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.