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Using the Mach 0.6 rule, that is 670 ft/sec. You can horse around on the wallace racing calcs and get a good idea. TFS 260 CSA is around 2.8, and that gets you to 670 ft/sec at 7800 rpm on a LS7. Again, its WR calculator, so not perfect, but it does tell you unless you are building for some higher RPM, the smaller efficient port is going to do best. It is difficult in your mind to separate high flow and high port velocity, since you would think increasing flow also increases velocity
A good high velocity port likely helps tame a cam too, which would explain why cathedral heads in general can tolerate more cam than the rectangles. But in contrast, they also NEED the larger cams, so it's a good thing they tolerate them better...
The entire debate boils down to usage. Street or race. Intended application. Two different average hp categories. You can only get away with so much on the street, meaning noise, RPM, speed, safety (gasp) etc. Driveability is far too much of a debate concerning what one deems acceptable, so that’s not what I’m talking about concerning street usage.
This discussion always drifts over to the race category, as it always does and probably always will. I live in NASCAR country, and the local popo here might allow us a little more room as far as foolishness goes, but even still, street is street, and laws are laws. 7k is doable in short bursts at times. You talk 8k on the street, and you will be heard for blocks, I’m talking 3/4 of a mile. Hard to hide from cops with a vehicle like that.
Race track on other hand, is a totally different build, cam spec, induction setup and chassis setup. No matter what you believe, those guys on Street Outlaws are not driving street cars. The show is staged and nothing about street racing is actually happening. I smell a new debate coming on shortly...
My wife has reported hearing my car from over a mile from the house. I would even go so far as to say a mile and a half, but I cannot say with absolute certainty I was a full 1.5 miles away when that happened.
How do you use something like coefficient of discharge to select a head?
The easiest way is to use a set of heads designed for a smaller bore than what you have. Keep the intake valve in the ~51% of bore range. This ensures that your valves are as far away from the cylinder wall as feasibly possible.
Without knowing the piercing points of the heads/valves and orientation to the bore, it's difficult to quantify exactly how much of your curtain area is shrouded by the cylinder wall, though.
It's why my theoretical 373ci uses a 3.9" bore, with heads specifically designed to be used on a 3.78" bore. If they flow 300cfm on the small bore, chances are pretty good that they will flow more on the larger bore, and be more efficient at the same time.
Noticed smokeys grammar & punctuation is on another level now. Nice! Still, as he stated himself he doesn't think he should be more diverse. He must think he is right all the time, with any car, in any situation, anywhere, no matter what. To another statement, who do you think is really LS1Tech limited? Everyone except you? Just so you know smokey.. think that there are many people, you have probably never heard of, that have been faster than you.
Carry on.
That's because he's not high today. But high or not he still has severe tunnel vision as far as his opinions on tech.
AND high or not, he still figures he's a legend in his own mind...
The easiest way is to use a set of heads designed for a smaller bore than what you have. Keep the intake valve in the ~51% of bore range. This ensures that your valves are as far away from the cylinder wall as feasibly possible.
Without knowing the piercing points of the heads/valves and orientation to the bore, it's difficult to quantify exactly how much of your curtain area is shrouded by the cylinder wall, though.
It's why my theoretical 373ci uses a 3.9" bore, with heads specifically designed to be used on a 3.78" bore. If they flow 300cfm on the small bore, chances are pretty good that they will flow more on the larger bore, and be more efficient at the same time.
I thought you meant Coefficient of Discharge like as a ratio of observed flow to area. That’s how I understood the definition of CoD which I still can’t grasp how it would be of any value to someone buying heads.
Originally Posted by G Atsma
That's because he's not high today. But high or not he still has severe tunnel vision as far as his opinions on tech.
AND high or not, he still figures he's a legend in his own mind...
KCS, if you have "X" flow at "Y" lift, it gives you "Z" curtain area.
That curtain area has a diameter, and if the cylinder wall is closer to the valve than the radius of your curtain area, it will reduce said area.
In the case of the theoretical 373ci example I have been using for the sake of this discussion, the 1.975" valve flows 298-300cfm at, let's say, 0.650" (Tony hasn't published any data sheets, so I don't know the exact lift he achieved 298cfm)... this produces a curtain area of 4.033in^2, which has a diameter of 2.27" or a 1.14" radius.
If the cylinder wall is within 1.14" of the center of the intake valve, it will reduce the curtain area. In order to prevent this from happening, and to use every bit of that available curtain area, you have to consider valve size and placement in relation to the cylinder wall.
Sure, it's possible that a larger valve may produce the same or more curtain area than smaller valves, even if a decent amount of its area is blocked by the cylinder wall. I see it as more efficient to use the entirety of the smaller valve's area, personally, though.
KCS, if you have "X" flow at "Y" lift, it gives you "Z" curtain area.
That curtain area has a diameter, and if the cylinder wall is closer to the valve than the radius of your curtain area, it will reduce said area.
In the case of the theoretical 373ci example I have been using for the sake of this discussion, the 1.975" valve flows 298-300cfm at, let's say, 0.650" (Tony hasn't published any data sheets, so I don't know the exact lift he achieved 298cfm)... this produces a curtain area of 4.033in^2, which has a diameter of 2.27" or a 1.14" radius.
