what duty cycle are injectors at their best?
 

Question: What duty cycle does an injector work at it's optimum? 70%?80%
The reason I am asking is if your have a programmable computer does it matter if your injector is only working at 50%? For example: would 550cc primaries running at 70% duty cycle be very much different than 850cc primaries running at 60% duty cycle? Are the spray patterns that different between the two?

I guess I should ask if there will be a noticable difference running 550cc primaries vs 850cc primaries given that I have a programmable computer?

They work great up to 80-85%, at which point they are reported to just stay open. They probably don't work too well at like 1% either, but I don't think that is your concern.

I don't think duty cycle is the relevant issue, but smaller injectors like 550s (still huge compared to most automotive injectors) are said to have a better spray pattern than larger injectors. The spray pattern is important for a good idle and low speed drivability. Of course that does lead us to the possibility that a larger injector might butt up against the low end of the duty cycle scale, but I am not sure if that is the case or not.

So, 550 primaries give a good idle and low speed drivability, regardless of their duty cycle. 850s are probably worse in these regards, but I am not sure how significant the difference is. It might be imperceptable, or it might make the car idle like crap, I'm just not sure.

-Max

my car idles fine with 850 primaries and large porting. rock solid 800rpm idle.

The problem is not the upper duty cycles, but the lower duty cycles. Injectors work fine down to 5% (that is what most people say), but below that they start to become inconsistant and that will give you a rough idle.

4% duty cycle is fine on a few cars I have tuned, but anythign below that you start to run into problems and have to bump the idle up.