GT 500KR Fuel pressure

[gt350bp]

Learned a bit of an expensive lesson today, that I thought I would share, to hopefully save someone any grief. I decided to use a 67 GT 500 pre-bent fuel line from fuel pump up to a 45 degree inverted flair that attaches to a Summit fuel line and pressure gauge. This makes a very clean installation for a double pumper carburetor. I then decide to upgrade to a liquid filled fuel pressure gauge. The issue started when I started up the car cold and the gauge would read
5 1/2 psi so everything was good until the car started to get warm. As the car came up to temperature, the gauge started to read lower and lower until it would read about 2 psi! To make a long story short, changed the fuel filter, installed a new tank and sender as well as cleaned and flushed the lines. Started the car up and the same scenario, as the car came up to temperature the fuel pressure was dropping until it would be 2 psi on the liquid filled gauge. Thought maybe the 10% ethanol pump fuel sold here in Florida was vaporizing as the fuel lines got hot Insulate the lines. Same result. Googled liquid filled fuel pressure gauge and guess what? A liquid filled fuel pressure gauge will change readings as temperatures change. Attached is a Summit Racing bulletin regarding this phenomena. Hope this helps someone in the future.

Don
gt350bp   

[Royce Peterson]

Ideally the fuel pressure gage, liquid filled or not, needs to be mounted remotely from the engine so that it doesn't get hot. Instead of screwing the gage into the carburetor fuel line you screw in a fitting to connect your 3/16" braided stainless flex line to. Racers generally mount fuel pressure gages in front of the driver out side the cockpit. Of course if you wanted you could mount it someplace under the hood, away from the heat of the engine. Say to the front of the radiator core support.

[Drew Pojedinec]

I run expensive ($60) liquid filled gauges. They have a vent you can lift to relieve pressure.
Pretty much the only way to get a perfect reading at temp.

I think AED brand gauge, but many industrial gauges work fine.

[gt350bp]

Drew,

Learned that yesterday by contacting Summit Racing. They have a gauge with the pressure release for $45 as you mentioned. Now that I'm comfortable that I have enough fuel delivery, I probably will plug the fuel line and remove the gauge from under the hood, as a double carb to air cleaner gasket was needed to provide air cleaner clearance to the pressure gauge. That in turn is causing a tighter seal to the ram air chamber.

Don
gt350bp

[pbf777]

Ideally the fuel pressure gage, liquid filled or not, needs to be mounted remotely from the engine so that it doesn't get hot.

     And also,........... fuel pressure gauges should not be located on the engine due to the vibration that can/will cause failure of the, to delicate to tolerate such, internal bellows/bourdon-tube/mechanism etc., which can lead to disastrous results!   

     Besides, what good is a fuel pressure gauge mounted under the hood, that can only be observed at idle, or even if the R.P.M.'s are raised, under no load, hence low fuel volume requirement,...........this proving not much of a test?     

     Scott.

[Royce Peterson]

Outstanding point. Fuel pressure is important when you are going across the finish line. Not too important to observe when the car is in neutral in your driveway.

Ideally the fuel pressure gage, liquid filled or not, needs to be mounted remotely from the engine so that it doesn't get hot.

     And also,........... fuel pressure gauges should not be located on the engine due to the vibration that can/will cause failure of the, to delicate to tolerate such, internal bellows/bourdon-tube/mechanism etc., which can lead to disastrous results!   

     Besides, what good is a fuel pressure gauge mounted under the hood, that can only be observed at idle, or even if the R.P.M.'s are raised, under no load, hence low fuel volume requirement,...........this proving not much of a test?     

     Scott.