Engine - Fuel System
Most people do not understand that increasing the fuel delivery will increase engine power if-and-only-if the
air flow into and out of the engine has been increased to the point that the fuel system is no longer able to
supply adequate fuel to maintain a 14.7:1 air fuel ratio. Adding more fuel above that ratio will do nothing but
make the engine sluggish and unresponsive. As long as the fuel system is not damaged or broken, without serious
engine modifications, there will be no benefit to increasing fuel flow capacity.
Diagnosing fuel delivery problems or inadequacies can only be done using a air-fuel gauge, fuel pressure gauge,
and injector duty cycle gauge. Without attaching those gauges to the vehicle and running the vehicle under
load, any discussion of upgrading the fuel system is likely to be a waste of time and money.
If the air-fuel ratio drops below 14.7:1 at high RPM, then the fuel system is not delivering enough fuel.
If the injectors go over 80% duty cycle, then they are not large enough.
If the fuel pressure drops at high RPM, then the fuel pump is not large enough to provide adequate fuel to the
In addition to this, the only way to change the fuel curve, and fine tune the fuel delivery to match the need
across the range of engine speed, is with a programmable ECM. Changing injector flow rates and fuel pressure
settings to correct a lean burn problem at high RPM, while using the factory ECM, is likely to cause rich burning
problems (sluggish and lethargic response) everywhere below those high RPM engine speeds.
Adjustable Fuel Pressure Regulator
An adjustable fuel pressure regulator can be used to fine tune fuel delivery if the inadequacy found in fuel
delivery is very small. Raising pressure slightly can increase the flow slightly.
As has already been stated, this has serious limitations.
The DOHC injectors will support 152 HP at 80% duty cycle.
The injectors are rather common and are available in many flow ratings (sizes) and available from AC Delco,
Accel, MSD, and many other sources.
The challenge with fuel injectors is to use a size that is large enough to run below the 80% capacity safety
margin at maximum engine load, but is still small enough to provide the correct amount of fuel at idle speed.
Grossly oversized injectors will not run anywhere near their capacity, but will also not be capable of spraying
the small amount of fuel required for engine idle. Selecting an appropriate and realistic size injector is
Large Bore Fuel Rail – A Waste Of Money
High volume or large bore fuel rails became popular for other brands of cars and engines, most often available
in a variety of bright colors. These are offered by some pretty reputable companies, all citing the ˝ inch
diameter round bore (.196 square inch cross sectional area), which is substantially larger than the original
used on those engines, and large enough to support 500 horsepower.
The Isuzu 4X series engines use a fuel rail with a square bore, which measures ˝ inch by 7/16 inch. But because
the bore is square, instead of round, the cross sectional area is .219 square inches. That’s 12% larger than
the high volume fuel rails sold as racing parts for other cars, and using those manufacturer’s power calculations,
the stock fuel rail of the Isuzu 4X engine will support 560 HP.
This becomes academic, because no one has ever documented a case of the fuel rail flow being a limiting factor
for the Isuzu 4X series engines. All of this just shows how an upgrade part for one brand of engine, may not be
available for another brand of engine, because the original part is more than adequate and needs no improvement.
The factory fuel pump is shared with numerous cars, including Mitsubishi 3000GT DOHC Non-turbo, so the factory
fuel pump will easily support 218 HP (naturally aspirated).
The fuel pump is a common size and shape, with numerous size upgrades available from Denso and others.
The typical default setting in the aftermarket is the largest size, which grossly oversized and is not
recommended, because it will draw more electricity, run hotter inside the fuel tank, and require more frequent
replacement due to the additional heat.
In the event that a larger fuel pump is actually required, seriously consider a small step up to an appropriate
size, instead of the giant leap to the largest size available.
Fuel Pump Swirl Pot
The fuel pump of the Geo Storm is mounted in the middle left side of the fuel tank, with the fuel pickup
pointing to the right, at the bottom of the tank and roughly in the middle of the tank. The pickup is
intended to be submerged in gasoline all the time. But gasoline is a liquid, and the tank is a large open
container without anything to prevent the liquid from sloshing to the left, right, front, or back, as the
car is driven around turns, accelerates, and decelerates. And when the tank is low, it is possible to
slosh all the gasoline in the tank out from under the fuel pickup, causing air to enter the fuel pump and
fuel line, which will cause the engine to cut out and temporarily stop running until the gasoline returns
to the bottom of the tank and is picked up by the fuel pump and pumped through the fuel line (as in when
the car exits the turn and returns to a straight line of travel). Most people would only experience this
if they allowed the tank to go nearly empty and drove around something like a interstate highway cloverleaf
with extended high speed turns. However, if the car is used for racing, especially autocross, and
especially if the car is equipped with R-Compound or sticky Street Touring class tires, and a suspension
stiffened with lowering springs or coilovers, this will occur whenever the vehicle is taken through
a long sweeping turn with less than 3 gallons of gasoline in the tank. This is an issue for racing,
where the driver wants to keep the vehicle weight as low as possible by filling the fuel tank with only
as much gas as it takes to run the race. (Each gallon of gas is 8 1/2 pounds of unwanted ballast).
The solution is installing a swirl pot in the gas tank. A swirl pot is a cup shaped container around the
fuel pump, with an additional "lift" pump, which keeps the swirl pot filled with gasoline and the primary
fuel pump submerged in fuel. During hard cornering, the fuel might slosh out from under the lift pump,
but this does not matter, because the swirl pot will hold a half-quart of fuel around the primary pump,
and keep it supplied long enough for the vehicle to exit the turn and the fuel to return to the bottom
of the tank where the lift pump will once again begin filling and maintaining the reservoir of fuel
in the swirl pot.
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