. I've got some ideas on my own, but I'm wanting to know if someone else has figured out a way for doing it on the cheap. Getting some piston volume into the chamber
I agree, the numbers on the cheap, are hard to reach with the big-port heads. I haven't done it. I have all the cores to do it. But because of the expense-to-fun ratio being so much higher than with a 360, I nixed the idea.
You might like to work the formulas backwards to see what falls out.
For instance;
A nice FTH street cam to run in a 318 might have an Ica of 62*
Say you already have your big-port iron heads at 72cc.
And you will run a 6.3cc gasket
say you want to get the Q down to .040 which will allow you to run 165psi on best gas.
Plug those into the calculator as best as you can, then start whittling the Scr down/up until the calculator spits out the 165psi.
That will give you an Scr number to target. Now all you have to do is shape your piston and place it at the correct distance from the deck to achieve the Required Scr
Here's an example;
Static compression ratio of 10:1.
Effective stroke is 2.61 inches.
Your dynamic compression ratio is 8.09:1 .
Your dynamic cranking pressure is 161.14 PSI.
V/P (Volume to Pressure Index) is 121
In this example, I inputted the parameters of 3.91/3.315/62* Ica/ and 900ft elevation. Then I played with the Scr to eventually get 161psi which was close enough to the target 165psi.
So the target Scr is 10/1
using the formula; Scr =(CV+cv)/cv where
CV is the swept volume of 652cc, and Scr =10, we get
10= (652+cv)/cv which is rearranged to cv = 652/10 minus1 =652/9=72.44cc
Which is the required TOTAL chamber volume. Therefore
72.44 less (72+6.3)= minus 5.855cc for the deck volume.
Now you just have to figure out how to manipulate the deck-height and piston crown, to get it, but when the Q goes to a bad spot, you gotta abort the exercise.
So then, because of the Q-problem, you gotta start with a reverse-dome piston that pokes a dome up on the far side of the piston, and whittle it down to fit. Then re-figure for the net -5.855 cc.
KB's solution is the Step-D cup piston
Now, if it becomes impossible or very expensive to make a piston fit, then you shave the heads to a new number and start over.
If you need to fine-tune it; you can play with the Ica a couple of degrees, or with the head gasket, but that will change the Q. And finally, you may have to settle for less cylinder pressure and cover the bottom-end powerloss with a higher stall.
fun with numbers
But if you find a piston that looks promising, and looks like it will provide an appropriate Q, you can work from the decks up.
One thing nobody talks about is slightly longer rod. I mean if yur buying rods anyway......
