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RE: Coatings and velocity
A couple of thoughts come to mind over the posts regarding heat "generation".
By this, let's go further into what's happening in the turbine section of
your turbocharger. Remember, you increase exhaust velocity=increase heat.
Turbo scrolls on the hot side can be thought of as nozzles and manifold
collectors. If you conceptualize unwinding the scroll this becomes an easier
A k24 for example has a very small nozzle, and a small exducer bore. What
this does is creates really high nozzle exhaust velocity (think of a nozzle
blowing the fan). The advantage of this is very little exhaust velocity
makes more boost sooner. The disadvantage of this, is that the backpressure
into the manifold, and the heat the nozzle creates, increases hot side temps.
Take an k27 turbo as our second example. It has a larger nozzle, and a
larger exducer bore. This creates less backpressure, needs more manifold
exhaust velocity to generate boost, but also creates less heat generating
that boost (since the nozzle size is larger = exhaust velocity is lower =
heat is lower). It needs more initial exhaust velocity to generate a given
PR, and will run at a cooler operating temp/PR.
Manifolds get hot, but a measure of the exhaust system shows the turbo is the
hottest. Why? Cuz the highest velocity=highest heat is IN the turbocharger
(nozzle), not in the manifold, regardless of A/R ratio. Think of it this
way, the collector on a turbo I5 is ~52mm (that's 5 runners into 1). When
that 52mm gets smaller IN the turbo, more velocity=heat is in the nozzle than
anywhere else in the turbo exhaust system.
Regarding the nerding on CC's. I advocate that you do smart things to make
your car go faster. CC the inside of the manifold has little measurable
performance results. A couple reasons why. Cast iron manifolds are really
thick, hence they trap heat by definition, and can even heat soak. If you
had a thin walled SS manifold custom made the CC sounds like a good idea. So
would really long manifold runners. However, when the longest runner to the
turbo is 12inches, you could spend your money more wisely elsewhere in the
system. Cheaper, measureable, and less work hours could be spent on the
bypass valve install in the 10vt cars. For 20vt cars, the RS2 manifold beats
CC the stock k24 manifold, Ned's RS2 copy Extrude Honed prolly beats both.
Again reread the rules of thumb: Increasing turbine speed alone increases
PR, not flow. If I were attaching a k27 to a 10vt motor, I 'might' consider
optimizing exhaust velocity thru a manifold with CC. For the stock k26, you
are so close to the surge line anyhow, the benefits are minimal, and not
CC works in theory. In audi I5 turbo applications, the theory also dictates
the gains to be minimal. I don't question the theory, but sure would like
someone to post up some data showing any measureable advantages. I couldn't,
the 0-60 maybe the wrong measure, so noted. However, if the WGFV is
operating more sooner (or less later, what I'd expect - remember the 10vt
WGFV adds boost pressure to the top of the WG, if you already have proper
boost/rpm the WGFV won't add), that means I should see more boost sooner in
the rpm range, which should affect 0-60 times. Since the autobox has a
torque converter, the results should be more apparent than in a 5spd car.
RECOMMENDED READING: Lots of books on turbocharging principles. Corky
Bell's book "Maximum Boost" although not my favorite, is certainly a more
welcome baseline on this list to some pretty complicated theory.