This blog is going to track the process of modifying my 2002 Subaru WRX in an effort to increase performance while maintaining reliability. This is going to be a learning experience for me and I hope this blog will become a good reference for those looking to do something similar.

Saturday, October 08, 2005

Great Article on Fuel Injectors

I came across a solid knowledgebase at XSpeed. which includes this great article on fuel injection on forced induction motors. I've copied and pasted it below and it's available here in its original form along with other articles.



The Low Down on Fuel Systems

Added On: 19th May 2005

Fuel System.

With all forced induction motors you have to make sure that where there is an increase in positive manifold pressure that there is enough fuel being supplied to prevent leaning out.

Fuel Injectors
There are two common types of fuel injectors, Pintle and Disc. Pintle injectors have a superior spray pattern to disc actuated injectors, but disc injectors are less expensive and generally flow large amounts of fuel easily. If possible, always choose high flow Pintle style injectors, as fuel atomization at anything other than full throttle (high velocity port flow) is superior, leading to better drivability and economy.
View our range of fuel injectors here.

Larger Injectors.
Generally if the injector duty cycle goes beyond 80% (meaning the injector is open and firing over 80% of the time), you should upgrade. Fuel injector performance can become unstable beyond this point, plus it doesn’t pay to have a proper sequential fuel injection system working as a simple always open spray nozzle. Of course upgrading their size is only one way to add more fuel, and although a very good idea, it works best on car’s that can be easily retuned to work with them. In the case of MAF cars (Mass Air Flow sensor), system recalibration is generally just a matter of changing the sensor (many of the available larger sensors offer multiple calibrations). For speed density fuel injection systems (the other type of airflow sensing), its generally necessary to have an ECU capable of recalibrating it’s fuel tables for different injector flow rates. This is a problem mainly for the Honda people out there (all of which come with speed density systems), in which case you should think seriously about giving Zdyne or Hondata a call. Both offer very nice programmable ECU systems capable of keeping a big time forced induction motor running reliably and strong. Or, you can read on and see a few other ways to increase fuel flow without changing injector size.

Fuel Pressure Regulators.
All fuel injected motors have high line pressures, generally somewhere in the area of 25-40 psi. Changes to the line pressure will have a direct effect on total fuel flow from the injectors, but it’s important to realize this pressure vs. flow relationship is NOT linear. By one estimate I have read, to double fuel flow through an injector you must quadruple the line pressure. However most fuel systems will not tolerate more than 60-100 psi of pressure (even with upgraded fuel pumps; more on this in a moment), so just know that changing fuel pressure works great until you need to add anything over about 50% more fuel, at which point you need to do something else besides simple line pressure changes. Most aftermarket fuel pressure regulators are simply adjustable with a screw (where you will need a fuel pressure gauge to know how much you’ve changed things), and are very easy to install. However in the case of forced induction motors, what you need is not the simple adjustable regulator. You need to find a rising rate Fuel Pressure Regulator that can vary fuel pressure according to boost levels.
View our range of fuel pressure regulators here.

Fuel Pump.
There are two common choices to what fuel pump setup you can choose to run, internal and external pumps. Internal (the pump is located in the fuel tank) pumps are quieter and sometimes easier to install than external pumps, being that they are more like factory replacements than add-ons. External pumps (which are located somewhere on the fuel line, usually as close to the tank as possible) usually offer the highest flow potential. Additionally, there are two things to remember when talking about pump choice and placement. First, pumps like to push fuel, not pull it, so always mount the pump as close to the fuel supply as possible. Second, fuel pump flow drops as fuel line pressure increases, and this is again not linear. Pumps are judged not only by how much fuel they can ultimately flow, (being Liters Per Hour), but also by how much fuel they can flow at given line pressures.
View our range of fuel pumps here.

Fuel Controllers.
Fuel controllers (I use this term to describe any electronic controller designed specifically for fuel tuning) come in a few different forms, some modifying ECU input data, and most modifying ECU output data. Every one’s purpose is to somehow manipulate injector pulse width to tailor fuel delivery, and they work very well to that end for WOT (Wide Open Throttle) tuning. What they offer is the ability to ultimately control injector duty cycle, and hence the ability to easily tune fuel delivery for maximum performance.

The only problem with fuel controllers is there usual lack of ability to be tuned for anything other than WOT engine operation, but since we like to keep the go pedal on the floor, this usually isn’t much of a problem J. Fuel controllers let us tune the fuel delivery curve with precision Fuel Pressure Regulators can only dream of, and in most applications they work so well that even solidly tuned combos will see power and mileage gains through their use. These relatively new inventions basically “piggy back” standard ECU tuning, letting the ECU do the fuel delivery work until full throttle is called upon, where they step in to modify ECU signal (in the case of controllers that modify ECU output data) or modify the airflow signal (in the case of controllers that “lie” to the ECU to get the desired fuel delivery from it) to let us modify fuel delivery. This is a very good idea because all that time spent tuning the stock ECU to work perfectly at idle and most throttle openings and RPM (other than WOT) is left intact, with our tuning only taking effect when things really start to stretch whatever setups the factory had in mind. All can offer fuel delivery tuning accurate enough for serious performance applications, and all can be very cost effective when compared to complete engine management systems.

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