when choosing an injector you want to around 80% of the duty cycle, it's easy to figure out your fuel needs as follows..
Lets say for example you are making 400 flywheel HP, most boosted engines run around 0.55 or 0.65 BSFC ( Brake specific fuel consumption) N/A motors run at 0.50.
Take the 400hp x 0.55 lb./hr-hr= 220 lb/hr max flow or at 100% duty cycle.
next divide the 220 by the # of injectors 8 which is 27.5 lb/ hr per cylinder.
now to find an injector that will run at 90% duty cycle, divide the original # by 0.90. 27.88 lb/hr /0.90 =30.61 or 31 lb/hr needed for this motor.
so you would need a 34lb or max of 39lb injector.
some quick #'s
A boosted motor running 400-450hp could run 34lb to a 39lb injector.
same motor making 460-550 could run 42lb to a 47lb injector.
fuel pressure/exhaust temps can also figure into this but basically the tuner will tune the fp so if the injector is to big it will only flow a specific rate, the problem lies with an injector that is too small, although a 60lb injector on a 300hp motor might also be a problem. I probally over explained myself but I'm sure someone could use the formula.
no real drawback to going with larger injectors as long as the setup is tuned properly. when you start getting HUGE injectors. i can see problems with the idle and low load tuning could be a little tricker since the PWs will be so small.
but I am using 42s and seem to be able to tune my car alright. and my car is N/A now. doesnt really matter anyways because the fuel demands at idle and light cruise are going to be pretty close to each other, whether you have a 331 N/A or a 331 with a turbo, the tuning for idle and driving around will be same same. but when you go under the boost is where you need the extra fuel (when you need larger injectors)
If you plan on upgrading in the future, buy large injectors. Unless you go so huge that you begin to wander into the 25% duty cycle area you won't even notice anything. I personally try to plan on a 50% duty cycle so I have room for growth. Just don't short-change yourself. Small is bad. Big is no big deal.
Try not to get so caught up with the 80% duty cycle madness. This number (or some use 85%) was intended as a MAXIMUM to allow for a bit of breathing room if you underestimate your power output. This also gives the injectors some time to cool.
Yes, injectors can be too big. Obviously an injector can only open and close so fast (1.7 ms is a good minimum injector pulse), called pulse width. If you use very large injectors, your idle pulse widths get very short. This can drastically reduce the mixture ratio control that you have during idle and cruise situations, and lead to very poor driveability and seemingly strange tuning behavior.
Keep in mind that every car is different, but the standard duty cycle seems to be 25 ms (100%). So the bare minimum for idle conditions should be about 7% duty cycle (1.7ms). Typical injector pulse width for an idling engine at normal operating temperature is between 2.5 and 3.5 ms (10-14%).
So if you plan on upgrading in the near future, you would do well to pull out a piece of paper and a calculator to figure out exactly how big you can go before exceeding your nominal idle characteristics.
One item of note, don't forget that you can play with fuel pressures to get an even wider range for your injectors.