I have a question about fuel pressure...

[StefanLee]

I just changed a few hoses and replaced the fuel filters in my GT due to a leak, and I figured I would measure the fuel pressure at the same time. I bought a fuel pressure guage, and hooked it up to the line leading to the Weber 36DCD (to the guage instead of the carb...).

When I ran the fuel pump, I got a pressure of about 1 psi on the guage. I blew into the guage with my mouth afterwards as hard as I could just for kicks and I got it up to 3 psi.

Is the fuel pressure to low? I don´t want to starve the engine. there is a device on the fuel pump that I suspect is a pressure limiter (the line goes from the pump to the device and from there to the engine) and I might bypass it altogether if I need to up the pressure.

With the line off the carb, the pump produces a trickle of fuel, not very little, but it doesn´t flow with force.

Stefan

[bigjim5551212]

You need flow and not pressure. Maximum pressure is 3.5 psi. You could use a Holley Red pump if you wanted but you have to have a good regulator. The Holley unit can not be trusted for low pressure. Thats why we promote the Malpassi Filter King. Its made for Weber and Dellorto carburetors. High flow, low pressure. $60 shipped.

A trickle is not very much. if you had the carburetor full of gas and then pinched off the line, the fuel would last about 3 minutes. So you need decent flow.

[StefanLee]

Ok, in that case, how much flow do I need? it´s easy to just put a measuring cup under the tube and measure it over time.

I see the point of having a fuel pressure regulator, I have a mechanical one on the line just before the carb. I need to know how much fuel needs to go through the carb so that I can adjust it properly or replace it or any other part of the fuel system if faulty.

Stefan

[RallyBob]

Originally Posted by StefanLee Ok, in that case, how much flow do I need? it´s easy to just put a measuring cup under the tube and measure it over time.

15-18 gph will feed nearly any carbureted Opel engine. Any more is redundant, but it won't hurt as long as the pressure is correct.

Bob

[GTJIM]

... around a quarter of a gallon a minute.

Watch it though, it is extremely flamable!

[heimue]

Originally Posted by RallyBob 15-18 gph will feed nearly any carbureted Opel engine. Any more is redundant, but it won't hurt as long as the pressure is correct. Bob

15 gph --> you make around 30 mpg on an interstate --> so you travel around 15 x 30 = 450 miles in one hour. I knew you were fast, but THAT fast???

Dieter

[StefanLee]

Yeah, I figured about a liter a minute would be enought (about a quarter gallon). What made me think though was if there is a minimum pressure needed for the carb, I just felt 1 psi was a little low pressure wise.

So if I get a quarter gallon minute from the open hose I really don´t have to worry about the pressure as long as it is not to high?

Stefan

[markandson]

All you need is enough pressure to fill the bowl, that's it, so it is the restrictions in the lines and the carb inlet that determine how much pressure you need at the pump to maintain the flow, you can't have flow with 0 pressure. Make sense or not?

[StefanLee]

Hehe yeah... I guess it doesn´t take all that much pressure to open the valve to fill the bowl... In hindsight maybe I was thinking a little backwards. I´ll make a precise measurement of flow tomorrow and see if I have to increase pressure AT THE PUMP, not the carb...naturally! I knew that, just not consiously!

Thanks for the feedback guys!

[newman27]

Just out of curiosity, do you have a clear fuel filter and, if so, what does the flow look like through there when the car is idling?

Matt

[jordan]

Also, your pressure gauge is not accurate because the maximum pressure a human can exhale is about 0.228 psi so @ 3 psi it is like 13x more than a human can physically exhale, so it is a super human or inaccurate gauge situation

[kwilford]

Originally Posted by jordan Also, your pressure gauge is not accurate because the maximum pressure a human can exhale is about 0.228 psi so @ 3 psi is like 13x more than a human can physically exhale, so it is a super human or inaccurate gauge situation

Yep, I have read that the "normal" maximum pressure generated by a person blowing is at most 16 cm of water column, or about 0.24 psig, and the maximum volume is about 4 litres/sec (about a gallon per second). As we get older, the maximum pressure and volume gets reduced to as little as half that.

In other words, if your gauge says you can blow 3 psi, but the fuel pump only is putting out 1 psi, then you don't have enough pump pressure.

[bigjim5551212]

You need at least enough pressure to overcome the needle valve and float.

[StefanLee]

Ok I´m trying to get to grips with unit conversions. I normally use metric standards but used psi since most of you guys are inclined otherwise.

I attached a pic of the guage I´m using, maybe I read it wrong but the fuel pump went from zero to one on the yellow scale to the right when I did the pressure test on the carb line...

I did a flow test today and the pump flows 0,8 liters a minute, or 0,21 us gallons. A little on the low side, but that should only starve the engine if run on pretty high revs for a longer period of time. Or?.....

Stefan

[bigjim5551212]

I would say that your pump is on its way out. a good fuel pump should give a litre of fuel in a few seconds. Should be like pouring milk out of a container.

[jvandyke]

Having recently played with this issue: 21 gallons per hour is plenty. As Rallybob pointed out and he would know. Pressure not more than 3.5 psi (Weber's don't like it high). If you're getting 21 gph I have to imagine your pressure is high enough, just worry about being too high, especially 21 gph through a 1/4" line, that would seem to be excessive pressure?

oh wait, ,21 gpm, is 12.6 gph, that's a bit low then,

[StefanLee]

I´ve just bought a new fuel pump, just to be sure. I don´t like the prospect of breaking down on the freeway due to a faulty fuel pump...

The new one is rated at 31 gallons per hour and max pressure of 0,25 bar. Should do the trick!

I´ll get back to you all on the matter when I have it installed and tested.



Stefan

[bigjim5551212]

Any results yet?

No news is usually good news.

[StefanLee]

Naw...not really. The new pump didn´t work, it wouldn´t suck fuel from the tank (and yes, I connected it properly) so I've hooked up the old pump again. I have a fuel return, so I restricted the flow in the the return line and thereby increased the flow at the carb. Now it is fully satisfactory. I´m going to keep it as it is since I have more pressing matters to attend to (and the pump seems healthy enough), for instance I have the dash and instrument cluster off to fix some electrical sisues with my instruments and redo the dash with vinyl.

But I will redo the fuel system when it is more appropriate.

Stefan