I have been trying to fix an overboost situation for a while now while breaking in a new engine. Old one blew a piston die to ...overboost. Anyway, it spikes over 15 PSI then the fuel cut comes in and scares the crap out of me. All I think is not another rebuild. Not to bore everyone with what I looked at, just found out about a restrictor pill in the line between the turbo and wastegate. It can be the wrong size and delay the actuation . So I pulled it out and drilled it open a few thousandths, maybe a few too many. So now 9psi max and no fuel cut. I know it is lower than the spec but for now it is a safe number. For $10 I ordered a new pill and hose and will open it up to .003 less which should get me about 12.
Why is this thing not common knowledge, everyone talks about the overboost solenoid and other stuff.
Ask an FD owner from about 25 years ago about boost tuning by playing with restrictor sizes... or what happens if you replace all the rubber hoses with silicone and forget to transfer the restrictors over.
Dempsey Bowling used to have plans on his Turbo Dodge site for a simple two step boost controller, using a vacuum solenoid controlled with an on/off switch, and multiple "Grainger valves" (adjustable orifice).
You're not treading new ground, but your brain is working.
I had an adjustable valve on my GLH turbo, just figured 15 years later they would come up with a better system that a brass slug with a little hole. The suby has a wastegate solenoid and a much improved computer than what we had 20 years earlier, why would it rely on a silly mechanical bit that costs $2?
Without knowing exactly how the Subaru boost control works, if it is an actively duty cycled solenoid to control boost in real time or what, there has to be a calibrated restriction somewhere in the line, may as well make it a $2 part rather than integrated into a more expensive component.
You also have to figure, Japanese automakers tend to be extremely conservative tech-wise, and Subaru was like the AMC of Japanese automakers, I think every chassis up to the middle of the last decade or so could share parts with its previous generation with only incremental changes made, to the point that I am pretty sure that the front struts and knuckles from a 2015ish car would bolt in to a 1985 GL and vice versa. If they could do boost control simply and cheaply, they would.
