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.

The (in)famous 3/16 boost control mod

Many a WRX enthusiast will have heard of the 3/16 mod. It's said to be a very cheap way of upping the performance of your stock, garden variety Subaru Impreza WRX. Unfortunately there's not much in the way of scientifically valid testing done on this modification, and opinions vary widely.

After a fair bit of research I decided to try the mod on my stock-as-a-rock MY02 Impreza WRX, and have been quite pleased with the results. However I will make the following disclaimer.... Fiddling with the boost control system on any turbocharged car is potentially dangerous. Overboost can destroy your engine in extreme circumstances. Make any changes AT YOUR OWN RISK.



What this mod does is give the stock WRX boost system a little kick. As I understand it, it allows the boost control solenoid to act faster and have more effect, which means that the boost comes on earlier and slightly stronger. It also means that if you overdo things, you'll end up with spiking, overboosting and underboosting, as the system chases its tail.

From what I've read about the WRX boost control system, it works like this... The wastegate is mechanically set at around 7psi. This is how much boost a car would run if the boost control solenoid was always shut.

The solenoid basically allows a small amount of air to bypass the wastegate, thus the turbo has to build up more pressure to have the same effect. The ECU thus controls the boost by opening and closing this solenoid. Solenoid open for longer means more air bypassed and more boost.



To avoid having to worry about the diameter of all the tubing in the solenoid circuit a restrictor is placed which gives you a set flow rate. Since this is the narrowest item in the circuit the restrictor is the deciding factor in how much air is bypassed through the solenoid.



Increasing the restrictor diameter means that opening the solenoid has more effect... or in other words the boost builds faster and potentially to a slightly higher level.

If the diameter is too big though, the boost will climb and fall quicker than the ECU can cope with, and it will overcorrect. The pressure will in that case be inconsistent and power delivery not smooth. You may get boost spiking to high levels and then cutting out as the WRX computer tries to compensate.

The replacement restrictor I sourced from my local hardware shop. It's original application was a hose coupler for a automated garden watering system. I then drilled it out to the required size and replaced the factory item.

Now I don't have a G-tech or boost gauge (although others with this mod have noticed boost arriving quicker and slightly higher).

Seat-of-the-pants-wise I found that on a hill that I'd previously have to down change to 3rd gear, I could now climb in 4th. I also found the traditional WRX Whoosh! arriving about 500rpm earlier.

All in all I've been pretty happy with the results, and have been driving the car with this modification for over a year now with no noticable down side.