LAST EDITED ON 15-Apr-01 AT 04:49 PM (PST)

So 80% is the desired figure. Great now how the hell do you figure out what duty cycle your at? Whats the formula for it? Whats the relation between increasing fuel pressure and injector duty cycle. They have formulas of how increasing fuel pressure affects injector flow rates but then the max hp capability formula require you plug in injector duty cycle again which cant be the same at 45psi as at 90psi (at least not with 235cc injectors). How does the ECU pulse width modulation on the injectors affect duty cycle and how the hell do you calculate its affect if any? Then even changing BSFC from .55 to .65 changes the numbers a great deal. The more I run these formulas the more they seem too freakin theoretical to put into absolute practice.
Even using the formulas with 100% duty cycle and what the injectors are "acting like" at high fuel pressure the hp #s still seem low (until you manipulate the BSFC number to make it right). All I want to know is a good size injector to run with a 4:1 regulator, 10psi and AFC but the formulas aren't really helping things (they have me running big ass injectors even with the 4:1 regulator but I wont be able to cut back base fuel pressure). If 235cc injectors work at 90+ psi why the hell do the furmalas make me require huge injectors to run a still high 85psi at 10lbs of boost? If you cant explain dont worry just suggest a f*cking injector size to use with 4:1 reg, AFC and star fuel system?

BTW explain Brake Specific Fuel Consumption (BSFC) while your at it.

Stan2gnt
Stan2va@aol.com
95 Talon ES-I-NOS but T soon!

:farfrommugen:
Yes I still own a 2gnt.
Damn right I've been here longer than U
klassic_liven@yahoo.com

Yeah, me too... I sure would love for someone to be able to help me sound as if I know a damn thing... Just glad you asked first Stan, now I don't feel so stupid...

DoN

'99 GS Sports Edition
Star Stage 2, CFDF Clutch
Greddy Manual Boost Controller
Greddy BOV and EVO Exhaust
Greddy TT and shift knob
AF/X Underdrive Pulley
AEM Big Brake Kit
B&M Edge Short Shifter
Eibach Sportlines with Tokiko Blues

'99 Eclipse GS Sports Edition
Star Stage 2
Greddy EVO Exhuast
AEM Big Brakes
Greddy Type S BOV
Pioneer DEH-P7200
Bazooka EL1500 running 2 MTX 10" Subs
Fosgate Separates throughout

Hey Guys,

Been out in Atl for the last two weeks wrenching on my car and not keeping up online. Back in England now so trying to play catch up. I am tired so if I make some mistakes give me some slack here :p I should be in bed.

Let me see if I can help you out Stan.

First I must say that I understand the duty cycle is in fact linear and the important factor is keeping the fuel system within a safe range of pressure. I could be wrong. If I am, someone please correct me. I would hate to give false information here. I think you might be looking at some of the relationships wrong here. Instead of looking at duty cycle vs fuel pressure, work out what the percentage is based on the new flow rate created by increased pressure. Injectors sizes are rated at a specific pressure level, usually around 40-50psi or so. The number being given is only when the injector is flowing at that specific rate. The best thing to do would be to calculate what size injector is needed for the theoretical amount of power you intend to produce and then workout how much fuel pressure that is going to take. Lets use the stock injectors then. Correct me if I am wrong, but I believe they are rated 235cc at 47psi, which would mean they flow 235cc when running 47psi of fuel pressure. Now, lets say the absolute theoretical max a stage II setup would make is 300hp, which is probably more than true. I will use a BSFC of .60 as this is considered a safe number even though I have heard less before. I think Corky Bell quotes .55. In fact, I think the equations used are meant to be pretty safe and should not be taken as the absolute last word. That being said, doing the math, (300*.60)/(4*.80)=56.25lbs/hr*10.5=~590cc So we need around 590cc to safely handle 300hp. So now you can use (590/235)*47=118psi required. That is a hell of a lot. Of course this is conservative and they are just numbers to use as reference. The HRC recommended psi for a Stage II setup is 110psi which would tell us that the injectors would now flow (110/47)*235=550 which is enough for around (550/10.5=~52.4lbs/hr) (52.4*.80)/.60=~70*4=280hp which is just about where a HRC Stage II setup is rated to make power. I am not going to go into detail about how fuel pressure effects fuel pump flow and yadda yadda right now as this is already becoming long, but that is an extremely important consideration to think about before people just start running off and cranking up the fuel pressure. Fuel pumps have a degenerating flow rate curve the more fuel pressure being run as they are alos rated at a certain flow at a certain psi which can cause some serious problems if their specified flow rate is exceeded. We also need to remember fuel lines and injectors actually do have a maximum recommended psi rating as well as the pump.

