Adjustable Fuel Pressure Regulator

Discussion in 'Fox 5.0 Mustang Tech' started by John Dirks Jr, Aug 28, 2013.

  1. When I do my intake swap I've decided to upgrade the stock regulator to an adjustable one. What is a reasonably priced unit I should look at? I don't need anything with a gauge port or anything fancy looking since engine dress up is not on my list. I just want reliable unit at a decent price.

    Which ones would you suggest? Is there anything in particular I should be looking at with regard to range of adjustability?

    My final setup will include 70mm MAF, 65MM TB with Explorer Intake, stock 19lb injectors, stock E7 heads, 1.7 full roller rockers, stock cam and full aftermarket exhaust.
  2. you don't need one with your setup. The engine is going to adjust the FP back to stock regardless of the setup- 39-41 with the line attached. If you want one anyway, I prefer the Kirban.
  3. Another vote for the Kirban
  4. Thanks for the reply.

    For the sake of discussion, I'd like to know how this works.

    First, lets not determine if my particular setup needs one or not. However, you mentioned that the engine is going to adjust the pressure to stock setting regardless of the setup. I have two questions to start with.

    1) Exactly what are the physical things that the engine (ECU I assume you mean) does to change the pressure?

    2) Are you saying that if I put an adjustable regulator and set the pressure to 35psi, the ECU is going to respond and override my manual adjustment?
  5. You're going with the stock cam and 1.7s, so your vacuum is going to be close to stock. So the vacuum on the regulator is going to be basically the same, therefore it will regulate as it would stock. The computer has nothing to do with fuel pressure regulation, it is all vacuum based.

    And I have a BBK unit and I like it. Haven't had any issues with it in 2 years, and I bought it used so there's no telling how old it is.
  6. The stock regulator is manifold-referenced. All it does is ensure that the fuel pressure remains ~39psi higher than manifold pressure so the flow rate of the injectors doesn't change. An adjustable regulator does the same thing, except you determine how much fuel pressure it supplies above manifold pressure.

    IMO, there are not many situations that warrant one. A good tune and the right injectors make them unnecessary.
    f8tlfiveo likes this.
  7. I get it. When mikestang63 said "engine will adjust", he meant manifold vacuum. I totally understand that now.

    Now, 65ShelbyClone has me a tad confused by saying that the regulator keeps the fuel pressure 39psi higher than manifold pressure. Its confusing because a naturally aspirated engine does not have manifold pressure, it has vacuum.

    Understanding that the intake manifold vacuum on a naturally aspirated engine is highest at idle, and lowest at full throttle, I can assume how the regulator reacts to those differences. You certainly need lees fuel at idle than you do at full throttle. So, my assumption is that when the regulator recognizes low vacuum (as in acceleration) it increases the pressure in order to deliver more fuel for the demand. Conversely, when it recognizes high vacuum (as in idle or coast) it decreases pressure since the demand is low.

    Do I understand this correctly now? If I'm wrong, slap me around and set me straight.
  8. Kirban. /thread.

  9. Ok.. Say you leave the stock FPR on. it is going to be at 39 -41 psi with the line on and around 34ish with the vacuum line off. So now you replace the stock FPR with an adjustable one. What are you going to set the FP to?

    The same 39 psi right with the vacuum line on. There is no reason to change it from stock.

    The only difference is the adjustable one has the provision to adjust the FPR up and down to whatever you set it to. NOT so that you SHOULD set it to anything other than factory. Once you install it, it works the same as the factory FPR. People use the adjustable FPR to overcome smaller injectors when they approach full duty cycle by increasing the FP, or if they run larger injectors by decreasing the FP but that is a band aid solution IMO. They do help when you tune a car and when you start adding a power adder.

    Now, for full disclosure I have a mild setup,- explorer top end, 70mm TB and MAF, headers, U/D, and I have a Kirban FPR- only because it came free with the other parts I bought.
  10. Kirban here too.
    In fact i got bad gas once and it screw everything up, tank, fuel pump, sender, injectors but the kirban kept ticking. Later replaced it after my tune to be safe,
    with another kirban.
  11. Kirban or oem non adjustable
  12. What if a factory pressure regulator needed replacing? In that case, it wouldn't be out of line to replace it with , say..... a Kirban adjustable, would it?

    Furthermore, consider the vehicle in question had some mods that were mild, yet carefully selected, might be able to benefit from a tad more fuel. Then, wouldn't it make sense to slap on a Kirban adjustable unit?

    Heck, you never know, the setup might even benefit by having less fuel, something that could also be accomplished with a Kirban adjustable regulator.

    You see the assumption is that one is always after more fuel. It could be the other way around.
  13. ....but, alot of that can be adjusted via tuning software too
  14. That wasn't your original question now was it. There are a lot of guys that can explain the fuel mapping and EEC adaptive properties MUCH better than I can, as to why unless you have a tune, if you start adjusting the FP manually, the computer will adjust the short term fuel table and eventually the long term fuel table back to what the computer is set to read. Now I have seen guys at the strip adjust the FPR up or down a few lbs to compensate for weather, etc., for everyone else driving the car everyday the stock setting is where it should be. You can ask the same question 100 times my friend, notice you are getting the answer. For 99.999% of the engines with mild bolt ons like the ones you have, the factory setting for FP is correct for everyday applications and the stock unit is just fine.

    But like I always say, that's why Baskin Robbins makes 31 flavors.
  15. Think in terms of absolute pressure. There is no such thing as a negative absolute pressure.

