Injector myths debunked

[Scot]

I see a lot of misinformation propagated here. I like people to be well informed and I'm happy to share 20 years of production engine calibration experience with anyone that wants. Knowledge is power.

1. Bigger injectors will not kill your fuel economy - IF YOU ADJUST YOUR CALIBRATION FOR THEM.

Yes, bigger injectors flow more fuel, but with proper calibration and closed loop fuel control, you will not notice a difference in fuel economy.

Here are the sort of calibrations available to tuners with respect to injector flow:

1. Flow rate vs. Pressure - I'll get to that later
2. Flow rate mult vs. volts - this compenstes the injector pulsewith based on system voltage
3. Flow rate mult vs MAP - this compenstes the fuel injected based on intake manifold pressure. It is harder to push fuel into a manifold with 20 psi boost than with 10 psi vacuum.
4. Flow rate mult vs IAT - this compensates fuel flow vs intake air temperature. In a port-fuel injected engine, there is an ever-changing "puddle" of liquid fuel. The size of this puddle - and what ends up getting in the cylinder - changes with temperature
5. Flow rate mult vs Flow - Injector flow rates are non-linear as they approach the min flow regions.

Flow rate vs. pressure:

This is the main table you adjust when you change your injector sizing. Basically, it is how much fuel the injector can flow (in grams per second) at a given fuel pressure. The array's axis is in "delta pressure" - but that's to let us know the pressure is in "gauge", not absolute. So, this array lets us know at a given fuel pressure, we get a certain amount of flow. The ECU can then calculate the required injector pulsewidth for the amount of fuel needed.

Now, if someone doesn't know how to properly calibrate big injectors, they might find a problem of running too rich at idle, and that would negatively affect fuel economy a bit, but it's pretty hard to mess up the basic conversion.

To be continued...

 

[River_Rat]

Out of curiosity, what kind of Injector Pulse Width are we looking at with 42# injectors running gasoline at idle?

 

[Scot]

No idea yet. I'll tell you after I install mine.

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[Preloader]

Great info. I've been thinking about starting ups a thread dedicated to LTFT education.

Can you explain how injector size would directly affect Fuel Trims? Or is there no relation if they are calibrated correctly?

 

[ScarabEpic22]

Scot,
THANK YOU!!!!

Gas engines DO NOT get a see performance or mileage increase or decrease with different injectors IF they are calibrated correctly in the ECM! If you're out of injector, you might notice some increase in hp/tq output IF you were not able to fuel correctly with the old injectors. This is all part of a PROPER CALIBRATION (not tune, calibration).

Diesels, well they're a completely different beast altogether...

 

[sx sonic]

Keep the education coming Scot!!!

 

[Twitch]

he

good write up,

remember a while back i had pulsing in my accel when boosting ? I have found it is only when the temp is below 30-25 degrees F. So im thinking it is leaning out ? sound like something hat could happen with a full tune ?

 

[Twitch]

??

 

[Larrymer]

Could be a ton of things, boost control, fuel, spark, etc. Send your tuner a log and let them sort it out. It's what you paid for after all.

 

[Stew]

No idea yet. I'll tell you after I install mine.

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To get you pretty close if wanting to go to 42 lb injectors, would it really be as simple as multiplying the values in those cells by an appropriate multiplier? For example, if the Sonic has 27 lb/hr injectors (not sure about the stock injector size) and you want to stuff 42 lb'ers in there would you just multiply all the values by 1.56?

If so that would be pretty easy. I know that injectors have many parameters which you can fine tune in the ECM, I'm just talking getting a pretty good starting point so you can start it up and do some logging.