do i need injectors - Page 3

cliffyk · 10-22-2009, 06:17 AM

Quote:

Originally Posted by teej281

So running around N/A, 42lbs would be the max you'd ever wanna go with hopes for the future??? What about running a really built 4v with ported cobra r heads and healthy cams???

The most air any 4.6L 4-stroke/cycle engine can consume, at 6500 rpm, is 528 cfm. At STP this is 2370 lb/h--at 100% VE (Volumetric Efficiency).

Make that 95% VE, the best you'll ever see running n/a, and it's 2250 lb/h. Now assume a 12.5:1 AFR. Each 12.5 lbs of that air will have 1.0 lb of fuel mixed with it, 2250 lb/h of air will need 180 lb/h of fuel at 12.5:1 AFR.

180 lb/h of gasoline, at 0.5 BSFC, equals 360 HP. For a 2V running at 85% VE it's 2016 lb/h of air and 161 lb/h of gasoline = 322 HP.

Even at 100% VE (unattainable running n/a), and 12.5:1 AFR, the max HP would be 380 HP--well within the 30 lb/h injector range.

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I'll say it again, you need to match the injectors to your engine's current power levels, so as to keep in the 75% to 85% duty cycle range. If you are running n/a there is no point in using anything larger than 24lb/h or 30lb/h injectors.

I.e. when you upgrade to another level you also buy new injectors (that's why they are made in such a wide range of capacities)--the tuning forums are full of people reporting poor fuel economy, poor idling, surging, stalling, etc. because they have oversize injectors.

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cliffyk · 10-22-2009, 06:29 AM

Quote:

Originally Posted by Black_GT_02

I plan on circa 550rwhp next summer. I'm on 42lb injectors now. Would I be okay just upgrading my fuel rails?

At 550 rwHP you'll be pushing 42 lb/h injectors up to 90% duty cycle or so--that's too high for my taste, YMMV...

As to the fuel rails, unless they are currently limiting the flow (which I doubt they are, see below), changing them would have no effect of any kind. Gasoline is not a compressible fluid, so it's entire volume and flow available at the fuel rails at any point in time has to equal the volume and flow through the entire delivery system--if you are still running the return less system.

I have no idea what the flow capacity of the stock rails is, however based on a quick look it would appear that their inside dimensions are at least the same as the ID of the fuel supply line, and probably larger--meaning that the fuel line ID becomes the flow limiting "weak link".

Chris98Gt · 10-22-2009, 06:31 AM

Wow, my poor 19lbers were probably screaming at me when I just had the pi swap, stage 2 cams, l/t and making 270rwhp lol.

cliffyk · 10-22-2009, 07:17 AM

Quote:

Originally Posted by Chris98Gt

Wow, my poor 19lbers were probably screaming at me when I just had the pi swap, stage 2 cams, l/t and making 270rwhp lol.

Yup, they were hitting 95%+ duty cycle--were you still running the stock 40psi fuel pressure?

I was pushing 19 lb/h injectors to deliver 30 lb/h on my supercharged '92 Miata--at 120 psi rail pressure and 14 psi boost...

teej281 · 10-22-2009, 03:16 PM

So, just a scenario, im pushing 10:1 compression, 350/300cfm ported cobra r heads, stage 3 n/a cams, longtubes, and all the goodies. Im assuming that would be over 400rwhp. So what size injectors would be needed to get to say 450rwhp n/a? 42lbs do the job?

cliffyk · 10-22-2009, 03:56 PM

Quote:

Originally Posted by teej281

So, just a scenario, im pushing 10:1 compression, 350/300cfm ported cobra r heads, stage 3 n/a cams, longtubes, and all the goodies. Im assuming that would be over 400rwhp. So what size injectors would be needed to get to say 450rwhp n/a? 42lbs do the job?

That's what the chart indicates, however you'll need to see peak HP at 7000+ rpm the get an n/a 4.6L pumping out 450 HP at the crank--to do this and have an engine that could last more then one 1/4 mile run would cost one whole bunch of money...

Chris98Gt · 10-22-2009, 08:47 PM

I assume it was, I didn't have an adjustable fpr at that time.

cliffyk · 10-22-2009, 09:17 PM

Quote:

Originally Posted by Chris98Gt

I assume it was, I didn't have an adjustable fpr at that time.

They were pumped near the limit then. What happens when the go static (100% duty cycle =, open all the time) is that any ECU demands for more fuel cannot be met. If the ECU is stupid (many are) it will call for a 100% duty cycle--which obviously cannot be met in the real world--and while it may think enough fuel is being delivered the truth s that it's not.

The result is LEAN and explosive detonation in every sense of the phrase...

teej281 · 10-23-2009, 01:04 AM

So ive been reading one of two things around...ford switched to 21lb injectors at the end of 2002 and that they just switched for 2002 in general. So just a question, do you think that my engine more than likely has 21lbs or 19lbs injectors??? Cus i fear that i keep hearing some pinging and i think that it is detonation. I mean with a slipping clutch, i put down 250rwhp and 263rwtq...and i heard if youre running lean your torque will be down. The a/f wasnt working at the time because it just read 15 the whole way through. But Im kinda worried now because I get valve noise when i go wot sometimes. So do you think that upping to 24's would give me enough room to be good for some longtubes or something??? And cliffy, is that chart for rear wheel horsepower or flywheel?

Black_GT_02 · 10-23-2009, 02:15 AM

Cliff, is there anything I can do with 95% fuel duty cycle to prevent me from replacing my fuel injectors to 60lbs? I'm a college student and ....welll... don't have too much money haha. I'm getting the internals really cheap and the main reason is so I dont have to constantly worry about throwing a rod? Obviously now fuel rails won't work but is there anything else you can think of? My fuel pump will hold 550rwhp but i see now that my injectors (42) will not.