Originally Posted by Rocket

What does 100% duty cylce mean?

Originally Posted by Injector Tech

Duty Cycle
Unline pulse width, which is onlyu a measurement of the amount of time the circuit is on, the duty cycle is a measurement in percent of the amount of time that the circuit is on versus how long it is off. A duty cycle that is too high will often cause damage to components. This is because a circuit that is passing current will need to dissipate heat, and if the circuit has a duty cycle that doesnt allow sufficent time at rest, the head will not be able to leave the circuit and eventually cause damage.
In tuning a custom EFI system, it is important to keep an eye on the duty cycle of the injectors so they do not overheat and fail.
It is possibly to calculate the maximum allowable pulse-width that an injector can be given in order to stay within an acceptable range of duty cycle for any given engine speed. It works like this:

First, divide engine speed by 60 to get revolutions per second.
6,000 RPM / 60 (seconds/min) = 100 revs/sec

Then, divide 1 second by the revolutions per second = seconds for one rev.
1sec / 100 revs second = .01 Seconds per revolution

Now multiply the seconds for one revolution by 2 to get the amount of time one complete engine cycle. (In a four-stroke engine it takes two revolutions to complete a cycle)

0.01 Seconds per revolution x 2 = .02 Seconds per Complete 4-stroke cycle.

This is also the amount of time it will take to complete one complete four stroke cycle, which is also the maximum amoutn of time we have beofre the nect cycle beings and we need to worry about filling the cylinder with air and fuel again. The amount of pulse width compared to the amount of time available to complete the engine cycle will give a percentage equal to the duty cycle. If the amount of pulse width required of the injector in order to supply enough fuel to achieve the desired air-fuel ratio produced an unacceptably high duty cycle, then you need injectors that can supply greater amount of fuel for any given pulse width. This is usually referred to as the injectors flow rate, and can be measured in pounds per hour, cc per min or grams a second.
For example, you can see that at 6,000 rpm it takes 20thousands of a second or 20milliseconds, to complete each four-stroke cycle. That means if we have an injector pulse width of 15ms at 6,000 rpm, then our injector duty cycle would be 75 percent.

15 (.015)milliseconds / 20 (.020) milliseconds = .75 or 75% Duty Cycle

As the engine speed goes up, the amount of time it takes to complete the four-stroke cycle decreases , so if the pulse width remains the same, then the duty cycle will increase.

Examples: I am using .0118 pulse width here for demonstrational purposes only, just for this example. I know that honda's Avg. Duty Cycle is near if not 80%, so i used .0118 as a median for the pulse width. So when i datalog my gsr i will edit this article using my actual pulse widths according to engine rpm..
VTEC X-Over

4400/60 = 73.3 Revs/Sec
1 Sec / 73.3 Rev/s = .014 Seconds to complete 1 rev
.014 x (2) = . 027sec to complete one four stroke cycle

Injector Pulse Width of 11.8ms or (.0118) (Stock 240cc's)
.0118 / .027 = 43.7% Duty Cycle. So at VTEC Crossover the injectors are firing @ 43.7% Duty Cycle.

Secondary Butterflies:

5700/60 = 95 Revs/Sec
1 Sec / 95 Revs/Sec = .010 Seconds To Complete 1 Revolution
.010 x (2) = .02 Sec To complete one four stroke cycle
11.8ms Pulse Width
.0118 / .020 = 59% Duty Cycle

Redline:
8200/60 = 136.6 Rev/s
1 Sec/136.6 = .007
.007(2) = .014
.0118/.014 = 82.4% Duty Cycle

Most will say that 85% duty cycle is the max. acceptable limit of normal injectors. If you find in order to obtain Your desired a/f , you have to order pulse widths that produce higher than 85%, then you may want to consider using larger injectors.