injector size

[jamie2370]

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

[cmracing]

Nope, depends on the EFI system you are using. A newer system, like Holley, and there are others, will let you time when the injectors open, but to do so they need to be much larger than "typical". If you were to use a 30 pounder or so, you wouldn't be able to change when the injector opens in relationship to the intake valve, you wouldn't be able to get enough fuel in the chamber at higher rpms. By over-sizing it, you have the flexibility to try different things. Again, only if your particular EFI system will let you adjust the actual "timing" of the injection events.

[jamie2370]

Thanks for the PM's guys. You know who you are. I appreciate the advise.

[Robin Lawrence]

Boy this question can have a ton of correct answers. Minimum would calculate to about 40 PPH.
Based on an estimated 600 HP and a rough .45 BSFC that is around 33 PPH.
Add another 20% margin since we don't like to run injectors above 80% duty cycle and you get near 40PPH.

As said above you can run a larger set that can be timed to cam intake valve events giving some advantages depending on camshaft/intake design. I have seen some where it was not worth any gains. The feature can really help the Idle in come cases.

If you want to run sequential injector strategy you will need a cam sensor. It will allow the engine to run bit cleaner at low RPM and when you go up on the converter. It's usually not an issue with the smaller injectors to run batch or bank to bank strategy. Many older combinations run batch fire at close to 100% duty cycle. Some of those guys still use fuel pressure adjustments to fine tune their EFI rather than adjusting the base fuel map.

Hope this helps

Robin

[ss3011]

Robin , Do you see a gain by running the injectors at 50 or 55 psi instead of oem 42 psi ?

[Larry Hill]

When the injectors are timed to valve events does that lesson the energy in the reversion wave. Will it generate a "sweet spot" with the reversion wave to pack the cylinder with A/F mixture? With the larger injectors, will droplet size suffer by being larger or will the droplets be close to the same size.

Trying to learn!

Thanks

[Robin Lawrence]

Originally Posted by ss3011

Robin , Do you see a gain by running the injectors at 50 or 55 psi instead of oem 42 psi ?

As in may scenarios my answer is "It Depends". Only testing at the different pressures will tell you if there is any value.
Typically I try to run injectors at their design fuel pressure. As you get away from the design pressure some injectors characteristics can change. An injectors opening and closing can be effected by a couple of things, fuel pressure and voltage. changes in these areas effect the way the injector reacts and can alter flow.
Most modern ECU have voltage compensations tables. Getting the correct "dead times" for a specific injector offset any voltage fluctuations and deliver what you have programmed in your base table.
As for pressure changes only flow testing at the various pressures can tell you what that injector will to a setting different than what it was designed for.
And only track testing can tell you if there is a benefit. (notice I didn't say dyno)

As drag racers we affected less by these variables. A race car that is driven at part throttle or through a wide rpm range will see a benefit. A street car will have better drivability with good injector setup data.

In my opinion you will see the most benefit in a drag race scenario by tuning the individual cylinders at wide open throttle through the RPM band of your combination.

I would used the injector timing tables to help mitigate the effects of a huge cam at idle. If an auto car this can also help when loading the converter.

Sorry if I got long winded

Robin

[Ed Wright]

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?

30lb would be plenty. I have made more power with my LT1 SS engine using 36lb injectors, at 60 psi. At your power level, I would use no more than 30lb. Myself, I would try 24lbs at a higher pressure. I have had best luck with Lucus injectors in my cars.

[KRatcliff]

I have been running 30 lb Ford Racing injectors in my LT1 for a while. I have an injector flow bench that I will test a few boxes to try to set match them. They seem to work pretty well for me. Generally have my fuel pressure between 52 lbs and 58 lbs depending on the air.

Also have some 60 lb injectors that I want to experiment with, but I have my hands full at the moment with other stuff.

[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