I do agree, but remember, it's a 625-650hp motor. May seem 'mild' to today's standard, but it's still 625-650hp. Any tune issues, and these pistons are less forgiving.

Anyhow, we want to make sure ring gap spec was chosen wisely, and to make note that there are a lot of Mahle 4032 pistons that have broke this way on other type motor, and many in the same spot. Yes, that relief is a pretty normal spot for any piston if detonation and/or too much fuel if.....but, again, just seams more Mahle 4032' have done this.

My IMHO, and repeating myuself, we still have to respect 625-650 as a lot of N/A horsepower. Because, quite frankly, it is.

I agree. At 1.3 HP per ci, thats a stout number for normal pump gas N/A. Not crazy, but still up there a bit. If this was a 572ci, we'd be looking at about 750hp. Healthy build.

To take a step back from the whole EFI tuning thing for a second, what are we looking at for cam specs on this build? Is there a cam card available? I know you mentioned you added lift. Also, what is the static compression, and cranking compression?

From Reher Morrison

One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.

This thread actually makes me wonder if some of the stock 496 piston failures when hopped up aren't just random piston failures or detonation, but rings butting and lifting the ring land. The production spec from Merc is 0098-.0161", that is pretty tight. I could see a motor built on the tight side of the spec butting a ring when wicked up. I'll make sure to measure after one of mine blows up lol.

4.280 bore. 2618 alloy Icon 778 piston (not 8.1L) http://www.summitracing.com/parts/uem-ic778ktd-std

I like the gapless top ring. I get no blow by at all now. Even if I plug the PCV, I get only a small amount. Used to get a huge amount that you could smell after a run.

EDit : they are pricey, but as a % of what you are spending, the increase is small.

When I talked with Total Seal, they recommended higher gaps than what was listed for any application on the sheet that came in the box. The reason was the sustained nature of the WOT runs vs the intermittent nature of circle track and the short nature of drags. IIRC, for blown drags the sheet recommended almost 0.010 LESS than they told me to run on a marine engine.

I know of a couple of stock 8.1L engines that have had a ring land lift just like this, so not just hopped up.

Just remember what the 8.1L was built for, not boats. As you know in our stringent emission world, tight pistons (this means higher silicone pistons if forged) and higher placed rings with close as possible gaps are the norm. Okay, more than a norm, pretty much a damn requirement. Any place for fuel to hide and not be burned is a no, no...emissions speaking.

Think closed cooling is just for 'corrosion protection' on these motors ? I say no. Tight clearances with hot pistons + cold cylinder bores = what ? Yup. OOps !

Rage,
I propose you get your engine back together and come down here and spend a week or so before you put it back in the boat. We'll beat the snot out of it on the dyno, where it hopefully won't blow up if something's out of kilter. In our spare time you can help me building the Cobra, or maybe I'll have it running by then. Even better! Between your afr meter and mine we can look at 4 cylinders at a time. You're retired, so time is not an excuse.

Bob

Rage did you take all the hard edges off the top of the piston before install? I usually run a tootsie roll over all the hard edges on the piston and smooth out a little. It doesn't look like this was done. Hard edges that see combustion can and will turn into hot spots acting like a glow plug and can cause issues. Doesn't look like the valve reliefs were smoothed out at all. On a small bore motor you really need to get that top ring away from the top of the piston. Where the piston broke is the weakest part of the piston. A lot more material in that area on larger bore stuff.