El Porto, 600 is single turbo territory for a 3 litre engine. What you're talking about isn't lag, it's boost threshold. You want 15psi at 2k and 7k and no turbo can do that. A R2S system, could, though. It's kind of compound, but kinda not. I love it, and have loved it since it was released maybe 6+ years ago? Keen to do that on my truck too. Keep the existing holset for the big one, and introduce valving and a tiny one for the other end.
In my experience with my large truck holset on a small petrol 4 cylinder spool time is negligible indeed. If you jump on the throttle at 5k, it's near instant. If you lean on the throttle at 3k it will *never* build more than a psi or so. This is NOT lag. It's boost threshold. You can plot possible boost vs RPM on my engine and it's not linear in the slightest. I know my holset will do 50+ psi, where it ends, who knows, but this should illustrate the point:
A curve like this is a fact of life for any traditional turbo setup, hence all OEM setups have tiny turbos, 'cept the mitsi evo and subby STi, and they're still pretty small. Putting on a tiny turbo pushes that curve down lower and makes it steeper. Instead of 12psi at 4k you have your 12 at 2500 and will make a few psi at idle RPM and more in between, etc. The down side is it chokes the top end. You can't really win if you want linear torque, except by going to an R2S like system.
Fred.
