injector size

[pbp1]

Ahhhhhh! One of my favorite subjects, injector sizing.

Long Post Warning!

I agree with what Robin and others have said here. This question gets more complicated as we try to answer it.

The simplest answer is that to simply supply the engine with the fuel it needs, you need about .45 pounds of fuel per hour, per horsepower and then add about 20% so that you avoid running the injectors at 100% duty cycle. So if your engine makes 500hp, 500 X .45 PPH = 225 PPH. Then add 20% and you get 270 PPH. This is the total amount of fuel that you need from all 8 injectors so now divide by 8 and you get 33.75 PPH per injector.

These calculations are based on "Rule of Thumb" constants like the Brake Specific Fuel Consumption value of .45. This is generally a good number, but we have seen brake specifics of .35 - .40. If your engine is more efficient, it will use less fuel, leaving you a little surplus fuel capacity (not a big deal).

Injector Phasing or injector timing is a strategy where you phase or time the injector event (the time between the opening and closing of the injector) in relationship to the intake valve's opening and closing events. I 100% agree with Robin on the benefits of this. Both FAST and Holley allow the user to Phase or Time the injector event. I have seen cases where good gains were found in doing this, and I have seen cases where a lot of effort and testing yielded little or no gains. The biggest question with this injector phasing strategy is when should the injector event happen in relation to the intake valve. Many have an opinion on this, but few have actual data, and even when there is data to support a theory, the results are not universal across different engine combinations.
In the words of Solomon, the conclusion of the matter is this, unless you have the time and money and are willing to exhaustively dyno test to find out what your specific engine wants, and, are willing to accept that for your application, there may be no gains to be found in this strategy, you might want to leave it alone.

As for injector size required to play with injector phasing, consider this. A typical race camshaft will have about 280 degrees of intake duration at .050" lift. Consider that if it takes 720 degrees to complete an engine cycle, then the intake valve is open (at .050") for about 39% of the engine cycle (280 / 720 = .3888 or 38.9%). What does this mean? It means that if your goal is to only have the injector spraying fuel while the intake valve is open (and I am not saying that is always the best strategy), then your injector duty cycle cannot exceed 39%.
In the above 500hp example, now, 500hp X .45 PPH = 225 PPH. To keep the injector duty cycle down at 39% (so that it is only spraying when the intake valve is open), you divide that 225 by .39 and you get 576.92 PPH. This is the total potential fuel that could be delivered by all the injectors, but you would only be operating them at 39% duty cycloe which gets the delivered fuel back down to the 225 that the engine actually needs. So, in this case, you would take the 577 PPH and divide by 8 and you would need a set of 72 and change PPH injectors.

Again, there are a lot of strategies that can be tried. Someone may want to spray all of the fuel in a time that is even shorter than the intake duration, another person may want to try spraying all of the fuel only when the intake valve is closed. One may want to push the injector event up against the intake valve opening, one may want to push it up against the intake valve closing, one may want to wait until the exhaust valve closes to eliminate fuel reversion.

I have introduced more questions than answers but the main take away should be that as stated, to play with injector phasing, you must have an injector bigger than what the engine actually needs!

Robin, you just thought you were long winded LOL

[cmracing]

Well pbp1, you certainly gave a very detailed explanation!

I find it funny when people think that running a 24lb/hr injector will result in a quicker pass than if he was to run an 80lb/hr injector, both running the same A/F ratio.

I had a friend make a pass with a set of 30's, swapped to a set of 66's, I had already made a fuel map that would be close with the larger injectors, car ran identical. He was very surprised because everyone said it would be slower. He then learned how to time the injector opening event with the larger injectors which he could not do with the smaller injectors.

The only real "disadvantage" to running too large an injector is the idle quality. Running sequential fueling really helps this situation though.

[Ed Wright]

I went to smaller injectors, at 60 psi, was due to a suggestion of a long time friend, who is a retired GM Calibration Engineer (“tuner”). Same reason the GM LS series engines went to higher (60psi) fuel pressure than the LT engines the LS series replaced. Higher pressure makes for a finer fuel mist from the injector, which is easier to fire. More consistent. There is some power & emissions improvements from injector timing as well. Large injectors are not required to find power there. My cam position sensor is adjustable. That is where I effect my injector phasing.
I also run mine in Sequential Mode.

[Ed Wright]

With a “Firing Order Swap cam”, be sure to program your Sequential Injection for the new firing order.

[Ed Wright]

In Seqential Mode, it’s easy to correct individual cylinder A/F ratios on the dyno. I have seen cars at the track with 8 wide band O2s. That would be interesting to log.

[KRatcliff]

Originally Posted by Ed Wright

In Seqential Mode, it’s easy to correct individual cylinder A/F ratios on the dyno. I have seen cars at the track with 8 wide band O2s. That would be interesting to log.

We can log all 8 O2s (9 actually) on my stocker and on our Sunfire SS going down track with our FAST XFI, but we don't do it all the time. They pull a lot of juice from the batteries.

[GUMP]

Originally Posted by jamie2370

Stocker, 350 tpi motor, Fast efi. What would be a recommended injector size? Am I being crazy by installing 60lb injectors?

Yes. Run a 24 lb injector.