I would go .020-.030 off deck.. It would increase compression.2-.3. Also bring piston closer to deck. Although some say quench isn't an issue so it's not needed, my theory is more of the space between. As in if the flame has to burn thru chamber of head past gasket into piston dish before it pushes piston down. Seems like wasted burn time and power loss. Less space to burn before it gets to piston will push piston sooner and with more force. Don't forget this stuff happens in milliseconds. Just my thoughts but it may be true on some level. Most builders do a zero deck for a reason.
Quench is always important, no matter the combustion chamber size, it's just that with the Buick 350's narrow bore in relation to its long stroke makes a quench platform not quite as needed as say the big blocks or any engine that has larger combustion chamber sizes. The larger the combustion chamber, the more quench will benefit it. Something to pay more attention to if you won't have a quench platform is the shape of the piston, and for the Buick 350, that's a dish. Focusing the flame travel toward the center of the piston is best. Polishing the combustion chamber also helps eliminate hot spots that will encourage detonation too. Any piston notches need to have the sharp edges smoothed down, which will often require weighing the pistons individually to make sure there isn't a weight discrepancy between them that would affect engine balance. You get the idea. Getting the pistons as close to deck top is ideal, but may not be practical in some applications, such as street engines on a budget. You'll still get some quench with the outer ring of the piston (the outer 'lip' on the outer perimeter surrounding the dish), just not as much; not to mention that quench has a zone within which it works--closer being better, but still has an effect even further down. Cam and ignition timing will affect where the quench is most used also, which may not be at its peak distance, but a bit further down from it. Remember that everything is in motion.
In the interest of different experiences... The sharp edges being THE most productive time spent on BOTH the piston tops and combustion chambers (especially after the machine shop surfaces them...) , and being tremendously important to eliminating a glowing hot spot... 1. You'll never weigh the differences from putting a radius on a piston, whether dome or dish edge, even if slightly different visually. As an example, a large valve relief in it's entirety is barely a gram of weight. You'd have to use a circular saw to do that to even be concerned. Definitely weigh them if they've not been balanced yet though. 2. The trend has been to keep or put some surface texture on combustion surfaces, after removing the obvious casting blobs and anomalies. Some of us program that right into our CNC's as the results are quite repeatable. Polishing does have it's benefits with thermal efficiency but the current thinking is that improved mixture motion contributes more to power production than the ability to hold higher levels of heat at the surface and it's contribution to the power cycle. I can't say that any tests have been done polishing the Buick 350 chambers vs dimpling or other surface textures but the difference is pronounced with any shape of combustion chamber that has been tested, AFAIK. Polishing is one way to help. There are other ways that might be less time consuming with slightly different benefits. (fair enough, Gary? )
Hey it's all good! Your contributions are greatly appreciated. You are a gift to the community. You selflessly keep giving great advice for all. Thanks Tony.
I came across a better written contribution that fits the discussion of combustion pressures and detonation recently. "Given a typical engine with 50% squish area ratio, and SCR values of 11.08. 11.36, 11.6, with clearances of 0.070, 0.060, 0.052, and normal camshaft events, we calculate the following: Compression temperature at 30deg BTC, to be: 530F, 534F,and 538F, respectively. The maximum squish velocity to be: 26.7 m/s, 30.1 m/s, and 34.3 m/s, respectively. As can be seen, the changes in compression produce very little temperature change while the changes in squish clearance produce substantial squish velocity changes. If it were my engine, I would use the higher compression and the higher squish velocity designs. Even more so if a longer duration cam were used. " David Redszus Apply what's needed for this and the other related threads
Not to risk being rhetorical but would that question change somewhat if those heads were milled the .060-.070" as often referred to and recommended by some members? What is the distance left at the area we are referring to in that pic? Squish and quench are not the same thing and I think you might be referring to the fact that the chamber shape shown has a larger quench distance than others commonly referred to.
Here is but one reference out of many potentials to get a basic description and recommendation for quench distance: http://www.hotrod.com/articles/ideal-quench-height/ Notice it says quench can be as high as .050 to .065, but they don't recommend anything over .060. Traditionally, 'quench' is referred to the areas that are flat on the piston and head, which of course the Buick 350 head does not formally have, which means it is referred to as an 'open chamber' head. There will, however, still be some quench involved as long as the piston is close enough to deck surface (and the head is close enough to the piston). This is what I was referring to in my previous post regarding quench. Even so, the smaller chamber size reduces the need for it as much as larger combustion chambers need it. This is my understanding of it, and my understanding could be off or in need of improvement! If so, I welcome any responses that would enrich my learning and understanding of what I already know. So from what I'm gathering, quench distance doesn't have to be a perfect .035 or it's completely useless, as seems to be the inference I read a lot elsewhere. The closer the better (within a safe distance for thermal expansion considerations), but farther away (up to a point) is still useful (as I said in my previous post). All that said, also consider that quench distance is measured when the piston is at TDC. Ask yourself: when does my spark plug fire, and at which point in relation to TDC (and quench distance) is the flame traveling and through how much quench distance? As Tony pointed out, also take into consideration the squish speed, which will affect all the above (not to mention the piston is moving the entire time all this is happening, so it's not staying at TDC so the optimal quench can do its thing while the piston sits there and waits on it lol) I'm sure there are others who could give a better description than I.
A pic of the most shallow part of the head chamber. This area is about 30 thou deep but on most pistons the only "quench pad" on the piston is the small ring on the outside edge. If the pistons are 50 thou in the hole, 40 thou head gasket then 30 thou for the depth of this area then you are WAY outside the effective quench distance range of 30-60 thou. :
And if you have a "zero" decked block, the piston's outside 1/4" circumference, is the only area at deck heighat...then the 0.040" gasket....then whatever you have left on the heads. (0.030" in your case Sean)....so it's still 0.070".....using steel gaskets knocks another 0.020" off, but compression goes up again...but you would have 0.050".
I'd say quench is pretty much moot on a typical mild rebuild with pistons sitting pretty far below deck anyway, so we're mainly talking about max effort (to obtain the most quench possible out of iron heads) to be able to squeeze a bit more compression out of it, but on these types of builds, you're probably going to have to use race fuel or at least a mixture of it with pump gas anyway. This is where the narrow bore and small combustion chamber comes in, where you can run higher compression even without quench, vs a larger bore and larger combustion chamber engine. This would make the Buick 350 more efficient using the engineering design it already innately has, regardless of quench, though having it would improve things even more, of course. The aluminum heads would really make the Buick 350 shine with all the improved features they have over the stock iron heads, one of which being the quench pad. With this alone, you could probably run dynamic compression ratios in the 8:1-8.5:1 range on 93 octane, though this has yet to be tested. This would transform even mild street engines using these aluminum heads by virtue of the compression ratio alone. I know Chevy small blocks with larger bores and combustion chambers can run 8.5:1 dynamic on pump gas, providing they have good quench, so this is definitely a reality for the Buick 350 with those aluminum heads utilizing good quench. We may even see higher dynamic compression ratios possible on pump gas.
