[N0DIH]

6 psi, no you don't. Even at 10 psi you really don't. Higher than that, yes, intercooling is recommended. Typically I think the rule of thumb is 12 and higher.

The most important thing is knock control above all and sufficient fuel supply (as you are now making HP for a larger engine, and the boost pressure will be the injector tip reference, hence the need for larger injectors to keep the pulse width in line, if you keep it down, you might be ok, but I could keep a close eye on injector pulse width, don't run out of headroom!). And to give additional fuel anytime you are positive pressure.

I would be concerned with catalytic converters surviving with much of a "lean" mixture (below .87v O2 under boost). They will get hot, enough they could be damaged. That is why GM defaulted so rich on so many cars at WOT. Protection. The thoughts of using a TBI for additional fuel is a good one, maybe the way to go is a modified factory computer or a MegaSquirt with knock control. So it will provide additional fuel and timing retard with good control. I can tell you from experience, turbos will ping badly with lean mixtures. It is death to pistons. It would be an interesting thought to go completely MegaSquirt. Does it support knock sensors? Buick has had a stand alone ESC module that fed the ECM with knock data. Maybe we can decipher what the HEI needs on the EST line to control timing, maybe go to a fixed timing under boost and override the ECM control? Need to find a knock sensor that is "tuned" to the 4.9L. The resonant frequency of the block when pinging is what you need to find and then find a sensor that is on that frequency. Being it is an Alum engine, that may be an interesting find. Heck, maybe evena N* sensor might be close. I have some freqs of some older GM engines (2.8L, 5.0L and 1.8L Turbo somewhere).

For additional fuel too, you might consider an 9th injector like the GN guys did (they had the 7th....). Create a ckt that will drive it and have a control you can tweak, and maybe even make it variable with boost pressure. That shouldn't be too hard to create. Likely woudn't need a big injector, and having it just after the turbo would enhance the adiabatic cooling effect, further reducing the needs for an intercooler.

Other things turbos aren't very fond of, tuned intakes (the 4.9L doesn't really have, right?). They will work, but it will make them more peaky. Give them some additional plenum area, this will be your "aftercooler". Check out semi diesel designs, easiest to see on the 6 cyl diesels. Like the Cummins turbo 505 CID inline 6 (I used to drive one from time to time, but my truck was a 855 CID non turbo, 1000+ lb/ft torque @ 1000 rpm). They use a large manifold area AFTER the turbo to create an aftercooling effect. The air gets compressed and is heated (adiabatic cooling/heating), when it is uncompressed, it cools, so the area after the turbo but before the valve is the area you can do some aftercooling in. Immeditately after the turbo, see what you can do to increase the plenum area. It is easier than an intercooler. And less lag. Too large plenum will create some lag too (just like a non turbo engine with too large intake, like a Victor 454 on a 396 with oval port heads...), so don't go overboard.

But for sure, no intercooler is NEEDED for 6 psi. 12 yes, 6, even 10, I wouldn't waste the deisgn time unless you have unlimted funds and time. As for a specifically designed engine for high output turbo, then I would design it in, more boost and more power Ar ar ar ar..... Not saying the intercooler won't help, it will, but not as much as it would if it was up at 15 psi. The adiabatic heating isn't that much. I'll see if I can find out how much it heats, it is calculatable.