Don't ask why I'm thinking about it but I've been thinking about how a rising rate FMU works and an AFPR. Mostly because I'm trying to figure out what will exactly happen when you put a rising rate FMU behind a 1:1 AFPR. My assumption is that there will be a cumulative effect on the rising rate of FP in proportion to boost pressure (ie 1:1 AFPR with a 3:1 FMU will lead to an addtional 4 psi of FP per 1 psi of boost). Now heres my reasoning behind it.

In the case of a standard FMU (Vortech FMU in my case and what I'm basing this off of) it is normally placed on the fuel return leading back to the tank. It has a boost source on the top. The boost pressure pushes down on a disc (of varying size/area) which then in turn pushes down on a "button". This "button" really is just decreasing the area of flow from the inlet to the outlet. Say with no boost pressure the fuel can flow from the inlet the out the outlet without any restriction. When the boost pressure increases, this "button" that gets pushed down restricts the passage allowing fuel to flow from the inlet to the outlet. Since we are dealing with a fixed volume (fuel hose, rail, etc) more or less the amount of fuel can't increase so the amount of pressure exerted by the fuel trying to get out of the outlet increase. Hence a rising rate. Now the nifty thing is by the FMU using varying size discs they can manipulate the amount the "button" is pushed down. Since boost pressure is in PSI and as you increase the size of the disc there is more area for the pressure to act against thus pushing the "button" more. Sound right?

Now for the AFPR (in my case an Aeromotive 1:1 AFPR and again I'm using it to base my decisions on it). Now it should have the same mechanics as the FMU for the rising rate part of it (not really the same mechanics but the thought process is the same). But it can be adjusted for idle FP (starting FP or whatever). The screw at the top moves the starting location for I guess what you could call a "lid". The Aeromotive has the inlets in from the sides and the outlet leaving from the bottom. This "lid" IF it could be moved down completely would cover the outlet and thus would not let any fuel leave (thankfully that is not possible). Adjusting the screw up moves the lid further away from covering the outlet and thus decrease fuel pressure (easier for the fuel to get to the outlet). And adjusting the screw down will create a smaller area for the fuel to leave thus increasing the FP. Now for the rising rate the boost pressure will move the "lid" a fixed distance per PSI of boost enough to give a 1:1 rise rate. Sound right?

Now with all that it's just basically like a garden hose that you end up squeeze. You have a fixed pressure in the hose. Now you make the hose outlet smaller the pressure in the hose goes up. Since in theory thats all it does have consecutive "kinks" in the hose would make the pressure go up in all previous portions of the hose. Now it makes sense to me but I could be completely wrong or I could've missed something I'm not taking into account. So what do you guys think?

Just for reference.. here is our AIM convo about it:

Dark0ne95: it's not going to be cumlative.
Avenger0080: really? you'll have to explain that
Dark0ne95: Which one is going to close faster?
Avenger0080: most likely the FMU
Dark0ne95: Which one is last in the line?
Avenger0080: FMU
Dark0ne95: Which one do you think is going to increase the fuel pressure?
Avenger0080: they both should
Dark0ne95: What makes you think that?
Avenger0080: working in reverse low pressure from return to tank, higer pressure between FMU and FPR, even higher pressure before FPR. As the FP increase between the FMU and FPR the pressure before the FPR will increase too ... the more you close of the route in the FPR the pressure before the FPR will be even higher
Avenger0080: so both should be work in conjuection with each other
Dark0ne95: The FMU is going to close more, increasing the pressure after the 1:1. The change in the valve of the 1:1 become unimportant because it's not really moving anything anyway, or not enough through the FMU for its change to effect the fuel pressure.
Dark0ne95: It's like you've got 2 valves: Feed-------------Valve1---------Valve2------
Dark0ne95: Close valve2 some. Pressure increases from the feed all the way to valve2.
Dark0ne95: Now close vlve1 less. What happens?
Avenger0080: pressure increase before valve 1
Dark0ne95: Nothing, all the presure is still being regulated by valve2.
Dark0ne95: because valve1 is closed less than valve2.
Dark0ne95: This is how my thinking goes.
Avenger0080: it doesn't matter ... that vale 2 is closed more than vlave 1
Dark0ne95: Sure it does.
Avenger0080: it's 2 seperate pressure differentials
Dark0ne95: If valve2 is closed 3/4 of the way, and valve1 of closed half way, the fuel would still have twice the area to flow through than it does at valve2. there would be no pressure increase.
Avenger0080: lets consider the scenario at the FMU. pre FMU lets say that you have 10psi and post you have 10psi. Now as you close it will increase the pressure before and decrease the pressure after right
Avenger0080: now lets look at the 1:1. same thing ... before lets say 50psi after lets say 10psi. now since the FMU upped the pressure after to say 15psi the pressure before the FPR has to increase as well ... more pressure in the return is basically a restriction so it will up the pressure before the AFPR
Avenger0080: now if you close the valve in the FPR some too ... the pressure before the FPR HAS to go up
Dark0ne95: Dude, you're going to be running like 15psi base pressure.
Dark0ne95: you'll have so much flow goign through there, ther eis no way you'll get 4:1 out of it because the 3:1 will be the one controlling the fuel pressure.
Avenger0080: actually I can't now that I have found out ... the aeromotive goes down toe 30psi low
Dark0ne95: Ah
Dark0ne95: I still say you'll get 3:1 out of it.
Avenger0080: See now I'm not so sure ... such small interactions make such huge diffences in fuel pressure ...
Avenger0080: this might need a real world test
Dark0ne95: yeah, but the FMU will be making much larger adjustments.
Dark0ne95: There may be a smaller hole at the 1:1, but all the fuel will be abckin up off the fmu.
Avenger0080: I'm not disagreeing with you on that but all the FPR has to do is bump it up 1 more psi
Dark0ne95: I'm saying i don't think it will.
Dark0ne95: Under boost, the FMU will be going all the fuel pressure control

