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fuel pressure regulators

PostPosted: October 31st, 2008, 10:06 pm
by grumpyvette
Volume vs. Pressure

FUEL Volume and FUEL pressure are two completely different things when you talk about fuel,supplied to an engine, but they must work together to give you the proper air/fuel mixture delivery. Both carburetors and EFI systems require the proper fuel volume to make the desired horsepower, but the pressure differences between the two are quite different. A carburetor setup requires very low fuel pressure—somewhere between 5 and 6 psi, while most EFI setups, require 40 psi too as much as 60 psi on the high side limit. This means the fuel pump you select needs are entirely different, for each system and regulating pressure is handled by different components as well. youll need too select a Regulator manufacturer who makes regulators specifically for each application.
A return-style fuel system keeps the flow of fuel through the entire system at a constant,rate, keeping the fuel cooler. Constant circulation of cooler fuel will also result in less tendency to get into conditions favoring vapor lock , than a dead-head–style system, as fuel sits in the lines and carburetor absorbing heat from the engine compartment.
Ive consistently had far better results with the bye-pass style regulators ,for several reasons, the non-bye-pass regulators allow more heat to build up in the fuel and tend to be slightly more prone to get the fuel temp up into detonation range, fuel thats cool absorbs some combustion chamber heat lower burn temps very slightly, you NEED a fuel pressure regulator with a BYE-PASS CIRCUIT from the regulator at the carburetors location back to the tank,If the pump is capable of more than 7PSI you should run a regulator regardless of the engine, simply because most carburetors needle and seat valves can not control flow over 7 psi and many can,t handle even 6 psi
The engine does not care how much pressure you push to the carburetor, but the have the carburetor will have a needle & seat valve that does, because it must maintain a near constant float level in the fuel bowls to have a consistent fuel/air ratio.
All the engine cares about is that the carburetor feeds it a controlled fuel air mix ratio between about 12.7:1-15:1 & you need to maintain enough pressure & volume to keep the carburetor fuel bowls full under all conditions, WITH OUT FLOODING or fuel starvation.
dead head regulators will frequently NOT maintain a constant pressure , it tends to surge and fall, and running a electric pump without using fuel that flows constantly thru it will USUALLY cause the pump to heat up and LOOSE some efficiency, in fact they commonly loose efficiency or fail if heated up, trying to pump the same fuel constantly, the flow thru a bye-pass regulator with its separate fuel return line to the tank allows the fuel to circulate and pull heat from the pump,the better quality, return style regulators are vastly more effective
WHEN TESTING<be sure the fuel pressure gauge reads correctly by comparing it to a second test gauge, these fuel pressure gauges are frequently defective

The minimum fuel line size (from the pump to the regulator) is dependent on the horsepower output of the engine (and/or Nitrous system) regardless of the size of the pump. fuel pressure regulators are normally placed as close to the fuel rail or carburetor feed inlet port as possible to maintain most of the fuel feed lines under higher pressure to reduce vapor-lock, high g -force fuel starvation and to reduce the chance of air entering the carburetor ,or injectors and keep a constant flow of cool fuel reaching the injector's or carburetor, because theres a constant flow of new fresh fuel being forced up to and some bye-passed thru the system, and fed to the engine
keep in mind TPI fuel pressure regulators maintain about 40-45 psi while carburetors require about 4.5-6.5 psi MAX, but in most cases the fuel pressure at the carb inlet port NEEDS to read 4.5-5 psi MAX and you NEED to set the float levels per the carburetor manufacturers instructions, having the pressure at 6.5 psi will in many cases cause the needle & seat valve to constantly leak excess fuel, into the carburetor while the engine runs
if your running a return style fuel pressure regulator it depends on the instructions that come with it and the number of ports,its usually mounted AFTER the two carb inlet fuel feeds
the pump feeds the fuel log, the fuel log feeds both carb inlets and the regulator mounted on the far end of the fuel log from the fuel feed bleeds off excess pressure to the return line to the the tank.the problem with all decent quality fuel line supply component parts is they cost more money, than the cheap low quality parts and generally take up more room and take a bit more time and thought to instal correctly
the regulator linked above can be adapted easily from carb to EFI use
Image ... d_sbs_sg_1
[ [color:red]BASIC CAR FUEL SYSTEM DESIGN[/color]
but on some models its mounted just before the fuel log on the port labeled "CARB" and the two other ports are labeled "feed" for the pump and "RETURN", for the return line



