Fin density is measured in FPI or fins per inch. Each fin surface counts as one fin meaning there are two fins per wave. Fin stacks start off as flat sheets of aluminum that are fed through a special machine where they are bent to the desired FPI per row height and length. Garrett manufacturers around 20 different air-to-air and air-to-liquid intercooler cores supporting a range of horsepower from 310 up to 1260. Each of those cores are different in length, width, and height to meet the needs of different performance enthusiasts.
Garrett Intercooler cores utilize a combination of fin density depending on the flow path of the core. For example, the cold flow path or the front of the intercooler commonly has a higher density fin count to allow for better thermal performance and cooling. Core width/ cold flow can range from 3 – 5 inches so higher fin density is critical with the short span the ambient air travels across to cool the charge air passages. Additionally, the cold air passages are not sealed or pressurized like the charge air passages, so your ambient air flows through, cools the hot passages and evacuates into the engine compartment.
Hot flow passages have a longer run spanning from 11-28 inches in some intercooler cores. The hot flow path is a series of sealed chambers with fins spanning the entire length. There is a lot of surface area for heat transfer to take place and for that reason, the hot chambers have a lower fin density. This also helps reduce pressure drop as the charge air travels from one side to the other.