Because the fuel pressure will be higher in front of the FMU then the 1:1 regulator requires, it will not accumulate. The FPR is set to X psi at ambient (0psi). As the manifold pressure increases, the FPR rises the fuel pressure. So, in boost, you have X + (boost) psi. It will not clamp the pressure any higher then X + (boost). If the pressure goes up, however, the FPR will have no effect.

-=B-=

I believe that the FPR and FMU are rather different animals.

Here's a pic of a FPR:


The valve stays closed until the force of the fuel pressure pushing up becomes greater than the force of the air pressure and spring pushing down. As the valve is opened, the fuel is allowed to escape and fuel pressure drops. The system will find an equilibrium point and the fuel pressure will be maintained until a factor changes (fuel pressure supplied by pump, spring preload, or air pressure).

I believe that the FMU offers minimal flow restriction at ambient boost pressure and under vacuum. That is why it can't function as a stand alone fuel pressure regulator. It only starts to restrict flow as boost pressure rises. I believe that it closes the fuel valve a certain amount as boost pressure rises. I believe that the FMU doesn't care what the fuel pressure is. It just adds a certain restriction based on the boost pressure.

I picture the fuel routing like this. At least thats how we hooked up Kirby's system on his '95. The FPR is the stocker in the fuel rail. We had a fuel pressure gauge on the inlet of the FMU.



When driving, the fuel pressure gauge read like 10 psi under vacuum and 0 boost. I believe this indicates that the FMU provides very little restriction under these conditions. But, as soon as you broke the setting on the FPR (50 psi), the fuel pressure on the gauge rose quickly (like 8:1). So, the FMU was just adding to the base fuel pressure. The base fuel pressure could have been 40 or 60 psi and I think the FMU would have added the same additional pressure (making it a cumulative system).

But, I can't quite make sense of exactly how everything works together. I have dissassembled FPR's, but not a FMU. I'd do some testing. Go put a FPR and FMU on your car and test it. You can do this in a parking lot. You can simulate boost using an air compressor. With the car running, you can apply "boost" to the FPR and FMU. Record the boost pressure and fuel pressure at several points. Then you can easily see how they affect fuel pressure at the fuel rail when used together. I've tested a few different fuel systems like this. I used a small DC air compressor and bleed valve shown below:



Corbin

'95 ESI-T
HRC+FMIC+ETC...



Gimme fuel...Gimme fire...Gimme that which I desire

that top diagram seems off.. reverse the fuel in/fuel out tags.. the pressure should lift the plunger off the seat and not act against the diaphram.

otherwise..

ya..

I have to get pics of the adjustable 95 FPR i made up someday..

-- WheatKing

I took apart a cheepo 1:1 and it had the fuel pushing against the diaphram. Inlet was on the side; outlet on the bottom. You'll also notice that the Aeromotive has the inlet lines on the side and the outlet on the bottom:



Corbin

'95 ESI-T
HRC+FMIC+ETC...



Gimme fuel...Gimme fire...Gimme that which I desire

Well, I had one of my co-workers give me a quick lesson in fluid dynamics and it seems that my assumption is partially incorrect. Depending on the difference in size of the upstream and downstream valve (oriface). Once the difference reaches a certain point the pressure will equalize prior to the downstream valve and that pressure will be maintained in all areas prior to it. It's not a cummulative effect that I hypothesized. More like a decreasing gain as the size between the 2 orifaces increases till there is no more gain.

Damn my head hurts. :shrug

ITYS.

Well not completely. But my co-worker explained it much better :P

HEY! being stupid is very NATURAL! maybe even an herbal pill for it these days. You can't get on his case for that. I mean look, there are lots of people that come in those neat slobbering cute packages every day!