if your, looking at the diagram,and asking what pressure the fuel rail feeding the carburetor is pressurized too its obviously controlled by the fuel pressure regulator just beyond it in the diagram, which is usually located near the carburetor and fuel rail, to maintain the indicated 5psi, beyond that is the return line feeding back to the fuel tank and that should be close to zero, the main feed line from pump to the first fuel pressure regulator which ideally is located on the inner front fender or firewall maintains the 8psi-12psi the fuel pump provides , keep in mind a fuel pressure regulator can only control the pressure between it and its pressurized feed source, by bleeding off pressure above the peak its set for, it has zero control past it, it only controls pressure between it and its pressure feed source ,in that diagram the first fuel pressure regulator is NOT mandatory in some applications, its the use of the secondary nitrous feed that makes it useful in the depicted application
IF you have a PLENUM OR RUNNER FLOODED WITH FUEL, ... gnosis.pdf
ok logically if your flooding fuel into the intake, its related to injectors leaking,or the fuel pressure regulator leaking or a vacuum line sucking fuel from some place like the fuel return line simply because thats how fuel can enter the plenum.
if you block the fuel return line to the tank from the fuel pressure regulator, and remove the fuel pressure regulator vacuum line temporarily, you should see the fuel pressure bump up to 40-47 psi , if not its most likely the injector(s) leaking, or fuel being sucked into a vacuum line, because your effectively eliminating a defective fuel pressure diaphragm in the regulator.
as always its a isolate and test procedure




viewtopic.php?f=32&t=596 ... _pressure/



viewtopic.php?f=55&t=1939 ... _pressure/


the manual fuel pump is working in direct relation to the engines rpm levels, an electric fuel pump is working against the rapid changes in inertia as the car accelerates and brakes, if you think shifting gears at high rpms won,t alter the fuel pressure curve if you don,t have a bye-pass style fuel pressure regulator and a fuel return like in use you've never watched a fuel pressure gauge on a car without those components

ideally all lines are 3/8" minimum but 1/2" id is far better
the electric fuel pump would ideally be mounted lower than the tank and as far back as practicable INSIDE the frame rails for crash protection, with a hot lead to the electric pump that only supplies electric power ,if you have oil pressure and the ignition keys in the on position, as electric pumps PUSH fuel,far more efficiently than they PULL fuel

BTW alcohol in fuel tends to cause aluminum to oxidize over time
as you don,t want the electric pump running if the engines NOT running.
the fuel filter should be mounted for easy access,as you should change it frequently.
the regulator should be mounted as close to the carb as easy access allows, with the return line back to the tank, ideally larger than the feed line for zero resistance to fuel flow
keep in mind the by-pass style regulators work by allowing all fuel flowing above a certain threshold pressure you set the regulator too,to return to the tank, usually that's set at 4.5-6 psi for carbs[/b] [/i]

keep in mind that the pressure sensor only makes the electrical connection once theres 5 psi of oil pressure in the blocks oil passages
"Ok G.V., need your input.
I intend to wire up 1 electric pump through an oil pressure switch (GM thought of it first that's how I got the idea). If the car sits dormant for a few weeks and the fuel evaporates in the carb the engine could be hard to start. Granted, you should be able to crank 5 psi long enough to fill the carb and start the engine but this adds wear and tear to the starter. Which of the following solutions would you recommend if any?
(1) Bypass the fuel pump circuit and power it through the start circuit so the pump powers up during engine cranking. This would need a diode so the start circuit is not back fed through the fuel pump circuit after engine start up which complicates the circuit.
(2) Wire in a toggle switch that powers the pump before cranking. Once engine starts, kill the toggle and allow the oil pressure circuit take over. The switch positions would be marked START (hot)and RUN (open). In a (heaven forbid) severe collision, the toggle could be activated (unlikely but possible) and defeat the safety built in to the oil pressure power up circuit.
What do you think?"

