Markybrisca

Hi, wondering if anyone has good knowledge of a standard setup and restrictor pill sizes. Basically, just removed a manual boost controller from my stock wrx bugeye and returned all boost plumbing to normal with new Subaru parts. Only problem is when i fitted standard restrictor pill,(1.2mm) i get ridiculous overboost to the point it bounces from 0.8 to 1.2 bar in 3rd and above gears when accelerating hard. I drilled out the restrictor to just under 2.0mm and all is good, 1st and 2nd gears get to 0.8 bar, 3rd gets to 0.9 bar and i now get peak 1.0 bar in 4th and 5th when accelerating hard. Only problem with this is it hits peak boost and holds perfectly but feels slightly flatter on acceleration obviously due to hole in pill! I have just bought 2 more pills from Subaru and drilled one out to 1.5mm and kept the other standard. I will fit the 1.5mm pill when the rain stops at some point and see if that gives me a better acceleration feel. Anyone else tried the pill size experiment and if so, what results did you get, comments appreciated Mark.

Splitpin

1.2 is the correct diameter, and 2.0 is a huge increase, so the obvious inference is that there's some other remaining issue causing your overboost.

Are you sure the car's totally stock, for example? We know it had a manual boost controller on it, so is it possible that you bought it with a decat exhaust that you weren't told about? If there's any uncertainty in this area it's worth eyeballing the downpipe and mid-sections to determine the presence (or otherwise) of the two post-turbo cats.

In addition, are you sure you've assembled the new boost control pipework correctly? Restrictor should be in the pipe run between the compressor outlet and t-piece, approx 30mm in the pipe from the tee. You haven't got the pill in the pipe to the wastegate actuator or something like that?

Markybrisca

Hi Splitpin, thanks for your reply. The boost plumbing and restrictor location are ok as i have a stock photograph from this website. As for my WRX, the only piece of non-standard kit is a Powerflow custom cat-back stainless exhaust system, the rest is standard kit too! The car reaches peak boost and holds solid at just under 2mm, but it would as this is obviously way too large! The only other possibility is that the manual boost controller was fitted along with a custom map of some sort, but as suggested before no mapper worth anything would fit a manual boost valve would they? Without spending some cash, there is no way of knowing if my car has been mapped though to my knowledge. I will try the 1.5mm restrictor tonight and see what kind of drive this gives me. Thanks for the advice/suggestions Splitpin!

One more thing, am i right in thinking that the Impreza`s ECU will pull back boost so it doesnt peak in 1st and 2nd gears when accelerating? thanks Mark.

Splitpin

Aaaaahhhh, right. Well, now things make more sense...

This is the key. When you said "stock WRX", your situation sounded odd. Now we know it's a stock WRX with a cat-back exhaust, the overboost is far easier to explain. Is the exhaust system a full decat? Would guess so from what you're describing.

Think you will find a few rare examples where reputable mappers have fitted manual boost controllers under very specific circumstances, but they're very much the exception that proves the rule, yes. With a newage there's also the possibility that some of the less reputable "mappers" may have had their hands on it. If in doubt, and given that peace of mind has a value, it might be worth putting a few quid or a couple of pints of beer in the direction of a good mapper and asking him to check whether the ROM image currently on the ECU is standard or not. Only a 2 minute job.

As above though, if you have overboost right now, and the car is decatted and it had an MBC, likeliest explanation is previous owner taking the common cheap power hike path.

Not quite. It doesn't pull boost back as such. The real explanation is that your ECU doesn't have any sort of gear-indexed compensation. Because, under natural circumstances, your turbo will develop more boost in the higher gears (when load on the engine is greater), you'll get more in fourth and fifth than you do in first and second.

The more advanced ECUs with in-gear boost compensation actually do it the other way - adding duty cycle in the lower gears to remove the disparity.

Markybrisca

Again, thanks Splitpin for your concise help. I have just fitted a new restrictor pill (1.5mm) and can see significantly more acceleration, boost now peaks at around 1 bar and holds well, 3rd gear acceleration is now awesome! My exhaust system still has the original cats on and the Powerflow has been fitted just after these, mainly for noise and looks i think! At the moment running a 1.5mm pill all seems good and is not a million miles away from stock. Will give a run down the motorway through gears tomorrow to check boost holds firm. Thanks Mark.