[KyleTownsend]

>> Intercoolers are a VERY good thing. As you correctly pointed out, they increase the detonation margin. All other stated reasons for having an intercooler are questionable at best.

Makes sense to me. The only caveat I would add is that intercoolers are a good thing on engines that need them. If the detonation margin is already sufficient for a given engine, then intercoolers are superflous.

>>You made some faulty assumptions, though. The addition of the intercooler does not increase the HP output.

I guess you are referring to the "Con" argument I citied. This was just a comment that an RTC instructor made that provided food for thought. I believe the core of his argument hinged on the fact that pilots may be tempted to use the POH power settings that apply before an intercooler is installed, to the same engine after an intercooler is installed. His theory was that if it is an "ISA+20" day, but the intercooler is providing 40 C of cooling, the pilot should apply the "ISA-20" power settings rather than the "ISA+20" power settings, and that he might not know this (although I believe that some (or all ?) of the intercooler manufacturers provide supplements that adjust the power settings appropriately).

>>That is logical, but incorrect since the increased exhaust backpressure lowers the volumetric efficiency enough to make the HP output just about the same with and without an intercooler. Most people neglect to include that effect in their calcualtions.

Yup. I hadn't thought of that.

>>The most detonation prone condition is with high IAT (without an intercooler) and with high CHTs and high internal cylinder pressures. It's not at cruise at 17,000 feet as you postulated.... it's on takeoff on a hot day with high CHTs. It is almost impossible to detonate these engines unless the pilot does something inappropriate with the mixture.

Now you are getting into an area where my ignorance truly shows. I have read more about detonation since my original post, and it is an amazingly complex and intricate subject. There are, in fact, a number of engines that operate at internal temperatures well in excess of the spontaneous combustion point of 100LL. Apparently, this has to do with the time-delta of the pressure rise vs temp rise. I was just thinking about where the maximum temperature rise across the turbocharger would occur. This will be much higher at altitude than at sea level (by a couple of hundred degrees, at least). At this point, I suppose I would ammend my thesis to specify the "critical point" as the altitude at which the turbocharger starts bootstrapping at 100% power (rather than 75%). I believe that this would yield much higher CHT's than 100% power at sea level, because the heating due to the turbocharger would be much more than the somewhat lower ambient temperatures existing at altitude.