Zenmervolt
Elite member
- Oct 22, 2000
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Which is what I said earlier. And what you repeated after I said it. And what hasn't been disputed by anyone so far in this thread. Why say it again?
ZV
ZV
I think it would be more accurate to say that non-OHC OHV wins at power/mass/cost, but not necessarily at power/mass.
Zenmervolt
Elite member
- Oct 22, 2000
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Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
Zenmervolt
Elite member
- Oct 22, 2000
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"OHV" is universally recognized as referring to pushrod designs, so specifying "non-OHC OHV" is redundant (and slightly pedantic). Regardless of how it is referred to, OHV engines are indeed lighter and smaller (physically) than OHC engines barring the use of some very exotic materials in the OHC engine. Given identical materials, the OHV engine will be lighter.
Like everything, I am sure that there are a few exceptions, but it's nearly universal that OHV is lighter than OHC.
ZV
It's the exceptions I'm talking about. If you cap the max RPM of the two types, OHV is likely to have a higher HP/lb ratio, but if you maximize displacement while keeping mass constant (for the purpose of this comparison), as you increase the redline, OHC will eventually surpass OHV in HP/lb because OHV will crap out before OHC does.
That's why I said OHV is not necessarily superior at power/mass. Also, I wasn't sure that everybody knew that OHV meant pushrods, since OHC is OHV but OHV isn't always OHC, so I threw in the non-OHC part.
EDIT: I don't disagree that OHV is lighter than OHC for a given displacement.
Zenmervolt
Elite member
- Oct 22, 2000
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FWIW, the Mercedes Ilmor 500I CART engines were pushrod designs and they ran 12,000+ RPM. Given a determined engineer, it's possible to design an OHV engine without flow issues at high RPM, and also to overcome the normal limitations of valvetrain mass.
By the time that redline was increased enough to raise the hp/lb of an OHC design sufficiently, the engine would be incredibly peaky and probably close to impossible to drive on the street.
For all practical purposes, the RPM limitations of a modern OHV design aren't going to come into play. However, the marketing benefits of OHC and the way that OHC allows engineers to more easily design the intake and exhaust ports tends to make OHC more popular. Also, in-line engines don't have a convenient location for an in-block camshaft like a vee engine does, so OHC is a natural choice for an inline engine.
ZV
Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
"Postcount++" perhaps?
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
:thumbsup:Originally posted by: MercenaryForHire
Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
"Postcount++" perhaps?
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
++
What? I refuse to believe that we are all just neffing.Originally posted by: fbrdphreak
:thumbsup:Originally posted by: MercenaryForHire
Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
"Postcount++" perhaps?
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
++
I refuse your refusalOriginally posted by: Howard
What? I refuse to believe that we are all just neffing.Originally posted by: fbrdphreak
:thumbsup:Originally posted by: MercenaryForHire
Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
"Postcount++" perhaps?
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
++
++
touchéOriginally posted by: fbrdphreak
I refuse your refusalOriginally posted by: Howard
What? I refuse to believe that we are all just neffing.Originally posted by: fbrdphreak
:thumbsup:Originally posted by: MercenaryForHire
Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
"Postcount++" perhaps?
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
++
++
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 36
- 91
You figured it out! Damnit.Originally posted by: MercenaryForHire
"Postcount++" perhaps?Originally posted by: Zenmervolt
Why not just edit the previous posting? Would be more efficient. Besides, why bother with reference when there's no dispute?Originally posted by: MercenaryForHire
My guess is for reference.
- M4H
ZV
Oh, and postcount++ for each of us for debating the worthiness of it.
- M4H
ZV
Actually, I think they were playing with the idea of a Caddy clone of the vette, and I think that had a northstar
Originally posted by: Toastedlightly
Originally posted by: Howard
Why can you get lots of low-end torque with pushrods but not with OHC?Originally posted by: fbrdphreak
No more uber-low end torque
I'd assume you could muster more by having far better paths for your intake and exaust ports due to not having push rods in the way.
You clearly have never actually seen the internal setup of a push rod engine - overhead cams are a much bigger obstacle to get around than push rods. Push rods are not in any position where they are in the way of an intake or exhaust manifold - they are down inside the cylinder bank. But then, you just assumed. Bad assumption
Why? Their OHV offerings are already quite good, where else does it have left to go? Great power, great reliability, low cost, good mileage? Maybe for an all out performance engine to get more out of the displacement?
No more uber-low end torque
[/quote]The most direct answer is to better suit the luxury and/or near luxury segment of the market. As excellent as the LSx motors are, OHC engines are inherently "smoother" running and a better fit for that target market.
4.6L DOHC V8 vs 5.0L OHV V8
That's not as funny as this picture though:
Miata engine + tranny in front vs 5.7L Small Block Chevy engine + tranny in back
exdeath
Lifer
- Jan 29, 2004
- 13,679
- 10
- 81
Originally posted by: Howard
Why can you get lots of low-end torque with pushrods but not with OHC?Originally posted by: fbrdphreak
No more uber-low end torque
It?s all about port velocity and cylinder filling.
