I asked my machinist about this when I dropped of my heads to be surfaced and Vac tested. He seemed to think it would hurt the quench effect and may hurt performance. That is why I started thinkin about custom pistons. The "tub" in a turbo piston is about 6mm deep. I was thinkin if you mill a tub in the NA pistons about 3mm deep you'd be there. The valve relief cuts are about that deep already.
how would it hurt the quench? it is providing a more even, circular path for the flame kernel to travel along as it combusts.. you want it to have the same distance to travel in all directions..
Obviously, we cannot make our chambers hemispherical.. but we should be able to do more. the machinist I talked to seemed to think it might be a good idea, but not looking at a bare head he couldnt tell if there was enough meat to do the requisite grinding.. "the machinist I talked to" WAS my buddy, same age as I am, so not as much experience as some... BUT, you know, I had to say it
Anyone else here who understands what we are talking about, and have anything to contribute??? I am REALLY curious on this, but have ABSOlutely no time, money, motivation, vehicle, parts, etc etc etc to even THINK about trying it....
If you do try the custom dished piston route, I would suggest making an eccentric dish, with a deeper "hole" just below the spark plug.. again, trying to establish as close to a spherical path for the flame kernel to travel on is a top priority in redesigning the combustion chamber..
Honestly, I am talking slightly over my own head.. BUT I have started doing ALOT of reading on www.hybridZ.org (a Zcar website) and those guys are REALLY REALLY knowledgable about all this stuff.... I mean, those guys are taking 240SX cylinder heads, cutting them up, and welding them together to make a DOHC crossflow head for the inline six engine that was put in the 240, 260, and 280 Z/ZXs from '70 to '83... seriously mind boggling stuff for a Zcar/datsun fan...
Anyhow, I for one am all for machining the combustion chamber (whether on the head side or the piston side) to slightly lower compression and improve quench for this high comp turbo build concept... we need more input from knowledgeable machinists though!!
I might try emailing a couple of pics of my cylinder heads to a guy on that forum with a link to this thread... along with a concise summary of these engines, and where we sit. Something tells me that a RAM performance copper headgasket is going to be a must in any situation for true reliability's sake, though.. If you are gonna go through all this to redesign the engine.. it really is a no-brainer..