In the last step, you read about the intake manifold. In this
step you're going to learn about Fuel Injectors and how to pick the appropriate
size.
Your motor is going to flow a certain amount of air depending on the
intake, cylinder size, boost pressure, RPM and other factors. The job of the
injectors is to spray the correct amount of fuel into the motor for that air. To
put is simply, the Injectors are just high speed electrically operated valves
that open and close to regulate how much fuel is supplied to the motor. The
ratio of open to close is known as the duty cycle. The longer the valve is open
and the higher the fuel pressure, the more fuel is sprayed out the
injector. In order to allow the injector to spray efficiently and to
provide some leeway in tuning you should size the injectors so that at the
maximum engine demand the injectors are within 45%-75% of their capacity. If the
injector is too large, then at idle it will open and close so quickly that Idle
will be poor and the engine will be hard to tune.
The flow rate of an Injector is usually rated in "Pounds of fuel it can
flow in one hour at a fuel pressure of 45 PSI. The "Pounds of
Fuel" rating system came about because to obtain the correct air/fuel
ratio, one could calculate the volumetric efficiency of an engine, and then know
what the total amount of air that would be going in, (in pounds), then all that
needed to be done is add in the one part fuel (in pounds) to obtain the 14.7
air/fuel ratio.
The general rule of thumb is that it takes about .5 pounds of fuel to make 1
HP for a naturally aspirated motor and .6 pounds for a Turbo motor.
Example: Say we have a VW engine expected
to make about 160 HP. That's 40 HP per cylinder so you would need at least a 20
pound/hour injector in each cylinder. But that's at 100% injector capacity.
Since it's best to stay within 45%-75% of the injectors capacity you need
to use a larger injector. In this example a 33 pound /hour injector from the
table below will work.
If you're turbo charged, you're better off
staying on the low side of the duty cycle range (45%). Here's why: Under
Turbo boost the fuel has to overcome the extra pressure in the intake manifold.
Injectors are rated by using 45 pounds of fuel pressure (PSI), so this means
that to maintain the same rated flow while under boost, the fuel pressure would
have to be 45 PSI higher than the boost pressure. A fuel
pressure that high would overburden the pump so what you want to do is use a
larger injector that can flow more fuel at a lower fuel pressure. An injector
operating at 45% of it's capacity should be a good starting point. In this case
a 45 pound/hour injector will work and you could lower your fuel pressure to 25
PSI to get enough flow.
| Tim's Injector Firing Tech:
On fuel injection, depending on the type of computer injector firing
configuration, the fuel pump must flow enough fuel for the total amount of
injectors that will be firing, at the fuel rail pressure. Stay with me here. For
example, some computers like on the new cars, use what is called
"Sequential" injection. That means only 1 injector will fire at a
time, so the pump only has to flow the total amount of one injector, at the fuel
rail pressure. Some systems use "Sequential Double Fire." That is to
say that two injectors will fire at the same time. The pump has to flow the
equivalent of the total flow of two injectors at pressure. Still others will do
what is called a "Batch" fire. That is all injectors on at the same
time.
To do a sequential system the computer needs a "Cam Position
Sensor," and also a "Crank Angle Position Sensor." This gets
expensive for a VW. Most EFI systems out there for a VW will use the
"Sequential Double Fire" technique. It is possible, under boost, that
this system could end up with all the injectors on 100% of the time. This would
require the fuel pump to match the flow of 4 injectors at once at max fuel rail
pressure. If the pump does not keep up, the engine will lean out. |
The chart below lists tested fuel flows for some injectors. It is not an all
inclusive list and the actual injectors you have will be slightly different. It
is best to spend the time to test your injectors to be sure they flow the
required rate on your vehicle and that the flow rate between each injector
matches within 4 or 5%. You will learn to test the injector flow on the
EFI controller page. Throttle body injectors on Fords will typically have
the higher flow rates (45 LB/HR).
