sizing a pre comp boost pressurised system

[sambeeb]

Hi,

I need some advice on a boost pressurised pre comp injection system. The engine is a VAG 1.8T BJX with a borg warner K03s turbo. This turbo pushes 12 psi peak in standard form and in its stage 2 flash tune guise pushes 20+psi (1.45 bar). The exhaust manifold has been ported and port matched and the wastegate pretty radically ported too. This along with the 3in dump should have reduced back pressure substantially, so would it be fair to assume it is not hot side limited but rather compressor limited and a good candidate for pre comp injection?

The aim is to increase the turbos upper range/efficiency whilst still running a generic flash tune where fuelling can't be altered and only global timing can be adjusted.

So here's what my train of thought has been so far:

1. I want to do a purely mechanical boost pressurised system using a spraying systems atomising nozzle. From what I can gather this will give excellent atomisation which will allow me to inject well upstream of the compressor which is all I have access for, and will also give variable flow

2. Based on the standard 2.5in MAF peak readings of 185g/s, if I inject at the recommended 2-3% of air, calculated on 2.5% of max MAF flow = 4.625g/s = 277g/min = 3.66 gal/h.

3. There are 3 spraying systems nozzles that fit the bill with working air/water pressures from 5psi up to 20psi. The SUE15B and SUE18B both give between 1.4-2.0 gal/h between 10-20psi. The SUE18A though is between 2.2 and 3.0gal/h between those boost levels. Given that I will likely inject only above 1 bar (my spool is already pretty aggressive and ZI don't really need to enhance that) which nozzle would be more appropriate?

4. I was going to run WM 50/50. I would have preferred water only from a reliability point of view but considering that I can't 'tune' for the injection other than adjusting global timing advance, then incorporating meth seems to be the way to go. Is 50/50 sounding about right for this kind of brief or is more meth needed considering that I won't be able to add much timing?

5. I have read that in these systems when under boost, the air orifice in the atomising jets can be large enough that they can actually represent a boost leak. I can't really see how that is the case - it may be one until vessel filling is complete and then stop being one surely. Either way i'll be running small diameter hose and may even use a solenoid/valve in the air line to be activated when the injectant solenoid is activated to prevent this. likely to be an issue or not?

6. One thing I'm unsure of is the best way to design the reservoir. I was actually going to use one of the VW/audi ball shaped coolant tanks since the design is baffled, will handle the pressures, see through, has all the tappings to use and will not be out of place in the engine bay. The only issue I could think of was that perhaps volume of the chamber may not be right. Is there any rule of thumb for determining what air:injectant volume ratio works best in a boost pressurised tank?

7. lastly from what I can gather I will likely not see an increase in MAF g/s even though the engine may be making more power ie the engine will draw in the same amount of air its just that (if successful) it will be using it more efficiently. I just wanted to verify this so that I can change the way I measure progress as to date I've always looked to MAF readings/curves as a way to gauge power increase - crude I know but free!

again, the aim is to run a system that will give me more mid range to top end on my maxxed out turbo that will have the best chance of working with a fixed fuelling tune. any help greatly appreciated.

thanks
sam