erik69&85

iTrader: (1)

Since there seems to be some misunderstanding about how the fuel pressure regulator in a fuel injected engine works (for third gens)... lets spend a minute and cover this. Remember that the fuel pressure regulator controls the opening of the bypass that sends fuel back to the tank. Like a pressure relief valve. Once the pressure reaches its setpoint, it opens to bleed off extra fuel to hold the pressure at a given setpoint.

As a little bit of background, I am degree'd and licensed mechanical engineer with 23 years of experience. Understanding this is first year engineering school stuff. Its not too hard to follow,

First, the position of the regulator diaphragm (and the attached seat that cover the bypass orifice) is determined by the relationship of two forces on the regulator. The regulator spring working against the fuel pressure plus the vacuum force on the regulator diaphragm from the engine signal. This is best stated by the following equation of forces:

Fpr = Ffr + Fvp

where:
Fpr - Force of the fuel pressure regulator spring
Ffr = Force due to the pressure in the fuel rail
Fvp = Force generated due to the vacuum signal from the engine

When the forces on each side of the equation are not in balance, the regulator seat moves to find equilibrium. We will consider a static situation, engine running at idle for this example.

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The force generated by the spring in the regulator is governed by the following equation:

Fpr = k * x

k is the spring constant which relates the displacement of the spring from neutral to the force it generates. Double the displacement, double the force. This value is based on the properties of the metal used to make the spring as well as how the spring is wound. The units are pounds/in.
x is the spring displacement from neutral. Neutral is the unassembled spring. It is neither compressed nor stretched. Most regulators have compressed springs at time of installation. Units are inches.

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The force generated by the pressure in the fuel regulator is governed by the following equation:

Ffr = Pfr * Aseat

Pfr is the pressure in the fuel rail in psig.
Aseat is the area of the pressure regulator seat that the pressure acts on in square inches. (this is the polished seat on most regulators you see)

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The force generated by the vacuum signal from the engine is governed by the following equation:

Fvp = Pvp * Adia

Pvp is the vacuum pressure in the engine in psig (convert from in Hg, 1 in Hg = 0.4912 psig)
Adia is the area of the regulator diaphragm that the pressure acts on.

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Putting all this together, the balance equation looks like this:

(k * x) = (Pfr * Aseat) + (Pvp * Adia)

If we want to look at changing the pressure in the fuel rail (Pfr), lets looks at the other items in the equation....

k - We can change how stiff the spring is in the regulator... if its stiffer, it generates more force, and it takes more pressure to open the bypass. If a spring ages and get soft due to lots of use, the k values drops, and it generates less force, so less force from the fuel rail is required to open the bypass and pressure in the fuel rail drops.

x - We can change how compressed the spring is. In adjustable fuel pressure regulators, like those from South Bay, this is exactly what happens. Turning in the screw pushes the back of the spring in and compresses it more, so it takes more force from the fuel rail to open the bypass. Fuel pressure goes up. The opposite happens when we turn the screw out, we uncompressed the spring and it generates less force against the seat and its easier for the fuel pressure to open the bypass. Fuel pressure in the rail goes down.

Aseat - if we drill open the orifice from the fuel rail to regulator seat, the fuel pressure acts on a greater surface area and generates more force, This allows lower pressure to generate the same amount of force and open the bypass. The pressure in the fuel rail drops. Not a good idea since you can't put the material back.

Pvp - If you increase the vacuum pressure from the engine, it generates more force to keep the bypass closed. If you jump on the throttle, the vacuum signal drops and the force on the diaphragm drops. The spring pressure takes over, closing the bypass and increasing the fuel pressure in the rail and to the injectors... this makes the injectors spray more fuel in the same open time which makes a richer mixture.. just what you want to accelerate the engine.

Adia - its kinda hard to change the size of the diaphragm in the regulator... so lets not mess with it. If you could make it bigger, the vacuum force would have more effect trying to open the bypass and decrease the pressure in the fuel rail

Hope this helps... just remember, fuel pressure in the fuel rail is governed by the amount of pressure generated by the fuel pump and how much we bleed out back to the tank.. Open the bypass, bleed down the pressure. Take a look below....this also explains fuel pressure control for boosted engines.