I would NEVER use a manual toggle switch,on an electric fuel pump they have a tendency to get thrown into the wrong position at times, causing all kinds of potential problems ranging from flooded carbs,engine stalling, dead battery's, burned out pumps etc.

if you want an over ride switch for the fuel pressure you'll want a BUTTON (normally open contacts)that connects the circuit only while the spring loaded button is MANUALLY HELD in the depressed position, similar to the older cars starter circuit buttons, thus the circuit works with or without the button depressed if the engine has oil pressure,and you have the OPTION to pre-prime the carb by MANUALLY depressing and holding the over ride button, momentarily ,sending the electricity to the electric pump to pre -prime the carb. but the fuel,system still works either way

THE DEAD HEAD STYLE REGULATOR...frequently not to be trusted
works with a spring on a valve that allows the valve to open once the DIFFERENCE IN PRESSURE between the sides of the regulator valves fuel lines has changed
think of it as a door that usually has 7-10psi on the feed side and you want lets assume 5.5 psi at the carburetor feed,
youll need to understand that the dead head regulator works on the DIFFERENCE in pressure between the feed and use sides so having 10psi on the feed and 7psi on the use side is almost exactly the same as having 12psi on the feed and 9psi on the use side to that style regulator, it uses a spring and residual fuel pressure to limit flow until the difference in pressure exceeds a set value.
as the fuel pump fills the line it eventually (fractions of a second )reaches the point where theres a volume of fuel past the valve with enough pressure to allow, BOTH the SPRING in the regulators valve and the fuel pressure past the valve to close the valve.... until the fuel pressure past the regulator is reduced to the point that the SPRING and the remaining fuel pressure/volume beyond the valve can not hold the valve closed and the valve is force open and held open until, that difference in pressure is restored. now lets launch the car hard, the pump that had maintained 8-10 psi to the regulator, 5.5 psi past the valve and the spring in the regulator is now fighting the fuel in the line feeding the regulators inertia, and the sudden drop in pressure as the throttle drops full open in the carb,what the pump sees is the full 8-10 psi or MORE the regulator sees a sudden drop off to near zero and it opens wide, if the fuel pumps able too it tends to flood the fuel bowl for a second then the valve slams shut, until the pressure drops off as you hit each gear the cycle repeats, the result is a surge in pressure and a rapid drop off in volume then a rapid flood of fuel that rapidly cycles as you go down the track
if you had a accurate fuel pressure sensor at the carb you'll see a rapidly cycling pressure/flow
if some crud gets stuck in the valve it cant close and your carb FLOODS OUT, because it must fully close every few fractions of a second to work correctly


because fuel flows strait thru at all times but as soon as resistance to flow exceeds the set pressure level the adjustable spring seat is seat at, the relief valve to the return line is forced open and all excess pressure and flow exits into the return line maintaining the set pressure but preventing any further increase, but never interrupting fuel flow , one other advantage of the return style fuel pressure regulator is its generally going to provide a lower fuel pressure temp being delivered to the fuel rails, or carburetor.
your ideally routing your fuel line well away from your engine and exhaust so your in theory not dealing with fuel line temps at any point much above 200F.
keep in mind that most engine compartment fuel lines are metal except for the last 12"-18" of flex tube, engine compartment temps commonly run in the 160F-210F range but if you correctly plumbed your fuel line routing and used a return style fuel pressure regulator the constantly changing flow of fresh fuel thru the fuel lines will absorb and transport a good deal of that absorbed engine heat out of the fuel lines before it reaches the carburetor or inspector fuel rails..the rubber or synthetic fuel line is a poor conductor of heat and there are insulated reflective covers available.

