Most if not all cylinder heads are one big block. Why don't some car manufacturer use a single cylinder head for each cylinder? What are some of the biggest drawbacks of this type of method?
This can be for any ICE.
There are a few factors to consider.
Ease and cost of mass-production. Individual barrels require individual assembly, whereas a cast piece of metal has a lot of things built-in and assembly can be automated much more easily.
Durability. Engines make vibration, that can't be helped. A solid piece of metal can both handle and channel this better than individual parts.
Cooling. Engines put out A LOT of heat. A solid piece of metal can conduct this away very easily and very quickly, but more to the point, the cooling system has "conduits" cast into the engine directly. These aren't hoses that can crack and leak, these are "channels" that are burrowed into the solid metal block.
Yes, individual barrels would allow you to swap an individual warped barrel out instead of machining the entire block, and a combustion upgrade would be as simple as swapping out all your individual barrels for larger ones instead of boring out your block, but none of these rank high on a manufacturer's priorities list. Low cost of manufacture and durability are.
Most modern car engines have overhead camshafts. Overhead camshafts pretty much oblige the use of a one-piece cylinder head. Imagine the difficulties of aligning the camshaft if each cylinder had a separate cylinder head.
It's easier to seal the cooling and oil passages of a water cooled engine if there are less joints. Less components also means faster assembly.
In the past, large castings statistically had greater possibilities of defects than smaller ones. This led manufacturers to design cylinder blocks and heads in groups of no more than 2 or 3 cylinders. As technology has improved this has become less of an issue.
Despite these advantages, separate cylinder heads still find use in air cooled motorcycle and aviation engines.
Separate cylinder heads is actually quite common on truck, marine and plant engines. There are a few reasons but the main reasons are:
Modular design, a manufacturer can just extend the block and add one or more cylinder head and piston. This saves the need for different tooling, stock and production lines. Saving money on tooling and production is a major concern as these engines are built in relatively small quantities compared to car engines, seeing as tooling for one engine could be in the hundreds of thousands if not millions of £/$ the amount per engine is considerable.
Ease of service and repair. These engines are expected to do millions of miles or hours and stick driver and operator abuse as well as harsh operating conditions. Things will break such as valves dropping, piston ring/liner damage and bearing failure. Time is money so being able to just remove one head and swap out the piston and liner or replace the head is vital.
There are drawbacks though and these are why engines produced in large volumes and performance engines don't use separate heads:
They only really work on push rod engines as a six cylinder engine would require 24 camshaft oil seals if it used over head camshafts and it would also require stripping all of the over rocker covers and require setting the entire top end up to remove one head.
It would cost more in high volume production, there becomes a point where the added parts list adds more cost than the tooling saving.
There is no need, cars and small commercials rarely give internal problems such as head gasket and piston damage so the added cost isn't justified as they probably will only do an 8th of the millage of a large truck.
Individual cylinder heads are the norm for radial engines used in aviation, as there is no advantage to casting or machining them together. Usually power to weight ratio performance is paramount over cost in these applications. Also these heads are usually air-cooled, and therefore simpler, as they have no coolant plumbing.
For automobiles, cost is a heavy factor. Until recently, weight was not. One of the highest cost factors is machine time. A design factor is the need for liquid cooling, now practically mandatory to meet emissions targets.
A multicylinder cylinder head allows the same machining operation to be performed repeatedly on each cylinder, moving the head a fixed distance each time between machining operations. Combining cylinders in one head also allows the coolant passages to be cast en bloc, which avoids fittings and plumbing, along with the machining time associated with accommodating the fittings.