what fuel pressure to run with adjustable FPR

SCGuru92

Registered User
What is the stock fuel pressure first of all?
What pressures are you guys running with aftermarket FPRs?
I don't feel like typing all my mods, so lets just say I have basically a Stage I motor with all the normal bolt ons.

I've played with it from about 38-48 psi, but didnt notice anything. So before I screw somthing up, I figured I'd get some input

Thanks
 
The EEC-IV system is set up to work on 39 psi.
Unless youve gone to Lucas injectors you dont need to run any higher pressure.
 
i run at 42psi on 30lb injectors. I did this mostly because i had these maxed out past their limit. I was running nitrous and getting 285rwhp. 42psi seemed its best. You really dont need to change that unit unless lucas injectors are used. They dont work well at stock pressure for some reason...their best i heard is somewhere around 42psi as well. I have 42lb injectors now so running a higher pressure actually hurts my track times. I have settled on a pressure of 40.5psi. This is the ideal pressure when i race because my nitrous is on most of the time. Having a regualtor on a stock car is more of a pain then a gain. I'd sell it and keep the stock one to be honest with you. I like mine...but i have a reason to use it.
 
I have to disagree. Unless your paperwork says otherwise, those injectors are like the MSDs and are rated to flow 36lb/hr@ 43.5psi in which case the pressure should be set for 43.5psi.

Paul
 
Assuming you are not running a EEC Tuner I would recommend staying with the stock fuel pressure of 2.7 bar or 39.5 PSI as well. From your screen name it appears you have a 92 SC. If you go to a 36# injector the EEC has no way of knowing it and will still be using the pulsewidth it has programmed for a 30# injector. If you jack up fuel pressure to the rated value stated by Paul you are just making the problem worse.

Now if you are running an aftermarket MAF calibrated for the 36# injectors then that is the only time I would consider trying the MSD rating value of 43.5 PSI and even then you may end up running rich or lean since you are fooling the EEC with the MAF signal if you don't have a tuner board.
 
I've got the C&L MAF calibrated for 36#.

My main reason for starting this post was that I have a lot done to the car, but I dont feel that it is working together as best as it could. No matter what I try, I can only get into the 14.4 range at the track. It justs frustrates me a little when I see some guys with a lot less mods done to their cars running much better times. I think I just need a little fine tuning. Not sure what the best way to go about it is, besides dynoing several times. And still, I dont know as much about these cars as some of you guys, so I wouldnt know where to start.
 
My statement is based upon assumption that anyone using larger injectors would already have appropriately calibrated the MAF through the use of a new tube or different transfer function accordingly. Remember, this thread started vague.The higher pressure is necesary to allow the injectors to flow their rated value. This is also vital to proper spray patterns as well according to MSD tech support. The injectors need to be ran at the pressure they are rated at for proper performance. Regardless of the EEC adaptive learning strategy, it does not dial out the additional pressure, it compensates pulsewidth to maintain proper flow. An often overlooked aspect of running higher pressure in the EEC controlled return fuel supply car is its need to have the fuel available on demand. Sure, the computer can and will lean things out if it needs to, however, when the temperature drops 50*+ because winter is coming and oxygen density is higher per given unit of air entering the motor as a result, I would rather have the fuel there upon demand of the computer. Remember LEAN IS MEAN especially on our forced induction motors. A buddy of mine learned this the hard way in his supercharged 80s stang when he didnt spring for an AFPR with the aftermarket injectors. He calculated fuel demand off of 50lb injectors which werent delivering rated flow at stock pressure and lost a piston on the dyno before getting the chance to add fuel.

Of course, a good tune is essential for the most power from your combination, but keeping the balance of parts in check is important as well.
 
Last edited:
"Regardless of the EEC adaptive learning strategy, it does not dial out the additional pressure, it compensates pulsewidth to maintain proper flow."


Not exactly......it compensates pulse width to maintain a match in AF from actual to what the O2 sensors are "seeing" for a given LOAD. Load calculation comes from mainly the MAF signal which in this case has been screwed with since there is a different tube in the aftermarket MAF. Thus regardless if you have the right calibrated tube...without a custom chip burn OR EEC Tuner or equivalent you are essentially in never never land as far as the EEC really knowing what is going on. This has been explained a multitude of times that you are asking for trouble if you start changing things and you have no way for the EEC to "understand" what changes have been made.

I won't add the fact that this fellow is probably running an aftermarket TB with the attendant problems (at least with most BBK units I have seen) concerning the TPS sensor hole locations.
 
