Kubs

Ok it is my understanding that the vacuum line on the regulator is there so that when you punch the throttle it pulls a vacuum and opens a diaphram in the regulator to let more fuel in at that instant. This is good when the regulator is in FRONT of the fuel rail inlet. In my case I am moving the regulator to the end so I always have a constant flow of fuel through the rails. Do I still need to hook up the vacuum line?

JackDidley

You should be able to tune around it. Add a bit more fuel in AE and PE. I ran mine with the pressure maxed out for a while so the vac was there but not doing anything. Pump was putting out 76# at the gauge all the time. Only did this because, injectors were too small at the time.

leesvet

Makes absolutely NO difference where you place the regulator...cross-over tubes carry flow to both sides...the regulator just likits what the pressure build up gets to be in the whole rail system so you don;t pop o-rings. Flow has access to the whole rail...the regulatos simply dumps off the extra.
how would it moving the regulator make any difference? Its hydraulics...fluid seeks any open path.
IF the regulator is inline, it is simply allowing the set pressure to get to the injectors..fuel always flows thru the rails. The majority of the fuel, (like idle or low rpm) that is not used is returned thru the return or low pressure side of the system...there is always fuel returning since the engine cannot possibly use all of the pumps output at lower speeds...even high speed the engine cannot burn as much as pumps can produce.

You do NOT have to have the regulator connected to vac as long as your bsae pressure is enough to cover accelleration demand, sustained high rpm operation without leaning (due to a momentary drop in system pressure)(which will be a "stumble" when punching the gas, and the inj can actually handle the increase in pressure. DO not attempt to make 22 lb/hr injectors spray 30 lbs/hr by jacking up the regulator pressure sky high..that is what blows o-rings, inj seals, and causes injs to spray wet instead of an atomized mist that can mix with high velocity air flow.

If you have a regulator that will adjust to 50 to 60 psi and injectors that can actually use the fuel, then d/c the vac. It must be able to supply more fuel than the engine is using at all times...but IF your engine is not that thirsty yet, leave the vac in place so it can bump the flow as rpm jumps. This is the built-in EFI equivelent to an accellerator pump.

I've done it both ways and I can tell you that the pump is far more important than d/c the vac line. If you have a pump that can keep up, then you need the vac line to give that bump up in pressure when the pedal is slammed. There ARE very nice regulators available that will let you get 100% of the pumps gpm and pressure but they are going to cost 2-$300 or more.

Kubs

Ok, let me clarify what I meant. I understand that the fuel is constantly flowing, but having the regulator in front of the rail makes the fuel sort of "dead head" into the regulator. What is needed by the injectors is sent to the rail and the rest is returned. When you stab the throttle the injectors demand more fuel, which means the regulator has to react to open more fuel to them. This takes a slight amount of time and is the cause of the "stumble" you mention. Having the regulator after the rail means the rail gets everything, and what is not used is bled down by the regulator. Fuel is constantly flowing through the rail at all times. If you stab the throttle the injectors are already getting all they can use from the pump, no hesitation or waiting for the regulator and eliminating the stumble. My original thought is that the vacuum is no longer needed with this setup because the boost the injectors need happens automatically. I just wanted to confirm my thoughts before plumbing it that way.

As far as the equipment, I am using a Walbro 255lph pump, and and adjustable Aeromotive regulator like THIS. Actually not like that one, that IS the one I am using. So from what it sounds like I have the parts to support the fuel needs, and not hooking up the vacuum won't be an issue.

Kubs

Thanks! I will not be the one tuning it. I will be sure to mention this to the person who does.

383vett

When you punch the throttle, the vacuum DROPS which causes the diaphram to relax the pressure on the disk in the regulator allowing for increased fuel pressure.

Kubs

Ohhh. So this function should work regardless of the regulator position?

