super impressed with the LSx engines - somewhat higher in displacement but not that heavy and very reliable. I have a Jag with a DOHC V8 and am scared to death of a timing chain failure (which ruins the engine). I have a Nissan DOHC V6 and that requires replacement of the timing belt every 60K miles at about $700 a pop. It used to be that simple OHV engines with hydraulic lifters were limited in RPM due to valve float but most DOHC engines now have RPM limits about the same as the LSX so that advantage is gone.
Racer
Joined: Apr 2006
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Overhead cam and overhead valve heads look the same on the combustion side.The difference is in the cam placement and valve geometry on the top side.The Ford Modular engines are a great example of why the ohc and dohc is overkill.They are a lot more complicated and less durable, for no gain. Hell, even the dealers considered them throwaways if they needed major internal work.For exotic engines, dohc is the way to go.Think motorcycle technology.
Melting Slicks
Joined: May 2000
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From: Brentwood TN
Now if we REALLY want to get serious about high rpms and valve geometry, let's talk about desmodromic valve systems that have NO springs. One cam opens the valve and another cam closes the valve. Use roller bearings on the rod and main bearing journals and you get an engine that is materials of construction stregth and intake and exhaust flow limited. Mercedes raced an engine like this 60 years ago.
Charlie
Burning Brakes
Joined: Jul 2004
Posts: 866
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From: Jensen Beach FL
As long as we’re getting rid of valve springs, why don’t we just go ahead and ditch the cams too? I’ve seen very interesting research where computer controlled electric solenoids are used to open and close the valves. Limitless possibilities as far as VVT. Very interesting technology. I think current automotive electrical systems will not support the electrical draw of a system like this however.
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Le Mans Master
Joined: Nov 2005
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As long as we’re getting rid of valve springs, why don’t we just go ahead and ditch the cams too? I’ve seen very interesting research where computer controlled electric solenoids are used to open and close the valves. Limitless possibilities as far as VVT. Very interesting technology. I think current automotive electrical systems will not support the electrical draw of a system like this however.
Le Mans Master
Joined: Nov 2005
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I guess it's really better that we didn't know 30 years ago that we couldn't buzz a small block like we did. I had a blueprinted 265 K-stock '56 Vette that regularly turned 7200 rpm through the traps. I was an NHRA tech inspector in the late '60s and to mid '70s and saw any number of small blocks running in various bodies in super stock that were spinning 12,000 and one that ran over 14,000 rpm through the lights.
Now if we REALLY want to get serious about high rpms and valve geometry, let's talk about desmodromic valve systems that have NO springs. One cam opens the valve and another cam closes the valve. Use roller bearings on the rod and main bearing journals and you get an engine that is materials of construction stregth and intake and exhaust flow limited. Mercedes raced an engine like this 60 years ago.
Charlie
Now if we REALLY want to get serious about high rpms and valve geometry, let's talk about desmodromic valve systems that have NO springs. One cam opens the valve and another cam closes the valve. Use roller bearings on the rod and main bearing journals and you get an engine that is materials of construction stregth and intake and exhaust flow limited. Mercedes raced an engine like this 60 years ago.
Charlie
