fuel injector duty cycles/flow question
 

i have an lt1 running a 230/236 cam, completely p/p heads, all the supporting stuff. i figure max flywheel hp i can expect is around 450.

i was low on cash when i was shopping for injectors so i settled for some sy/ty injectors. 32# or so rated, since i have an a1000 on the car and they are pintle injectors i dialed up the fuel pressure to 60# to step up the effective injector sizing.

using a calculator i came up with 35#/hr for 450 hp. 32# injectors running at 60psi should have that covered. but how does this affect the duty cycle?

will i be ok with this setup, should i go higher on the fuel pressure?

thanks
mike

What pressure were your injectors flow rated at? That will determine what they flow at 60psi.

If they were flow rated at 43.5psi (3 bar):

(60 / 43.5)^0.5 = 1.174
32 x 1.174 = 37.6 #/HR

If they were flow rated at 39.15psi (2.7 bar):

(60 / 39.15) ^0.5 = 1.238
32 x 1.238 = 39.6 #/HR

If you increase the injector flow rate, the duty cycle will drop proportional to the increase in injector flow rate. If you were at 100% DC with the 32's operating at 43.5psi, the DC would drop to 85% with the injectors operating at 60psi.

The actual injector size required is a function of BSFC (brake specific fuel consumption) and desired duty cycle. BSFC will be determined by the efficiency of the combustion chamber, and the efficiency of the tune. Typical injector sizing programs recommend a BSFC assumption of 0.500 #/HR/HP (normally aspirated). That's fine for a factory tune, but a bit conservative for a good set of heads, well designed quench and a custom dyno tune. You might see a BSFC below 0.45 with the right combo.

If you take a "middle of the road" BSFC of 0.476 #/HR/HP, and then use a target DC of 85% (I wouldn't use anything higher than that), you find that you can approximate the minimum injector size for your engine by multiplying flywheel HP X 0.07.

450 flywheel HP x 0.07 = 31.5 #/HR

You might be marginally OK with the 32's operating at their rated fuel pressure, but you will be safer with them operating at a higher fuel pressure.

wish i followed all the math but it sounds like i should be safe then running at 60 psi. i plan on having a dyno tune done, i need a 58mm tb and i wanted to be sure the injectors would do the job before i paid a dynotuner $500 to see what they can do. kinda a waste of money if the injectors are undersized.

i couldn't find what pressure the injectors are rated at, i poked around the sy/ty boards and some people say they were rated at 32# and some say 33# noboby mentioned the pressure they were rated at.

heads were done by me, i figure a dyno tune will tell me how good i did on them. i've been running a patched together tune taken from others for the same cam. i did some modding on the fuel tables for the increased injection but really want to get this car to a tuner and see where i'm at.

thanks for the quick reply.

you should be just fine my buddy has 383 with stealthram making 450hp with 30lb injectors at 60 psi fuel pressure.

cool thanks, really then i just need to tidy up a few things, get a new tb on there and take it in for tuning!