383vett

The only way I see a fuel pressure regulator working is if it placed after the injectors. This allows the fuel rail to be pressurized to provide fuel to the motor. If it is placed before the injectors, the line between the fuel pump and regulator will be pressurized and the injectors will be receiving "exhaust" fuel so to speak and will have very little pressure going to them.

whitebrow

My understanding is that the vacuum line to the regulator was to compensate for the vacuum in the intake so that there is an even differential between the fuel pressure and intake pressure. If you have the line hooked up and can rely on that differential being constant, than you know the injectors are flowing at a constant rate. Without that reference, you'd be dealing with higher flow during vacuum and lower flow at full throttle.

Kubs

On an inline style regulator yes it would work that way. Some regulators have one inlet and two outlets. One outlet goes to the rail the other is return.

You make a good point. I think I am going to hook it up.

383vett

The reason for the vacuum line is because at idle, there is less need for fuel. Instead of having a higher fuel pressure (which is needed at wot conditions) and having the computer trying hard to shorten the pulses of the injector, the lower fuel pressure will allow slightly longer injector pulses and allows the use of a injector adequate for wot applications to still idle correctly.

JackDidley

The LS cars have the regulator built into the fuel filter. No vacuum line to it. Just a return line to the tank.

jt351

This statement is totally correct, and placement of reg does not matter. a engine with vaccum is actually trying to pull full in, so lets say 15" of vacuum and 30psi is same flow as 40psi and no vacuum(WOT) IT is just easier to tune around, that is why throttle body injection above throttle plates do not use a vacuum referenced regulator. Later more modern is returnless fuel system because of emissions and had to be tuned around from factory, actually a band aid. the superior fuel system is is still return style with vacuum regulator.

Cliff Harris

The fuel pressure is inversely proportional to vacuum. High vacuum = low fuel pressure. Low vacuum = high fuel pressure. You get low vacuum at WOT, which is when the engine needs more fuel pressure. You get high vacuum at idle and cruising when the engine is using little fuel.

If you look at the locations of the regulator and the fuel lines, you'll see that the fuel feed line enters the fuel rails at the front and the regulator is in the rear and comes AFTER the fuel goes through the fuel rails. It bypasses excess fuel back to the tank.

I'm pretty sure the LT1s use an inline fuel pressure regulator, so there is no return line. I know the C5s have an inline FPR.

Orr89rocz

Small difference in pressures in the intake manifold on a n/a motor likely wont need the fuel pressure regulator to be "adjusted" by intake vacuum. You can tune around that no problem but vacuum lines are easy to hook up so its up to you. Only concern is if that regulator diaphragm breaks, fuel comes out the vacuum port. Whether you want raw fuel to dump out onto the engine or dump out thru the vacuum line into the manifold is up to you.

On a forced induction motor, you need a line to the manifold for boost referencing. because 43 psi fuel pressure trying to spray against 20 psi intake manifold pressure on a higher boost application isnt gonna cut it Boost referencing makes 20psi on the regulator to make 63psi at the rails to maintain the differential pressure of 43 psi

Kubs

Thanks for all the input guys. I ended up drilling a hole in the intake for the regulator vacuum. I figure if I end up not using it I can just cap it. better to drill it now before I put the intake on.

Chuck Tribolet

I hope you tapped that hole too.


Chuck

rodj

LT's are identical to TPI
Supply, reg and return

Kubs

coupeguy2001

I read in a fuel injection book that the reason for the increase in pressure is not for flow, but during acceleration and higher engine speed, the time it takes to get to the cylinder is shortened with less manifold vacuum, and the higher pressure assists in the injector nozzle spray pattern to help the atomization in a shorter time span. ie the faster the engine turns, more fuel needs to be atomized in a shorter time allotment, but if you are going faster, there is more fuel that needs to get delivered, and thus, might have a problem with wet flow in the port.
If the fuel spray cone was more atomized, it wouldn't take as much work to mix it with the incoming charge of air changed by increased atomization.

I hope I got it right. Haven't opened that book in a long while.