Oh the memories.... "Cow magnets"

Interesting. My skepticism would be how OEM's and race teams, who literally have spent a billion or more dollars on designing combustion chambers, didn't discover this. (including Formula 1.) But if it works for people on the street...

Traditionally, adding sharp edges to combustion chambers enhances the prospects for detonation. So should this diminsh the likeliness for detonation, how it does so would be fascinating to learn. I guess the turbulence effect keeps fuel away from the edges?

Or is this another 100mpg carburetor/cow magnet in the making? It would be great if there were a simple mod like this that provided so many benefits though. Is there a formula to determine good placement of the grooves, or this is done by eyeball?

Interesting, for sure! 

This appears to be wacko at first but then you read the ton of information and sites out there from guys who've done it and are converts.  A lot of MG guys too.  I'm sure you engine guys know about this already.  Also called a "squish groove".  My heads' off now and since I can't read of a case of damage anywhere I might just try it in the next couple weeks.

The idea (easy and free): Dremel a groove(s) in the squish area, pointed generally between the intake and exhaust valves.

Alleged benefits:  Significant MPG and torque improvement.  Lower temps, smoother idle.  Allows you to make lean AFR and timing adjustments that normally would cause detonation.  Crazy ignition advances allowed and low-octane fuel, without apparent longterm damage.

What it does:  Highly mitigates detonation, pinging and preignition by getting rid of unburned fuel.  

Problem it solves:  As the piston compresses in the compression stroke some unburned fuel hides in the 'squish area' outside the combustion chamber.  This is the flat area of the head you see inside the gasket.  This fuel is apparently the primary cause of preignition/detonation/ping because it's not always burned 100%.  When the piston is all the way up, apparently the flame front doesn't always get all the way back into the squish area.

The groove helps two ways:

1.  Squirts out the unburned cloud of fuel to the burn/quench area around the plug.  A primary source of detonation is removed.

2. This squirt creates combustion chamber turbulence which A) pulls in more unburned fuel from around the chamber, and B) ensures more complete burning of the total mixture as the flame front is distributed better. 

Downside: Can't find one yet.

There are many sites and threads ... MGexperience , Speedtalk , Turbobricks , Vtwinforum , Herning , Jeremiahsviolins , GM Performance , HotRodders.com

Looks legit enough to be worth a discussion here.  Yeah I know it sounds too good to be true,  but again there's a whole lot of guys out there talking about it.