If the cylinder wall is within 1.14" of the center of the intake valve, it will reduce the curtain area. In order to prevent this from happening, and to use every bit of that available curtain area, you have to consider valve size and placement in relation to the cylinder wall.
Sure, it's possible that a larger valve may produce the same or more curtain area than smaller valves, even if a decent amount of its area is blocked by the cylinder wall. I see it as more efficient to use the entirety of the smaller valve's area, personally, though.
Would this "curtain" area help promote swirl on the backside of the valve?
I think the backside valve design, undercut stems, and the seat/throat/bowl blend have more to do with swirl than curtain area.
The curtain area is the umbrella of air that cascades off the open valve. Any swirl on the backside of the valve has pretty much already happened by the time it releases off the valve into the curtain area.
In theory, the umbrella should be circular, just like the valve. But if it's too close to the cylinder wall, that part of the circle is cut off/ blocked.
What real world difference does any of this make? I don't know. Air in the cylinder is air in the cylinder, but it seems a lot more efficient to use 100% of a 4.033in^2 curtain area, rather than 75% of a 5.4in^2 curtain area... even though they are the same.
Y'all say the lq4 block had poor bay to bay breathing. How does the ly6 block compare? It's gen 4 so I'm sure it's different I just never payed any attention to it. Just curious since I have an ly6 in my car.
I think the backside valve design, undercut stems, and the seat/throat/bowl blend have more to do with swirl than curtain area.
The curtain area is the umbrella of air that cascades off the open valve. Any swirl on the backside of the valve has pretty much already happened by the time it releases off the valve into the curtain area.
In theory, the umbrella should be circular, just like the valve. But if it's too close to the cylinder wall, that part of the circle is cut off/ blocked.
What real world difference does any of this make? I don't know. Air in the cylinder is air in the cylinder, but it seems a lot more efficient to use 100% of a 4.033in^2 curtain area, rather than 75% of a 5.4in^2 curtain area... even though they are the same.
What about some of the posts in other threads about using offset pins a la BBC to unshroud the valve and still run a larger valve? Or castings that move things around to prevent shrouding?
Using the 51% rule, LS7 valve should be 2.10, and most run 2.20-2.25. I seen some 2.3 out there
What about some of the posts in other threads about using offset pins a la BBC to unshroud the valve and still run a larger valve? Or castings that move things around to prevent shrouding?
Using the 51% rule, LS7 valve should be 2.10, and most run 2.20-2.25. I seen some 2.3 out there
I think it's great that offset dowel pins exist and work. Anything that opens up new options has potential to open up new ideas and provide the community with new knowledge.
I personally will probably not ever sacrifice the exhaust side just for a larger intake valve, but I highly value the ability to evacuate the exhaust just as much as I understand the need to get air into the engine. That's why I picked heads with a 82% exhaust ratio.
I also use 51% of the bore for intake valve diameter in order to keep the valve from getting too shrouded by the cylinder wall. For people seeking power/rpms more than efficiency, the proper percentage is probably closer to 53-56% of the bore. It's all about what you hold as the higher priority.
The ONLY car ive seen that actually perform like a 416ci LS3 headed motor with a GM intake and a hydraulic roller is this one....
The rest of them seems to fall very far from the tree.
These are good times but trap very similar to what mine 10.58@131 (LS3) and hio 10.7@130(LS6) bolt on cars did (with rockers). Cecil is one of those forgiving tracks. I want to get up there but it is 4 hours for me with traffic
Last edited by lazerlemonta; 06-14-2019 at 01:52 PM.
These are good times but trap very similar to what mine 10.58@131 (LS3) and hio 10.7@130(LS6) bolt on cars did (with rockers). Cecil is one of those forgiving tracks. I want to get up there but it is 4 hours for me with traffic
Thats impressive. Those vettes are light as a feather.
KCS, if you have "X" flow at "Y" lift, it gives you "Z" curtain area.
That curtain area has a diameter, and if the cylinder wall is closer to the valve than the radius of your curtain area, it will reduce said area.
In the case of the theoretical 373ci example I have been using for the sake of this discussion, the 1.975" valve flows 298-300cfm at, let's say, 0.650" (Tony hasn't published any data sheets, so I don't know the exact lift he achieved 298cfm)... this produces a curtain area of 4.033in^2, which has a diameter of 2.27" or a 1.14" radius.
If the cylinder wall is within 1.14" of the center of the intake valve, it will reduce the curtain area. In order to prevent this from happening, and to use every bit of that available curtain area, you have to consider valve size and placement in relation to the cylinder wall.
Sure, it's possible that a larger valve may produce the same or more curtain area than smaller valves, even if a decent amount of its area is blocked by the cylinder wall. I see it as more efficient to use the entirety of the smaller valve's area, personally, though.
David, how are you calculating curtain area? Flow has nothing to do with the equation, nor does a cylinder wall come into play here. Formula is...Valve curtain area = valve diameter x 0.98 x 3.14 x valve lift.
06-13-2019, 06:31 PM