GeneralDisorder

iTrader: (1)

That's all well and good, and yes there's a ton of variables, etc. But from the perspective of the fuel injection computer, you want the effective pressure across the injector to remain constant - for each pound of boost pressure, you better have a pound of additional fuel pressure or pretty soon you will push fuel back to the tank. The computer ASSUMES that the pressure is constant. The regulator is absolutely a crude device for this, but the ideal situation is that it maintains a static pressure across the injector. Of course in practice it will not be exact because we're dealing with a crude mechanical approximation device (a spring, and diaphragm surface areas, etc).... some are better than others at properly regulating.

I understand (probably better than most here) what you are saying, but the ECU's simple programming doesn't allow for the dynamic inaccuracy of the mechanical regulator. It assumes a static pressure across the injector of 43.5 psi at all times. Regardless of vacuum or boost. The regulator is designed to provide this within reason. The regulator will not be capable of this with perfection, but I can say that my SouthBay adjustable regulator is 43 psi with the engine off, and right around 32.5 at idle. So it's pretty dang close.

Here's a link to a fuel pressure regulator manufacturer's web site detailing the operation of a 1:1 fuel pressure regulator. Ideally they should all perform this way. The whole point is to remove the variation in flow at different manifold pressures. If the engineers wanted to tune with varying pressure across the injector they would have just omitted the return line and ran the pump at 50+ psi and tuned around it (there exist cars built this way).

https://www.turbosmart.com/news/how-does-an-fpr-work/

GD

Vader

Other missing variables are the dynamic fluid flows through the regulator lines, ports, seat, and disc. The regulator was designed for an approximate range of nominal flows. When flow volume increases beyond that design point (whatever it is) the restriction of the ports, lines, seat and disc increase dynamic loss which actually "assists the spring" and skews the regulation point. Thus, installing a high volume fuel pump can negate the effectiveness of the regulator in maintaining constant or proper fuel pressure.

GeneralDisorder

iTrader: (1)

I'm running a 255 LPH turbine pump from Deatschwerks and an adjustable regulator on my TPI and have no issues with fuel pressure control.

GD

GeneralDisorder

iTrader: (1)

Here's some recent scope traces that I've marked up to indicate the fuel pressure. These are at idle with manifold vacuum of 9.9 psi (20.1 in/HG manifold vacuum).




Now - 42.3 minus 9.9 is actually 32.4..... there is likely a bit of measurement error and possibly some vacuum loss through the supply hose.... not to mention all those other variables pointed out above having to do with mechanical inaccuracies of the crude spring/diaphragm pressure regulator. But it's REAL close to being fuel pressure minus manifold pressure as I've previously stated. This IS what the ECU expects. If you have radically different numbers than this (after doing your own math with fuel pressure and manifold vacuum) you have some kind of issue and should look into why.

GD

erik69&85

iTrader: (1)

There certainly are many things that affect the fuel pressure. We just want to be sure not to mislead others who have less of an understanding that its simply about vacuum pressure and this directly subtracts fuel pressure. Its just not the case. But it is interesting that the numbers happen to be close. I agree that the ECM thinks it has a steady pressure, that exactly why increasing the pressure helps increase the mixture. And yep, they are relatively crude devices but very effective in what they do. My LB9 engines run pretty steady with fuel pressure. With all brand new components... 45 psig with no vac and 37 psig with vac... right on the expected 8-10 psig drop.

Boosted engines are whole other story.... something i don't get into and is not stock third gen so I was not going to go into it here.

With high volume pumps, (with any pump)... if there are restrictions, the pump "rides up" on its pump curve. All pumps have a performance curve that relates flow to head (pressure in engineering terms). As it rides up the curve, flow is changed into pressure. If a system naturally has restrictions, it starts higher up on the curve and moves less flow. High flow pumps can move more flow against a stock restriction, but they will always operate somewhere on their curve. In engineering terms, its where the pump curve crosses the system curve. A high flow pump may or may not generate more pressure. It is entirely dependent on the pump impeller design. Some have a flat curve and will not generate much pressure, regardless of flow.... others are steeper and generate LOTS of pressure... which doesn't do us any good in this situation. As Vader points out, high flow creates a higher pressure drop through the system which drops pressure between the pump and the rail and if its high enough, the regulator may not have much to do.... the relationship between flow and pressure loss in the system is the system curve.....

A generic set of pump curves...in this case, they have relatively similar curve profiles. A system curve would start in the lower left and curve to the upper right...

QwkTrip

iTrader: (22)

Thumbs up to Erik, That's a good post right there.

