1. Hemi Head Engine

2. For nearly 40 years, Chrysler has been aggressively promoting their hemi head engine. For you gearheads, do you know WHY a hemi is supposed to be better? In my experience, virtually no one seems to actually know! I wonder if Dodge and Chrysler salespeople even know?
A hemi head is actually a (somewhat) hemispherical head. Virtually all the other styles of overhead valve engine heads have relatively flat pistons and heads that have a relatively shallow recess in their heads, for the combustion to occur. Remember the roughly 6 cubic inches that must remain at TDC? With a 4" diameter cylinder, that equals roughly 1/2" in cylinder height, near the sides near zero and near the spark plug may be 3/4 inch. Now, a cylinder has to have both an intake valve and an exhaust valve, both in the head (in overhead valve engines, the most efficient designs). The flat shape of the usual combustion chamber limits the diameter of those valves, to well under HALF of the entire distance across the piston. An engine with 4" diameter pistons can therefore not have intake or exhaust valves which are larger than about 1.5" in diameter. By the way, the INTAKE valve is always larger in diameter than the exhaust valve. Do you know why? It is because the EXHAUST is DRIVEN OUT by the upward motion of the piston, while the INTAKE is SUCKED IN by the downward motion. It turns out that devices that suck air cause a lot more turbulence, and so it is less easy to do. The larger intake valves are therefore needed to provide the SAME necessary flow rates for the cylinder to be most efficient.
The hemi head uses a VERY deep combustion chamber, so that the distance across is about half the circumference of a circle (1.56 * diameter) rather than being only slightly more than the diameter. This allows a lot more available space for the two valves. The valves tend to therefore be at odd angles to benefit from this added size. The SINGLE actual advantage of a hemi head engine is that it has much larger diameter valves! This allows the fuel-air mixture to get in easier and the exhaust to get out easier. Bigger valves is a very good thing, and the hemi head design is the simplest way to provide the space for really large valves.

3. Since the hemispherical chamber is so tall, a flat-top piston would allow too much remaining volume for a good compression ratio, so all hemi head engines have to have dome-top pistons. So if you ever see a relatively flat-top piston, it is from a non-hemi, and a significantly domed piston is always from a hemi. (An engine can have flat-top pistons replaced with slightly domed pistons to increase compression ratio, but that is a very different effect.) Also, if you happen to see an unusually large valve, it is likely to have come from a hemi engine.
So, a hemi is not "magical" or anything, but merely is a design that permits bigger valves for better engine breathing. There is no other significant advantage of it. And, actually, the domed piston somewhat interferes with airflows and makes it less likely to get really uniform distribution of the gas-air mixture, and really good removal of all exhaust products, so some of the benefits of being a hemi are given up in exchange.
You may be aware that there are some newer engines that have four (smaller) valves per cylinder. This provides the improved breathing of the hemi while not having the disadvantages of domed pistons. But the engine is much more complex, and expensive.