Turbine housing AR effect on tune

I am just finishing up a project upgrading a Turbine Housing from a .82AR V-band inlet to a 1.0AR twin scroll T4. This include fabricating a turbo manifold.

Since this obviously significantly reduced the back pressure seen by the engine, What effect/changes should I expect with the tune?

Here's a picture of the new manifold for your viewing pleasure.

generally a larger turbine housing gives less exhaust restriction and allows slightly more timing if you are limited by detonation. normally youll lose spool but going from single to twin scroll you may not be too far off.

I can see it going both ways with the timing. Allowing more heat to escape each cycle would allow for more timing, but having less exhaust "recycled" and more O2 to burn would seem to make for a more robust combustion, needing less timing. It may turn out that it's a combination of both depending on the RPM and load.

Going to have to suck it and see - don't forget that the larger turbine housing will also tend to produce less boost at low rpm and more at higher rpm, so that should also affect the tune for both timing AND mixture.

https://www.garrettmotion.com/ru/news/newsroom/article/turbo-tech-how-to-turbo-and-the-difference-an-a-r-makes-for-optimizing-your-turbo-system/

Thank you for that link.

Besides being very informative, It was equally entertaining.

Because I started with a V-band inlet turbine housing, I had to fabricate a fully divided manifold to match the new divided T4 housing. So I made a much greater change than just replacing the Turbine housing. I was concerned it would deaden the spool somewhat, but the butt dyno can't tell much difference. It definitely freed it up at higher RPMs though.

I haven't had much opportunity for testing But shortly I'll get on that. Going to get it on a dyno, but the pandemic lock down may push that back a bit.