i think my motor may not be getting enough fuel. im running longtubes, 306 cam and ported lt4s. what fuelsystem/injector setup will keep my motor happy? wanting to spray 150 on this setup as well. thanks in advance.

 

Fuel demand is determined by flywheel HP and brake specific fuel consumption (BSFC). A quick way to estimate the minimum required injector size is to multiply peak flywheel HP x 0.07 (this is for normally aspirated applications only).

Example:

500 flywheel HP x 0.07 = 35 #/HR injectors.

Without the HP, there's no way to tell you what size injectors and fuel pump you need.

Adding N2O usually does not affect the injector size. A wet system adds the extra fuel externally, not through the injectors. A dry system pushes the extra fuel through the injectors, but the typical LT1 dry kit raises the fuel pressure when you spray, to supply the extra fuel. You size your injectors based on the HP without the N2O.

 

will the stock pump take good care of 35# injectors?

 

What do you mean "take good care of?"

 

only a few ways you could interpret that i thought. well supplied with fuel is what i meant.

 

Did you read injuneers post?

"Without the HP, there's no way to tell you what size injectors and fuel pump you need."

 

yes i read it, i didnt realize that hp was a factor when asking if a stock fuel pump could maximize a 34# injector. forgive my ignorance. ive been messing with high hp diesels for the past few years and im trying to switch my brain back over to gas.

 

I think I understand what you are asking, and yes the stock fuel pump should be able to keep up and supply the 34# injectors. It is debatable on how much hp the stock fuel pump will support, some say 350 rwhp, some say closer to 400 rwhp. I say just upgrade it to a Walbro 255 lphr pump and then you dont have to worry about the stock fuel pump keeping up.

 

And of course you can't measure any fuel pump's capability by referencing REAR WHEEL HP. You have to look at FLYWHEEL HP.

 

(thanks again injuneer!)

 

Wouldn't the PCM adjust the amount of fuel spray through the injector?

 

Of course it would, but that's at a predetermined fuel pressure. If the pump can't provide ample pressure, it will run out of fuel during periods of higher fuel demand.

 

As noted... if the engine needs 250 liters/hr of fuel, and the pump can only supply 190 LPH, there's nothing the PCM can do with the injectors to make them spray fuel that doesn't exist.

Similarly, if the injector is too small, the PCM will try to set pulse widths that exceed the amount of time it takes for 2 revs of the crank (= greater than 100% duty cycle), and the injector will be constantly open, and not supplying adequate fuel. Once the injector is open more than 85-90% of the time, the flow deteriorates and the spray pattern fails.

 

so if my build is projected to lay down 430rwhp n/a, i wont even come close without a larger injector and pump correct?

 

Yes, you are correct. The stock injectors and stock fuel pump will not be able to keep up with that demand. Keep in mind though, as injuneer pointed out earlier, to determine correct fuel supply, you need to figure flywheel HP. So if you are planning 430 to the rear wheels, you will be pushing ~ 500 to 550 HP to the flywheel, depending on your transmission and other drivetrain losses.

There is some really good info in this thread. Not to mention there are some good engine and racing calculators on this site as well.