2001 Non Egr Cummins Isx , the wastegate is completely useless imho !

The best solution is to put on a non wastegate turbo. Cutting off the waste gate from activating might seem right, but you can do damage to that particular turbo. i have never been a fan of "shade tree" modifications to a turbo. Just do it right and get a non wastegated turbo as a replacement and end your troubles.

 

effect of exhaust manifold backpressure makes it open much earlier than when pressurizing only intake side.

I don't understand what kind of valve there is but my guess is it shouldn't be leaking except if it is bleed valve used to raise boost pressure.

 

Ok... let's look at this again. A non wastegate turbo is still better overall that trying to rig a waste gated turbo. And the non waste gate versions come is just about any size and flow rate you could imagine. All that is needed is to look at the volume flow of the exhaust and evaluate the housing size and compressor wheel you would need to get maximum potential from that turbo on your application. There is plenty of info available and the more specialized diesel shops could point you to a product that would get the job done. Don't keep your focus so narrow.

 

But with non-gated housing you have only 2 choices: 1. Poor downlow torque and response, good power uphigh 2. Good downlow torque and response, overboost, high egt and poor power uphigh

 

If you were to "tie off" the wastegate function, you have the same deal. You have a turbo that is acting just like a non wastegated one, but not the effectiveness of a non waste gate one. Like I mentioned, there are any number of compressor wheels that can be installed in a non waste gate turbo to overcome some of the limitations you mention. There is a balance that can be obtained to match what you are doing and how you are doing it. Your response still exhibits too narrow of a thinking on this. It is captivating how so many engines have done extremely well over the years using only non wastegated turbos. When they are custom matched for the engine and the application, they do a much better job than the OEM cookie cutter mold type of turbos that get put on. The only reason for the OEM going with a waste gate turbo in the first place was to cover all the applications that the engine might be faced with from local daily p&d type of work, to primarily highway running. Sure it does cover all those bases, but a waste gate turbo, by having to cover all those bases, does none of them near as efficiently as a properly matched non waste gate turbo.