I'm building a procharged 350 tpi motor. How much will the stock injectors take before needing a change? Fuel rails? I believe mine are aftermarket cause they are bugger then my friends 88 Gta 350 cars.

Also I plan on making about 500-550 rwhp.

Stock injectors wont support that power. you will need more like 42's and a boost referenced regulator. Maybe even crank up pressure, more than likely going to happen with a procharger kit since I beleive they have a FMU for fuel pressure. So if you use that you may beable to get away with slightly smaller injector but still dont think stock injectors will make it.

If you have money for the procharger kit, please upgrade the injectors. Old stock injectors have no business on a build like that.

I used the stock fuel injectors (22lb/hr on a mildly built TPI) along with the FMU that came with my P1SC procharger, and the car topped out at 372rwhp. That was running 12 psi of boost and 75+ psi on the fuel rail. Stock injectors didn't like it very much and they died within a few months. You definately need a larger injector:

Injector size (lb/hr) = (FWHP x BSFC)/(#cylinders x duty cycle)

FWHP = flywheel horsepower
BSFC = Brake Specific Fuel Consumption
Duty Cycle = Max on/off time of the injector

BSFC for naturally aspirated combinations typically falls in at .55-.50 lb/HP
BSFC for forced induction combinations typically falls in at .50-.45 lb/HP

Both of these can change slightly based on the volumetric efficiency of the engine. These are pretty safe estimates for these calculations.

Duty cycle should always be kept under .85-.80%. Higher duty cycles will not give the windings in the injector enough time to cool down before they overheat. I have seen people go 100%, but they typically do it for very short periods of time. To be safe, try to stay at 80%.


Injector size (lb/hr) = (FWHP x BSFC)/(#cylinders x duty cycle) =

FWHP = (Injector size lb/hr x #cylinders x duty cycle)/BSFC =

FWHP = (22lb/hr x 8 x .8)/(.45 lb/HP) = 312.9 FWHP


Typically we run rail pressures of 45psi, and the following equation shows the relation of fuel pressure vs. flow volume:

New Flow Rate = SQRT(New pressure/old pressure) x old flow rate

*Note* Stock injector flow rating is at 43.5 psi rail pressure.

Lets say your FMU increases the rail pressure from 43.5 psi to 70 psi under boost. That makes your 22 lb/hr injectors act like they are:

FR = SQRT(70/43.5) x 22 lb/hr = 27.91 lb/hr

Plugging that into the original equation yields:

FWHP = (27.91lb/hr x 8 x .8)/(.45 lb/HP) = 396.94 FWHP


Keep in mind that you should really minimize the pressure you run at the rail. Much like running too high of a duty cycle, the additional pressure makes the injector windings overheat. The best way to do this is to use a fuel pressure regulator that is boost referenced 1:1. Therefore, if you run 12 psi of boost, it will add 12 psi of fuel pressure.

Hope this helps you determine the right size injector for your application. Another thing worth considering is that chances are the stock injectors you have on the engine probably have substantial mileage on them now. Why risk a good engine with worn injectors? Replace them with some of the right size and you will be much happier in the end.

What are you guys doing for the computer?? I was thinking about ls1 computer swap but what else is out there?

I personally run 84 lb/hr low impedence injectors with a FAST XFI ECM. Others use the factory ECM and burn PROMs to handle various high impedence injectors. If you want to use low impedence injectors you will need to use some kind of aftermarket controller as the factory ECM alone can't source enough driving current. If you go with the LS1 swap, you can use LSEdit software to change the injector size. Not sure, but I think the LS-series PCMs can only handle high impedence injectors as well. Someone more familiar with LS based ECMs should be able to get you the answer.

LSx is high impedance and they have aftermarket drivers you can add on to drive low impedance.

One popular factory ECM is the 749 ecm that can be found in the GM turbo trucks from 90-93 or so. Syclone / Typhoons. Also can find them in the early 90 turbo sunbirds by pontiac I believe. It can handle both high and low impedance injectors.

I run a factory 730 ecm, which is found in the 91-92 fbodys. I run turbo truck code thats modified abit. Called code $59. Runs my high impedance siemens 80lb injectors just fine.


Bonus to doing the LSx conversion is that the majority of tuners out there can tune those setups. Not many people are doing TPI based stuff anymore.
Advantage to keeping factory 730 or 749 ecm is that its cheap.

I have considered bigstuff3, fast, megasquirt, the ls1 conversion I like the ls1 just cause you can tune it easy.

I run the 730 computer also with code59 for boost, it's dirt cheap and seems to work pretty good, plus for yourself you wont need to do much to get it setup. Just make sure you have a wbo2 hooked up to the computer to make tooning easy.

The XFI is pretty easy to use.

As far as injectors go, I would at least use 42's.

I recently installed a P1SC on a F150 and while extremely impressed with the procharger, there are two glaring things that I didnt like. For one, the whole FMU thing.....nothx. Secondly, the fuel pump is pretty damn loud. So much so that the owner of the truck wants me to replace it with a Bosch 044 (at his expense.)

One more suggestion, for your power goals, a D1SC might be a better idea. Same power on less boost is a happier engine.

Can the stock fuel rails handle this? Or will I need aftermarket? What is a good intercooler? Fmu?
Good fuel pump? Should I go external?

As for the trans can a 700r4 be built to handle this? I don't have experience with them. I would like to keep overdrive and such

I havent had good luck with built 700's as of late, so I might be partial.

Thanks for the info