Jim Writes:
<< They were from Brazil and they told me that they flew their IV-P's at 33,000 feet with no problems.>>>

FL330? Really? Maybe after having your wings waxed.

My IV-P has a TIO540 Lyc with 8.7:1 pistons and max MAP of 35" (360 Hp on the Barrett dyno @ 2700 RPM).  Measured critical altitude is FL280. Back during the flight test period, pre RVSM, I took the old girl to FL300 but she was not happy. Flight controls were very sensitive, engine was hot, mags were starting to misfire, cabin altitude was in the O2 needed range, and this was one occupant, half fuel and no baggage with a 2,188 Lb empty weight. Note to self FL280 is operational max. With the RVSM requirements, anything above FL270 does not matter as you cant get there from here.

If you have a low critical altitude then here are some things you may wan to check:

Air cleaner. Is it rated for the volumetric flow you are asking? As you climb the air cleaner flow increases and the delta P across it rises causing the compressor inlet pressure to drop below ambient. Full power air demand is ~500 CFM at sea level. You need ~2.4 times that at FL280. I use a pair of K&Ns that are rated to 800 CFM each.

Right turbos. Early TSIO550  had turbos that tended to stall. The fix was a restrictor on the turbine outlet. The real fix was the right size turbos.

Leaky Wastegate (or controller out of adjustment).

Exhaust leak.

Pop-off valve leak.

Poor cooling flow through intercooler.

There are others but that is where I would start.

FYI, typical performance numbers for N170BR are 275KTAS @ FL240 & 75% power (2,500 RPM, 28" MAP, 21.5 GPH, 1,500 TIT, 385 CHT, 195 Oil Temp). Yes, I run ROP with a BSFC of .49 BUT I just logged 1,000 hours on the Lycoming with all original parts. Cylinders, rings, valves and compression is still 72 -75 PSI. Pay for fuel or pay for cylinders.

Regards
Brent Regan