More 3/16 boost control mod testing

Well after my last post and some discussion with various people, I decided it was worth doing a simple test to prove or disprove my theories about the 3/16 boost control mod.

My understanding about the boost control system as it applies to my MY02 Subaru Impreza WRX is as summarized as follows.
- The wastegate takes its pressure feed from the intake manifold (post compressor).
- The wastegate valve is set to around 7 psi.
- All other things excluded this gives a maximum boost pressure of 7psi.
- The pressure feed line contains a T-piece that feeds the boost control solenoid.
- This solenoid allows the ECU to bleed some pressure prior to the wastegate, thus giving more boost pressure overall.
- Inline with the solenoid is a restrictor.
- The restrictor is used so that the system is not sensitive to the internal diameters of all the piping and connectors, but rather to a single minimum point.
- This restrictor tends to govern the effect of the solenoid.

What the 3/16 mod is supposed to do (if you can believe what you read on the internet) is to enlarge the restrictor, and thus allow the solenoid to take effect quicker, and possibly also give a higher boost level from the Mitsubishi TD04 turbocharger.

If you know anything about control theory (and why would you! It can be amazingly boring stuff) you'd know that any system with a feedback loop can by pretty sensitive to changes. If you alter a part of the system you risk creating a situation where it's chasing its tail. In this case, if the boost rose too quickly, the ECU would peg it back, which would happen too fast, then it would open it up again. The result would be jerky and inconsistent power delivery, which has in fact been reported by others experimenting with this mod.

How to test my theory?

I figured the safest way to test things was to block the line to the boost control solenoid. If my theory was correct, this would cap boost at around 7psi and effectively give the WRX about ~120kW instead of the full whack 160kW. Why 120kW you may ask? Well 7psi is about 0.5 atmospheres (or 0.5 bar) which means that the engine will be getting 50% more air/fuel mixture than under natural breathing, which in broad terms should relate to 50% more power than a comparable naturally aspirated motor. This means that at 7psi the WRX is effectively running a 3.0 litre engine, and at full noise running around 14psi its effectively running a 4.0 litre, which is born out when you compare output figures of various cars.

Armed with all this knowledge and no small amount of consternation I pulled the restrictor from the hose, wrapped it with plastic and plugged it back in, thus blocking the hose completely.

Driving the car was quite interesting.

The characteristic WRX power delivery was altered quite a lot. The power came on smoothly up until around 3000rpm just like before, then... it just kept coming on smoothly. Instead of that grin-making whoosh that usually arrives, it just kept pulling strongly. It certainly felt stronger than a naturally aspirated 2.0 litre, and closer to a 6 cylinder in feel, but nothing like the usual WRX rush.

The turbocharger was definitely still working, because since I have the intake resonator removed I could still hear the spool up, and the factory BOV sneezing occasionally. However my theories seemed to be proved correct. The Mitsubishi TD04 turbocharger seemed to be making about 7psi with the hose blocked, and around 14psi (or whatever stock levels are anyway) with the restrictor in place. If the line was unplugged completely I'm guessing that the turbocharger would generate as much boost as it could, which is a pretty cheap way of upping your power, as long as you don't mind most likely detonating your engine.

My next tests will be to try some different restrictor sizes, or fit an adjustable valve. Stay tuned for more.