One of the things that a lot of people don’t totally understand is how the boost control system works on your turbo Subaru WRX or STI or in fact, many of the other turbo Subarus such as the Libertys, Legacys, Foresters, and such.

An easy way of obviously gaining more power out of the Subaru is by simply turning up the boost, but what risk do you take? Are you risking your factory warranty? Are you risking the reliability of your engine? Are you risking excessive fuel usage and all those things combined together? The boost control system in the Subaru is incredibly complicated and has a lot of safety features built in.

Meaning it not only controls the boost to give you maximum performance, but it also controls the boost to protect your engine against damage should the ECU detect that something is not working correctly. Now if you have bought an after-market boost controller, whether it’s a pneumatic one or a electronic one, you take away one of the huge benefits and the fundamental features of the factory ECU and its ability to protect you and your engine from either catastrophic engine failure or risking some type of damage.

Now what can actually happen is, for example, you drive your car, go to your local petrol station, fill it up with what you believe is the right fuel for your car, and unbeknownst to yourself, the tanker delivery of fuel put the wrong type of fuel in the bowser’s pump tank underground, and of course, you end up with a car that’s not running as good as what it should be or giving you the right amount of performance.

You may not be aware of this because you’ve paid good money for the right amount of fuel or the correct octane fuel, which is obviously typically a 98 style octane fuel for your Subaru. You go for a drive, put your foot into it and all of a sudden you hear this horrible noise coming from under the engine. It might be a failed piston or it might be just piston detonation.

If you’ve got an after-market boost controller, the boost controller will continue to deliver the correct amount of boost for what you’ve chosen to set it for. But in the case of your factory boost control system, the ECU will detect that you’ve got the wrong fuel in the car to a certain degree, because it will have detected knock or engine ping from the knock sensor on top of the engine, and will start pulling out ignition timing.

If it continues to knock and ping, the ECU will then start trying to pull out boost. The thing that you need to consider is that obviously what you want to have happen, even though you might not be totally satisfied in losing performance but it’s certainly a lot better way than losing an engine.

If you’ve got the boost control turned up and adjusted through the factory ECU using a software program such as EcuTeK and getting your EcuTeK unit to adjust the factory settings, you can have the desired boost that you want with the related engine performance and ignition timing and fuel delivery matched to suit, but also the fantastic benefits of the safety settings that Subaru spent millions and millions of dollars in designing to protect your engine.

So there you have it. There’s a quick tip on how to both upgrade your car and not risk damage to your engine at the same time. If you’re fitting an after-market boost controller, be aware that there is a lot of risk involved and you need to be conscious of how you choose to drive your car and what you feed your beast as far as what type of fuel you’re putting into it. If you want the safe option with also a good boost control map at the same time, you can use that advantage through the factory ECU.

And one point that I’d like to point out is the factory ECU boost control is very, very advanced and very, very accurate. You’ve got the obvious opportunity of getting a much better, smoother boost control delivery, and in fact, the factory ECU, if tuned correctly with the right boost, will bring the car on boost earlier and better than 99% of the after-market boost controllers.

If you’re comparing a factory ECU-tuned boost control map to, say for example, a pneumatic boost control map, then it is light years ahead because pneumatic boost controllers are nowhere near as good as electronic boost controllers, and after-market electronic boost controllers are not as good as the factory boost controller, which is, of course, electronic as well.

Article By Brett Middleton
Subaru Performance Handbook Blog