I'm looking at a couple options for (forged 4032) pistons and getting ready to pull the trigger to get this 3 monkey circus driveable again after the silent but deadly spun rod bearing. Boost level should remain around 10-12 psi. L9H at 4.070, stock heads, D1SC, LLT2 VVT cam from GPI, LT's.

Both are Mahle TBC, or ceramic coated tops, and I'm shooting for a stockish static of 10.5-10.7. Option 1 is a Mahle Flattop yielding 10.2 static. Option 2 is a Mahle dome yielding 11.1 static.

I know the tuning window for 11.1 is much smaller (and domed may present other issues?) than the 10.2, but will the ceramic coating on the piston top only offset that?

"Premium" fuel in my parts is 91 clear, I have access to e85 but not distributed enough regionally to run it full time.

 

Play it safer with flat tops and lower compression. Everyone wants to have a max tune on a daily driver, I like factory style tunes that you know you can hold the gas pedal down with no worries.

 

Yes, I agree. If you want a tad more compression just mill the heads a bit.

 

especially with a boosted setup, you'll want to get your cam specs to calculate the dynamic CR.
case-in-point: i took my stock long block LS1 + TVS that was making 525h/540tq @ 11psi and rebuilt the top end while spinning the blower faster. bumping SCR up to 10.6 but running a cam with more overlap yielded me nearly the same DCR as before except now pushing 10psi & laying down 650h/630t... and then i spun the blower even faster for 13psi. still runs fine.

 

My 408 is around 9.5 to keep it safe on pump gas. With my compression that low I really don’t have to worry about fuel quality.

 

illustrative numbers from my before & after

stock 2000 LS1 + TVS2300 @ 11psi




after ported/valved 243s, a bigger cam, and increasing blower speed



so as you see here, even though i bumped up my static compression up almost a half point, i've got lower overall cylinder pressure than before.

 

Definitely go with the flat tops. Not only is the "lower" compression more compatible with the combination of boost and 91 octane; but flame propagation with domed pistons isn't as efficient and the poor quench can actually increase potential knock. The coating will help keep the piston cooler and reduce heat transfer into the rings in either case, but it won't help the quench impact with the domed.

And if you really want to use coatings to control knock, doing the chamber and valves will be beneficial. You can get probably 2/3 the effect by doing just exhaust valve faces if cost is an issue (they are typically the hottest are of the whole combustion chamber). That will also help exhaust valve and valve seat longevity. Smooth all chamber edges to prevent hot spots, and don't forget to drop the spark plug heat range and run a slightly tighter gap than stock.