Aubrey, I couldn't agree more about the slot type being more efficient but as you describe, there is a danger in the exhaust valve relief area. As one who has noticed lateral valve movement, that scares me some. Enough to use clay to verify orientation clearance. Not just slot type pistons but on everything. The picture of Skips JEs show little clearance for a big cam but on the other hand, less sharp edges and more of a "flatter" piston top. I think all the experts agree a flat top piston produces better flame spread compared to a raised dome or even valve reliefs. So, the down side to his pistons is the lower dome height meaning lower c.r....... Has he enough c.r. for his needs? Yes, great dyno numbers. Could the hp be more with a higher dome and higher c.r. ? Yes. Could he install a cam with more duration? No way, without some work on the pistons. A real balancing act as to what is best for each engine. The Ross pistons I used in my engine had .150 ex. valve reliefs. I ordered them that way. I assumed .150 would be plenty for the ex. Wrong. Mock up showed way too little clearance, so , back to the machine shop to have reliefs deepened an additional .080.......... Now, the cam is .735 lift and 272 @ .050 ex. A traditional .650 lift,
250@.050 would have been a different story. The dome height of the pistons are about .650. Tried to get all the c.r. I could for race only. A piston with .400 dome height would also have required a lot less ex. valve relief. Again, all about balance. Whats best for a particular application. I'll post a pic of my pistons and you will notice no problem with ex valve lateral clearance. Them suckers are deep and wide. Downside? Less c.r.......... But, some peace of mind. I have a habit of erring on too much clearance. Cost me hp for sure, but I try not to gamble with parts banging into each other. I have collected a list of many sets of pistons that I have used over the years and some from other members, comparing dome heights. Its been interesting to me but boring to most. From what I can tell, 11 to 1 c.r. and lower usually have a .350 to .450 dome height. .450 to .550 11 to 1 to 12 to 1....... Now, that is a rough way to look at it because there are other factors determining final c.r. Bottom line? a normal street build 348 or 409 with 11 to 1 ( ordered) or less will be furnished by the piston maker with sufficient valve clearance. Its only when you want to move up to more c.r. and big cam that one can get into trouble.
And learning.
