Posted for "Barry Gardner" <barrygardner@mindspring.com>:
I asked some of those same questions about the relationship between thinner
air above and the requirements of a turbo. Here is my own compilation of the
wisdom from Corky Bell and Hugh MacInnes on the subject. The spreadsheet was
intended to assist with turbo selection, so its main focus is on calculating
pressure ratios at higher altitude. User variables are highlighted in green.

Whether seven pounds of boost can normalize pressure up to some critical
altitude is a matter of defining "normalize." If one starts with the
normally aspirated horsepower as between 146 HP (factory specs stock) to 160
HP (with mild porting of a six port engine), then the seven pounds of boost
can compensate for some of the thinner air. But if one has a goal of rotary
takeoff power more like an O-360 or IO-360 (i.e., 182 HP factory specs for
'86-88 or 200 HP for '89-'91), Mazda calculated those numbers with ALL the
boost full on at sea level. So maintaining that power up higher requires
still more boost to compensate for the thinner air.

Like others, I have never found compressor maps for the Hitachi OEM turbos
that came with RX7s of any generation. I've resolved to switch to a TO4 of
some kind because the OEM turbo is likely not a good choice for the higher
pressure ratios required to maintain sea level power up high. Without a
compressor map, you have no idea if you are pushing the compressor side of
the turbo into the surge range. Additionally, if you've read any of the
chatter on the RX7 auto listserves, the internal wastegate design gets
easily overloaded at higher boosts, leading to uncontrollable overboost and
blown apex seals. I've gotten the impression from the auto guys to whom this
has happened that overboost happens quickly and that backing off the
throttle is hard to do fast enough.

While some people have said they'll run without a wastegate and regulate
their manifold pressure via the throttle, I'm a bit skeptical that I want
that additional responsibility included in the load on the pilot. After all,
if I'm taking off on a high density altitude day and the trees at the end of
the runway are getting bigger faster than I'd like, I really don't want to
have to ponder how far I should push my manifold pressure in order to escape
a crash on takeoff. I know that with the adrenalin pumping, I'd ram that
throttle forward--and that might inadvertently lead to both compressor surge
AND the crash.

Others have proposed use of a blow-off valve, which admittedly would work.
However, in that same scenario, my suspicion (without experience, I might
add) is that the blow-off valve vents ALL my boost--not the situation I'd
want when pushing my rotary motor to give me all she's got.

So I've resolved to bite the bullet on the added pounds and exhaust
complexity of a wastegate and get a bigger turbo with a known capacity.

Barry Gardner
Wheaton, IL