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Re: IC's the long and the nerdy

In a message dated 97-06-16 15:55:17 EDT,
Gross writes
<< >If one has to address a trade off in a turbo/intercooler application, ie,

 >do you go for more pressure or more cool, which way is most 
 >productive?  I realize this is not quite an apples-to-apples swap in 
 >terms of efficiency and cost, but which factor has the greatest influence?
>>Eric writes:
>> One interesting side effect of a cooler intake charge is actually LESS 
>> boost.  Since your exhaust temp is lower the exhuast gases have a lower 
>> velocity and energy content to spin the exhaust turbine. A nice side 
>> effect is the piston temps are lower reducing the chance's of detonation. 
>>  You could get that EGT back by cranking in a little more ignition 
>> advance, which happens to have the nice side effect of giving better 
>> throttle response and faster spool up on the turbo.
 >Do I spend my money on a bigger turbo, or do I try to find better ways to 
 >cool the air mass that I've already compressed?
 >>Well if the turbo is making lot's of cold air then the turbo would be a 
 >>good thing.  But if your just going to crank up the boost on a stock 
 >>turbo I'd look at the intercooler........
Scott will add:
Cold air is the key, increasing boost isn't, period.  Gross, the easy answer
is:  Accepted rule of Charge Air and motors: Supercharged (incl.
Superturbochargers) Internal Cumbustion Motors achieve the highest volumetric
efficiency with the highest density at the lowest pressure.   What you don't
factor with boost, is heat soak, which can give you short term boost, but not
long term, ala k24 in the 20vt motor.  Great for 'overboost' and short
blasts, but not for any length of time.  An intercooler is only 1 part of a 5
piece puzzle.  The others are hot and cold sides of the turbine and their
respective housings.  Too small a hot side, heat soak and hiend power drain
are the effects.  Too large a hot side and the exhaust velocity required to
spin the blades increases.  Too large a cold side, the exhaust velocity
required makes for a "late bloomer" (ned's beast comes to mind).  Too small a
cold side, the heat is higher for a given pressure.

Not sure more parameters need to be addressed before Eric's blanket
statements regarding velocity and effects become truisms.  Assuming the stock
k26 turbo, this 'might' have validity.  Timing advance can be dialed in to
'compensate' for lack of velocity (creating INTERNAL heat), tho the hot side
turbine could be modified to 'compensate' cheaper and more effectively
(EXTERNAL heat).  But why?  On a turbo motor, this is not necessarily a good
thing.  Increasing the speed of a given turbocharger increases the Pressure
Ratio NOT the Flow = increases HEAT.  So say that you need to increase
exhaust velocity to make up for the increase in Charge Air density, post IC,
is not really correct.  That equation is IN the design of the computer and
wastegate in Absolute Pressure terms, not in the exhaust velocity.  You are
increasing volumetric efficiency, that's all.  Since a hot side turbine is a
wing, by definition, one could easily modify it's power output by changing
the design of the wing.  Extrapolating, this is exactly how the RS2 gets all
that power and flow from a k26 turbo in fact at cooler pre IC charge air
temps too.  

Cooler intake charges doesn't necessarily mean less boost.  IC design, or
efficiency is more the key.  The stock 2 pass IC's on the 5ktq's aren't very
efficient, comparatively speaking.  So, if one increases the IC efficiency,
one can maintain the same boost level AND have a cooler charge.   On audi
computers boost is an absolute number, controlled by wastegate and computer
at the Intake Manifold.  So, one could put a 4foot by 4 foot IC before the
manifold, and boost pressure would still be 1.5 with a stock computer box
(absolute) and 2.0 (or more) for the 'stage II' boxes.  The WG would stay
closed longer to make up for the pressure loss in the IC.  The fact that this
might be out of the efficiency of the turbo MAP is most important. 

 For instance, lets assume a 2.0bar (stage II) MC application, stock k26
turbo at 0 feet altitude 4psi Pressure Drop across IC.  At 4700rpm, a k26 is
at maximum efficiency (72%).  Between 4700 and 6000 rpm the computer will
compensate with more WG pressure, which will keep the efficiency of the turbo
at 72%.  However, this will have the effect of spinning the turbo faster to
achieve that same absolute boost level. Heat soak will rear it's ugly head
faster with this mod.  Jump to 6000 rpm, and you have heat soak (overspin)
AND a turbo that is losing efficiency making up that 4psi Pressure Drop, in
fact the turbo is now operating at close to the 65% level, which is the
accepted MIN for targeted turbo efficiency.  So, me, I don't buy totally into
the IC equation.  It helps, but in the grand scheme of things, the turbo is
just as much a problem, and with a larger IC, it becomes one quickly.
 Especially with a stock k26 turbo.  Mods just add 'fuel to the fire', so to

Presssure Drop across the IC is key to getting the proper IC setup.  Most
dedicated IC applications will give less than 2psi drop across the IC at a
given PR, more than that and you are asking the wrong turbo to do the wrong
work, since a turbo MAP gives boost pressure AND flow and is MEASURED pre IC,
not post.  The computer and WG operate and measure post IC, so hence a
problem develops in the design.   IC flow on a 2.2 liter motor really is
irrelevent in the grand scheme o' things, we are small displacement, low
flow, especially compared to most Spearco applications (Ross' 1380cfm comes
to mind, a 10vt needs maybe half that at WOT/WOB). Pressure drop is key, as
is improving it's efficiency, especially with a stock turbo.

So, the summary?  IC design is tricky for the k26 stock turbo, too much PD
and the turbo will lose top end power and heat soak from overspin.  Not
enough (0> psi) means you are creating heat instead of eliminating it, so 0
is the target, 2psi PD prolly max for design purposes. Computer accuracy is
just as key to the equation as well.  Most stage II boxes operate way in
excess of a 2.0 PR, far above the design parameters of the turbo MAP and the
Pressure Transducer itself.  A reason Ned et. al. are scrambling to put
3.0bar Pressure Transducers in computer mods.  No control beyond 2.0 (and
actually less than that, since 4.9v is constant output before 2.0PR on the
10vt boxes) really doesn't help the turbo in the equation.  In fact, overspin
is common, especially at higher altitudes, since PR = Ambient pressure +
Boost pressure/Ambient Pressure, altitude has a more detrimental effect on
heat soak and overspin. 

Increase the efficiency of the IC, or increase the efficiency of the turbo,
or best yet, do both.  Increasing the efficiency of the IC doesn't require a
new IC necessarily.  Just a better design, the later 200t have it.  That's
what I would do with a stock k26.  Increasing the efficiency by size, may
dictate a visit to your friendly neighborhood KKK turbo dealer....  Hey, why
not?  :)

^The short and the easy here.  The long a complicated, btdt.
More nerding available, hope this helps.  Be 'cool' folks.

Scott Justusson
'87 5ktq/w
'84 Urq