19doug90

is there some magic formula? some mystical folk lore? come on guys is there a real way to figure out how many lb of an injector you need?

sofakingdom

iTrader: (1)

Don't know much about "magic" or "mystical folk lore", myself... I don't think motors care much about any of that either.... I'm strictly a "facts" kind of guy, which fits in well with this sort of thing, since it's all numbers and physics.

Take the engine's BSFC if you know it (Brake Specific Fuel Consumption - a measure of lbs of fuel per HP per hr); if you don't, or can't get a reasonable estimate from some kind of dyno sim software, use .45 as a starting point for a good hot-running street motor. Multiply that by how many HP you expect to generate, that'll tell you how many lbs of fuel per hr your engine will need. Divide by 8 (if you're building a motor with 8 cyls, or by some other # if not), that'll tell you how many lbs per hr each cyl needs. Divide by the highest duty cycle you'd like to see at rated fuel pressure, .85 is a good # to use. Voilą!! Injector size. Tweek the various constants and estimates according to your better judgement. Keep in mind, you'll get better results with injectors that are slightly on the small side with higher FP, than with too-large ones.

19doug90

so im making 275 hp at the wheels, expect it to be more around 285 with some other stuff being done
lets say thats around 325 at the motor?
i truly dont understand BSFC but ill read up on it

using your given number for it at .45 x 325hp / 8 = 18.3

thats way too low, so what gives?

i'm running 22's in the car right now, think thats probably a little on the low side, but im at the point right now where its, ya 24's or 26's would probably be good, but thats not specific enough for me, id like to be able to figure out exactly the right injector

sofakingdom

iTrader: (1)

BSFC is basically a measure of the engine's efficiency; it's the amount of fuel required to produce 1 HP continuously for one hour. The more efficient your engine, the lower that number will be. For typical street builds it comes out to something in the .45 - .50 range; for a REALLY efficient motor, a NASCAR motor for a racing example or a LS1 as a street example, it can be way down into the .3s. A poorly built street motor might be well up into the 5s; that would be one that's so mismatched that it's NEVER efficient. Let me create a TERRIFIC bad example here: A stock 76 Vette short block with a Performer RPM cam and a Torker 2 and a Holley 750 vac sec out of the box, 1¾" headers with 3" collectors. :barf:

Desktop Dyno and Engine Analyzer and some of those softwares will estimate that for you. Or, if you've had your motor on an engine dyno, it's a piece of info they'll almost always produce and supply you.

275 at the wheels is probably closer to 350 at the crank, with an auto.

350 x .45 / 8 /.85 = 23.2

24 is probably the best choice.

Try some of the FoMoCo 24 lb/hr ones.

19doug90

275 at the wheels with a manaul

it got a basic tune done but needs to go back to get more work done. Its running with the 22's at about 45 psi right now, dont think the regulator would go any higher then that. Thats probably enough until i buy some better heads.

motor is probably also not the most efficient right now.
little too much cam for the head...altho

okay a different topic, obviously you want peak lift of your cam to be right around the peak of what your heads will flow. My iron eagle heads only flow to around .520 lift, and my cam with 1.5 rockers is right around .515 lift so thats perfect.
but how does rpm effect that? my motor made peak power at 6000 rpms on the dyno, and my heads probably dont supply anywhere near enough air at that rpm? unless lift is all that really matters? i dont fully understand. I dont think my combo is that inefficent, but its probably far from fantastic.