Suggestions for new design Aluminum Mopar SB clean slate (kind of) cylinder heads
Could you give the long answer please?
The long answer is it depends on several things.
One being RPM. A cylinder head for say 408 inches at 7000 RPM may still be good for 340 inches at 8000.
It also depends on cam timing, which is also RPM dependent. If you are using SFT lifters you get screwed pretty easily because you can’t get enough lift before the lobe becomes too aggressive. You can get there with a roller lifter (BTDT) but its a bit harder to do with the roller.
As section increases the flow numbers go up more (let’s say as a percentage) at higher lifts than they do at lower lifts. As an example, I don’t care who you are or how you do it if you look at the same size intake valve (lets say 2.100 because that’s about the right size valve for a 4.04 inch bore) the increase in air flow at lower lifts (lets say lower lifts are .050-.350 for a cam in the .700 lift range) is increased much less than the air flow above that low lift. It is what it is. You can force more air past the valve at low lifts unless you put a blower or hair dryer on it.
The increase in flow really starts showing up at .19 L/D and up. Or close to that. So you need to open the valve more to use the increase in air flow or you’ve just pissed away a bunch of money. I will say the goal of any head designer/porter should be to get the flow to continue to increase all the way to 1.00 inch lift. I have done it a few times but a head with a conventional port location and tight pinch won’t do it. At least I can’t get one to do it. Even if you don’t use all that lift, the flow curve increasing is a clue that the port is working.
At any rate, once you increase the cross section and get more flow and you need to add lift. You quickly learn that even with a roller lobe you can get to the point of the lobe being so aggressive that you start needed Comp Eliminator style spring maintenance.
Not good for a bracket racer or even a heads up car if you don’t need it.
I learned this the hard way on my W5 heads. I developed the ports to net .800 lift. When I finally got the ports in shape to do it, the best I could do was a lobe that netted me .736 lift. And that lobe would need springs every 150-175 runs, depending on what RPM I shifted at. Had I wanted to cut that down to 100-120 runs, I could have netted .775 but the math said it wasn’t worth it. To actually net .800 lift I would have been in 40-50 run area and I didn’t have the budget for that.
Like I said, the best answer is the short answer and that is it depends.
Just because a head is “big” doesn’t make it a bad head and just because a head is ”small” doesn’t make it a good head.
These engines are so valve and induction limited it’s crazy. With a correctly sized port you could run far less seat to seat timing with much higher lifts, make more power and go quicker and faster.
All of this is nonsense to the bracket racer, but in actual drag racing it matters. Unless we are talking fast bracket racing like Top Dragster or Top Sportsman. Those guys are far less bracket racers and are closer to heads up cars.