ENGINE HEAT AND THE RESTRICTIVE FLOW are both good reasons to avid using fuel filters like this one pictured below in the engine compartment
braided stainless covered flex line rated at 300 psi and 300f is a good idea, adding a heat reflective tube cover also helps reduce fuel temps
2 fuel pressure diaphragm
3 fuel flow chamber and pressure route chamber
4 boost reference connection
5 pressure adjustment
8 return line valve and exit

in the diagram fuel pressure is supplied by the fuel pump and enters the port on the upper left it fills the area ABOVE the DIAPHRAGM in area 3 , as the pressure rises it eventually reaches the point where the flexible DIAPHRAGM 2 forces the valve 8 lower and away from its seat ,allowing fuel to flow thru the return line back to the tank, until the pressure above the diaphragm drops allowing the seat spring to close the valve, this effectively causes the return line to constantly get a very rapid flow of fuel as the pressure above the diaphragm causes the valve to the return line to keep bleeding off any pressure above the springs resistance, this insures the pressure stays very consistent at the carb entrance port.
the boost or plenum vacuum pressure line 4 from the plenum adds or subtracts resistance to the diaphragm movement to open the return line as boost pressure rises or vacuum increases, thus fuel pressure stays consistent with air pressure the carb sees, on some versions, but the adjustable spring pressure makes it possible to set the minimum fuel pressure thats being maintained to feed the injector fuel rail or carb feed input port, as long as the fuel pump can at lease maintain or exceed that pressure

the far superior, bye-pass style regulator functions in a totally different manor
assuming the same set-up but you replace the regulator with a bye-pass style regulator, the bye-pass regulator works by opening a valve too a much lower pressure path for the fuel to return to the tank,,the open fuel return line. anytime the pressure exceeds the 4.0-5.5 psi,you've set it to, so the fuel line to the carb can only see a max at that 4.0-5.5 psi. now the pumps sitting there potentially supplying at 8-10psi just like before, but it can never exceed 4.0-5.5 psi because the bye pass regulator bleeds of any and all excess flow volume,the pump supplies. but lets look at your launch, if the pressure drops to 6, or 7 psi nothing changes at the carb, if it increases to 10 or 12 psi, nothing changes at the carb,if it drops to to 4.0-5.5 psi that you set the regulator bye-pass circuit too or less the valve to the bye pass line will close , but the fuel route to the carb inlet remains consistently open, but that excessive drop in pressures seldom a problem, it the sudden changes in pressure and over pressures that happen when you suddenly change the fuel flow required or the (G)loads on the system that potentially screw things up,the bye-pass regulator style regulator isolates the carb and maintains the desired 4.0-5.5 psi to the carb FAR MORE CONSISTENTLY.
this style regulator design depends on a fuel return line size large enough that there will be no significant resistance to fuel flow past the regulator, in route back to the fuel tank, because any pressure in the return line tends to reduce the regulators accuracy.
now lets assume the spring gets a bit weak over time or the adjustment gets set at 4 psi in error, with the bye-pass style you'll probably never notice ,if you had a accurate fuel pressure sensor at the carb you'll see a rock steady pressure/flow, at the 4psi.
should some crud get stuck in the valve and it can,t close 100% ,NOT MUCH HAPPENS, because its normally OPEN not closed,
if you check you'll see MOST EFI systems are BYE-PASS regulated designs also due to control and reliability issues
[color=#00BF00]return style typically have 3-4 ports[/color]

but on the dead head the cycle just gets about 20% more erratic and more frequent in the cycles, further weakening the spring over time

read ... a-4309.pdf

btw your fuel pump tends to run under less stress and run cooler with a bye-pass style regulator also ... -13301.pdf ... ter-NN.pdf ... 0036-1.pdf

most people don,t understand that the return fuel line to the tank will ideally have very low restriction to flow for the fuel pressure regulator to function correctly, if theres a restriction to flow it reduces the bye pass fuel pressure regulators accuracy and consistency.
most guys use a smaller diam. return line thinking that theres less fuel flow due to the engine use of fuel, that's not generally 100% accurate because if the fuel pressure is to remain consistent at a set pressure & volume the fuel pump must produce more fuel pressure and flow volume even at peak rpms than the engine can burn and even under high (G) loads, meaning that the return line needs to be fairly large and unrestricted to reduce the resistance, to flow in the upper rpm ranges with manual pumps that work in relation to engine rpms and all the time with electric pumps


Dead heads require a bigger rated pump than return style, hp for hp. They don't call them dead heads for nothing One source recommends -6 or 3/8 lines up to 375 hp, 550 with -8. Make sure the rest of your system is up to the task.Running a return style regulator is a pain, but worth the effort. Your pump will thank you by only working hard when it needs to, your pressure is stable, fuel cooler, and you can concentrate on the next problem. John

