This is a good primer, applies to all turbocharge applications.

Marc Wiese

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From: Walter Atkinson walter@advancedpilot.com
Subject: Intercooler Philosophy

Oh, what the hell... somebody's gonna ask. This from George Braly at TATI:

For a year, I spent a good portion of my time re-engineering an intercooler
installation and obtaining FAA approval for the changes.
Very careful instrumentation and measurement. I have more recently done
more
of that with a different engine on the engine test stand (see
www.engineteststand.com <file:///\\www.engineteststand.com>).

I learned a lot. I am a strong supporter of intercoolers, so some of what I
am going to say may sound strange at first.

So, here goes: Frankly most of the conventional wisdom on this subject -
like a lot of the now famous LOP/ROP discussions - is simply not true.

The practical application results in an outcome that is rather different
than everybody anticipates. It is largely a myth that 30" of MP AFTER the
intercooler gets you more HP than 30" of MP BEFORE adding the intercooler.
In fact, a poorly executed intercooler installation can - - and often does
-- result in LESS horsepower at sea level than the same engine without the
intercooler.

I don't know who started the whole theory of de-rating an engine after an
intercooler, but it sure as the devil was not somebody who was measuring
the
actual engine torque - accurately.  Rather, they were just following the
theory as you outlined it and making the calculations, etc. (And, yes, I
have heard a story about one after market intercooler outfit that claims to
have put a torque meter on before issuing the reduced MP instructions based
on that result... but until I see the data, I will have a very hard time
with that notion.)

The big problem is that in the euphoria over the large and very beneficial
drop in Induction Air Temperatures (IAT) that one gets with an intercooler,
the "engineers" forget all about something else: How much of the "good
stuff" (i.e. good, cold, high density air) you can get into the cylinder on
each intake stroke - - also depends on how much of the BAD STUFF (i.e.
exhaust products) you got out of the cylinder on the previous exhaust
stroke.

The ratio of the new "good stuff" to the theoretical maximum "good
stuff" is
called the cylinder volumetric efficiency VE. Normally aspirated engine
values are up around 85 to 92% of the cylinder's displacement - - although
with turbocharged engines, the number will substantially exceed 100%.

When you add an intercooler to a turbocharged engine and leave the MP
constant, you place a restriction in the intake plumbing. That means that
the compressor discharge pressure is now two or three (or 4 or 5 or
6 in one case) inches of Hg higher than the wide open throttle MP!!!

In order to generate that extra pressure, the compressor has to work
harder.
And that means that the turbo has to work harder. And that means that the
wastegate is closed a bit more. And THAT means the exhaust back pressure
increases and that reduces the cylinder volumetric efficiency.

Result? The improved number of molecules you get into the cylinder due to
the denser air is just about perfectly offset by the reduced VE.
Thus, no net increase in useful airflow through the cylinder. And the
result
of that is no net increase in horsepower at the same MP.

Now... having said THAT - - in my view, it is almost criminal for
anybody to
operate a turbocharged engine without an intercooler.

There are excellent old SAE research papers that show a HUGE improvement in
the detonation tolerance of these engines by use of even a modestly
efficient intercooler. These improvements are not trivial. They are
substantial.

[Rule of thumb: every 10 degree F decrease in induction air temperature
decreases engine octane demand by one octane number.]

Installing an intercooler means that you are very, very much less likely to
inadvertently cause detonation during a moment of inattention during a busy
high power climb in an IFR environment - - when you accidentally fail to
have the mixture rich enough. Lots of other benefits.

For the same reasons, it lowers peak cylinder pressures substantially and
that seriously promotes reduced exhaust valve temperatures and greater
valve
and cylinder longevity.

Last, and this is something almost nobody appreciates, adding an
intercooler
results in much better fuel atomization by your fuel injectors during high
power operation.

[I wonder why that is?]

Like I said, the "conventional" wisdom on this subject is seriously flawed.

The benefits of intercoolers are sort of over promoted for the wrong
reasons, and vastly under promoted for the right reasons, all at the same
time.

They are generally a good investment.