You want to run the minimum boost on the SC 6psi is more than enough. If you run more the SC will rob more power from the engine, also if you have the SC at 10psi to get 20psi at the SC outlet the TC will be pumping 6psi with a large TC and that flow rate the TC will be very inefficient. You can also IC between the TC and SC with the SC only running 6psi no need to IC
I would run a large TC and use the TC to provide the bulk of the boost. The large TC will spool very quickly due to the exhaust flow created by the SC.
Yeah it's like a scale - SC boost vs turbo boost - but it's SO hard to say what's best to run without actually just trying combinations and monitoring boost pressures.
at one end of the scale you have:
Most boost through the SC:
Advantages:
- turbo doesn't need to spin as fast, and hence is not a restriction in the exhaust as much..
- run lots more timing and make more power due to above reason
- keeps exhaust temps a lot cooler
- *probably* spools the turbo a bit quicker
- if the engine takes more timing, it might not need to be pushed as far towards det. in order to make same/more power than most boost through TC. - this means a 'safer' state of tune
Disadvantages:
- more power draw on the motor
- more strain on the charger, belt, etc.
- potentially high intake air temps (pending cooler efficiency etc - would want to monitor these)
Most Boost through the TC
Advantages:
- supercharger doesn't rob the engine of power as much
- less strain on supercharger
Disadvantages
- more exhaust backpressure & temperature
- potentially longer spool time
So you need to find a point somewhere between all those two, by experimenting with pulley size, and monitoring all the pressures, while being able to monitor power output, and see how much more timing the engine will take. - Only then can you really get a clear picture of how it all works
On my setup, I've used a bypass valve which essentially allows compound boosting, until the turbo is ready to make more boost than the blower, in which case the boost 'adds' - until the blower switches off, and the turbo takes care of the rest of the work. I have been thinking about keeping the SC on for the whole run, but I don't know how much more power I can make on pump fuel, and whether my magnetic clutch pulley would hold at those pressures (unlikely) - I have gone with the bypass for now because the charger would be bloody loud if on all the time, and I doubt the pulley would hold at the boost pressures i'm aiming for - so really, I've essentially just used the charger to spool the turbo up (and not taken advantage of the whole backpressure thing
). The fact that the charger is electronically switchable is what makes my setup able to pick and choose where and when i want the charger to engage (via ECU) without serious side effects on drivability, or complex valve setups.
Regardless, once the car is back together, i will try book a dyno for a whole day, and experiment with (all of the aforementioned params), until i have dyno proven power, torque and boost curves, and some more (maybe new?) theories.
And having said that - even without having found the optimal charger pulley size, etc - check out the difference in boost curve PRE and POST charger! - If i use compound boostage (the next plan) all the way through till full boost, I predict full boost can be achieved by like.... 2500-3000 rpm.... watch out rods!