Tighter Quench = Horsepower Loss NA?

[ptuomov]

How about the fuel was liquid enough to wash that area clean.

I always thought that when there is more fuel than air in the area, the area ends up dirty. When there more air than fuel, the area ends up clean. Oxygen is Shiva, the destroyer of worlds, that gets rid of anything carbon based or organic. Not pretending to be an expert, so correct me if I am wrong.

[DrillDawg]

You know what you know, but if the piston is squishing out liquid fuel it's going to wash that area. The fuel injector spray keeps the back of the bowl clean and the "squishing out" of liquid fuel washes that area. We are talking about a properly set engine, not some slug, lol.

[statsystems]

You know what you know, but if the piston is squishing out liquid fuel it's going to wash that area. The fuel injector spray keeps the back of the bowl clean and the "squishing out" of liquid fuel washes that area.

If it's clean there is no combustion there. I figured that was settled science. No fuel is washing carbon off the piston and make power and live long.

[DrillDawg]

There is combustion residue above the top ring on most pistons so there is no reason the same residue would not be all over the top of the piston because that's where most of the combustion is taking place, so now you have to ask what's washing the residue away, it sure is not oxygen. You have to look where the liquid fuel is going that's hitting the back of the bowl and running down the cylinder once it gets past the valve.

[groberts101]

There is combustion residue above the top ring on most pistons so there is no reason the same residue would not be all over the top of the piston because that's where most of the combustion is taking place, so now you have to ask what's washing the residue away, it sure is not oxygen. You have to look where the liquid fuel is going that's hitting the back of the bowl and running down the cylinder once it gets past the valve.

Maybe the flame front consumes the bulk air/fuel mass in the most homogeneously mixed areas first and never really makes the full burn into the perimeter/crevice area(shrouded)? And either fuel puddling/insufficient AFR isn't conducive to full burn.. and/or oxygen content quickly falls after being wicked towards the main flame front? Similar to the soot creation when quickly capping a burning flame?

[DrillDawg]

The Mopar big block engines that I build usually end up with 0.040 quench. Either zero deck the block and use a 0.040 gasket, or put the pistons 0.005 down and use a 0.036 gasket. That is just a recipe that works with off the shelf parts. That combination makes good power, has decent BSFC (less than 0.450) and doesn't seem to cause any long term issues. I suppose at this point I'm not interested in trying anything tighter and I don't see any reason to run anything looser. The top quench area is usually shiny after dyno testing which tells me that the combustion isn't fully uniform, but it doesn't seem to be a big problem. Here is what a typical BB Mopar looks like after some dyno pulls. This is a pump gas 470 inch engine that made more than 750 hp. You can see a little bit of shiny area at the top edge of the pistons.
DSC_0436 (Large).jpeg

To me the top and bottom moon shaped shinny spots are from piston rock and the lower shinny spot with the black above/next to it is from puddled fuel and rich burn from that fuel. Those shinny/black spots are right below the intake valve.