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What
is the purpose and/or advantage of an intercooler?
The purpose of the intercooler is to remove the
heat in the air charge that the turbo/supercharger
puts into the charge when compressing it. There
are two advantages: Reducing the heat in the air
charge increases the charge density (more molecules
of air per cubic foot), thus increasing the potential
for making more power. Reducing the heat decreases
the tendency of the combustion process to knock
(detonation).
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How does the intercooler
affect the power output of the engine?
Power is dependent on the
density of the air charge. By decreasing the temperature
the intercooler increases air charge density, therefore,
the power is increased. Typically, the magnitude
of the increase will be between 10 and 20% for the
average (street) boost pressures.
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Is some intercooling
better than no intercooling?
No. It depends on the design
of the intercooler, and there are two factors involved;
efficiency (how much heat is removed) and the flow
restriction (lost pressure) created by the presence
of the intercooler. Regardless of the efficiency,
if too much pressure is lost, then the intercooler
is either useless or can actually decrease performance.
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Are there varying
styles of intercoolers?
Yes, two basic styles: Air-to-Water,
whereby the Charge-Air is cooled by Water, and Air-to-Air,
whereby the Charge-Air is cooled by (ambient) Air.
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How can an air-to-air
intercooler be more efficient than a water based
intercooler?
There is an overwhelming quantity
of ambient air available to cool an air-to-air core
relative to the charge air thru the inside of the
intercooler (The iced down water intercooler is
the only exception to this argument.). At just 60
mph, with a 300 bhp engine at full tilt, the ambient
air available to cool the intercooler is about ten
times the amount of charge air needed to make the
300 hp. Whereas the water intercooler largely stores
the heat in the water until off throttle allows
a reverse exchange. Some heat is expelled from a
front water cooler, but the temperature difference
between the water and ambient air is not large enough
to drive out much heat. Another way to view the
situation is that ultimately the heat removed from
the air charge must go into the atmosphere regardless
of whether it's from an air intercooler or a water
based intercooler. The problem with the water intercooler
is that the heat has more barriers to cross to reach
the atmosphere than the air intercooler. Like it
or not, each barrier represents a resistance to
the transfer of heat. The net result; more barriers,
less heat transfer.
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What are the relative
merits of an air or water-cooled intercooler and
which would suit my purposes best?
This depends on the circumstances.
These circumstances are; street use, drag racing,
or endurance racing (more than two minutes).
Street use: The air-to-air intercooler will prove
superior in efficiency when sized properly.
Drag racing: The short spurt of power allows the
iced water to cool the charge air to below ambient
temperature.
Endurance racing: The air-to-air intercooler is
clearly superior due to the shorter route of getting
the heat out of the air charge and into the atmosphere.
Endurance racing would preclude the use of ice water,
thus negating the singular advantage of the water
intercooler. Further, the air-to-air intercooler
is (virtually, see comments below) maintenance free.
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What factors are considered
in configuring an intercooler?
Use of the vehicle: note the
answers above.
Space: When insufficient space exists for an air
intercooler of adequate internal flow area, then
a water based unit is the better choice. A water
intercooler usually reverses the flow paths through
the core and puts the charge thru the big face,
thus its flow area can be quite large even in a
small overall package.
Type of core: There is very little distinction between
core types with respect to efficiency. Charge air
flow: This is the factor that determines the intercooler
size.
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Does one style core
cool better than another?
No, almost no difference.
With three decades of testing intercooler’s,
we have found no appreciable difference between
any core style or manufacturer. Keep in mind, that
the merit of a core is it’s efficiency versus
its internal drag characteristics. When sized for
a tolerable flow loss, virtually all cores will
produce essentially the same efficiency results.
Perhaps a core with slightly less flow area per
linear inch, or one with longer tubes, will need
perhaps 5% more tubes to equal the best of intercooler’s
with regard to flow loss and efficiency. Not a very
important difference.
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What are the variations in core construction used
in intercoolers?
