Three things:

1. You MUST retune for that header. No excuses.

2. What is the primary diameter of those headers?

3. You are confusing LAG with BOOST threshold. ELH can improve transient response but rarely reduces boost threshold.

Matching primary diameter to turbo is VERY important. The VF48 is a small turbo that requires quite a bit of exhaust pressure to be effective. Smaller primary diameters maintain faster exhaust flow which in turn helps spool.

This is conjecture without knowing the details of your header, but you've probably killed spool by going too big.


EDIT: ah ok. This is basically an eBay header, probably zero R&D. There is a reason reputable brands design with CFM analyses and are flow optimized

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Thanks for the info Zax. I do have a retune booked, but I'm thinking I'll go back to the OEM header considering boost threshold is my priority here.

I'm going to wrap them first and see if it makes a difference.

It's unfortunate to hear that they're an eBay spec header as I bought them from a reputable exhaust shop, but that's just how this stuff works sometimes.

Edit: just eyeballed the header and it looks the same diameter as oem

 

UPDATE -

The O2 sensor on the up pipe was super loose, causing air to leak, it was also pure black.

I cleaned the sensor and screwed it in all the way (tightly) and the car runs as it's supposed to now

The boost threshold is almost the same as the previous hearers. There might be a tiny bit of extra spool required for full boost? I can't really tell but let's assume there is for anyone potentially buying these headers.

Problem solved. Tune will still be happening

 

Well the tune needs to happen. Changing the exhaust manifold requires a retune.

 

Booked in for March 16