Questions about EXHAUST manifold inside diameter and turbo drive pressure

Hoping some "Diesel Tuners" see this.
Last year, I replaced the stock turbo on my '07, 5.9 with a Garrett stage one with the billet compressor wheel. Tom at Turbo resource set it up for me. At the same time, I also changed the exhaust manifold with a two piece DDP exhaust manifold. The DDP has larger inside diameter porting in the manifold than stock.
My truck is a 2007, 5.9, G56. I run a Smarty JR at the 70 hp setting and the engine is stock. The very best boost I can get is 30 psi at close to 2900-3000 rpm in fifth W.O.T. and I feel that I should have better boost at lower RPM. Tom has been a help in his and continues to tell me that I should be getting closer to 38 psi.
I pressure tested my intercooler and no leaks. I see no apparent leakage at the intake cover on the head but I am not sure what kind of gasket or sealer Cummins used to seal it.
I have wondered about putting back the STOCK exhaust manifold as the I.D. of the manifold is smaller than the DDP manifold and in my thinking, the smaller manifold would increase the exhaust gas velocity and help to drive the hot side of the turbo faster at lower and higher speeds. Many turbocharged diesels I see, light duty as well as heavy duty seem to not have real large exhaust manifolds on them.
I have the O.E. Cummins gaskets to change the manifold but wanted to ask some of you guys first. Otherwise, I am considering having Tom rebuild my stock turbo and going back with it.

 

I think you may be barking up the wrong tree,I have had at least 4 different manifolds on my o5 and have never noted any change in max boost between them.

 

Thanks Bob, Are you running stock or non stock turbo?

 

non stock right now,it's a one off from a friend

 

Any suggestions Bob? I realize one thing against me is that My truck is a stick. With the 6 speed, just about when the boost kicks in good, I have to let off and shift and start all over again. I have checked just about everything I can think of.

 

Look for a boost leak. Only thing exhaust manifold volume will affect is the rate boost is build, When i had a small boost leak i would only clear 32 in 4-6 gear. Fixing the leak pushed boost to 35+ under same conditions. Doesn't take much of a leak to bring down boost when the exhaust side doesn't have restriction. My leak was the boost gauge line.

 

Thanks. I'll check that. I have a made up plug to hook an air supply to and it fits into one of the boost hoses. I'll pressurize the air horn toward the head.

 

Pressurize from the intake to the turbo all the way through the system. The leak could be on the charge air hose also.