The variations are in the
style of manufacture of the core material. A “Bar
and Plate”-Style Core is made by brazing up
a layered structure of flat plates separated by
small rectangular spacers. A “Tube and Flange”-Style
Core is created by extending a series of tubes through
a header flange and brazing the assembly together.
For heavy duty applications, pressure spikes up
to 250 psig and rugged environments, ”Solid
Extruded Tube”-Style Cores are available.
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What are the differences
between short tubes and long tubes?
The longer the tube the greater
the pressure loss accompanied by a slight increase
in efficiency.
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Does the length of
the tube affect efficiency?
Very little. The most heat
comes out of the tube where the temperature difference
between the inside and the outside is the greatest.
That exists in the first couple inches of the tube.
The last inch of the tube, wherein the charge temperature
is rapidly approaching the cooling media temperature,
will transfer very little heat, thus being of minor
use.
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What factors affect
efficiency of an Air-to-Air Intercooler?
Frontal area: This is a rapidly
decreasing function. If the proper core size is
used, then doubling it will definitely not double
the efficiency. More likely, doubling the core would
raise the efficiency about 5% and cost twice the
necessary amount and add substantially to the weight.
Plate area: Plate Area (the sum of the Core-Plate
Area which is exposed to the Atmosphere) is directly
proportional to the frontal area and the thickness.
Thickness, however, is a double-edged sword. With
the greater thickness, the plate area increases
but less ambient air can penetrate the thicker core
to offer cooling.
Ambient air quantity: It is very important to insure
that air coming in the snout of the car will actually
go through the intercooler.
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Are there any improvements
that can be made to the system for improved efficiency?
Yes, several small factors
influence the efficiency. A proper duct is probably
the single most beneficial thing that can be done
to an existing intercooler. Positioning in the main
stream of ambient air is crucial. By comparison,
a taped up intercooler with no ambient air flow
will offer only about 20% efficiency.
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What ranges of efficiency
can be expected from an intercooler?
A typical air-to-air intercooler
for a street application achieves between 60% and
70% efficiency, an excellent/optimum design for
road racing can approach close to 90% efficiency,
but requires an adequate "budget!”
Typically, a liquid-to-air intercooler achieves
higher efficiencies than an air-to-air intercooler,
starting at 75% efficiency and reaching peaks of
95% efficiency. Another advantage is the optional
use of ice as a coolant, which is the only way to
reduce the charge-air temperature below the ambient
air temperature.
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How does one measure
the efficiency of the intercooler?
The efficiency is defined
as the ratio of the temperature removed from the
air charge by the intercooler relative to how much
temperature is put into the charge by the turbo/supercharger.
For example: If the turbo/supercharger puts 150
degrees F into the charge when compressing the air,
and the intercooler removes 110 of those degrees,
then the efficiency is:
Eff = 110 / 150 = .733, or 73.3%
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What factors affect
the pressure or flow loss?
The internal flow area is
the major controlling factor. Tube length is the
second biggest consideration, as a tube twice as
long as another will have nearly twice the drag
at the same air velocity. Tube entry and turbulator
density play small roles and can be considered insignificant.
When configuring the orientation of the core in
a given space, always position the core to offer
the shortest length tube and the most number of
tubes. Clearly, this optimizes the internal flow
area.
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What ranges of pressure
loss can be expected? And what is acceptable?
For good solid performance,
the pressure loss across the intercooler ought to
be kept to less than 1.0 to 1.5 psi. If any pressure
in excess of 4 psi is measured, then the intercooler
is not suited for the job and certainly harming
the performance.
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Does one core style
offer less restriction than another?
There are a few fine points
that mean little. Essentially, they are all the
same if the flow area is the major design consideration.
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What is flow loss
thru an intercooler?
Flow loss is what is measured
in the pressure loss and is the restriction presented
to the smooth, easy air flow through the system.
Essentially, the drag. It is measured by a pressure
difference between the air charge entering the intercooler
to that exiting the intercooler. This flow loss
is due to the aerodynamic drag offered by the shape,
the net area of the tubes, the length of the tubes,
and the density and style of the turbulators.
