Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Sun, 06 Oct 2002 21:34:04 -0400
Message-ID: <redirect-1799089@logan.com>
Reply-To: <13brv3@mchsi.com>
From: <13brv3@mchsi.com>
Subject: RE: [FlyRotary] Re: turbo sizing common sense.....
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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.
-------------------------

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.


-------------------------
Whether seven pounds of boost can normalize pressure up to some critical
altitude is a matter of defining "normalize."
-------------------------

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.


--------------------------
Additionally, if you've read any of the
chatter on the RX7 auto listserves, the internal wastegate design gets
easily overloaded at higher boosts,
--------------------------

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 :-)

---------------------
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.
---------------------

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)