From the rim not the pad.
The pad may have to be machined off to set the Q.
But the volume of the Q-pad, after it has been milled, will need to be figured into the equation to determine the Scr.
The volume of the dish has been given, but there is no accurate way to determine the volume of the finished pad by itself, so you will have to use the .500 down fill-volume method.
But be of good cheer, the uncut volume of that pad is likely only between 1.5 and at most 2.5cc
The rim will need to be at the deck for 10.4Scr, I think
With an .027 gasket, that Q-pad needs to end up just below the surface of the deck about .013, for Q of .040, But if the rim is at the deck, you'll just cut the pad flush to the piston rim for a Q= gasket thickness, or .027 in this case. Or increase the gasket thickness by the amount of the pop-up.
If the Scr works out to 10.4, then with than ICA of 71*,the Dcr works out to 7.9@158 psi, great for iron heads.
How did I get an ICA of 71*? I averaged the durations and subtracted twice the LSA to find the overlap,which came to 69*. I gave half of it, 34*, to the intake , and determined the ICL to be 108.5. This fixes the ICA at 71*. That's a heck of a cam.
Looking at that piston, I'm guessing to make 20cc in the dish, that it would need to be about .085 deep or more. If you measured it at .057, then the rim must have been above the deck, some estimated .028 inch; and that would all have to all come off, to make room for that .027 gasket..... .or a gasket at least .028 thicker chosen. Or some combination.
But for aluminum heads, this is not the way I would do it; I like my 185 psi. But then if I had a stroker, on the street, even at 158psi, the bottom end is already stronger than most chassis will be able to handle, and the bonus is it will burn almost any old crap gas.
But; to get 180psi, you would need to get the Scr up to around 11.5 for a Dcr of 8.7@179psi. And that's where the problem is . Your swept is 857cc, and to get 11.5, you would need a TOTAL chamber volume of just 81.6cc. You can make about 76cc with an .039 gasket, but the FLAT tops would need to be down a bit, depending on the volume of the eyebrows. It would be very easy to have too much compression. I'd estimate about .030 down. But this makes the Q to be .069, so it is as you said .
The tops of those flat-tops would need some custom milling, to leave a Q-pad up near the deck. 180psi would make a killer bottom end, certainly more than any streeter could likely handle, and would be waaay overkill. So
A little less cylinder pressure would be fine. The trouble is ,with flat-tops, how to get it; you'd have to sink the pistons even further.
I feel your pain.
IMO, the dished pistons would be the way to go, but less cam would pump up the cylinder pressure. Unfortunately even 170psi, would take a LOT less cam, at 64* ICA. That's 7* less on each end so about 270 intake duration. And the bottom end would be ludicrously strong, posting a VP of 168.
If you really want to run that very large cam, AND aluminum heads, AND make the big pressure, I guess the best solution is to order custom pistons, with the correct compression distance, the correct Q-pad height, and the correct dish volume. They're not that much more money, are they? And think of all the machining costs you'll save.
But that's a heck of a cam.
I suppose you could compromise with a little less duration AND a tighter LSA, and sacrifice some overlap. The math says a 274* cam, with an LSA of 106 and installed straight up would have an ICA of 63* , close enough. This could be had with 65* of overlap. And, as a solid flat tappet, at maybe 235*@.050... Mr Wallace says Scr of 10.4/8.45 Dcr./172psi and a whopping 171VP
Decisions, decisions.....
But before I would get too excited, I would revisit the deck height, this time measuring to the rim,not the pad and not the bottom of the dish. Ideally it will be at or above the deck.
Does this look like your dished piston?
