Engine - Engine Cooling
Cooling can be an important upgrade if, and only if, the engine is developing more heat. This would usually be
due to leaning out the fuel mixture with a programmable ECU, raising the compression ratio, installing a larger
engine, or adding a turbocharger or supercharger.
A larger radiator holds more water, and weighs more. This means that the car is carrying more weight. An external
oil cooler also adds weight in additional oil and the metal of the cooler itself. With a car as light as a Geo
Storm, every pound counts, and these things should not be done unless the engine needs more cooling capacity and
the power improvement is large enough to more than compensate for the added weight.
DOHC Heat Exchanger for SOHC Engine
The DOHC engines have an oil-to-water heat exchanger installed at the base of the oil filter on the back of the
engine. This can add a safety margin for the SOHC engine, and adds nearly no weight to the vehicle, making it
possibly the only compromise-less upgrade in this list.
External Oil Cooler
The turbocharged Impulse Turbo RS AWD uses an external oil cooler to cool the added heat burden of the turbocharged
engine. In general, oil coolers are used as a band aid when it is not possible to increase radiator size. The
engine is cooled with water, and lubricated with oil. The only reason to cool the oil is when there is not enough
water capacity to cool it correctly.
For naturally aspirated engines, if additional engine cooling is required, it is best to rely on the engine coolant
system and upgrade to a larger radiator.
External oil coolers are best used only with forced induction applications, and best left until after the largest
radiator that will fit, has been installed. If the water coolant system remains inadequate, then install a larger
oil cooler. Setrab and B&M offer thick, racing style oil coolers which are very similar in design to the Impulse
RS Turbo AWD oil cooler, and the B&M Supercooler line provides several good options for larger size coolers.
Three Sizes of Factory Radiators
There are three different sizes of radiators used in the Geo Storm and Isuzu sister cars.
The cooling capacity of a radiator is determined by its core size (width x height x thickness). Turbocharged
engines require proportionally larger radiators, because they make more heat per unit of power produced.
For reliable operation, the 4X engine requires radiator core volume of 1.76 cubic inches per horsepower
for naturally aspirated use, and 2.86 cubic inches per horsepower for turbocharged or forced induction use.
- The DOHC and SOHC Manual Transmission cars use a radiator with a .63 inch thick core (228 cubic
inches). This size radiator will safely support about 152 HP naturally aspirated, with a manual transmission.
- The DOHC and SOHC Automatic Transmission cars use a radiator with a .93 inch thick core (337
cubic inches). These radiators have an internal passage to run transmission fluid through, in order to cool the
transmission. If used on a manual transmission car, with the transmission cooling loop plugged and unused, this
size transmission would support around 220 HP naturally aspirated.
- The Impulse RS AWD Turbo and Gemini JT191S Turbo radiator with a 1.26 inches thick core (457
cubic inches). Turbocharged engines produce more heat per horsepower unit produced, so while the radiator
supports 180 HP with a turbocharged engine, it would support about 300 HP naturally aspirated.
As a side note, the body of the Lotus Elan M100 Turbo (which shares the Isuzu 4X engine) was designed with a
steeply sloped hood and sharply pointed nose. This presented several challenges and limitations to the engine
compartment layout, not the least of which was the elimination of the top mounted intercooler and blow off valve
assembly. Specific to engine cooling, the radiator of the Elan M100 is grossly undersized, with a core volume
of approximately 336 cubic inches, which will support 117.5 horsepower in turbocharged configuration. This is
the primary reason that the Elan M100 is so notorious for engine overheating problems and blown head gasket problems.
To support the stock power level, the Elan should be upgraded to a radiator to a 1.75 inch thick (44 mm thick)
core. For any increase over the stock power (boost) level, the Elan should be equipped with a radiator core of 2 inch
thickness (51mm thickness) or larger.
Aftermarket Replacement Radiators; Two Sizes
The aftermarket replacement parts suppliers make two different sizes of radiators for the 1.6 liter engines.
There does not appear to be a replacement radiator available for the 4XF1 1.8 liter engine, which is one more
good reason to change the 1.8 liter over to use the
1.6 liter two
belt system, oil pump and oil pan, rear mounted alternator,
and water inlet fitting arrangement and radiator hoses.
