Originally Posted by brainsboy
I understand all that but it doesnt really answer my question. Building for the street or strip or for a grocery go getter isnt going to make any difference if quench is based on the maximum piston rock and I build it for average.
If I take the average and build for .035 then that makes the max piston rock at only .024"..... Are quench numbers usually based on using the average of piston rock (center of piston front and back locations not the top and bottom) or based on using the maximum piston rock? Do they take into account piston rock when calculating for min quench numbers?
It is the amount of squish/quench and clearance between the piston and head that is the question. A race engine is usually configured around the minimum possible clearance with 2 assumptions about race engines those being winning requires taking the engine to the absolute limits. Two, a race engine sees a lot of disassembly and internal maintenance so to a certain extent potential clearance issues that develop as the engine wears will be found before they become destructive. For street engine that's expected to wear for several 10's of thousands of miles without internal maintenance, one does not push the clearances to their limit. In this case the engine is not blueprinted so significant dimensional variance can exist between cylinders, so setting up one cylinder at the minimum may not be adequate at another cylinder.
The problem of squish/quench amounts is dependent upon area and clearance. Clearance gets a lot of talk, but area is seldom mentioned. There are two purposes to squish/quench. Squish is the ejection of mixture from the far side of the cylinder toward the spark plug. This serves to stir the mixture getting air and fuel molecules into close association and to break up any globules of fuel. This stirs and shoots the mixture into the space by the spark plug increasing the density of mixture before the plug. This increases the likelihood that the spark will catch the mixture on fire and once lit speeds the burn and takes the burn to a complete reaction which maximizes power and minimizes fuel consumption.
The other function is quench, this is the result of having a considerable amount of surface area enclosing a small volume. This removes heat from the far-side mixture so it does not self ignite before the flame front gets there. If it does self ignite there are at lease two pressure waves in the cylinder which when they collide make the typical pinging sound associated with detonation and/or preignition. This is the sound of the piston being hammered to death.
So far as I have seen there is no magic formula to determine the precise amount of squish/quench an engine should have. I've not seen a formula or rule of thumb in either Ricardo's or Taylor's writings.
By experience and documented tests we can see that the amount of squish/quench in terms of a function is directly proportional to cylinder diameter and spark plug placement. That is the larger the cylinder the more area of the combustion chamber needs to be devoted to Squish/Quench. This also holds for the further from the center the spark plug is located the more squish/quench is needed.
All this can be is an optimization of conditions. In the end there is so much resistance to detonation a given fuel has. The best you can do is get the most energy from the fuel without blasting the guts out of the engine. But you can't just get better and better performance from the fuel by making the squish/quench step bigger or closing the clearance. Ideally a modern combustion chamber is about 1/3 squish/quench and 2/3's valve and spark plug pocket for a wedge chamber. The spark plug located as close as practical to the center but needs to be off set enough for valve clearance. A hemi or pent roof chamber offers better spark plug placement but usually has a less effective squish/quench which is now a narrow band around the circumference of the cylinder.
Piston shape is very important, for a wedge a tight chamber with a flat top piston is ideal but usually creates too much compression. A D dish design allows the compression ratio to be tailored and keeps the dish under the valve pocket. The OEMs have a bad habit of using circular dish pistons because of cost advantages they fit both sides of the engine where D dishes have a separate left and right orientation and therefore from a cost stand point only half as many of a design orientation can be made. The circualr dish reduces the close locating of the piston crown to the squish/quench deck reducing the functional area. The bottom of the dish may sit from .08 to .1 inch lower than the crown, seriously limiting the amount of closure clearance. These things significantly reduce this function in most production engines which pushes the owner operator to compensate with a higher grade fuel or more of it as in rich mixtures, or compromising the ignition timing thus throwing unburnt mixture out the tail pipe power and efficiency going with it.
Gotta run, I'll pick this up tomorrow if you want.