660's will only solve a fuel shortage issue. Meaning if you are getting knock from running lean you would need the increased fuel. If the root of the issue is the superheated air, then you don't actually need more fuel.

The hot air is actually a smaller mass of air and would need less fuel than an equal 'volume' of cold air (not sure if my terminology is correct there but the theory is sound).

Bascily you want the most total mass of air. 22psi of hot air can actually be less actual air than 16 psi of cold air depending on the application.

Two things I can think of is the shaft speed of the turbo, its effeciency range (not just peak efficiency but where its actual power production actually drops off) and the quality of you intercooler set-up.

Hope that helps.

Oh, and that chart's been around a while. Not sure how much I buy into it as gospel. Good reference/rule of thumb but not the be-all-end-all IMHO.

The other thing to look at is igniton timing. If your getting to much timing, that will also induce knock. It could be that your not using up enough of the 550s and ending up in a "low load" timing map. This results in to much ignition timing which will come up as knock. Richening up the SAFC might fix this, and make you think that you need more fuel when it is really just a tuning issue.

Some guys have said that going down to +3 or +4 has helped their high-end knock...not sure exactly the thinking on that one, though. Maybe it has something to do with what ncgalant is saying about the "low load" map. Do you know what your injector duty cycle is at when you're getting this knock?

We have to remember though that with piggy back systems we do not have complete control over the timing or the fuel for that matter. Richening up the mixture with the SAFC or MAFT may serve to decrease timing and richen the mixture, but in the end the ECU has final say based somewhat on what we tell it or how we fool it. Immediately and over time it will retard timing as it continues to see knock. It will change the fuel and timing maps to whatever it sees as best for the engine based not only on the MAS input that we have control of, but also on O2 sensor input and knock sensor input, which we don't have control of with a piggy back system.