AFPR Pressure??

fastsc92

Registered User
What should I set my regulator at to start with. I have stock injectors and a 255lph pump. I just want to take most of the guess work out and see what PSI works the best. Anyone wanna share what they set theirs at for stock injectors??
 
I would start with 40 psi static (with vacuum line off ) and don't go any higher than 44 psi. If it responds well to the extra fuel, get larger injectors and re-calibrate the MAF to match so that you can run stock fuel pressure.

The higher static pressure can cause you to think you have plenty of fuel, but you can still run lean under boost and high rpms, because without a forced induction fuel pump the flow is greatly reduced as fuel pressure is increased.

David
 
so when i tune this FPR....the vacume line has to be off?? And I should start at around 40psi??
 
Yes, 40 psi with vacuum line off and plugged with engine idling.

It's more of an adjustment that tuning. I'm still not clear on why you are installing the adjustable regulator. You really shouldn't need one with the mods you are listing.

They are helpful for fine tuning 38# and larger injectors when you don't have a tuner or custom chip, but can get you into big trouble if your just jacking the pressure up on stock injectors to simulate larger injectors.

BTW, What brand of regulator are you planning to install?

David
 
I got a kirban from MN12. I think i might be pushing these stock injectors to the max. With a 73mm MAF, 75mm TB, 5% pulley, Raised top, and a zex nitrous kit. I just dont want to run lean. I cant afford new injectors right now and i thought that if i increased the pressure a tad...then it would increase fuel output a small amount to make it a little safer with the nitrous. It'll only be a 55 shot..and a 65 at the max, but is there any other way to play it safe with N20? I havent even opened this FPR yet so I could always return it.
 
I was concerned before you mentioned nitrous, now I'm thinking your asking for trouble if you don't install a 255 lph forced induction pump and raise the pressure to around 45 psi static before spraying.

If you don't want to risk toasting your engine, spend a few bucks extra and get some pulls on a dyno with a wide band o2 to determine your A/F ratio.

At wide open throttle it should not be leaner than 13.5:1 or richer than 11.0:1. You will get the best power if you can maintain 12.3 -12.7:1 air to fuel ratio. I would also pull the octane plug to retard the timing until your sure it's not pinging.

Nitrous without enough fuel = cutting torch on your pistons.

David
 
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Now i'm not sure what to do with all this stuff. Without the nitrous on.....am I maxing out the injectors? Cause the nitrous is only being used at the track, and because its a ZEX kit, when the nitrous turns on, it raises fuel pressure to add the increase of fuel. I just wanted to be safer and raise the pressure about 2-3 PSI from the stock 38.5PSI.
Also, with that octane plug, when i pull that plug, does it instantly retard the timing 4 deg. or does this only happen at WOT? IF so, I'll wire a switch up to it and use with when the nitrous is on. And its only going to be a 55 shot.
What are my options here......?
Use the regulator and raise pressure 2-3 PSI?
get 38lb injectors and a MAF and keep the stock regulator?
Keep the stock regulator and just use the octane plug?
Drive the car into the river??
 
I just think you need more fuel...but since I have never installed a nitrous system, I'm not really qualified to make that judgement.

I would suggest that you read this thread on the subject and then decide what you want to do.

http://www.sccoa.com/sccoo/Faq/nitrousoxide.htm

The article says that without any mods a stock pump and injectors will support a 60 HP shot, but with mods use a 255. I don't think that is enough.

David
 
you dont think what is enough??? I do have mods, but not major mods, just some simple bolt on's. The zex kit says it works with factory fuel systems without modifications, but I have installed a 255lph pump.
Also, I've been doing some research on AFPRs. Let me get this straight....when the vacuum line is hooked up to the regulator, for every psi of vacuum...there is one psi of fuel pressure dropped? This is quoted from my AFPR instuctions: "...the fuel pressure should drop approximately 1 psi for each psi of vacuum."
This doesn't make sense to me, shouldn't the psi increase when the vacuum increases? When you're at high RPM there will be more vacuum, and it will decrease fuel pressure? OR am i looking at this wrong?
 
vaccum is higher at idle then any other point in the RPM band, the instructions are right but explain it opposite of what happens in an engine, as it revs the vaccum decreases and the fuel pressure increases. so the less inches of HG (vaccum) the more fuel pressure. u notice how the boost gauge goes from inches of HG (vaccum) to lbs of boost as u rev, kinda like going from negative to positive vaccum.
 
The regulator instructions deal mainly with a normally aspirated engine (89-93 5.0) in this case. On supercharged engines the signal or refrence line also raises fuel pressure with boost. For every pound of boost, fuel pressure will increase a pound.

Vacuum will not lower the fuel pressure one psi for every inch of vacuum. It's more like 1/2 a psi for each inch of vacumm with a maximum reduction of 10 psi.

