Injector Cleaning Service, Cruzinperformance Options

[JeffBowlsby]

Rich Atwell is a great guy and a casual friend of mine. Anyone here in the bay area would do well to get to know him, he is very personable, I recall he lives on the peninsula - Sunnyvale or is it San Carlos maybe?. He is mildly interested in 914s too becasue of the similarites with his bus, but I have not persuaded him to bite.

Another variable to consider, is not only the flow rates and service pressures, but also pulse width and there is another variable but I cannot recall it off hand (dwell time?). I understand that aftermarket EFI systems calculate fuel requirements and control AFR by pulse width modulation (PWM), eliminating things like the CSV as a positve benefit. So really there should be several suitable injectors to select from if committed to a swap.

Mac at Emerald Perfomance also knows of other injectors that work, look him up and see what he has found. I recall a conversation with him recently that indicated he knew of inexpensive injectors that would fit our cars.

[lapuwali]

QUOTE (bowlsby @ Feb 16 2006, 06:22 PM)

Rich Atwell is a great guy and a casual friend of mine. Anyone here in the bay area would do well to get to know him, he is very personable, I recall he lives on the peninsula - Sunnyvale or is it San Carlos maybe?. He is mildly interested in 914s too becasue of the similarites with his bus, but I have not persuaded him to bite. Another variable to consider, is not only the flow rates and service pressures, but also pulse width and there is another variable but I cannot recall it off hand (dwell time?). I understand that aftermarket EFI systems calculate fuel requirements and control AFR by pulse width modulation (PWM), eliminating things like the CSV as a positve benefit. So really there should be several suitable injectors to select from if committed to a swap. Mac at Emerald Perfomance also knows of other injectors that work, look him up and see what he has found. I recall a conversation with him recently that indicated he knew of inexpensive injectors that would fit our cars.

Injectors inject using a pulsewidth, which is not, strictly speaking PWM. PWM is used on some ECUs for current limiting.

Basically, the injectors are on or off, and the time they're on is the pulsewidth. The length of time they're on, their flow rate, and the pressure across the injector (fuel rail pressure - manifold pressure) is what determines how much fuel is injected. Pulsewidth goes up with airflow into the engine to maintain the desired mixture ratio.

Duty cycle is the other commonly seen term, and is simply the percentage of time the injector is open. The maximum is obviously 100%, in which the injector never closes, but is injecting continuously. This will overheat most injectors, and to allow enough cool-off time, injectors are typically limited to a duty cycle of 80-85%. This effectively sets the maximum amount of fuel the injector can flow under normal use: 85% of the flow rate at the rated pressure differential.

There is also a minimum injection time set by the physical properties of the injector, which normally takes about 0.8-0.9ms to open and 0.8-0.9ms to close. So, if you define the pulsewidth as the time from when you signal the injector to open and when it actually closes, you have a minimum pulsewidth about 1.6-1.8ms. The injector can't flow any less than this other than to flow nothing at all.

This is why injector sizing matters, as you may not be able to flow little enough fuel at idle with an injector that's too big, or you may not be able to flow enough fuel with the injector open 85% of the for full load.

The PWM is used with low-impedence injectors to limit the amount of current flowing through the injector. PWM snaps the voltage off and on faster than the injector coil can react to it, lowing the average voltage seen by the injector, and thus limiting the current. This is strictly a trick for digital ECUs, which can't easily control voltage beyond "it's on" or "it's off". This PWM doesn't actually have anything to do with the pulsewidth that controls the open time, and thus the fuel flow.

So, pulsewidth and duty cycle aren't really variables to consider when selecting injectors, other than observing the 85% duty cycle limit. This is the reason for the 0.85 term in the fuel flow v HP calculation I gave above. Max pressure ability, fuel connection type (hose-barb, O-ring, or side feed), and flow rating are the important variables. Most injectors have the same size nozzles (so they fit into all injector spigots), though not all of them have the same size coil section, which can matter with some spigots. There's also pintle type (pull back or push forward), though this is pretty esoteric.