2019 STI - How It Should Come from the Factory

The owner of this beautiful 2019 Subaru WRX STI brought the car to us bone stock and asked for more power.  Nothing crazy, as he uses it daily and didn’t want it to be “too loud or flashy”.  Seeing as this was his first Subaru, we walked the owner through the modifications we suggested and why they would be a good start.  The goal was to make the car feel like it should have been setup from the factory - better but not compromised -  He agreed and we got started on the project!

We began by heat-wrapping a Cobb catted downpipe with DEI Titanium heat wrap.  This will keep the heat in the downpipe where it needs to be and not radiate out into the engine bay – which not only heat-soaks the intercooler and engine, but it can degrade surrounding  sensors and electronics. The catted downpipe is ideal for keeping exhaust resonance to a minimum in the cabin of the car, and it cuts down on emissions and fumes – win-win!  We topped that off with a Perrin heatshield, since the stock shield does not fit over the larger aftermarket downpipe.  This is very important to shield the intercooler from direct turbo heat, and something often overlooked on modified Subarus. Seeing as how the owner wanted it to look and sound mostly stock, we kept the OEM catback.

The next up on the list was an aftermarket 3 port boost controller.  This allows a much finer control of boost through the factory ECU over the stock 2 port arrangement, which can be overwhelmed at more-than-stock boost levels.  We also installed a high flow fuel pump, which is also E85 compatible.  An upgraded pump can flow a lot more fuel than the factory pump can, which is important as the factory pump can also be overworked as fuel demand rises with increased boost. Working a pump too hard can cause it to overheat and fail – which can leave you stranded at best, and with a blown engine at worst.

Using a Cobb V3 Accessport we then custom tuned the STI on the dyno to maximize the upgrades we installed – final results were 306whp/356wtq - very good numbers for a mostly stock STI!  It drives like a stock STI but with a lot more oomph, and it sound stock, perfect for the owner!

So you may think this is where the story ends…but as with most car guys….that wasn’t enough.  So about a month later the owner returned and wanted a bit more power – but wanted to keep it all safe and still keep noise and “flashiness” to a minimum.  So we set to work to oblige with the owners’ requests once more.

We upped the ante with a Tomei EL header – The Equal Length header replaced the un-equal length exhaust manifold that comes on the factory Subaru EJ25 engine.  The equal length arrangement is much more efficient (which spools the turbo better) and not only makes more power when tuned, but it can also reduce cylinder temperatures as well, making for a happier engine overall.  We heat wrapped the header in the DEI Ti wrap to keep the heat in as we did with the downpipe and installed it into the car along with the Tomei 3 bolt up-pipe (which was also heat wrapped).

Next on the list was a set of Billet TGV housings – which replaces the factory TGV housings. The factory housing has a divider plate cast into them, as well as a set of butterfly valves.  These are used mostly for  low speed air flow tuning, however, they create an airflow restriction at higher RPMS and boost levels – so replacing them with smooth bore Billet TGV housings can increase horsepower and efficiency. The intake manifold needs to be removed in order to replace them, and when installing the new housings we use 3mm phenolic spacers.  These give a little bit more room to work with under the intake manifold and also help reduce heat transfer from the hot engine heads to the metal intake manifold.

The extra room helps for the next modification – aftermarket fuel rails and lines. The factory rails and lines can create a restriction or a resonance that creates lean spots throughout the rev range, and while this can be seen on some stock cars it is more prevalent with higher boost levels and even more so with upgraded injectors.  The factory injectors also run out of flow capacity with more boost, even on the factory turbo.  So we upgraded to larger injectors, better fuel rails, and PTFE fuel line to eliminate all these issues. We run these new lines under the intake manifold to keep a factory look – but the result is much more fueling capacity to the engine. An adjustable fuel pressure regulator is added so we can control fuel pressure to those rails, and we also installed a Cobb Fuel pressure sensor.  The FP sensor allows us to monitor fuel pressure, something the factory ECU doesn’t do, it also allows for safety measures to be built into the tune – so in the event fuel pressure drops the ECU can compensate with a higher injector duty cycle – potentially saving the entire engine. A very good thing to have for a small cost!

A silicone turbo inlet was installed to increased airflow to the turbo, and then the entire intake manifold is replaced with new gaskets.  At this point we installed an Air oil Separator, which intercepts air coming out of the breathers from the engine and separates the oil from it.  Blow-by is common on these engines, which causes oil to be pushed out of the crankcase, which is normally recirculated back into the intake tract for emissions purposes. The problem is that the oil coats the inside of the turbo, intercooler, and intake manifold, and makes its way into the combustion chamber.  This lowers octane and can cause detonation - This is bad.  The AOS traps the oil before it can be circulated back into the intake – maintaining the clean emissions but also saving the engine from potential harm.  A very good item to have on any modified turbo vehicle!

At this point it was time to make the trip to the Dyno again.  We achieved excellent results, with an increase in power to the tune of 327whp/391wtq – on the stock turbo! The car feels absolutely great on the street now, with a huge amount of power over stock. The great part about the mods chosen for this project are that it still looks and sounds stock – exactly what the owner wanted – Mission Accomplished.

This is how the car should come from the factory!

Circuit Motorsports is a performance auto shop located in Orlando, Florida.  We specialize in performance upgrades, ECU tuning, engine builds, and fabrication.  If you are interested in performance upgrades or a full build consultation contact us today!

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