ITBs + Turbo with Speed Density ECU 2

[wgknestrick]

I am in the process of designing/fitting a set of roller barrel ITBs (Individual Throttle Bodies) to my Subaru STI and was looking for some insight on some things. This is something that is underway and going to happen so please don't try to talk me out of it, or say it won't make any HP, etc. That is not the point here.

My question is reguarding tuning of ITB systems. Most NA guys switch to an Alpha N based fuel map (meaning load is in terms of throttle position, TPS) instead of MAP/speed density/MAF. This is because of the near ATM MAP (very non linear) values read from the MAP sensor when the throttle is opened. MAP is fine (but lower) on idle, then pegs to around 0psi (ATM) for the upper 75% of throttle. This is because you now have 4x (or # of cyl)of throttle opening leaking the vacuum away.

Alpha N works great for NA apps to offset this, but I am running a turbo, so I NEED speed density to account for boost. The one thing helping me is that I would be in the upper PSI levels at WOT, unlike a NA app where they are in a flat part (0psi) of the fuel map. I have good resolution in the high load sections unlike NA guys, but I still have this window (where most driving occurs sadly) that will be difficult to map fuel around 0psi

I've read of Miata/Honda guys making a vacuum manifold or accumulator from each port, then tapping the MAP into that. The theory is that the accumulator dampens out some of the noise from each port. This is trying to replicate the a normal plenum that has one throttle on it. The problem is that I don't want to make this too big because it affects my throttled volume (main reason for the ITBs in the first place).

ITBs are used with turbo systems in 2 OEM applications, RB26DETT and the SR20 GTiR engine, both Nissan motors. They both however run MAF based loading systems. There are tons of guys that modifiy these engines and I would guess someone has made speed density work with the OEM ITBs. Please note my stand-alone ECU cannot run MAF loading.

So about what volume do I need in this vacuum accumulator for a 2.5l engine, and what ID ports should I connect to this? I was leaning towards a port ID of .7mm, but if anyone here had suggestions it would help a great deal.

Here are some Solidworks pics of my design:

http://users.rcn.com/wgknestrick/itb transparent.jpg

http://users.rcn.com/wgknestrick/itb section.jpg

http://users.rcn.com/wgknestrick/TURBO ass'y.jpg

http://users.rcn.com/wgknestrick/TURBO ass'y2.jpg

Vac accumulator would be connected to the hose barbs from each cylinder port.

 

[pmrobert]

Thanks! You might also need a restriction in the line as well, placed before the accumulator section. MIG tips are good places to start. Basically, you're creating the mechanical equivalent of an electronic RC analog filter circuit.
See

http://en.wikipedia.org/wiki/RC_circuit

. Hydra's good stuff - you should be able to empirically reach a nice resolution with a little work and butt-dyno experience.

 

[nhrones]

Have you concidered the simple addition of a 470 microFarad electrolitic capacitor accross the map signal. This would filter the signal without much lag.