Equal length vs. Unequal length headers

subaru-sti-equal-vs-unequal-header-manifoldWhile 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, 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 runnerĀ 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

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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.

Read more about EGTs

4 Responses to “Equal length vs. Unequal length headers”

  1. […] a Tomei Expreme unequal length manifold on my sr20 and it actually had quite a nice rumble to it. http://rusubaru.com/equal-length-vs-…ength-headers/ my old car with the tomei manifold __________________ Stillen G3/Pulley | PPE LTH | Invidia […]

  2. […] gas to the turbo. Equal length header is a must have for any higher horsepower Subaru WRX STI. Equal length vs. Unequal length headers | R U Subaru? – Tuning Forced Induction Cars and Alcohol Inj… Sound is NOT the primary difference between equal and unequal length headers. It is a side […]

  3. mitch says:

    Will installing an UELH cause the car to run more lean?

  4. Zach says:

    I read this article. I would still keep the unequal over the equal. Watch this Video. Sounds too much like a ricer with equal. Cant loose that Scooby growl. http://www.youtube.com/watch?v=OrLasJvUwZE