When modifying WRX/STIs, it is important to plan for the type of the aftermarket Subaru exhaust manifold (header) your will be getting. Unequal length header will allow you to keep the Subaru Boxer rumble sound, yet unequal length header is a poor choice for high horsepower applications where you want to maintain equal Exhaust Gas Temperatures among all cylinders and safe turbo inlet temperatures.
Since header runner for cylinder 4 is the longest on unequal length headers (on WRX/STI), you have the highest EGT in that runner and on cylinder 4. This is because hot exhaust gases travel longer in the longer runner. Cylinder 2 runner is the second longest. Since you are continuously subjecting cylinders 2 and 4 to higher heat than other cylinders it is very common to see various cylinder 4 and cylinder 2 failures in Subarus.
Shorter runners of cylinders 1 and 3 provide for lower EGT on those cylinders. However, since the temperature drop is less than in cylinder 2 and 4 runners, higher-temperature exhaust gases get transferred to the turbo, subjecting the turbo to additional heat stress.
Equal length manifold (header) smoothes exhaust pulses of the Boxer engine, thus changing the exhaust note to smooth vs. burpy. Some refer to it as a “ricer sound”, however it is really not that bad. Equal-length header is optimal for maintaining equal EGT between all cylinders and delivering a constant, lower-temperature exhaust gas to the turbo. Equal length header is a must have for
any higher horsepower Subaru WRX STI.
TIP: on unequal-length manifold, install the EGT sensor probe within 3 inches from the head on the longest runner. (cylinder 4 runner for Subarus). On equal-length manifold, you can install the EGT probe on any cylinder runner, still within 3 inches from the head.