**Twin Turbo 67 Dart Project**

I found some interesting reading on Injector Sizing

Sizing the Injectors:

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 or supercharged, 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.