    Atmospheric pressure is approximately 29.91inHg absolute at sea level. That is, atmospheric pressure is 29.91inHg above the zero-pressure vacuum of space. The reading on a vacuum gauge is some pressure below atmospheric. Suppose your engine pulls 14inHg of vacuum at idle. 29.91 - 14 = 15.91inHg, which means the manifold absolute pressure (MAP) is actually 7.8psia.

    The fuel pressure regulator is there to maintain 39psi above whatever is in the manifold....below atmospheric pressure, above it, whatever. This ensures that the injectors' flow rate remains constant all the time.
  16. Eh. I have a full HCI and I have my regulator set within the factory 39-42 PSI range. COULD it benefit with more/less fuel? Probably, but for me it would be better suited with EEC tuning because I have a calibrated MAF and larger injectors. Someone running stock injectors, MAF, and cam really doesn't need an adjustable regulator.

    But if you want one that bad or it gives you peace of mind to have one, then get it! Don't let us on here hold you back from setting your car up the way YOU want it.
  17. And the answer is...

    The fuel pressure regulator in 5.0 Mustangs is a shunt regulator that works in parallel with the fuel injection system. The regulator bypasses fuel back to the tank to maintain a constant 39 PSI to the injector tips. A constant pressure insures that the computer will always have the same flow rate to base its calculations on.

    The 39 PSI pressure is measured at 29.92 inches of atmospheric pressure to get the proper flow rate. But the pressure inside the intake manifold may be higher or lower than the atmospheric pressure outside the intake manifold. These differences would cause the flow rate to change and mess up the computer’s air/fuel calculations.

    As the vacuum inside the intake manifold increases, the effective pressure at the injector tips increases. Conversely, as vacuum inside the manifold decreases, the effective pressure at the injector tip decreases.

    Some math to illustrate the effect:
    39 PSI at 20” of vacuum inside the manifold works out to be 49 PSI,
    since the 20 “ vacuum/2 = 10 PSI that you add to the base fuel pressure.
    That gives you 49 PSI at the injector tip.
    39 PSI at 5” of vacuum inside the manifold works out to be 41.5 PSI,
    Since 5” vacuum/2 = 2.5 PSI that you add to the base fuel pressure
    That gives you 41.5 PSI at the injector tip
    39 PSI with 10 lbs of boost inside the manifold works out to be 29 PSI.
    That gives you 29 PSI at the injector tip
    That reduces the flow rate and explains the need for higher pressures on engines with pressurized induction.

    Since intake manifold vacuum and pressure plays havoc with the pressure at the injector tips, what has to be done to get it back in the magic 39 PSI range? That’s where vacuum applied to the back side of the fuel pressure regulator comes in. Remember this: unless you have some really poorly designed or trick plumbing, vacuum is the same throughout the engine’s vacuum system.
    Apply 20” of vacuum to the back of the regulator and the 49 PSI pump pressure with 20” of vacuum at the injector tips drops to 39 PSI.
    Apply 5” of vacuum to the back of the regulator and the 41.5 PSI pump pressure with 5” of vacuum at the injector tips drops to 39 PSI.
    Here’s another side effect: apply 10 PSI boost pressure to he back of the regulator and the normal 39 PSI at the injector tips increases to 49 PSI. That overcomes the 10 PSI in the intake manifold to give you 39 PSI at the injector tips. Pretty clever of these engineers to use intake manifold vacuum and pressure that way.

    Simply stated, intake manifold vacuum adds to the effective fuel pressure at the injector tips. Apply the same vacuum to the back side of the fuel pressure regulator, and everything balances out. Add pressure to the intake manifold and the effective fuel pressure at the injector tip decreases. Apply the same pressure to the back side of the fuel pressure regulator, and everything balances out.

    Now you know why to disconnect the vacuum when making fuel pressure measurements.
    #17 jrichker, Aug 29, 2013
    Last edited: Aug 29, 2013
    NTO_GUY likes this.
  18. Guess what?

    The car had been running real good when I started this thread. I have not done anything to it but it started acting up today. It is acting like its starving for fuel. If you drive gently or accelerate very slowly it acts totally normal. The moment you try harder acceleration it falls on its face. It even pops a little bit back through the intake at times if you floor it. The second you lift off the throttle it resumes to normal behavior. These symptoms are consistent. There are no CEL's

    Also, it's beginning to run a tad on the hot side. Could an extra lean mixture be contributing to the extra engine heat? Super lean could also explain poor behavior on hard acceleration, correct? Maybe these symptoms are tied into my delayed starting issue I've been trying to track down as well.

    What do you think?
  19. If you pull the hose off the FPR does it smell like gas? If you put your finger on the FPR port does it hold any vacuum. Perhaps the diaphragm is bad. SEE? It may have known you were talking about it and now you have to change it. lol Can you hook up a FP gauge to it.

    That or the timing is too retarded which will give it similar symptoms.
  20. Thanks for the tips.

    I will check the hose for gas smell and see of the regulator holds vacuum this afternoon. I'll also pick up a pressure gauge to use for diagnostics. If the regulator appears ok, is it also possible it could be the pump?

    I seriously doubt its the timing. I recently set it to 12 BTDC. Plus, if you give it short snaps of the throttle the response is good. If the timing were retarded that bad, the initial throttle response would be quite sluggish too.

    The way its acting is, normal grandma driving it seems perfect. Or, you can gradually increase throttle and take it as high as you want on the rpm band. But, stomp down on it, it stumbles and falls on its face. Its exactly what fuel starvation under high demand would do.