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How/why is the flow
loss significant?
The net result is the production
of power. It is hugely important because the power
required to drive the air thru the system must come
from somewhere. Depending on whether the system
is turbocharged or supercharged, will determine
how much power is lost from the restriction.
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Are there other factors
of flow loss, in the intercooler assembly, rather
than just the core?
Yes, entry into the intercooler
inlet tank and the smoothness of the exit tank.
The adjoining tube assemblies, their length, size
and bend configurations are all part of the flow
loss total.
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If the boost is raised
is it necessary to increase an otherwise proper
intercooler?
Very seldom. While the loss
through the intercooler is proportional to the flow
(CFM) squared, unlikely the change will be of a
magnitude that requires a bigger intercooler. If
dramatic changes in flow are created, say 50%, then
the flow loss would increase by 1.5 squared, or
2.25, and that would prove excessive thus strongly
suggesting a larger intercooler.
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Can an intercooler fail? If so, what are the failure
modes?
Water-to-Air intercooler Systems
have many obvious failure modes; Pumps, leaks, hoses,
reservoirs, corrosion and even a lack of maintenance,
all can contribute to a failure. The Air-to-Air
intercooler Systems are hard pressed to fail if
properly built for the job; running into solid objects,
like other vehicles, is probably the only significant
cause of failure.
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Is there a boost pressure
limit for intercoolers?
Intercooler’s can fail
from pressure if not specifically designed to operate
at that objective pressure. The failure mode is
definitely not like a bomb going off, rather a failure
is manifest in cracked seams, and is induced by
repeatedly flexing the material of the end tanks.
A panel of the cap, if large, subjected to high
pressures, and in-adequately stiff, will flex in-and-out,
or “oil can,” until the edges of the
panel fatigue and the seams begin to crack. (Pressures
up to 15 psig are usually safe for all designs)
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What is the importance
of a leak in an intercooler?
With the water intercooler,
a leak in the main cooler core could induce a significant
problem into the engine. External leaks are just
annoying, but not likely to be harmful. The air
unit must have a very large leak before it can cause
any problem whatsoever. If pressurized and placed
under water, an air intercooler will need to “billow”
bubbles before it causes any noticeable problem.
A dozen small trickles of bubbles would be entirely
meaningless to performance.
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How can an intercooler
be protected from corrosion?
The water unit will need water
passage protection offered by standard anti-freeze.
The air intercooler only needs the outside protected
from corrosion. Powder coating is perhaps the most
durable/cost-effective solution to corrosion. The
charge paths of either style intercooler will be
protected by the oily vapors from the engine breather
that are pumped through the system.
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Can an intercooler
be painted?
Certainly. One might find
a small loss of efficiency if the core is painted,
but likely this would be less of a difference than
the repeatability of measurement. By all means,
paint the end tanks, but preference suggests the
core remain exposed.
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Can an intercooler
be repaired?
Repair depends on where the
problem is and how badly it is damaged. Its much
the same as “can a fender be repaired?”
In most cases, an able fabricator, a band saw and
a heliarc welder can do wonders.
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Is there a maintenance regimen for an intercooler?
The water based intercooler
will need periodic attention to the water level.
Further, it needs anti-freeze for cold weather use,
corrosion and pump lubrication. An occasional leak
inspection would be advised.The air intercooler
needs de-(smashed)bugging when washing the vehicle.
Tube hoses and clamps need periodic checks. If the
air intercooler develops minor leaks, this is usually
of no consequence.
Perhaps every 10 to 15K miles, the internals of
the intercooler element ought to be washed out with
a solvent to remove the accumulated oil residue
and grime.
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What is a life span of an intercooler?
With minor maintenance, essentially
unlimited. Except for the water intercooler’s
pump, of course.
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Does an intercooler
carry a warranty?
Yes. All Bell Intercoolers
Cores and Assemblies are guaranteed for one year
against workmanship failures.
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