I had a problem in that I couldn't boost above 12psi.

Boost would build and then plateau towards 12psi...


Checked EBC settings in MS... no problems
Checked for boost leaks (and found a few)... helped but not much
Tried removing the external WG and using a block-off plate...nada
Was told I had a 'peashooter exhaust' so removed it...



BOOM!
(I saw boost spiking so I got off the gas and I'd found my issue)

I'm sure this would be obvious to a lot of people here but it took me some reading as my misunderstanding was that if my DP was 2.25" then that's the biggest restriction in the exhaust system and in fact it wasn't. The further back you go the more the exhaust gases cool and expand so a 2.3" cat-back is much more of a restriction than 2.25" pre-cat. (It's also very possible that the muffler was a restriction too.)

Reading around I found that this restriction can also cause overheating issues, knock and a few other problems.

I hope that this helps someone else with the same limited boost problem.

Anyway, the restrictive cat-back is back on the car so the neighbors don't hate me and I've ordered a BEGI 3" Turbo cat-back. Should get a controlled 18psi before long.

 

Clogged cat.

 

I'm sure a clogged cat would have a similar effect but there was no cat on the car in either log. 2.3" test pipe in the first, open DP in the second.

 

Very interesting. FYI... Gasses don't expand when cooled

would you say that this was truly an A-B test? The only change was the removal of the test pipe?

 

Perhaps I should have said 'expand and cool' rather than 'cool and expand'. Although neither really implies a causal relationship just that they are both happening.

Pretty much. There are of course other minor variables.

After removing the exhaust (dp back) I reached boost higher than 12psi for the first time after many other changes and trials. I am very confident that the limit was entirely down to the restrictive exhaust or muffler.

 

Congrats on the find. It is a bit of common knowledge on this forum that it miatas love 3" exhaust for turbo setups. I think it is great that you posted your experience so that people can find solid proof when they search.

 

Thanks! I hope this saves some others the time and frustration I had with it. More boost!