By Lance Lucas
Event: R&D Dyno Testing
Location: COBB Tuning Surgeline
Ambient Temp: Mild - 60's
Elevation: Sea Level
Car: 2010 WRX - COBB Tuning R&D Vehicle - "Ketchup"
Tuner: Lance Lucas
Dyno Info: Mustang AWD-500-SE
Transmission: OEM 2009 5MT
Target Boost: Wastegate Pressure (0% WGDC)
Engine/Power Modifications: Stage2+SF (COBB Turboback Exhaust, COBB SF Intake/Airbox System). COBB 44mm EWG Kit w/atmospheric dump installed for EWG tests.
Tuning: COBB AccessPORT with v331 OTS maps (Stage2+SF 91 Octane with WGDC set to 0%)
After finishing up a pretty big round of pending OTS updates for the 2009-2014 WRX, we decided to put our R&D WRX through some more rigorous testing regarding common modifications. Much like most customer cars, when equipped with full Stage2+Intake hardware, our car suffers from very severe boost creep. Even when it's not bad enough to cause a P0244 CEL, the fuel injectors are run right to the ragged edge from 5500 RPM to redline. Anyways...meet Ketchup the 2010 WRX. Look closely at the front passenger side of the bumper and you'll see the pressure measurement hose extending out and towards the dyno cart:
One of the more interesting but rarely-covered topics in turbocharged tuning is that of exhaust gas back pressure, or EMAP (exhaust manifold absolute pressure). On a Subaru, this is generally measured somewhere between the exhaust manifold collector and the turbocharger itself.
Positive pressure in the exhaust manifold (a positive pressure differential between turbine inlet and outlet) is required to drive the turbocharger but also represents a trade-off against an engine's efficiency level and ability to displace air. The less pressure the engine has working against it when the exhaust valves are open, the happier the engine tends to be. In a perfect world, we would be able to create boost w/o restricting the engine at all. Physics don't allow this unfortunately so we work to balance them. In racing scenarios, aiming for an EMAP/Boost pressure ratio of 2.0 or lower is a very general rule of thumb. Just some general statements here and the topic is well-covered in a variety of tuning and automotive/SAE texts.
The goal of these tests were to test our kit for its relative ability to eliminate boost creep on Stage2 & Stage2+Intake on the 2009-2014 WRX. More importantly, in our previous testing, we found that EMAP was *very* high on the 2009-2014 WRX at high RPM, which is a contributor to their strong tendency for detonation when fuel quality is not great. Knowing this, we wanted to compare EMAP on both the stock IWG and the EWG configuration. We then calculated the relative pressure ratio between exhaust and intake manifold with each configuration.
All of these runs were completed with the boost control system disabled (wastegate duty cycle set to cap of 0%) to demonstrate the boost system's performance on the IWG and then the EWG. The EWG spring pressure chosen for this test was .9 bar.
This data was collected using a stock manifold that has been tapped at the collector for the AEM Exhaust Back Pressure kit with sensor logging via the Mustang Dyno PowerDyne controller/software.
Stage2+SF hardware vs. Stage2+SF+EWG hardware, both at wastegate boost levels:
- On the IWG, EMAP/Boost ratios exceed 2.0 at high RPM. In other words, to produce ~12.5psi of boost at redline, pressure in the exhaust manifold is ~26psi with the stock internal wastegate.
- On the EWG, EMAP/Boost ratios are 1.5-1.6 at high RPM. In other words, to produce ~12.5psi of boost at redline, pressure in the exhaust manifold is ~19.5psi with the COBB 44mm EWG kit with atmospheric dump.
- EWG with atmospheric dump: muy bueno for power and efficiency purposes!
In the future we hope to extend this testing out to include recirculated EWG kits, upgraded turbos, different turbines and/or A/R housings with the same compressors, etc. Thanks for reading along!