Now for your setup. A problem I see first is that we do not really know how much power a Star setup produces at 10psi. Does anyone know? I have heard about 220 at 12psi on stock internals, so maybe around 200 or so? This is to the wheels and the formula realy is asking for flywheel so again it is all guessing which means you are correct in your assumption that the formulas can tend to be purely theoretical unless you actually have specific numbers to work with, but it is better than just guessing. I would imagine for what you want, just get as big as you can and then mess with the fuel pressure to adjust their size. If you have no real intention of building the bottom end and running a lot of boost later, I would imagine a 440 would be perfect. A RC 440CC is rated at 43.5psi so for kicks lets figure it out for 300hp. We can still use the 590cc as being a necessary size so, (590/440)*43.5=~58psi needed which now sounds very sane and safe to me. Running these injectors at 4:1 at 10psi and most likely idling at around 35psi would produce more than enough for what you need. By using the AFC, you should be able to lean them back down as needed with an EGT and an A/f to help. Of course a dyno with a wideband 02 is still my preferred method

BSFC - the amount of fuel you are using per hp per hour.
This exact number for your specific car can be figured out with dyno numbers.

Oh, and some info I came across the other day - these are part numbers for Mopar performance injectors - not sure if they will physically fit our rails or not, but here are some part numbers..

Part Number lbs. per hour
P4452804 36
P4529495 42
P5249452 53


P.S. - I hope you plan on retiring the NOS before you do this That would be a quick way to an engine rebuild.



MuRiX
97 Eclipse GS HRC Stage II
And a whole lot of other mods...
89 Accord LSi - yes it's mine
http://murix.home.icq.com/index.html

05 Mazda RX-8
06 Lotus Elise

LAST EDITED ON 16-Apr-01 AT 01:59 PM (PST)

LAST EDITED ON 16-Apr-01 AT 01:54 PM (PST)

No I dont plan on retiring the nitrous (how would the nitrous moderator look without nitrous). I plan on adding an inline pump and converting to wet nitrous which should be fine. And thats the main reason I dont want to use the 12:1 regulator. With the inline and Walbro's ability to maintain fuel pressure (12:1 reg +10psi=165psi of pressure), Both pumps might not be able to raise pressure to 165psi but surely enough to have me running rediculously rich if the injectors dont break first. With a 4:1 regulator fuel pressure will be 85psi at 10psi of boost. 85psi I can work with. The figure I use is 300hp @ the flyweel/about 250hp at the wheels (the actual figure was 235hp at the wheels @12psi but I'm optimistic I can produce 250hp @10lbs). Wet nitrous need not be included in these formulas as its will be working independent of injectors and the two pumps will be more than enough to maintain fuel pressure with a 50hp shot and 10psi (with a 4:1 or smaller ratio regulator that is).

I thought about it later and I think your right (I think I was barking up the wrong tree). duty cycle should be in relation to % of hp capability being used regardless of fuel pressure.

Stan2gnt
Stan2va@aol.com
95 Talon ES-I-NOS but T soon!

:farfrommugen:
Yes I still own a 2gnt.
Damn right I've been here longer than U
klassic_liven@yahoo.com

LAST EDITED ON 16-Apr-01 AT 12:18 PM (PST)

ok the pulse width that the ecu sends to the injector is almost the same thing as the duty cycle. depends on rpm too, but think of it this way:

if your injector is spraying for .1 seconds and closed for .1 seconds and repeating that cycle, it is running at a 50% duty cycle. meaning it is spraying half the time. if it is spraying for .15 seconds and closed for .05 seconds, it is running at 75% duty cycle. if it is spraying constanty and never closes, it is running at 100% duty cycle. it is a safe practice to pick an injector size and fuel pressure so that the injector doesn't have to work more than 80% of the time. this allows room for error and ensures you won't run lean at max power/max rpm.

unless you dyno your car and have it measured, you will never know the exact BSFC of your engine. it's probably a good guess to use 0.60 for turbo engines and 0.50 for n/a engines. not precise, but close enough for our purposes.

now here is where i'm going to be working on some assumptions. to calculate injector size, we have to know the max HP the engine will produce at the FLYWHEEL. i'm going to guess that your 10psi will give you about 260 hp to the flywheel. my calculations will be based on that.

i'm also going to assume that the fuel regulator set-up you have will allow you to LOWER your base pressure. we'll use 35psi as the NEW base pressure for you.

with a 4:1 riser seeing 10psi, it will raise your fuel pressure 40 lbs to a max of 75psi.

now i calculated that your engine will need 511cc injectors to support 260HP. these numbers are from the formulas on RC Engineering's site and are suspected to be overly safe. maybe by as much as 20%. if you multiply 511 by .8 you get 408. now this is just a theory of mine, but that's probably closer to the amount of flow your engine will actually require than the 511 from RC. we'll be careful and only take 10% away from RC's 511 which gives us 459cc.

injectors are rated for flow at 43psi and an 80% duty cycle. so we just have to find out what injector will flow at least 459cc's at your max fuel pressure of 75 psi.

a 370cc injector will flow 488cc's at 75 psi. at your lower base pressure of 35 psi, it will act like a 333cc injector. your s-afc should have no trouble at all trimming that down for your ecu to handle.

if i've made any wrong assumptions about your engine or estimated power output, let me know and i'll run you some new numbers.

