From 12 - 9 -12 now I am 7 - 4 - 7.

Ok I just changed my muffler but loosing 5 Psi is not from the muffler.

I can hear a loud leak .........ssssssSSSSSS...... from the dash when I'm driving.

Must be some leak from the turbo.

Where can I look please?

 

Failure of the stock Y-pipe connector is a common cause of boost leakage.

Go here for more troubleshooting:

http://www.scuderiaciriani.com/rx7/boost_problems.html

 

Yeah, I'd check the Y-pipe and also the tubes leading too and from the intercooler, they tend to split with age.

But start with the simplest thing first. Is your muffler tightened? Could be exhaust leaking.

 

I had a very similar problem which turned out to be one of the large rubber hoses coming off the air pump. It had a large split from age, and it was only a $20 part.