fuel pressure change with elevation.. help

[krom99]

I have a fuel pressure regulator with a pressure guage and it changes pressure as I increase in elevation? At 5200ft I have it set at 46psi and if I go to 6000ft It changes to 43-44 psi. I checked it when we got down to the trucks and the pressure changed to 48psi. Not sure why it is changing as it only changes with elevation change. M7 EFI. Thanks for your help.

 

[Arctic Thunder]

Somebody correct me here but....

Your pressure gauge is working on "gauge" pressure which changes with elevation. So if you set your pressure at 40psi at 2000ft elevation, it will read 37psi at 8000ft elevation. You will loose about .5psi/1000ft elevation. But the absolute pressure would be the same.

Bottom line if you set your fuel pressure at see level for every 1000 feet of elevation the gauge will show .5 psi less. But the actual line pressure will be the same.

I know I said the same thing twice but does that make sense?

If you want to know Exactly what pressure you are running you need the absolute pressure not gauge pressure.

Thunder

 

[Dam Dave]

get a PSIA gauge- absolute pressure gauge

 

[krom99]

who makes a PSI guage that would replace the fluid filled one that comes with the Bikeman pressure regulator?

If I keep the water filled guage, at what elevation do I set the pressure? sea level? Bikeman recommends 46psi.

Thanks guys for all your help, just do not want to cook my pistons if the pressure drops below 42psi.

 

[Arctic Thunder]

What elevation are you at?

I would set the pressure at your elevation. Assuming it is between 2-3000ft.

Then when you check your pressure on a ride and you are at 8000ft. Expect the gauge to read 2-3 psi less. But you will still have the SAME fuel pressure but the gauge will read differently.

Like Dave said. If you have an absolute gauge it won't change with elevation.

Lastly, one more time. As you change elevations your pressure is the SAME, the gauge reading is different.

Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.

Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.

Thunder

 

[Dam Dave]

I think your worried about nothing, set it and forget it



http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

 

[Dam Dave]

who makes a PSI guage that would replace the fluid filled one that comes with the Bikeman pressure regulator?

If I keep the water filled guage, at what elevation do I set the pressure? sea level? Bikeman recommends 46psi.

Thanks guys for all your help, just do not want to cook my pistons if the pressure drops below 42psi.

its liquid filled, pure glycerine usually, water would freeze

 

[krom99]

where do I get an absolute guage?

The reason I am worried is that the guy I bought the sled off cooked his pistons running at 42psi.

So what thunder is saying I should be able to set my pressure at sea level at 46 psi and when i am at 6000ft it will still be 46psi? no matter what the guage says?

I would like to get an absolute guage to replace but can't seem to fine anything on the internet.

Thanks Thiunder and Dave for all your help.

 

[Coldfinger]

Wouldn't the indicated gauge pressure rise as altitude increases? The outside air is less dense.

When I take the ATV's from 1200' up to 9,000', the normal 4psi tire pressure goes up to like 8psi or more. Deflate them down to 4psi for running at 9k. Then by the time I get back down to 1200' the tires have 2psi or so, looking like they are half flat.

??

 

[Dam Dave]

Altitude and Air Pressure

SEA LEVEL = 14.7 PSIA
5000 feet = 12.23 PSIA
10'000 feet = 10.1 PSIA

its all in this link I provided
http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

 

[Dam Dave]

something to be aware of when buying gauges for pressurized fuel systems, they are typically a safety gauge, if they fail they will contain the fuel, a regular press gauge could fail and spray 42 psi gasoline all over your hot engine, not good.............. I bet that liquid filled gauge supplied with your regulator is a safety gauge, just stick with that and learn how to use it as is. PSIA is over rated