How much are we talking for a custom chip burn or an EEC tuner? My funds are quite diminished.
Yes I have the BBK TB. What are the problems you were talking about?
 
At no time did or am I implying that a simple tube swap is going to solve all the problems, all I am saying is running stock pressure if the injectors are rated with higher pressure is asking for trouble based upon my experience. The last line of my post agrees with what you are stating Duffy with regard to a proper tune.

Derek, the TB from BBK often exhibits improper TPS voltage readings if you do not modify your TPS to compensate for the poor quality of the BBK unit. Check your TPS voltage at idle and WOT with a digital meter to confirm you are within spec. If you plan on doing mods in the future, go with a tuner and have a file written for it by Brian Herron or Fred H. If you want to go with a chip and pay for reburns in the future, Brian can do that for you as well.

Paul
 
All of the BBK TB's I have seen have the mounting holes for the TPS sensor not drilled in the asame place as the stock TB. The effect is the TPS is rotated in orientation with repect to the shaft of the TB which has the butterfly valve connected to it. The effect is the EEC "thinks" the Throttle plate is in a different location than it actually is. I suspect they did this to "force" the EEC into Open Loop / WOT fuel tables sooner with respect to actual throttle position. I believe BBK thought there was a chance of the engine going lean as they suspected (correctly) that their TB woulds allow the engine to draw more air so they wanted to make sure you got into the enriched fuel tables programmed into the EEC before the throttle actually was "full" open. This has caused problems for some since it can cause the TPS signal when the throttle is closed to be out of the expected calibration range of the EEC and then it throws a CE light and code.

The overall effect normally is those without a EEC Tuner or custom chip run too rich at something less than WOT.

EEC Tuner board....check www.mn12performance.com site. Never had a custom chip...can't say.
 
Just a point of info....I have 50# MSD injectors in 92. It runs fine....I use the stock FPR and have an EEC Tuner, 75mm TB, ZR Intake and a host of other mods.

Another problem with AFPR's is they seem to fail all the time. Reports have been given on Kirban, Aeromotive and Holley units all having their diaphrams rupturing.

Now if you screwed with the size of injectors and the MAF why would you want to further complicate things by running an unreliable aftermarket AFPR at a pressure other than stock with no way to "tell" the EEC what you are doing?
 
Your TPS voltage should be around.98 or under one volt.

On the BBK you need to enlarge the screw holes to adjust.

Wider the throttle is open,higher the voltage.Should top out
at just over 4. volts .

If you can get your SC on a dyno ,with a o2 readout.You
can send your dyno sheet to apten to get a chip burned.
www.apten-us.com

RANDY
 
I agree with Duffy on this, its time for an EEC Tuner.
Paul there are basicaly 2 different types of injectors used in the automotive world at present.(not counting any cold start injectors) The first type is the Bosch type. They are the type used by far the most in the stock automotive world. They are what Ford,GM,Chrysler and a multitude of foriegn car makers use.
Holley,Accel and MSD injectors are also Bosch type.
Where the problem comes in is that each of these suppliers of injectors rate the flow for there injectors at a pressure that they want to use to rate them at. Its a marketing ploy more than anything else.
The Lucas injectors were first developed by the Rockchester div of GM because GM didnt want to pay any royalties to Bosch for using there design. After several years of working with the Lucas style injectors GM threw in the towell because they never could get them to work as good or trouble free as the Bosch units so they sold that part of the buisness off. I beleive Delphi is the owners and developers of Lucas injectors now if I'm not mistaken.
Point of all this is any Bosch style injector will work just fine on 39 psi fuel pressure.
He stated he has Holley injectors I am very good friends with one of there engineers that goes around to all NHRA events to help any of the big dogs in drag racing with any fuel related problems they may have during an event. He and I have had this conversation before so he pretty well lined me out on the injection scene.;)
 
I didn't know GM had there finger in on the Delphi Lucas injectors.
I may half to throw them away now.And they work so well to.
Why did you have to bring that up Mike.:D
 
Last edited:
Sorry Randy! LOL!!! But as I said GM could'nt get them to work as well so they sold them off. Delphi figured it out though because they do work much better than they used to.

Hey if there working good for you I would'nt get rid of them!
True the first ones were horrible but the new ones seem to work very well.;)
 
All above may be worth something, but if you want to be safe get it on a dyno with a wide band 02 sniffer hooked up and adjust away until you find a safe place between 12:1 & 13:1 AFR under a full pull. Not only will you be safe but you will find a safe power level....my .02
 
Back
Top