Help on Fuel Pressure

[M-I-K]

Hello All from Italy!

i have a BDR COBRA with a Roush 427 R Engine


i have recently changed the mechanic fuel pump on my Roush 427R with an electric pump, using a direct line with no return (i.e. gas tank-filter-pump-fuel reguator-carb); the carb is a quick fuel 850.i have also have connected the gas line to the rear of the tank in order to not have fuel lacks in acceleration
The car works great and i have no fuel leak but i noticed that the fuel pressure on the gauge is not stable
The Pump (As the rest of the line and the carb) is brand new, and right dimensioned for the engine



when the car is cold is very high
becoming hot it goes down

actually i can say that we start from 11 (at the start) going down to 4 psi (very hot)

I thought that with fuel regulator and pump the pressure of the gas shold be the same always, and i do not understand the reason of this behaviour, maybe some one have suggestion/impression or even an ecperience to share, as you can understand there is not a lot of people here to discuss with regarding cobra and 427 engines.

Many Thanks

[Gaz64]

Are you using a liquid filled gauge?

Liquid filled fuel pressure gauges are highly inaccurate due to temperature change.

What regulator?

[M-I-K]

Hi
yes the gauge is liquid filled, i tried with more than one but all of them shows nearly the same dynamic. Do you reckon some alternative for the gauge?

The fuel regulator is a quick fuel, as for the carb..

[M-I-K]

I read now that liquid filled gauges are extremely sensitive to hot

so wich the correct pressure i have to take? with cold engine or hot engine? (i think cold but need reassurances)

[Gaz64]

If you must have a fuel pressure gauge under the bonnet, a non-liquid filled version is way more accurate.
The liquid filled versions are not vented to atmosphere.
All bourdon tube mechanical gauges need to be vented otherwise they become inaccurate as the ambient temperature increases or decreases from the assembled temperature.

Set your fuel pressure at 6-7 psi engine cold, ambient of 25 degrees C.

If the gauge then experiences temps of 80-90c, the gauge may read near zero.

I would prefer to run a more expensive stepper motor electric in-cabin gauge 270 sweep with a rail mounted pressure sensor.

[M-I-K]

Thans you Gaz, i had exactly your idea
i will keep the liquid underhood gauge just as ref and i will mount in the cockpit a (far more expensive sigh!) electrical gauge of Autometer (that is the brand i have for all the cockpit)

[Tim Brewer]

Just set it after a small drive and unless you are having any fuel problems, forget about it.

[Tom Wells]

M-I-K,

Do not keep the liquid filled gauge under the hood.

Here's how I know this: First Coast Cobra Club • View topic - Liquid filled gauges - lesson learned!

If you must, put it somewhere there's no heat.

I was lucky - the carb percolation/overflow didn't cause a fire You may or may not have the same luck.

If you require a gauge under the hood, get a non liquid filled one. They're inexpensive and much safer to use: http://www.summitracing.com/parts/aei-15632/overview/

'nuf said,

Tom

[M-I-K]

Thanks Tom
really i read yesterday the pressure @ cold and was pointing to 11 psi
the quickfuel carb is big enoygh to sustain this high pressure but the risk is always behind the corner
I will set up the pressure at cold between 6 and 7 as suggested by the producer and i have already ordered an electrical fuel pressur gauge from Automotive to install inside the car...that should set up the issue...