Looking at a cheap stealth exhaust set up so planning on purchasing a second hand sti exhaust and a decat downpipe for the forester. 3 questions that I can't seem to answer

1. What is the bore size of the sti exhaust 

2. will any bore size downpipe fit the sti exhaust e.g. 3" down into 2.5" mid

3. My car has 3 cats, 1 in the up pipe and 2 in the down pipe by removing the 2 downpipe ones will my car fail emissions 

i understand id be more beneficial removing the uppipe cat and then sports cat downpipe but Im planning on swapping the turbo next year which will make changing the uppipe easier so for now looking at a quick easy job with a cheap decat downpipe  

The bore size is probably 2.5,

Unless the dp is tapered to 2.5 it won't be a straight for,

You could get away with removing one of the dp cars and it may pass depends on how good they are, sit mod for the up pipe cat is to gut it. The main reason is when they break up they tend to go through the turbo 😢

If it's an impreza then the rear section will need a bit of work to make it fit

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