I drive a 2003 Tiburon 2.7, turboed with a Garrett GT30R turbo, with a 38mm TiAL wastegate.

The turbo was installed a little over a year ago, putting out 310 WHP at 0.75 BAR of boost (about 10~11 psi) on premium gas.  The wastegate has a 0.6 bar spring, which I raise using a manual boost controller. At this setting, full boost was reached at around 3600 RPM.

A few months ago, boost started going up slower, and not reaching full boost until over 5000 RPM.

It's a big job to take everything out, because of the cramped engine bay, so I just drove the car easy for a few months, until last week, when I had to replace my clutch.

I decided to get to the bottom of the problem while I was replacing my clutch.

I found some exhaust leaks, which I thought was the root of the problem, fixed them, and assumed the car would be okay.

Well, it's not. Right now, at 4000 RPM, the car is only reaching around 0.3 BAR of boost, which is about 4~5 PSI, and to even get to 0.5 BAR, I have to go above 5000 RPM.

I thought that the boost controller may be the problem, taking it out, so that I could get 0.6 bar of boost from the wastegate, but this is not the issue.

I'm at a loss as to where to look for possible boost, vacuum, and/or exhaust leaks.

Can anyone give me some pointers on what may be the issue?

I'm thinking the turbine may be blown, but when I spin it by hand, it spins freely, with no noticeable shaft play, and there's no smoke coming out of my exhaust.

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The best way to check for boost leaks is to make some sort of air fitting adapter that you can use to couple the inlet of your turbo to an air compressor.  Then pressurize the entire system with at least as much boost as you plan to run and listen for leaks.You need to make sure all the inlet valves are closed when you do this. Not so easy if this is an OHC engine.

Check the blow of valve first. It is a likely source of inlet leak.

This whole discussion about whether or not engines have very little overlap vs. some overlap is absolutely moot because as far as the OP is concerned, it should be extremely easy to find a crank position where both intake and exhaust valves are not open at the same time (ie. as long as no piston is within about 10-20degrees of TDC then you're fine) and even then, I really doubt the whole overlap thing is an issue to begin with.  If a boost leak is indeed the root of the problem, it has to be big enough such that a GT30R struggles to make 5psi on a 2.7L engine!  In which case, it must be a MASSIVE leak.  I guarantee it will become quite obvious the instant the air is turned on regardless of whether or not any air is leaking out the exhaust!
Sorry for the rant, I just can't believe how overly complicated some people are making this simple problem.

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