STi Downpipes
Text and Photos: Sami Sharaf
Some Photos: Apeworks
Testing: Sami Sharaf
Dyno Facility: XS Engineering
The first thing that comes to our mind when testing downpipes, aside from "area under the curve" power and torque production, is how much the boost level is affected by the addition of an aftermarket downpipe (and the removal of the factory catalytic converter.) In many cases - more so on vehicles that aren’t turbocharged from the factory - the addition of a larger diameter downpipe equates to more boost pressure. This isn’t necessarily a good thing as many turbo kits are tuned to the bleeding edge at a preset maximum boost level; after all, that’s one of the primary reasons that people purchase aftermarket boost controllers in the first place. With that in mind, we also charted the maximum Manifold Absolute Pressure (MAP) numbers to see if power came from enhanced flow, or from increasingly higher boost levels. A quick glance at our MAP chart shows that there were no signifi cant changes in MAP pressure, meaning that the downpipes tested produced power by reducing backpressure, not by allowing increased boost pressure. For the precision tuner who needs complete control over as many tuning variables as possible, this is an ideal situation.
We also noticed that, as far as peak numbers are concerned, the Invidia unit displayed the most increase. Design-wise, the Invidia unit shares the divided outlet flange that separates the wastegate and turbo exhaust housing ports with two other units tested; the other three incorporated a single port that included all spent gases. Therefore, we can’t say that one of these designs is better than another, as most of the downpipes performed in a consistent manner.
One of the main concerns of Subie owners lies in the placement of the o2 sensor in the downpipe. An inadequately or improperly placed sensor can cause a Check Engine Light condition, seriously hindering power production and general overall performance. All of the downpipes tested had some resolution to this problem, be it an o2 extension wire set or a properly placed o2 bung in the downpipe body itself.
If you’re a Subie owner and you’re in the market for an aftermarket downpipe, any one of these units is sure to fi t your bill as far as appearance, performance, price, and durability are concerned. Choose wisely and Stay Tuned!
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