Let's share some more info:
Steelmesh....The one thing you need to understand is that I was one of the very few people to put an M62 onto a 2.5 litre v6 probe gt more than 15 years ago. I would say (regarding experience with an m62) I have the MOST experience with this supercharger than most people you are talking with. In fact the gen of my m62 was older than the one you are using thus probably less efficient.
I invested 200+ hours over 12 months (didn't do anything over the winter doh) where I built a functional 500 bhp Turbocharged Solstice that I raced then converted to a M62 Twincharged Solstice before your very eyes. And I am very proud of this fact and I'm sure my mommy is too
I think supercharging the probe is unique and a cool project, so I hope your mommy was also proud of you.
On my 2.5 litre I spun the supercharger fast enough to make approximately 8-10psi of boost. This netted me two separate dynos of exactly 245 whp. The stock 2.5 litre v6 made about 150 whp so at 8-10 psi I was gaining 95whp. I KNOW for a FACT based on rpm calculators that EATON put out at that time for the m62 that I was running the supercharger very close to it's efficiency limit.
Slowbalt shared this:

Keep in mind that the max efficiency you will see out of the M62 is around 10psi and 460cfm which is spinning the supercharger around 14,000 rpms. More than that and you are raising heat exponentially higher than cfm which is closing in on a net loss in hp.
Based on the 5th Gen Map above, your supercharger was very efficient. Did you use an LSJ M62 specifically? Was it ported?
Maybe it's not directly related but something I thought of: I heard very recently from a racecar builder (HP Academy interview) that for max output applications the compressor map is completely useless because they literally fall "right" off the map, pun intended.
The roots blower doens't build "boost" like a turbo. It is essentially a big air pump and the way it gets boost is to push more air into the intake than the engine can handle (excess air=boost amount). In essence, with a roots blower...You could have a 2.4 litre with stock exhaust make 10psi then put an after market exhaust on and make 8psi but still make more power with the performance exhaust.
1) Same intake runners
2) Same intake valves
3) Same hotside
You either know the answer or not: Why does 25 psi "behind the intake valve" flow less air with the Twincharger setup versus 25 psi "behind the intake valve" with the Turbo-only setup.
25 psi of more dense 85F air entering the combustion chamber with Twincharger = less airflow
25 psi of less dense 123F air entering the combustion chamber with the Turbo-only = more airflow
This is not very intuitive, please stop and think about what I am saying. Hot air is less dense, Cold air is more dense. 1 cuft of Cold air has more molecules than 1 cuft of Hot air.
Let's not pretend we actually know what we're talking about and write essays with no real points but mostly speculations. I am still waiting for a PT engineer at work to answer this exact question in 1 sentence, that is the person I will pay attention to. Until then I'd like to figure it out myself because that would be cool. To me it sounds like if my temperature scaling is messed up, that could explain this whole thing and I would move on from it and leave it in the dust. Beyond faulty instrumentation is deeper analysis of the system, I like to draw things out to start sometimes it can solve the problem sometimes it's useless. And it's kinda funny that I don't actually have a ton of real tangible data points for my exact application / hardware either, so when my situation allows it I will take the opportunity to log more data for fun fun analysis. I have the power of the data, that should make people suspicious that I have potential to know quite a lot, very soon.
The other thing you need to realize is that you are using a compound setup with the turbo feeding the supercharger. So...it's not about making more power with the supercharger...It's about making more power throughout the powerband. If you made 450whp with just the turbo...you had a decent amount of lag as a result. The supercharger (even at 10psi) is allowing that turbo to work a LOT easier in a lower part of the efficiency map to make near the same amount of hp.
So again....I need you to understand that measuring boost is only telling you a very small part of the story. Maybe you are making less overall boost, but, if everything is done correctly (no leaks etc) you will be making MORE power over a wider range of the power band.
Of course agreed except I disagree with calling it a "compound setup" only because it's confusing and a term which multi-turbo setups identify with. Twincharging technically uses Superchargers and Turbos.
For quite a while I have been very open to saying stuff like "It's my anti-lag device" and "It won't add peak HP, just area under the curve" when talking about twincharging my car, like those are the lines I'll be using at car shows and tracks when showing it off to strangers.
So again....I need you to understand that measuring boost is only telling you a very small part of the story. Maybe you are making less overall boost, but, if everything is done correctly (no leaks etc) you will be making MORE power over a wider range of the power band.
What made you think I didn't understand that boost is relative to many factors? Did you notice I have been focusing on MASS AIR FLOW and mentioning boost as a reference point? The reason is because mass air flow is a very good indicator of air flow, air flow is a very good indicator of fuel flow, fuel flow is a very good indicator of energy potential of the intake charge.
Also, it might feel slower to you because in the straight turbo setup you have lag...when you floor it there is some time before the turbo gets up to speed then when it does it's a big hit. With the supercharger you are at a higher hp and tq level when you floor it than when you were just on a turbo. The reason it doesn't feel as fast to you is because there is no lag. In your mind what would feel faster to you? Floor it start out with 150whp then bam turbo kicks in and you're up to 450whp, or floor it start out with 250whp then turbo kicks in and you're up to 450whp.
It feels slower because I am literally down on power and the SC is causing parasitic losses. You have an interesting imagination otherwise
