How about a hemispherical combustion chamber for R-1820?

[stoxm73]

Since hemi-head has an excellent surface-to-volume ratio, minimal heat loss to the cylinder head, and room for two large valves. Besides, its spark plug locates at or near the center of the chamber to promote a strong flame front. Theologically, these allow squeezing more power from the same chamber volume comparing to regular design.

Its major drawback is that it can not apply more than two valves. However, most radial design can only have two valves too. So, it looks reasonable to apply hemi-head on radial engine design.

If an engine like R-1820 were updated with hemi-head, is it possible to squeeze 100 hp more while using 87 octane gas?

 

[z42]

Since hemi-head has an excellent surface-to-volume ratio, minimal heat loss to the cylinder head, and room for two large valves. Besides, its spark plug locates at or near the center of the chamber to promote a strong flame front. Theologically, these allow squeezing more power from the same chamber volume comparing to regular design.

Its major drawback is that it can not apply more than two valves. However, most radial design can only have two valves too. So, it looks reasonable to apply hemi-head on radial engine design.

If an engine like R-1820 were updated with hemi-head, is it possible to squeeze 100 hp more while using 87 octane gas?

If you look at the combustion chamber shape and how the valves are splayed on a R-1820, I'd say it pretty much has a hemi head, though without the official Chrysler marketing stamp of approval.

We also had a thread recently about WWII aero engine combustion chamber shape at Combustion chamber shape and valve angles in WWII aero engines

 

[Shortround6]

If an engine like R-1820 were updated with hemi-head, is it possible to squeeze 100 hp more while using 87 octane gas?

As Z42 has shown, the combustion chamber was pretty much hemispherical and had been since pretty much Day One.
That appears to be fairly early version of the engine.

This is a later version with much greater finning.
Later version got even more.
On air cooled engines if you don't have enough fin area it doesn't really matter what the shape of the combustion chamber is.

The "Hemi-head" dates back to before WW I, over 10 years before Chrysler even became a company, Although Walter Chrysler had been making cars for 20 years.

 

[yulzari]

With really large 2 valves it becomes more a pent roof on which case the period, and modern, position in to give an engine 4 valves and a pent roof chamber. What the aero engine has that road racing ones rarely do is to have a variable supercharger boost so that the hemispherical head can combine the shape with a decent effective compression ratio. You still have to lose the heat from those two large, heavy valves.