I fine it interesting what Chevy did with combustion chamber shape.
Chevy-
1955 to late 50's. A closed chamber wedge. SBC
Late 50's to early 60's. No chamber at all. 348, 409
Early 60's to early 70's. Closed chamber wedge. SBC
Early 70's to ??? Open chamber wedge. SBC and BBC
Early 90"s to ??? Super small chamber. SBC (40 something CC's)
The "new" SBC. Very small siamese kidney bean chamber.

Sometime in there they went with a cloverleaf but I don't rember the engine?

Are they ever going back to a no combustion chamber design like the 409? Makes sence to me. Any compression you want with no big lump in the way of the flame.

My 2 cents. You can't beat a Hemi for power on "fuel." You can't beat a wedge for power on gasoline. Why? I have no idea? But I have seen it over and over at the track.

 

The Hemi combustion chamber design had such a large chamber volume and large surface area to volume ratio that it's design led to a very slow burn rate despite having a symmetric layout and a centrally-located spark plug.

It would need so much spark timing advance (up to 45 degrees BTDC!!!) because of the slow burn and a huge piston dome to achieve decent static CR's which increased even more surface area causing a vicious cycle.

Engine tuners for the hemi would keep on advancing the timing because they saw that they achieved more and more hp gains, as they advanced more and more. Meanwhile cylinder pressures were going through the roof as they advanced more and more. Then suddenly the engine would grenade with the next increase in advance. These engines were twitchy at high rpms (shorter time for flame front travel) and would grenade out of nowhere as the spark timing was advanced just that little bit more.

 

Grooved combustion chambers
Here's a little reading material for you OMR.

 

Wow, good reading for everybody!!!!
I have seen flame grooves in HC pistons but never in the combustion chamber.
Thanks for the link mystereid!!!!
And now I'm late for work and will get fired and it's all you guy's falt!!!

 

Swirl port technology has been prevalent in automobile engines since the mid-80s. The principle creates high velocity swirls and vortecies to assist the fuel in vaporizing and homogenizing with the air. The cylinder heads of most modern engines are divided into 2 segments: the squish area, and the quench area. The squish area squeezes the air fuel charge and forces it into the larger quench area on the compression stroke. Squish creates high velocity streams inside the cylinder to vaporize and homogenize the fuel in the air.

As the spark plug fires and the flame front traverses outwardly, this pressure wave will travel down the groove to form high-pressure jet stream. At this point, the cylinder head and piston are still cool enough that this jet stream isnt dangerously hot.

Consider that the heavier elements in gasoline vaporize at 435 degrees F. at atmospheric pressure. Also consider that when the piston comes up on the compression stroke, this 435 turns into upwards of 1000 degrees F. under the extreme pressure. The liquid gasoline will go where there is the least activity in the cylinder between the cylinder walls and the piston. This center groove shoots a jet stream down along the edge of the piston to blow this liquid fuel back out into the high velocity activity area of the combustion chamber where it can combust and produce power.

Since the DS cylinder head already incorporates high swirl design and thus if the one groove would be added, it would be primarily be to clean out the ring lands of the liquid fuel.

The shape of the piston and cylinder head have their greatest affects on combustion efficiency when the piston is within about 10 mm of Top Dead Center (TDC). When the piston is more than 10 mm away from TDC, the cylinder head and pistons are just distant and innefectual walls bordering a large open expanse. So the groove does its work within this 10 mm range. On the compression stroke, they force the compressing air/fuel charge into the quench area via higher velocity jet streams to better vaporize and homogenize the fuel. On the power stroke, they guide the explosion through these same jet streams but in reverse to create vorticies, to clean out the ring lands, and to some greater or lesser degree further excite the flame front.


I have been thinking of this technique for some time, but the pentroof DS head is a very good design and with two spark plugs, I am not sure how much benefit the groove will have on power. I am totally convinced that the enhancement to a larger quench area is the way to go.

Thanks for sharing,
OMR