Assume a BSFC of 0.55 and gasoline at 6.25 lbs/gallon:

hp x 0.55 = pounds of fuel burned per hour


600hp x .55=330lbs
330lb /6.25lbs per gallon=52.8 gallons an hour minimum fuel used

A LINK WORTH READING ... _pressure/

a fuel filter with an easily replaceable filter element, a clear transparent drain, remotely mounted low and near the fuel tank,for easy access and maintenance helps fuel system durability

Re: fuel pressure regulators

PostPosted: November 3rd, 2008, 6:02 pm
by grumpyvette
thats a good question for this forum,
Ok Ive got one and have tested several of them.

viewtopic.php?f=55&t=1030&p=1912&hilit=fittings#p1912 ... index.html ... index.html


heres my take on them, short answer, they are a big help but a P.I.T.A. to set up and use if your not going to semi permenantly install them on your car.

IVE come to use reading spark plug condition,

use of a good timing light and vacuum gauge , a fuel pressure gauge, to verify your fuel pressure under load, and use of a GOOD HIGH TEMP INFARED THERMOMETER (THIS ONE)

as a very quick to use and accurate set of tools. :thumbsup:


well your main concern when tuning an engine is to keep the all the cylinders running aproximately the same ratio and at about 12.8:1 for max power up to about 14.7:1 for low emmissions and good mileage, AND WHILE A A/F GAUGE IS A GREAT ASSET, ITS EASY TO DAMAGE, AND ITS BEST INSTALLED IN YOUR CAR , NOT USED AS A SHOP TOOL.

so whats the advantage/disadvantages

a fuel air meter uses a o2 sensor, if you place it in the header collector it gives an AVERAGE of all the cylinders on that cylinder head,If theres an (X) installed close to the dual collectors reversion pulses can occasionally even give data from the other side of the engine, so in theory and in practice you can have two cylinders run lean and two rich and the AVERAGE tends to look RICH to the O2 sensor as it SEES unburnt fuel, if you place it in the individual primary header tubes you either need eight O2 sensors (VERY EXPENSIVE, and keeping the wires from burning or grounding outs a TOTAL P.I.T.A......IF you don,t succeed you destroy the O2 sensor and need to replace it.) or you need to be constantly swapping very hot and fragile O2 sensors and bung plugs constantly, but with the IR thermometer you can almost instantly see which cylinders are running hotter or cooler and adjust the jets or look for vacuum leaks, or other CAUSED for the TEMP DIFFERANCE, ETC, far faster too get all the cylinders running at approximately the same temp, indicating the same fuel air ratio, youll be amazed at how close the temp follows the fuel/air ratio, and you can confirm it with plug condition and the other test equipment. run any cylinder too lean and detonation can break rings or melt pistons, run it too rich and you can wash the oil off the cylinder walls and ruin rings/scuff pistons, you need to verify the fuel flow rates and pressure at the carb, or fuel rails the injectors are fed from,and you need to know whats going on in EACH CYLINDER not the AVERAGE of all cylinders.

SO, if your going to install a decent wide band fuel air ratio meter on your car thats fine, its going to be an asset to your tunning skills, if you install the indicator/gauge inside the car and weld in a couple extra bungs in the collectors for tunning and wide band O2 sensors which are a big help, but you will quickly find that its a P.I.T.A. to use it for tune ups on all your buddies cars with the welding collector bungs and installing plugs and O2 sensors while the IR thermometer route is fast and very simple and you can confirm with oplug reading the condition of the engine.

YEAH! theres meters that you can stick in a tail pipe, but they read THE AVERAGE, not the individual cylinders ,
think about AVERAGEs
AS my old physics proffesor once said,
IF, I pour molten lead in your front slacks pockets and pack your butt in solid with DRY ICE,.... ON AVERAGE your comfortable:D


bits of useful info on these

IVE generally found no problem with any corvette LT1 or TPI efi if it will quickly build a minimum of 38-40PSI on start up and MAINTAIN at LEAST 38-40 psi for a MINIMUM of 10 minutes after the engines turned off, 40-42 lbs is what the factory tests want and expect so if your getting 38-40psi your pressure readings indicate a normal, perhaps even better than average pressure reading, and more than expected pressure retention,
IF you suspect a TPI has a defective injector that's leaking or not flowing fuel,or partly clogged...You can also test this, to a degree, with a fuel pressure gauge
you can install a fuel pressure gauge on the fuel rail, and turn on the ignition key then watch the gauge, it should jump to 37-42 psi then remain fairly steady for at least 5 minutes before gradually loosing pressure.
If you can - and this is a pain in the butt to do, temporarily- remove the injectors. Leave them on the rail, but remove the rail. from the intake itself, This requires removing the majority of the TPI (P.I.T.A. but required).