OHC designs have a larger cross sectional valve area thus allowing more air flow at higher speeds which allows for more power on the top end. This isn't possible on OHV engines because it would be too complicated, if not impossible, to route 4 pushrods and rockers, etc, through the small space next to and above each cylinder. OHC engines are pointless without going more than 2v per cylinder, as you gain nothing but the disadvantages (weight, complexity, package size, etc). You also have a limit to how big you can make 2 valves and still be able to fit them in the cylinder without colliding and making them light enough to reciprocate fast enough. Thus OHC allows for higher quantity of smaller valves to increase the total valve area and allow for more horsepower at high RPMs.
On the low end, the large cross sectional area reduces gas flow velocity. Because the displacement of the cylinder is constant, the pressure/vaccum drawing air into the cylinder is constant. With constant pressure, you can vary the flow area to vary the velocity. When you put your finger in front of a garden hose you are increase the velocity by reducing the area; the mass flow rate and pressure remains the same.
High mixture velocity is necessary to efficiently fill the cylinder on the intake stroke by relying on mixture momentum and scavenging to pack as much mixture into the cylinder as possible when the engines pistons are not doing a good job of that on their own (i.e.: at the low end of the RPMs). With a OHV pushrod engine you have higher port velocity at low RPM, due to the smaller cross sectional area of less valves. What happens is there is momentum in the intake stream caused by the higher velocity, such that cylinder filling continues even as the piston has already stopped moving down. In fact the intake valve remains open and the cylinder continues to fill as the piston has already started to move up. This allows more mixture = more power. Port velocity is crucial to low end torque. However OHV engines typically cannot rev as high or make as much power on the top end as OHC engines due to the more restricted flow rate.
OHC engines with more than 2 valves per cylinder trade off this low RPM port velocity for high RPM breathing at the cost of low end torque to gain high end horsepower. Most OHC engines use variable intake runner systems that block half the intake ports below a certain RPM and open them at higher RPM to get both good low end torque and top end horsepower. The best of both worlds. Ideally, a OHC engine should produce more power on a smoother power band than a OHV engine, all other factors held constant. But engines like the LS7 prove that you can make *any* engine do whatever you want if you throw enough money, exotic materials, and engineering into it.
Better yet is forced induction (specifically a belt driven supercharger) on a 4v OHC design. You get the high flow rates at the top end, with the blower to take full advantage of the increase flow rate. But also at the low end of the scale, you have the blower pressurizing the intake, so that even with the larger cross section, as soon as those valves crack open the tiniest bit, the mixture rushes in at high velocity due to the supercharger forcing it.
Thats how my '03 Cobra is set up and it works wonders
4v OHC engines love superchargers.
exdeath
Lifer
- Jan 29, 2004
- 13,679
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By the way, the only reason GM even makes the 32V DOHC Northstar is because people who buy Cadillacs demand 'more advanced' and 'exotic' technology. They don't need it, it's completely a psychological comfort for the consumers of that market segment.
Due to people being completely clueless about engines, some people think overhead cam multi valve engines are deemed as somehow automatically better and more advanced than pushrod engines in every way just because its 'newer' as far as they know.
If you ask me, a pushrod OHV 2V engine with torque on the bottom end is more suited to move these heavy luxary cars where the owners just want to push the gas and go and don't care about revving it to next Sunday. They aren't supposed to be sporty and high revving noisy air breathing monsters. It would also be in a smaller package and allow for more passenger or cargo space.
There are applications for both types of engines. For example, multi valve high revving free breathing DOHC 4v engines respond to forced induction much better than 2V pushrod engines, where the smaller ports that give high velocity in the low RPMs becomes a severe air flow restriction at high RPMs. Technically, that is true, but then again look at the top fuel dragsters running 40 psi boost with 14-71 blowers on 2V pushrod V8s making 8,000 HP... so who knows.
Due to people being completely clueless about engines, some people think overhead cam multi valve engines are deemed as somehow automatically better and more advanced than pushrod engines in every way just because its 'newer' as far as they know.
If you ask me, a pushrod OHV 2V engine with torque on the bottom end is more suited to move these heavy luxary cars where the owners just want to push the gas and go and don't care about revving it to next Sunday. They aren't supposed to be sporty and high revving noisy air breathing monsters. It would also be in a smaller package and allow for more passenger or cargo space.
There are applications for both types of engines. For example, multi valve high revving free breathing DOHC 4v engines respond to forced induction much better than 2V pushrod engines, where the smaller ports that give high velocity in the low RPMs becomes a severe air flow restriction at high RPMs. Technically, that is true, but then again look at the top fuel dragsters running 40 psi boost with 14-71 blowers on 2V pushrod V8s making 8,000 HP... so who knows.
THe OHV vs OHC debate (as pointed out in several posts) is a marketing issue, not an engineering one. There are good applications for both designs, it is simply a matter of what the consumers demand in a particular model.
That said, GM engineers are stupid. Forget the extra torque, GM needs to learn how to make music from its engines. How many more people would buy a corvette if it sounded like an F430?
That said, GM engineers are stupid. Forget the extra torque, GM needs to learn how to make music from its engines. How many more people would buy a corvette if it sounded like an F430?
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