EFI-DIY
From there to the tail pipe its a 4" system orginally off of a duramax. Got 25' of pipe, hangers, muffler all kinds of bends .

It sounds like you are using a stock Duramax muffler?

If so, that will cause some back pressure & cause your turbo to spool up slow.

Two cents thrown.
MBHD

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12 port SDS EFI