Hoot

P.S. -- murix, a 235cc injector running at 118 psi will flow 372cc's. not 590. besides, i'm not a fan of ultra-high fuel pressures to get more flow. i'd rather do it the way i explained above. i know it's more expensive, but i figure if somebody wants to step up to stage II or higher, it's the price you pay.

...I'm just beginning to understand total fuel systems myself.

I assume that 110psi is safe(or rather the limit), so PLEASE CORRECT ME IF I'M WRONG. So if I wanted 320HP, arbitrary number, at the wheels(~411HP Crank) then 440cc at 90psi(fuel pressure obviously) would suffice so long as the fuel pump could handle(which obviously it can) the extra fuel.

In terms of max injector duty cycle well injectors use electromagnetic actuation(what the hell does that mean ), well electricity turns on and off electrical magnets but only so fast. At anything higher than %80 you could run into mechanical defficiency. Either to lean a condition because it can't spray quick enough and/or too rich(combustion chamber flooding only bad on small engines about <2L).

So Hoot, or anyone, my question is what is the SAFE MAX (I emphasize both words) daily driving limit of fuel pressure? Now I need to understand the pump in relation to why the Star system is SO inneffiecient at 10PSI(even though people run that everyday).

Thanx

Skrilla
'97 Base Talon NA
"Technology: The Only Replacement for Displacement"
"Peace and Turbo Grease"

*Skrilla
"Honest officer, I thought you were trying to race me"
Talon: 1st 2.4L
'97 Prelude
'04 350Z

Originally posted by etx
Good luck man! Project 'STFU Brian your talon is dead meat' should be on it's feet soon!

i ran the formulas at RC for the 320 hp you want and came up with 630 cc/min. like i was saying, i think that's a grossly over-safe estimate. i'm betting you'll need less than 550 cc/min for 320 hp. 440's at 90psi would flow 650 cc's. way too much i think. probably enough to support close to 400hp.

...and the OEM fuel pump won't even flow at 90psi. the walbro only flows 35 GPH at 90 psi. that comes out to 550 cc/min to each of 4 injectors. it couldn't feed the 650 cc's anyway. so keeping the fuel pressure down as low as possible with reasonable sized injectors is all we're trying to do.

440's at 70psi would flow 560 cc's. that's probably where you wanna be at. and i like the 70psi number better than the 90psi number anyway. at 70 psi, the walbro is capable of feeding four injectors flowing as much as 750 cc/min. a nice safe margin.

what's the MAX-SAFE pressure limit? nobody that i know of has ever tested our fuel systems to the point of busting a hose or making a fitting leak. i'm sure that's not the weakest part of the system anyway. it's the fuel pump's inability to flow at those high pressures and there's some concern about the fuel injectors operating properly at those pressures too. if i had to pick a number i'd say <90. 70-80 would be my target range.

i'm sure you'll be using an afc, since you're looking at 440's. and an a/f and egt are a must. so if you find that the 70psi i recommend isn't enough you can always take it up to 75 or 80, but i think 70 will be fine. if you have to adjust the afc into the +% on the top end, then it's probably time to raise the fuel pressure. otherwise, you're going past the recommended 80% duty-cycle limit.

doh, i just re-read your last post and saw that you wanted 320 AT THE WHEELS. my bad. i calculated for 320 at the crank. in that case you'd need up around 650 cc's like you said. to get that you would have to run the 440's at 90psi. if you're gonna do that, it would be best if you got the inline pump from HRC. i can't see a walbro flowing that much. just about everything else i said holds true though.

Note to self: call eye doctor. LOL

...I did it!

I wanted to make I did the fomulas correctly, and it appears so. Thanx again. I kinda figured I would need HRC and/or the holley higher performance pump. Ya you said that 440cc would support 400HP at the crank so that works out. That's a good starting point.

Skrilla
'97 Base Talon NA
"Technology: The Only Replacement for Displacement"
"Peace and Turbo Grease"

*Skrilla
"Honest officer, I thought you were trying to race me"
Talon: 1st 2.4L
'97 Prelude
'04 350Z

Originally posted by etx
Good luck man! Project 'STFU Brian your talon is dead meat' should be on it's feet soon!