Drilling into piston top

The whole concept of changing piston and combustion chamber shape, is to reduce the distance that the flame must travel in order to propogate through the charge.  Absolutely everything... and I mean everything... about  combustion chamber shape...

Let's put it this way... consider a combat firearm-  the three most critical functions that occur, are a charge coming in, the charge igniting, and the charge being removed.

A piston and cylinder are no different... but depending on many things, including intended operating speed of the engine, some factors are much more important than others.

The concept of a dished piston in a flat-topped head, is exactly the same as a flat-top piston in a hemispherical head... (except... that it's upside down).  The shape of the head compresses the mixture into an area that, instead of being flat like a pancake, is round and small like a ball.  In both cases, the fact that compression is going in three dimensions, the mixture is being jostled around good (turbulence).  This disperses fuel well, but also causes the flame DISTANCE to be very short, at least, when the cylinder is close to TDC.

In order for 'drilling holes' to have a positive effect, many other things would need to happen- your compression height, for instance, would need to be increased, so that the 'sides' of the piston edge get up, basically against the head (without contact, of course).  In order for it to yield a short flame speed, the 'holes would have to be extremely wide, and without sharp edges, so as to not wind up 'shrouding' itself.  Shrouding prevents proper discharge of spent gases... and sharp edges cause hot spots that cause detonation.

A shaped chamber means nothing when the spark plug isn't well located.  Likewise, a shaped chamber is irrelevant without having it match the motion and location of the intake and exhaust valves.  As noted above, many guys machine 'pockets' into the piston top- this is solely to provide critical clearance to intake and exhaust valves while at TDC exhaust/intake stroke, to prevent a collision under high lift.

It may help to understand this by considering the combustion chamber shape of a flathead engine... with the valves off the side, the flame path and airflow is a hideously contorted mess, with the valve area being a terrible waste-of-space.  There were a few engine designs, however, where the piston was shaped to a wedge, and the spark plug placed just off to the side of the valves, to move the entire combustion chamber area volume to a point concentrated near those valves, and the piston's TDC clearance to head was very small... thus, the charge was all crammed into the 'pocket' directly above the valves.  This worked great for getting the charge consolidated around the spark plug, and it was pretty darned turbulent, but in the long run, the engine design didn't breathe well at high RPM because the valves were basically buried in a sock drawer.

DrAllis wrote: The most important benefit (to me) of the basically "flat" cylinder head  design is the fact that it never (under normal use) seems to crack...   ...Quick overhaul times and long service life by design !!

When I was a kid, my Grandfather's neighbor pointed out that all it took to do an in-frame rebuild on an Allis, was a patient man with a flat file, a straightedge, and some cranberry juice.

At the time, I thought he was insulting the tractor, but I kept my mouth shut and ears open.  Years later, after I'd spent plenty of time with machine tools, and even more time with files, I realized what he meant.  The engineers kept the design so that all the machining processes as uncomplicated as possible.  A good quality flat file in the hands of a talented guy can make a very flat surface.  Add a good straightedge and some indicating dye, and you can hand scrape it like a machine tool.  I eventually figured out that the neighbor used to do exactly that with valve faces on steam engines... he used the Cranberry Juice as his indicating dye... he'd take a swig, spit some on the face, and lay his straightedge on 'em...

On a V engine, just doing a simple skim of the gasket surface becomes complex geometry, because it affects the geometry of the intake manifold...

And to add to the mess...  the V engine is a 'crossflow' design- intake on one side, exhaust on the other.  From an airflow perspective, performance guys prefer the crossflow, but the side effect is that one side will be substantially warmer than the other... and Dr's note about cracking is much easier to understand.

Another (kinda unrelated, but notable) things... a crossflow design has the advantage of a lower fuel/air charge temperature... this means higher charge density.  From a peak power perspective, this IS somewhat important, but fuel atomization occurs best when the fuel is warm enough to evaporate fully before getting compressed.  In a day-to-day working engine, running at anything other than full throttle, it's difficult NOT to send unburned fuel out of the exhaust, so warming the mix a little before sending it into the chamber gets that evaporation done.  Having intake and exhaust ports in close proximity seems counterintuitive for extreme performance, but for variable duty of an industrial/ag environment, especially on a cold damp day, it really helps improve operability to have them nearby.  Not only do fuel droplets not burn, MOISTURE (from the air) which has condensed into droplets (due to pressure drop coming through throttle/venturi) become a 'barrier' to flame path... and result in incomplete combustion.

Yesterday afternoon, I recognized an excellent demonstration of the effects of 'shrouding'.  I had an old can of PVC cement... it'd turned into unusable gak, but still had plenty of aromatics in that can.  I have a dumpster, which gets loaded into a compactor truck... and rather than dumping things like that into the trash, I tend to make sure that the flammables are burned off, so they don't put anyone down the line at risk.  I took off the lid, pulled out the goo'dup brush, set the can out in the driveway, lit the brush, and dipped it into the can.  Of course, the vapors caught fire, but instead of burning fast, it just had a little flame barely out of the can.  Why?  Because the fire only exists where there's both fuel and air... and since the can was full of fuel, it was only burning where air contact appeared.  Without a source of oxygen INSIDE the can, it would burn no faster.  This is just one example of  'shrouding'.

Probably same one you have, Mine was Charles Fayette Taylor: The Interal Combustion Engine in Theory and Practice, Second Edition, Revised.  C 1960, 1966, 1985 by massachusetts Institute of Technology, 1977 (first ed) 1985 (2nd Ed) 1998 (9th printing)  M.I.T. Press, Cambridge, Mass and London, England ISBN 0-262-20051-1.  Got it (both volumes) sitting on the shelf by my desk here.

Suffice to say that while it was written by Professionals, and taugh by Professionals, the guys that learned the most, weren't the guys who studied it in a room with a chalk board in front... it was in a dusty shop with a beer fridge by the door, and waterbrake dyno in the back room.  ;-)

Most of the principles are very easy to understand, as long as they're described using a relateable context.