To what extent is a desireable quench zone rendered less effective by chamber coating, piston coating and the use of forced induction?
My percieved value of the quench is to allow a final surge of mixture towards the center of the bore. Minimizing crevace and surface area to concentrate the center of the explosion in a homogenous swirling mixture that is the primary benefit of the whole .035 quench right? The air movement across the face of the piston helps to even out hot spots that may spark pre-ignition is another valueable byproduct.
So what are the chances of the fuel droplets from an injector [that just passed a hot intake valve being carryed by a intake charge from being compressed to say 175 degrees] puddling out or being centrifically spun onto the cylinder wall? Granted your not always on the boost but doesn't a properly advanced ignition run fairly cool at part throttle.
How much can you shift the bulk of the chamber volume to one side of the chamber or the other without adverse effects on piston slap and rocking. This imbalance will try to rotate the piston around the pin as well as push it down right?
I was looking over some -9.2 cc reverse dome pistons from my engine that threw a rod last spring. All have most of the visible wear on the oppisite side of the d-dish. The rod appears to have snapped just below the piston pin first and flailed around and disintegtrated. The piston apeered to have stayed relatively close to the top of the cylinder judging by the number of valve contacts on the crown and relatively intact skirt. The pistons were installed with the correct side clearance [.004 as i recall] and the rings were installed with additional gapping for a supercharger. I am making a chamber shape mold of the piston dish and chamber together and it appears that about 75% of the chamber is on the dish side of the pin, opposite side shows all the wear on the skirt.
I am deducing from the "clues" that i had a piston that was a little loose in a slightly out of round bore that had a tendency to slighly rock, slap, bind whatever you call it. A stress riser was close enough in that cheapo summit prolite rods that it opened right up and let go in a big way in the top of second gear. Should i blame the cheap rods or the chamber shape that trys to **** the piston as soon as the plug fires.
I seems to me the optimum chamber shape for a lt1 style chamber and with the wish to lower compression, would be a long deep valve relief trough as close to the pin as possible and slightly larger near the exhaust valve to center it right on the spark plug. in addition to the trough a slight dish going out to within .200 of the circumference. leaving the outer rim or the crown thicker for the ring lands and the quench area to move a bit of air quickly from outer edges toward the center 80% of the piston dish. Then using the shallower quench areas to steer the mixture into the trough in unison with the heads shape to tumble the air in the trough.
I guess my thought on the chamber shape i had been using is that it doesn't sound like a very efficient idea from a "pumping efficiency" point of view to get the piston within yea far from TDC then trying to move most of the chamber volume to one side of the cylinder, knowing this will increase piston slap and possible large loss of sealing from rings not being parellel to the bore. Another D cup piston with a bigger dish would lower compression and a longer skirt would give a more stablity from rocking, but would make a bigger imbalance side to side and can't good practice were reliability and endurance are very important. I don't know much about cylinder wall thrust patterns, offset pins and harmonics as they pertain to this but i was hoping someone who has done some old fashioned "piston down in the hole farther" to lower compression for a blower, and also has done a perfect quench ht type blower motor, to let me know their impressions on the tuning differences and the durability,wear of the different methods. Is it even worth worrying about quench with a coated combustion chamber and forced induction. I noticed Arias and Ross both have 1.4xx compression ht pistons for forced induction setups. Any feedback

 

I don't see how coatings make a difference (and I have 'em). But I guess that there is might be somewhat less of an advantage to having good quench on a blower motor. But why not have it? It sure can't hurt, IMHO.

As far as some of the issues you raise wrt piston rocking, etc: makes a certain amount of sense, but there are so many motors running with pistons having the flat quench area with a "D" shaped cup (a D-cup!) and no problems that I can't see it as much of an issue.

I was very surprised when looking at the Ross piston catalog to see some blower/turbo psitons with a circular "cup" and no quench area. If that's what you are referring to and someone could explain it, I'd also love to know more about it.

Rich Krause

 

Just recently rebuilt the 318 out of my dads Dakota (a bit too much nitrous) - it as those same dished pistons with no quench area as you describe. It suprised me first when I saw it. Anyway, it turns out if you run a large enough quench then you also escape the tendency to increase pre-ignition, etc. that a mid-quench motor can cause (though at the expense of emissions quality to some extent). It's apparently called a "non-quench" design. Factory on the 318 and 360 dodge motors was between 80 to 100 thousandths. I just wrote it off to dodge being strange. The factory rods are forged units that weigh just under 800g each! On the other hand the pistons are POS cast units (not even hypereutectic), but with a compression height of around 1.6" (9.6 deck height). I just came to the opinion that dodge is strange all around. The extra deck height makes stroking to enticing to ignore though


Chris