Hello all --
Hindle_Az has hit the basic truth: It isn't HP that matters on using an intercooler, it is boost. And more boost means higher intake temps, which means lower CFM, which means less HP benefit from the boost -- and much more likely detonation.
Here's an example using the equations of Bell and McInnes for a turbo. The physics of a supercharger is the same, so the trends below apply:
Assuming a compressor efficiency of 73%, ambient temp of 75 degrees, an intercooler at 75% efficiency and a 1 psi loss, at sea level, here is the comparison of outputs from a theoretical turbo engine --
Non-Intercooled Intercooled ----------------------------
Boost Intake T CFM In T pct CFM pct
------ ---------- ------- ------ ------ ------- ------
0 75.0 - 272.1 - 75.0 - 100% - 253.6 - 93%
1 88.8 - 283.3 - 78.4 - 88% - 270.4 - 95%
2 101.9 - 294.3 - 81.7 - 80% - 287.0 - 98%
3 114.5 - 305.1 - 84.9 - 74% - 303.5 - 99%
4 126.7 - 315.7 - 87.9 69% - 319.9 - 101%
5 138.3 - 326.1 - 90.8 - 66% - 336.2 - 103%
6 149.5 - 336.3 - 93.6 - 63% - 352.4 - 105%
7 160.4 - 346.4 - 96.3 - 60% - 368.5 - 106%
8 170.9 - 356.3 - 99.0 - 58% - 384.4 - 108%
9 181.1 - 366.1 - 101.5 - 56% - 400.3 - 109%
10 190.9 - 375.8 - 104.0 - 54% - 416.1 - 111%
11 200.5 - 385.3 - 106.4 - 53% - 431.9 - 112%
12 209.8 - 394.7 - 108.7 - 52% - 447.5 - 113%
13 218.9 - 404.0 - 111.0 - 51% - 463.1 - 115%
14 227.8 - 413.2 - 113.2 - 50% - 478.6 - 116%
15 236.4 - 422.3 - 115.3 - 49% - 494.0 - 117%
In case it doesn't line up (sorry, when I type this it looks fine, but when I preview it it jams up), column 1 is boost (range 0-15 pounds); column 2 is NON-IC intake temperature; column 3 is NON-IC air flow (DFM); column 4 is Intercooled intake temperature; column 5 is IC temp as a % of NON-IC temp; column 6 is Intercooled CFM; column 7 is Intercooled CFM as % of NON-IC CFM.
In the end, CFM is what matters -- we've all heard that engines are air pumps, and the more air you flow, the more HP you make. So, if you just look at the last column, you see that somewhere between 3 and 4 pounds of boost, you hit breakeven for CFM (IC CFM pct ~ 100%). Below that boost level, the pressure loss due to extra IC piping eats up more CFM than the IC can produce by increasing air density. By six pounds of boost, the IC set-up is colling things enouch to increase air density sufficiently, that it is pushing 5% more air than is the non-IC set-up, meaning something like an extra 5% in HP, too. Whether that bit of extra HP is worth the IC troubles, is a question of cost:benefit.
But look also at the Intake Temp comparisons. That gives you an idea how the IC helps with air density (cooler air is more dense), but it also gives you an idea on why people talk about IC's helping with detonation. At six pounds of boost the temp differences are already big -- the IC is running at 63% of the non-IC temp, or about 55 degrees cooler. At the theoretical 15 pounds of boost, the IC temp is HALF that of the non-IC temp, a diff of around 115 degrees. This is why Spearco (intercooler manufacturer) says an IC allows you to run the same octane level at 3-4 pounds higher boost without detonation.
Now not all IC's will operate exactly this way... A more efficient turbo, a less efficient intercooler, a hotter ambient temp, will give less of a difference. A better IC will give a bigger difference. Other variables include different elevations, changes in humidity, backpressure, etc., etc., etc.. But the general rule of thumb that at somewhere around six pounds of boost the IC will give noticeably better HP, and will have _less_ of a tendency to detonate.
Last thing: Different supercharger designs have very, very different efficiencies. I don't keep up with that side of forced induction, but 50% and 60% efficiencies are not uncommon with some of the older designs. That means IC's will be even _more_ useful for a supercharger. Maybe even at 4 pounds boost? I don't know.
Hope that helps.
--- Barrett
like for turbos) such as a Paxton, to make her go quicker and faster. 
