Equal length vs.
Unequal length headers
While this information is not specific
to Subarus, we will explain the difference between equal length and
unequal length exhaust manifolds using the Subaru example.
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. Equal length header will allow you
to maintain equal Exhaust Gas Temperatures among all cylinders and
turbo inlet temperatures.
Since the 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.
Shorter runners of cylinders 1and 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, which could be the reason for
unequal length headers having peak power gains in midrange and feel
more powerful than Equal Length headers.
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". Equal-length
header is optimal for maintaining equal EGT between all cylinders and
thus delivering a constant, lower-temperature exhaust gas to the turbo
for their linear power gain.