You would only be spraying the nitrous when you are at wide open throttle and under boost. The kit can only raise pressure so much...if you are running with 15 pounds of boost, then you already need 55 pounds of fuel pressure without the nitrous. If you don't have a forced induction pump....it isn't going to flow much more fuel. The standard 255 pump flow drops like a rock as the pressure increases.

David
 
David N.

I would like your opinion on something that ties into this whole fuel thing. I am still having detonation problems and what I was going to do next is install a 255 pump and #42 injectors. I have a 94 with 3/4 top, headers and exhaust, C&L 76mm MAF, 85 MM TB, SCP 3.5 inlet tube with the K&N filter, Magnum powers inlet, IC fan, March 3 piece UD pulley set, 89-93 SC pulley(I had a 10% and was getting allot of bucking and allot more detonation and up to 15lbs of boost), Magnecore wires, ported and polished heads, valve job, 3.27's, and stock injectors and pump.
I have not run a dyno with a O2 band yet so I don't know what my fuel/air ratio is. I have an autometer gauge installed and it goes to rich at WOT if that helps out at all and only goes lean at heavy decel, other thean that it just bounces back and forth from slight lean to rich while driving.
Will the larger fuel pump and injectors help and should I go with a forced induction with or without a FPR?
Will a chip help and should I have the O2 dyno before I order one?
Will a chip help without a new pump and injectors?
My RWHP and RWT listed below are from prior dynos before the March pullies.
Any ideas would be great.
The only thing I have left to do is the pump, injectors, FMIC,
 
fast92,

First off, please tell me your name...I hate responding to people when I don't know what to call them.

Your car may be fine....I'm just telling you that installing a dry nitrous system with a questionable fuel supply is playing with fire. I can't tell you exactly what to do. I've already given you my opionion on what fuel pump you need, to insure adaquate fuel delivery. Some people may say that is overkill, but it's much safer to be too rich than to run lean and toast your engine.

Being that your still a teenager, I'm sure this was an under investigated impulse buy..and now you want a quick answer on what to do. Here is the quick answer....Put the stuff you bought on and raise the pressure to 45 psi static, don't spray until you are on a dyno with a wide band o2 sensor. Most places will give you three pulls for $100. If you can't keep the AF ratio under 13.0:1 when spraying then you better stop until you get more fuel.

Get more fuel by installing the forced induction pump or using larger injectors (38s or 42s) and running stock pressures with the 255 pump you already have. BTW, stock fuel pressure is 39.5.

94skinflint,

Ditto on the name thing please.

I have an autometer gauge installed and it goes to rich at WOT if that helps out at all and only goes lean at heavy decel, other thean that it just bounces back and forth from slight lean to rich while driving. Will the larger fuel pump and injectors help and should I go with a forced induction with or without a FPR?
Will a chip help and should I have the O2 dyno before I order one?
Will a chip help without a new pump and injectors?

IMO, The Autometer A/F ratio gauge is a nice lightshow. I had one that did the same thing...totally useless because it gets data from the o2 sensors which can't read a wide enough range to tell you anything. I've have seen several indicate pig rich, and the engines were actually dangerously lean (mine included).

Yes the larger pump and injectors will help, and are probably long overdue. The forced induction pump cost a little more but fits in the stock location and is capable of flowing much more fuel under higher pressures. It insures that fuel flow doesn't slow down under max boost when you need it most.

If you planning to get a custom chip burned you don't need the adjustable FPR. Apten can burn the chip based on your mod list and get it close. After the chip is in, make a couple dyno pulls with a wide band o2 and e-mail them a copy of the sheet to determine if a re-burn is required. I would assume yours may take a couple re-burns to get it dialed in.

Most cars at your mod level gain around 20-25 HP just by getting a good dyno tune. I re-tune mine after each major mod, like ported blowers or FMIC. Here is a link to Brian Herron's company (Apten) that does the tuning on my car.

http://www.apten-us.com/

David
 
Sorry Dave my name is Jay. I did alot a research before i bought the nitrous, but i guess it wasn't enough. I wouldn't mind buying a forced pump, but i just put this new pump in 2 days ago...lol, so i couldn't return this one. This one has a max psi of 58, and the forced pump is 67psi. I will install the regulator, and i'll leave it at around 43psi static. In the future I will end up getting new injectors. What should I do about the A/F? Is there a guage i can get? How bout the wide band o2 sensors? What are the stock ones and where can i get wide band ones. Thanks a lot for your help dave...i've delt with you in the past and you've been a great help.
 
Jay,

Glad to help. I think your only option on the wide band o2 is going to a dyno that has one. It's very common and usually the dyno operators only charge $10 extra to use it.

Take a look at the dyno charts in the most powerful SCs section to see what the A/F ratio curve usually looks like. I'm not sure what it looks like when spraying nitrous, but I would assume it needs to be about the same if not a little richer.

David
 
gotcha..thanks a lot man. Looks like this is going to b=go back together and them i'm off to the dyno and see what this thing does.
 
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