Fuel pressure gauge isolator

[xb-60]

Got a question regarding fuel pressure isolators.

The isolator I'm talking about is the type that is required for a 'mechanical' gauge so that fuel is not plumbed into the inside of the car.

Question is - if the gauge is an SW 0-10psi gauge, will an isolator that is described as suitable for "15psi max" read correctly when connected to the 10psi gauge
(or will it only read correctly on a 15psi gauge)?

Cheers,
Glen

[Gaz64]

Glen,

I would say it would read correctly.

It is a diaphragm which converts liquid fuel pressure to air pressure.

Try to keep both lines as small as possible, i/d and length.

Gary

[CompClassics]

I believe there is fluid on both sides of the isolator, otherwise you would end up with a false reading. Fuel on one side and glycerine (?) on the other to the gauge.

[Jim Vander Wal]

I'd guess that the psi rating is not determined by the gauge rating but by the elastic capabilities of the diaphragm/isolator and the pressure ratings of the plumbing and connectors.

The Auto-Meter unit I have uses antifreeze from the diaphragm to the gauge. It's an all liquid system.

Jim

[xb-60]

Sounds good.
Thanks for the replies.

Cheers,
Glen

[Gaz64]

Originally Posted by CompClassics

I believe there is fluid on both sides of the isolator, otherwise you would end up with a false reading. Fuel on one side and glycerine (?) on the other to the gauge.

Yes, that may be true, thinking about it now,.

Almost any non-combustible non-corrosive fluid could be used.

There will be small amounts of air, depending on how fussy one is to bleed it out.

Gary

[CompClassics]

When working with a low PSI system such as those for Webers (3 PSI max) I’m not so sure that you would get an accurate reading.

[Gaz64]

Originally Posted by CompClassics

When working with a low PSI system such as those for Webers (3 PSI max) I’m not so sure that you would get an accurate reading.

The only way to know, would be to do a before and after test?

Gary

[xb-60]

Gauge isolators have a bleed screw so that the air (which is compressable) in the fuel line can be evacuated. That would help the gauge accuracy, particularly with a low fuel pressure Weber setup.

That said, my initial setup has to use fuel injection, but my eventual aim is to have a Weber setup. That configuration needs to be legal, so there will be some hoops to jump through first.

Cheers!
Glen

[Gaz64]

After reading some instructions, they all say the isolator needs to be mounted at the same height as the gauge to allow the gauge side fluid to be bled.

Hence why many installations are mounted up high on the firewall.

Gary

[xb-60]

With respect to height, Autometer states "If not mounted level with each
other, the fluid weight will cause a slight accuracy error"

They also say "Isolator must be mounted in engine compartment. (DO NOT mount on firewall per NHRA/IHRA rules.)"
I assume that not mounting on the firewall is to further isolate fuel from the driver.

Cheers,
Glen

[Gaz64]

Originally Posted by xb-60

With respect to height, Autometer states "If not mounted level with each
other, the fluid weight will cause a slight accuracy error"

They also say "Isolator must be mounted in engine compartment. (DO NOT mount on firewall per NHRA/IHRA rules.)"
I assume that not mounting on the firewall is to further isolate fuel from the driver.

Cheers,
Glen

It is a bit confusing.

Must be mounted in the engine compartment.

Of course we all know that's obvious.

Can't be mounted on the firewall.

But must be mounted at the same height as the gauge.

I'd give up on that idea, and go for something like this:

https://www.competitionsupplies.com/...pressure-gauge

I've seen many stepper motor gauges, accurate as you need, and only a tiny wiring harness to hide.

Gary

[CompClassics]

I have used electric fuel pressure gauges with good results. Unfortunately my latest project a Continuation Cobra required me to have a custom built gauge with logo that matched the other gauges made and the only fuel pressure gauge the manufacturer would build was a mechanical gauge. If you think about how the components have to be at the same level it would be implied that the hoses would also have to be at the same level with no deviation up or down.

[cycleguy55]

Originally Posted by xb-60

With respect to height, Autometer states "If not mounted level with each
other, the fluid weight will cause a slight accuracy error"

They also say "Isolator must be mounted in engine compartment. (DO NOT mount on firewall per NHRA/IHRA rules.)"
I assume that not mounting on the firewall is to further isolate fuel from the driver.

Cheers,
Glen

First, how much is a "slight accuracy error"? If it's 0.05 PSI it's not going to be an issue.

Second: I believe the NHRA/IHRA rules are about not mounting it on the passenger side of the firewall - the engine compartment side s/b fine.

[Gaz64]

From the 2017 NHRA rule book, section 20:

1:5 FUEL SYSTEMS
Location: All fuel tanks, lines, pumps, valves, etc. must be
outside of the driver’s compartment and within the confines of
the frame and/or steel body. Cool cans, fuel-distribution blocks,
etc. must be located at least 6 inches forward of the
flywheel/bellhousing area on rear-wheel-drive (RWD) vehicles,
and on opposite side of flywheel/housing area on front-wheeldrive (FWD) vehicles.
Fuel pressure gauge isolators, with steel
braided line, may be mounted on firewall.

Seems they have changed their mind at some point, and not told Autometer and other gauge manufacturers.

Gary