No surprise - the Northstar is old and needs replacing. Their OHV engines will be around for quite some time still.
Originally posted by: freebee
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?
Originally posted by: exdeath
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.
Originally posted by: Baloo
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
Originally posted by: freebee
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?
Originally posted by: Toastedlightly
Originally posted by: Baloo
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
Ah. Yes, I have taken engines apart. I am a grease monkey. The pushrod must go through the head. This blocks potential air flow. Over head valves don't go in the head one bit. Air flow is superb in those.
For example.. http://www.capriracing.co.uk/images/MarksV8EngineBuild/rebuild/pushrodadjust.JPG
Originally posted by: OS
OHC can additionally work around low intake gas velocity by opening only one intake valve at low rpms, and then opening both at higher rpms
Originally posted by: exdeath
Originally posted by: Toastedlightly
Originally posted by: Baloo
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
Ah. Yes, I have taken engines apart. I am a grease monkey. The pushrod must go through the head. This blocks potential air flow. Over head valves don't go in the head one bit. Air flow is superb in those.
For example.. http://www.capriracing.co.uk/images/MarksV8EngineBuild/rebuild/pushrodadjust.JPG
Uhm... the width of a pushrod is way smaller than the casted space between ports
You've obviously never seen a head like the 351 4V (2 valve pushrod head) with its HUGE rectangular ports. The ports on these heads are larger than the combined twin ports per cylinder on many OHC heads. The pushrods don't get in the way at all. And the head is still smaller than a OHC head.
For example, http://www.boss351.org/images/cylhead.gif