Since you're asking about cylinder head modifications, here are some pictures of the work done on the X-Prize Sonata. For starters, work was monitored using a Super Flow SF-1020 SB flow bench. The head was flowed stock to establish a baseline. The basic concept I was targeting was to have a 4-valve-per-cylinder head deliver "Swirl", and not just "Tumble". I found 2-valve-per-cylinder heads deliver better low-end torque and fuel economy, and 4-VPC heads flowed more total air for better top-end horsepower; swirl versus tumble. Another quirk with the Hyundai 2.4 engine was extremely low valve lift; 0.17"! The amount of air capable of flowing through the valve-seat gap at that lift was minuscule compared to the port sizes. I decided to reduce port sizes to boost velocity.
Second objective -- along the lines of "Swirl" vs "Tumble" -- was to bias the ports so that one intake port would admit the air-fuel charge at an almost straight-in-at-the-tangent direction, while the other would try to come in perpendicular to the 1st one. Initial testing was done with modeling clay to simulate the effect on the flow bench. I raised the floor on one side and lowered the ceiling on the other. This forced the charge high on the one side and low on the other.
After getting the basic shapes I wanted, the clay was removed, the ports thoroughly cleaned, and 2-part epoxy replaced the modeling clay. I used 5 tubes of epoxy on that head! That is the most I ever used on any head. [Unfortunately, I used either too much epoxy or the wrong epoxy, as some of the chunks broke loose from the port walls a couple thousand miles later.]
Since I couldn't use epoxy in the exhaust ports, I had to use a bit of creativity to induce the swirl effect.
The final intake ports were treated with my Powre Lynz. I extended this concept to the intake valves as well. Exhaust valves were polished, then electroplated with a catalyst to enhance ionization of the combustion charge.
Though barely noticeable in the pictures, I radiused all sharp edges in the combustion chamber to take advantage of the Coanda Effect to allow the expanding combustion charge to more easily roll around to the back portions of the combustion chamber.