Remove the rail, but leave it connected to the fuel lines.

Next up, put a strip of cardboard under each bank. Then, have a buddy turn the ignition key on but not try to start the engine , you should hear the fuel pump run for 3 seconds while you look to see what happens. You should get an immediate spray of fuel.(all should be about equal and no injector remains dripping fuel) now have your buddy Turn off the key, while you watch closely to see which injectors are leaking.
in many or most cases engine dieseling after the ignition is turned of is caused by a combination of a bit too rich of a fuel/air mix being drawn in and a bit too much heat in the combustion chambers this is common when a holley carbs transfer slot is badly adjusted to expose too much slot at idle or carbs with the float level set too high, or a fuel pressure regulator pressure set too high or a blocked return line on the fuel pressure regulator, or at times indicates a vacuum leak


BTW on carburetor equipped engines 5.0psi--5.5 psi at the carb inlet port is about ideal

Re: fuel pressure regulators

PostPosted: March 14th, 2009, 12:42 pm
by grumpyvette
if your running a carb, you need to have a correctly set up fuel system, one of the more common complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make float adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE that's located before the pump intake point



these cheap ones, that are dead head style seldom work well

much better

Ive never seen a dead head style fuel pressure regulator keep consistent pressure levels.
you need to think thru your fuel supply system, line size etc. while in an ideal world you run a similar size to the pressure feed line or a larger diam. line as a return line back to the tank so theres nearly zero flow resistance,you don,t need to run the return line back to the fuel tank, on a street car if your not all that familiar with setting up a fuel system with an electric fuel pump that didn,t come from the factory with a return line PROVIDED your under about 400hop and your feed line is a 3/8" line, its not as good, or effective to jury rig it but you sure won,t be the first guy to do it either, but it can be run to a TEE Imagein the line on the feed side of the electric pump

Re: fuel pressure regulators

PostPosted: March 14th, 2009, 12:42 pm
by grumpyvette
if your running a carb, you need to have a correctly set up fuel system, one of the more common complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make float adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE thats located before the pump intake point


these cheap ones, that are dead head style seldom work well

much better


High-flow for your fuel system.

When using high-pressure fuel pumps, a fuel pressure regulator must be installed to prevent carburetor flooding. These Mallory fuel pressure regulators feature housings fully machined from 6061-T6 billet aluminum, and specially designed high-flow fuel passages. These features, combined with a quick-acting diaphragm assembly, make them extremely efficient. A mounting bracket is provided to allow quick and easy mounting.

Re: fuel pressure regulators

PostPosted: March 14th, 2009, 12:42 pm
by grumpyvette
if your running a carb, you need to have a corretly set up fuel system, one of the more comon complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make floart adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE thats located before the fuel pump intake point (NOT NEARLY IDEAL, BUT FUNCTIONAL)
READ THRU THIS AGAIN ... g-140.html


these cheap fuel pressure regulators with only an in and out port , that are dead head style seldom work well, IN FACT IVE RARELY SEEN THEM WORK AT ALL!

much better





In the fuel injector sizing, always use a safety margin between 15-20%. , if the engine requires a 40 lb injector youll want a slightly higher rated injector sizem remember carbs generally operate at about 5 psi, fuel injection runs in the 37psi-80 psi range depending on the application.

Fuel Hoses & Routing
Even with proper injector and fuel pump sizing, a fuel system will not flow adequately unless the hoses that deliver the fuel to the fuel rail are of sufficient size and are routed properly. On systems that use the PNP version of the AEM EMS, there is no need to replace the fuel delivery hoses unless the engine is heavily modified.