- A Manual Transmission Size, with an approximately .63 inch thick core, is actually less common,
because it is specific to only a percentage of the cars. (Aftermarket manufacturers prefer reducing inventory by
offering one item for an entire model line). If the OEM manual transmission radiator is providing sufficient
cooling capacity, and weight is a concern, this is the replacement that should be used.
- An Automatic Transmission Size, with a 1 inch thick core, is more common, and
usually offered as a replacement for all 90-91 model cars. Be very careful to verify the "core" thickness,
and not confuse the end tank thickness which is typically 1 1/4 inch. The aftermarket suppliers appear to
have substituted the thicker automatic transmission core for all vehicles, as the additional, unneeded cooling
capacity will not harm the manual transmission non-turbo engine. These transmissions will support 220 HP
with a manual transmission and non-turbo engine, so they make a good upgrade for non-turbo race engines and
provide a lot more redundancy in the cooling system for stock engines.
- There are no 1 1/4 inch thick Turbo spec aftermarket replacement radiators. The
turbo model seems to have been completely ignored by the parts suppliers.
An aluminum radiator would be about 25% more efficient, due to the better heat
conductivity and cooling ability of aluminum. This would be a worthwhile consideration for a thinner radiator
with lower weight for a naturally aspirated engine, or for higher horsepower turbocharged engines.
Isuzuperformance is the only company to have ever offered full OEM size aluminum radiators for Geo Storms.
Half Size / Half Width Radiators – A Bad Idea
Radiator cooling capacity is based on the volume of the core (length x width x thickness). The length and width
also determine how much air will flow through the core and cool the water contained within. Whenever there is a
choice, wider and longer is more desirable, because it will provide the best cooling. A popular “upgrade” has
been using a half-sized or narrow radiator from another make and model of car. These are usually justified to
make more space on the front of the engine to fit a larger turbocharger. This is a bad idea. Reducing radiator
capacity by using a smaller radiator leads to overheating problems, blown head gaskets, and warped heads. It is
an especially bad idea with turbocharged engines, which make much more heat per power unit produced, compared to
naturally aspirated engines. The factory Isuzu Impulse RS Turbo uses a 1.26 inch thick radiator core, that
calculates out to 457 cubic inches of core volume, and this size radiator is too small to provide cooling for much
more than 180 HP. The popular half size/width radiator (from
Robie the Robot), with a 2.25 inch thick core,
calculates out to 407 cubic inches. This is smaller than the stock Impulse RS Turbo radiator, and struggles to
support 160 HP.
Half size/width radiators are simply foolish.
Radiator Cooling Fans
The factory electric fan on the back side of the radiator works very well when in good condition. But, more
modern designs and multi-fan assemblies are available from Flex-A-Lite and Spal. A fan with a built in shroud
will pull air through the radiator more efficiently and provide better cooling. Flex-A-Lite’s low profile product
line offers a two fan assembly which fits the Storm radiator very well.
Hood Scoops and Speed Holes - A Bad Idea
Fiberglass manufacturers have sold the fashion-fiction of hood scoops as engine cooling for many years. The truth
is that forcing air into the engine compartment through a hole in the hood is not going to cool the vehicle at
all. Engine cooling is done by the radiator. Forcing air into the engine compartment through the hood, increases
the air pressure in the engine compartment, and offsets air space inside the engine compartment that should be
occupied by air that has passed through the radiator. Anything that reduces the airflow through the radiator is
bad for cooling. So it is no surprise that those people who have put air scoops on their cars, are also the same
people complaining about overheating problems.
Engine cooling can be enhanced with the use of extractor ducts and vents. Examples of these include louvers
facing toward the rear of the vehicle, rear facing vents in the hood or fenders, the gap left at the trailing
edge of the fenders commonly used with wide body conversions, and raising the rear edge of the engine hood to
allow a gap at the base of the windshield. These openings allow air to more easily exit the engine compartment,
or even pull air out of the engine compartment as air blows over the vehicle when it is in motion. All of that
air exiting the engine compartment has been pulled through the radiator, enhancing engine cooling. Air passing
through the radiator will actually form a Venturi, flowing faster and at a lower pressure, further enhancing
radiator cooling efficiency.
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