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Posted for "Barry Gardner" <barrygardner@mindspring.com>:
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.
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I didn't get a spreadsheet with the message. If it won't make it through
the list, can you email me a copy directly. I'd like to see it.
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Whether seven pounds of boost can normalize pressure up to some critical
altitude is a matter of defining "normalize."
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Just to be picky, there really is only one definition for "normalize", which
is to maintain sea level pressure at altitude.
I wholeheartedly agree that the stock turbo will be inadequate for producing
significant boost at altitude, but don't underestimate what you gain with a
stock turbo. Say you have a 150 hp NA engine at sea level, then you add 7
psi of boost. Now you have (just making these numbers up) 200 hp. That's
quite a payoff for about $400 in low mileage used parts that bolt right on.
Now go to the VFR ceiling (with O2) and you still have about 150 hp. Now go
to a cruise alt of say 10,000 ft, and you'll still have about 2.5 psi of
boost over sea level, so maybe 165 hp. Compare these numbers to an IO-360,
and you'll see that you're coming out ahead. Just food for thought.
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Additionally, if you've read any of the
chatter on the RX7 auto listserves, the internal wastegate design gets
easily overloaded at higher boosts,
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Boost creep does seem to be a problem, specifically with the series 4 turbo
because it has a small wastegate. The S5 is much larger, but it probably
isn't large enough to keep boost at 0 when it's open. The big question is-
how much boost will the engine make with the wastegate wide open. I've
asked the question here, and on the RX-7club forum, but no one seems to have
an answer, so I guess it will be a ground run test. These wastegates can be
ported, to reduce the problem, and I may just do that up front. Of course,
I don't mind making a few psi of boost with the wastegate open, I just don't
want to make 10 :-)
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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.
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A reasonable approach to be sure.
How long till you're going to be running, and what are you building?
Cheers,
Rusty
Turbo 13B powered RV-3... Be Afraid :-)
1993 RX-7 R1... Stock (for now)
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