I've noticed quite a few questions regading the necessity of an Engine Management system (EM), based upon certain upgrades. Rather than searching in 27 different threads to find out, I'm going to try to create an all-inclusive thread.
With that, I'm going to (hopefully) clear up any misconceptions...
1) A catback exhaust (CBE) does almost nothing for your car except make it sound cooler.... you do not need an EM
2) An aftermarket intake "can" cause your car to run lean (too much air, not enough fuel)... so yes, you should be tuned. The exception would be an SPT intake, as Subaru claims they are safe to run on the stock engine/ECU. However, I would get tuned anyway.
3) A Turboback exhaust (TBE) includes a bellmouth Downpipe (DP). The DP allows your turbo to boost higher, and produce more power. With that, if your ECU is not telling your injectors to supply more fuel, and adjust the timing, to compensate for the added boost.... then once again, lean conditions and boost spiking occur. Yes, you need an EM.... period
*** Note, if you have a downpipe with no EM, don't complain when you blow something up.
4) An aftermarket top-mount intercooler (TMIC) with do very little for your performance... it will however, give you greater protection from heatsoak. No, you do not need a tune.
5) An aftermarket front-mount intercooler (FMIC) gives you added protection from heatsoak, but because the forced air has so much further to travel, it has been known to cause a bit of added turbo lag. The general rule of conscensus is that you don't need one unless you are over 350 whp. If you choose to use one anyway, then yes, you should be tuned.
5) a BOV will do NOTHING for you.. at all. In fact, if you don't get a recirculating BOV, you'll probably end up hurting your performance. Either way, you do not need a tune to install one.
6) On pre-06 WRXs, an aftermarket catless uppipe will give you added performance, and in many folks' eyes, is necessary preventative maintenance to ensure the catalytic converter doesn't get sucked up into the turbo. It will allow your turbo to spool up quicker, but generally will not cause lean conditions or boost spiking. For an uppipe only, no, you do not need a tune.
7) lightweight pulleys are the equivilent of removing weight from your car, you do not need a tune.
8) Turbo upgrades..... whatever it is, yes, you need to be tuned.... I can't tell you how many stock WRXs I've seen that bolt on a VF39, an 18G or a 16G turbo and drive off without doing anything else...... then complain about why their car doesn't run right.
Please... if you have anything else to add.... please do. Just seems like we get a lot of the same questions over and over again.... trying to help here and steer people in the right direction.
An EM should be the VERY FIRST item you buy for your car.... and I think just about everyone will agree with me on that one. Yes, they are expensive... but you only have to buy it once, it's infinately upgradeable, and most importantly, it's SAFE.
Hope this helps.