NEVER route fuel hoses through the interior of a car. Put bluntly, this is a dangerous thing to do. Whenever possible, use a delivery tube to make the connection from the pump discharge to the filter in the front of the car. The lines should be rated to withstand at least twice the maximum pressure of the EFI system.

Using the above parameters of our sample engine with moderate boost, we expect to see pressures in the 65-70 psi range. This will require a line with at least 140-psi rating (most AN hoses exceed this by a large margin). When routing fuel lines, it is imperative that they are protected from road hazards and the exhaust system. The fuel line should NEVER be routed near battery cables. Use clamps to secure AN hose every 15 inches, or 24 inches if a rigid tube is used.

The following table will help you determine which hose size is correct for your application: These sizes are based on a nominal fuel pressure of 40 psi.

Fuel Delivery Hose Sizes
Gasoline Powered Engines
Up to 499 HP .344” hose -6AN
500 - 799 HP .437” hose -8 AN
900 – 1100 HP .562” hose -10 AN

Methanol Engines
Up to 499 HP .437” hose -8 AN
500 - 799 HP .562” hose -10 AN
900 – 1100 HP .687” hose -12 AN

Re: fuel pressure regulators

PostPosted: March 21st, 2009, 6:46 pm
by grumpyvette ... g-140.html

If you don,t have an adjustable fuel pressure regulator on your fuel injection or carb equiped performance, engine.....
why not stop guessing and install a fuel pressure gauge to monitor whats really going on under the hood,and install an adjustable return style fuel pressure regulator with a return line to the tank, so YOU can control EXACTLY what pressure levels the carb input port sees!
being able to control, and verify exactly the pressure and volume of fuel at the carb, or injector fuel rail eliminates most of the guess work.
as an example
JACK recently stopped by to say his reasonably new carb would not maintain the carb float settings all the time, a quick check with a fuel pressure gauge showed he had 7 psi at the carb ,feed line, and that was overloading the carbs needle seat assembly most of the time, adding a return style regulator set at 5 psi cured is intermittent miss and occasional fouled spark plugs he had been putting up with for almost a year, it never occurred to JACK that the problem started to get worse after he upgraded his fuel pump, simply because the problem was happening with the old fuel pump, just not as frequently..

YES IN MOST CASES A CARB APPLICATION CAN RUN WITH NO RETURN LINE , OR A FUEL PRESSURE REGULATOR INSTALLED, BUT YOU WON,T TEND TO GET CONSISTENT FUEL FLOW VOLUME AND PRESSURE AT THE CARB WITHOUT A RETURN LINE STYLE FUEL PRESSURE REGULATOR, and you would be amazed at the number of tuning issues that are avoided with a decent return style fuel pressure regulator installed in a fuel supply line system , like inconsistent float levels, seemingly non-working fuel metering at idle, and high rpm lean or rich conditions.The manufacturers would NEVER spend the money on fuel return lines if it was not almost mandatory to getting the car to run consistently and pass emission testing under some conditions.
its kind of like carrying a spare tire , a jack in the trunk,or wearing a bullet proof vest, 90% or more of the time you could get bye without them, but under some conditions not having them becomes a HUGE problem

Re: fuel pressure regulators

PostPosted: April 2nd, 2011, 8:21 am
by grumpyvette
On the TPI and LT1 corvettes, camaros, etc. theres two fairly common fuel pressure regulator problems, when the fuel pressure regulator starts causing problems, the first symptom is hard starts because the fuel pressure never gets high enough to supply full fuel pressure, the second failure results from a leaking diaphragm and you get fuel leaking in the vacuum lines
that's not un-common, Ive seen several corvettes with both TPI and the LT1 engines with fuel pressure regulators that would not maintain the standard 37-42 psi but that didn,t leak, externally, the diaphragm may be good preventing leaks but the spring allows fuel to flow below the normal minimum of 37-42 psi required for easy starts and consistent tuning on the MPFI engines
obviously a fuel pressure regulator should maintain a minimum working fuel pressure and that needs to be verified with a fuel pressure gauge. adding a small fuel pressure gauge and an adjustable fuel pressure regulator to your engine makes diagnosing and tuning far easier
IF YOU DON,T CURRENTLY have an ADJUSTABLE fuel pressure regulator on your TPI you NEED TO INSTALL ONE
In some cases a defective diaphragm in a TPI fuel pressure regulator allows fuel under pressure to enter the vacuum lines and raw fuel flows into the plenum.