GeneralDisorder

iTrader: (1)

It is good stuff. Useful to know and keep in the back if your head at all times - especially when doing diagnostics. But the ECU needs to be satisfied and for that we want the most stable pressure that matches what the ECU desires - which is 43.5 psig across the injector. The way you achieve this is subtracting manifold pressure from fuel pressure. You test with accurate equipment and if these conditions aren't met or close to being met then further investigation into the cause is warranted. Because at the end of the day, the ECU must be appeased or no one will be happy.

GD

QwkTrip

iTrader: (22)

But nobody really cares, GD, because that's not what this thread is about. What you're doing is bleeding arguments from another thread into this thread and messing up this one. All the guy did was make a post explaining how the fuel regulator works, and that's fine!

GeneralDisorder

iTrader: (1)

Yes but he provided only theory. Which is interesting and good to understand but also generic. I was providing Third Gen specific information that relates his theory to reality and what it all means for the layperson third gen owner.

GD

QwkTrip

iTrader: (22)

That's fine because it's HIS DAMN THREAD!

You're not as altruistic as you claim. Just argumentative in my opinion.

QwkTrip

iTrader: (22)

That was too harsh, I'm sorry. All I'm saying is give people space to talk. Not everything needs a GD or QwkTrip follow up.

erik69&85

iTrader: (1)

Sorry....physically, saying you subtract the vacuum pressure from the fuel pressure to get the new fuel pressure is like saying I have 5 gallons less fuel because I drove 100 miles.....yes they are related, but there are many factors... It is NOT a direct relationship... What you are saying is NOT technically correct. That math is not correct.

This isn't theory, it's application...

Ah, the internet.....

Chases away all reason...

​(Here is why long breaks from this site are needed....just trying to help with my 18 years of experience on three third gens....time to go again.)

GeneralDisorder

iTrader: (1)

Probably not. But the underlying theme seemed to be to draw into question the "rule of thumb" about fuel pressure vs. manifold pressure from another thread and I felt it necessary to counter that with some actual data acquired from my third gen. Us mechanic's and tuners understand that there are underlying engineering principles attached to these devices that may not match our "general rules" but in the real world we have to use some approximations because we (our customers) don't have the budget to bankroll an engineering study of their fuel system.

GD

GeneralDisorder

iTrader: (1)

No one is going to bankroll an engineering study of the fuel system. In the real world we get it close and we send it. We use approximations just like in the Classical Mechanics you are using. To you our methods are not close enough - to a scientist YOUR methods aren't close enough because you didn't take into account the quantum mechanics or the special relativity or whatever. And to a theoretical physicist we're all wrong because we didn't account for dimensional shift or string theory or whatever the new flavor happens to be now.

Yeah - my math isn't right. Totally. Neither is your's..... besides the fact that you haven't got any numbers to actually *do* the math with. It's all formuli that we haven't got the data to input into for even solving any of it. Mostly because it's unknown or unpublished or considered proprietary. And the biggest problem of all - NO ONE is going to pay to find it out.

So what exactly do you suggest we do with this knowledge? Throw out all our approximations (that work, mind you), toss our hands in the air and go home till someone funds an engineering study of proprietary GM fuel pressure regulators designed in the 1980's?

It's important to remember these principles. But that doesn't make it actionable. And besides - most of these 30 year old piles have bigger problems than the unknown engineering characteristics of their FPR.

GD

ploegi

If the pressures are correct when doing my testing, I really don't care what else is happening. There are specs listed in the service manual for static pressure, idle pressure, and pressure vs. manifold vacuum. If the numbers are close, that's good enough. If they aren't, it's time to go looking for WHY. The answer is usually quite simple, blown diaphragm, dirty filter, or blocked vacuum line. (or vacuum leak, or anything similar.) All the way up to a failed/failing pump. Fuel system issues are generally easy to fix. (if you consider changing a pump, "easy"...... )

In all reality, the ECM can tolerate *some* variations from what pressures *should* be. Being it's a feedback system, (O2 sensors, and all that) when in close loop, the ECM will attempt to compensate for low fuel pressure, and if it can't, it will set a code. (Ok, it *should* set a code.....) ALL of these systems are crude by todays standards..... But, they WORKED. And reasonably well, for the time they were produced.

Knowing how the system is supposed to work is indeed good information to have. Greatly appreciated.