jerrod75110

I took my car to the track after the run I opened my hood and noticed my fuel pressure was hung around 3 psi. after driving home it returned to normal. I mounted a fuel pressure gauge in the car to see what was going on. The fuel psi is set on 7 psi, under normal driving it drops to 6 sometimes and at the track it drops to about 4.5 at wot!

I have a hose in place of the intank factory fuel pump.
a blue pump, stock lines and a Mallory return style regulator
the filters are fresh

any ideas?

FRMULA88

Fuel pressure does not equate to fuel volume.

4.5 psi at WOT for a carburetor application is just fine. You only need enough fuel pressure to keep the bowls supplied with fuel.

At idle you want no more than 7 - 7.5 psi, too much fuel pressure will damage the needle & seat.

for a drag race car make sure you install jet extensions on the secondaries, this will make sure the jets will stay immersed in fuel when you launch the car.

With all this in mind the stock fuel line may not deliver the sufficient volume of fuel to the carb that will depend on how much HP your engine makes.

A healthy 383 should have no less than -6AN (3/8) fuel system. Better yet would be -8AN (1/2) from the tank to the pump and regulator then -6AN from the regulator to the carb bowls.

The stock line is 5/16 (-5AN ) and was designed for a stock application. Eventually it will be the bottleneck to supplying the correct volume of fuel for a high performance carbureted engine.

jerrod75110

Thanks for the reply! I have always read on here the stock fuel lines are 3/8 and the return is 5/16? I know the ls1 guys are making big power on factory lines with big fuel pumps. My car traps 92 in the 1/8 and 117mph in the 1/4 so it doesnt seem like its falling off but if I can improve that with a better fuel system im all about it. I would like to put a nos on the car so I wanna have my ducks in a row!!

FRMULA88

5/16 supply 1/4 return
Don't trust a lot of what you read on here, always do your homework.

The LS1 guys you mention are running EFI, (high pressure)
not carb (low pressure) there is a difference.
The high pressure EFI pumps they are using are made for that purpose.

With a carb setup bigger fuel lines are mandatory to prevent fuel starvation when the car really starts launching hard.

If you want to add a N20 system (and upgrade you stock fuel system) a good way is to sump the stock gas tank, run a 2nd pump and fuel line for the fuel enrichment. There is room to mount the pump(s) & filter(s) under the car on the bulkhead behind the rear seat.

When I had my projection EFI on my 383 I used the stock lines but sumped the tank for the fuel pump pick up with -6AN and -8AN with Holley blue pump for N20 enrichment fuel. I Never had an issue for 10 years.. then I switched to the 421 and went with -10 fuel line to the regulator -8 to the carb bowls no nitrous.

jerrod75110

Sounds like I need to start saving! Lol. When I upgrade should I still run a return line back to the tank? And I know a sump need to be at the lowest point. Will it clear a muffler in the factory location? Thanks

FRMULA88

A return line is always a good idea versus dead heading to the regulator

It keeps the fuel circulating which prevents vapor lock (great for street machines) and helps to bleed off excess fuel at idle (great for race & street machines)

You can also run a return on the nitrous fuel line but most people dead head it because the line is not in use 90% of the time.

the sump will clear stock cross flow muffler but you may have to rework the stock heat shield.

mw66nova

iTrader: (13)

factory fuel lines are most definitely 3/8" feed and 5/16" return. i've measured

no need for a return type fuel pressure regulator on a blue pump setup.

FRMULA88

Matt,

Did you measure the OD or ID of the pipe? For our purpose we care about the ID.

So if you measured 3/8 & 5/16 OD. the IDs are actually 5/16 & 1/4.

I am using SAE units by convention, the fuel lines and fittings on a 3rd gen are actually metric.

mw66nova

iTrader: (13)

admittedly, i measured OD, but have used -6 and 3/8" compression-to-AN fittings in the past, so that's where my measurements have come from.

i'm using -10 to feed my pump from the sump, and -8 from the pump to the regulator on my car for whatever that's worth. the stock fuel lines are not enough in my opinion.

FRMULA88

Reference Material for O/P and others use.

This is for Mallory fuel pumps & regulators but applies to any fuel system design.

Note the fuel line sizes / HP applications.

Of particular historical interest is the OEM 3rd gens were 225 HP max (TPI cars) and the GM never intended these cars to be "upgraded". The bean counters saved money weight wherever they could so nothing was "over engineered". the ECM also kept most hands from tinkering.

A perfect example is the lack luster 7.5" 10 bolt rear end good enough for 225 HP and not much more.. same with the stock 700R4 trans,, and yes even the fuel system and mini- spare tire.

These cars were the products of the 70s energy crisis.

No matter how much we love them these were not high-performance machines in OEM form compared to modern standards.

Why would you use the stock fuel system in an engine that now makes twice to 3 times the original HP ?

FRMULA88

I did the same thing in the past; Not enough money and the know it all attitude that comes with youth.

but we both learned.. scary how similar how setups are actually.

FRMULA88

It would help if I posted the material huh?

jerrod75110

What's the best way to hook the return line into a stock gas tank? Especially two return lines if you the one for nos!

FRMULA88

If you sump the OEM tank for new supply lines you can easily rework the old pick-up assembly cover.
Drill the cover for a bulk head fitting and new -6 or -8 return. (Street/ strip)


Don't forget to vent the tank also. The stock vent is a bit on the small side but may be OK for a race only application. You can use the old 5/16 supply as your vent and plumb it with -6 (that is what I did) -6 minimum or -8 would be better for a street / strip car. If you want a larger vent line, you can install a bulkhead fitting on the pick-up assembly cover for this purpose.

For safety install a roll-over valve on the vent line.

Finally trim and cap off the old lines coming out the pick up cover that you are not going to use.

This keeps the car looking relatively stock... another option is remove the OEM tank and install a fuel cell..

jerrod75110

Thanks for all you help FRMULA88!