The stock fuel pressure for 1985 to 1987 C4 engines with TPI was 36 to 39 psi with the fuel pressure regulator vacuum hose connected, and 47 to 48 psi with the vacuum hose disconnected. For the 1988 to 1996 C4 corvettes, the stock fuel pressure is 40 to 42 psi with vacuum hose connected and 47 to 48 psi with the vacuum hose disconnected.
swapping out the stock non-adjustable fuel pressure regulator requires removing the plenum but takes only about 20 minutes and its easy to do, it allows some minor tuning adjustments which tends to help power.ESPECIALLY if the stock FPR is leaking

be VERY SURE you order a adjustable fuel pressure regulator that comes with a NEW matching diaphragm as many kits assume youll re-use the stock diaphragm

if you swap to an adjustable fuel pressure regulator on your TPI fuel rails it makes sense to install a small fuel pressure gauge on the fuel rail so you can easily verify your fuel pressure, keep in mind raising the fuel pressure tends to richen the effective fuel/air ratio slightly while reducing the pressure tends to lean it out
slightly, but remember the computer and the oxygen sensors try to maintain the emission friendly 14.7:1 fuel air ratio ... ProductId=
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INSTRUCTIONS ... a-4315.pdf
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All you need to do is buy any 1/8" NPT Fuel Pressure Gauge rated at 0-60 psi. to mount on your fuel rail, and a 1/16 /1/8 NPT adapter
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YES!!!! you REALLY SHOULD READ THRU ... pg%20D.htm ... gnosis.pdf








BLACKZ51 posted this info

How to Install a Fuel Pressure gauge on an L98 TPI.

-what your going to need: -Guage (most are 1/8npt)
- An adaptor that will screw onto the -4AN schrader valve on the fuel rail and convert it to 1/8Npt.
-A tool to remove the Schrader valve.

Cost for this project:
$2 for schrader valve tool.
$17.00 for Fuel Pressure Guage
$ 20.00 for fittings (elbow and 4an to 1/8 npt.)
Total Cost $39

Step 1: remove the Fuse that says FR or FP 1. this will disable your fuel pump and relieve the pressure so you dont get fuel all over the place. (thanks oldvettefan!)

Step 2: remove the cover on the fuel rail, and then with at tool remove the schrader valve. have a rag handy to catch the little fuel that spills.

Step 3:A little teflon tape doesn't hurt, I wrapped the threads then screwed on the new fitting. then did the same for the elbow and gauge.

Tighten everything and then your done! replace vaccuum lines if you needed to move them.


I did this first so I can monitor fuel pressure settings before I go ahead with installing the AFPR.
I found that I had the stock 47 psi at idle with vaccum disconnected. and 39 or so with it connected.

Re: fuel pressure regulators

PostPosted: April 26th, 2011, 7:06 pm
by grumpyvette
most EFI systems also use fuel pressure regulators but on efi fuel pressures generally run in the 37psi-80psi range with mpfi systems

Re: fuel pressure regulators

PostPosted: August 28th, 2011, 6:21 pm
by grumpyvette
Instructions, testing a stock fuel pressure regulator
Things You'll Need

Fuel pressure gauge


Locate the fuel system test port on the fuel system rail. The rail resembles a tube running between the fuel pump and the throttle body (or carburetor) and has what looks like a tire air valve stem with cap. Be certain you do not mistake the AC access tube for the fuel system rail. If in doubt, trace the line in both directions to be sure it is not connected to the air conditioning compressor.

Unscrew the cap from the fuel system test port with your fingers. If the cap sticks twist it gently with the pliers.

Screw the hose fitting of the fuel pressure gauge onto the test port. Set the gauge where it will not be vulnerable to moving parts of the engine, and start the vehicle.

Read the fuel pressure gauge, and then remove the rubber vacuum line attached to the throttle body (or carburetor) assembly. If the reading on the gauge does not increase at least 5 psi after the vacuum line is removed, the fuel pressure regulator is faulty.