Downpipe Turbo Subaru FAQ
2. Bellmouth: Completely open design.
3. Split bellmouth: Similar to bellmouth only with a divider inserted to separate the wastegate.
4. Divorced or Twin Dump: Separate exhaust and wastegate piping that connect further downstream.
Which downpipe construction method is least expensive? Generally, the blank plate design is the least expensive. The other designs, depending on manufacturer, are all around the same general price range.
Can I gut my downpipe like people do their uppipe? Yes. Others have used a 2 foot section of bent rebar and a hammer and reported the de-catting process takes 30 minutes total if the scoobymods directions seem too labor intensive.
Can I use an STi downpipe? Yes. There is very little difference between the STi and WRX downpipes though. Using the STi downpipe is fine, but do not consider it a significant upgrade. Differences in HP between the two are likely to be very tiny (5HP or less is a guess) and not worthwhile.
What other differences are there with downpipes? The other differences are as follows:
1. High flow cat or catless.
2. Overall pipe diameter. Sizes range from 2.5, 3.0, and 3.5 inches. 3.0 and 3.5 are generally only necessary for higher (300-400+ HP) applications.
3. Length of downpipe, some are stock length and others are longer to remove the 3rd cat.
4. Propietary length. Some have manufacturers have unique lengths or are JDM length which requires manufacturer specific length or JDM specific length components after the downpipe.
5. Flanges. Some are manufactured with a donut gasket and some with a flat flange, the end user needs to research what bolts up to what before purchasing.
Which downpipe has the best gains? There is no irrefutable evidence that any downpipe has better gains than another. The consensus, if there is one, is they are all within 5HP or less, gain wise, of each other.