Question On Boost Referenced Mechanical Fuel Pump
- Thread starter mateopdx
- Start date
mateopdx":2ns4s81e said:
well you can do 1 of 2 things, I guess you could use a check valve OR you could do a vacuum source that doesnt see vacuum such as the turbo compressor (some have a nipple sorta thing on them) or at the carb hat.
A
Anonymous
Guest
A check valve wouldn't work, anyway. It would pressurize the pump and keep it there. You don't want that.
You want pressure to rise with boost, and ALSO more very importantly, pressure needs to fall as boost goes away. Only way to get that is to get the signal from the hat.
You want pressure to rise with boost, and ALSO more very importantly, pressure needs to fall as boost goes away. Only way to get that is to get the signal from the hat.
I went electric. solved alot of problems I was having at the time (running a stock mech pump) plus now I am setup for EFI in the future
XPC66":1zfxkked said:
I know with my fuel pressure regulator a vacuum signal will actually lower the fuel pressure.
kirkallen143
Famous Member
XPC66":1xvtv2j8 said:
Yes, I have run my fuel pressure down to 3.5 - 4 psi w/ the turbo and had no problems running lean when boost was applied. I do this to help when I am going 4-wheeling with the inclines and off-cambers (i also lower my float level). This is on the holley carb and it has not done me wrong by any means...yet.
Kirk
I'm still not clear on how this works...you're talking about running a line from the pressurized carb hat to the vent on the backside of the mechanical fuel pump's diaphragm - to pressurize the "atmospheric" side of the pump so it can make enough fuel pressure to overcome the boost pressure?
Most of the mechanical pumps I've seen, the backside of the diaphragm is not only open to the atmosphere via the vent (where you put the pressure line?), it's also open to the inside of the block via the pump arm and the link that connects it to the diaphragm - so how are you not losing pressure to the inside of the crankcase???
Most of the mechanical pumps I've seen, the backside of the diaphragm is not only open to the atmosphere via the vent (where you put the pressure line?), it's also open to the inside of the block via the pump arm and the link that connects it to the diaphragm - so how are you not losing pressure to the inside of the crankcase???
lightnin-boy
Well-known member
jamyers makes an excellent point. Every mechanical pump I've ever seen to open to the crankcase from the atmospheric side of the pump too. This being the case, boost referencing a mechanical pump would be a waste of time...
Am I missing something?
Kirk
Am I missing something?
Kirk
Alright - think about it.
The way a carburetor works, all the fuel pressure does is keep fuel in the bowl.
To low of pressure, and the flow isnt enough to keep the bowl full. Too high of pressure and the flow will overwhelm the needle and force the float down, overfilling the bowl.
The needle+seat is a valve, that is controlled by the float.
In a NA application, the bowl is at atmospheric pressure, and is kept there by the bowl vent.
The Vacuum in the venturi of the carb (lower that 1atm) sucks fuel out of the bowl via the various passageways.
In a turbocharged application, you reference the bowl vent to the carb hat - so the pressure in the bowl is the same as the pressure into the carb. This means that the ratio of venturi pressure (absolute) to bowl pressure (absolute) stays the same.
The reason you need to increase the fuel pressure into the carb under boost is that, as you use fuel, the bowl needs to be re-filled.
If the pressure ratio between the fuel (absolute) and bowl (absolute) becomes to small, or goes negative - you will not get fuel flow into the bowl and will starve the engine of fuel.
So If you are at 1 atm (14.7psia) and your fuel pressure works "right" at 5 psig (19.7 psia) then, as you add 1 lb of boost, you need to add 1lb of fuel pressure to keep the ratio correct. There is probably some wiggle room in this, as an example I have no fuel pressure increase and my ranchero runs mostly o-k (because I am not draining the bowl under boost, yet)
But if I were to stay boosted for a long period of time (more than a few seconds) my bowl would drain and I wouldnt have the deltaP between fuel line and bowl to fill it up.
Infact, I probably have negative - so as the float opens the carb starts to flow BACK into the fuel line.
Get it ?
The way a carburetor works, all the fuel pressure does is keep fuel in the bowl.
To low of pressure, and the flow isnt enough to keep the bowl full. Too high of pressure and the flow will overwhelm the needle and force the float down, overfilling the bowl.
The needle+seat is a valve, that is controlled by the float.
In a NA application, the bowl is at atmospheric pressure, and is kept there by the bowl vent.
The Vacuum in the venturi of the carb (lower that 1atm) sucks fuel out of the bowl via the various passageways.
In a turbocharged application, you reference the bowl vent to the carb hat - so the pressure in the bowl is the same as the pressure into the carb. This means that the ratio of venturi pressure (absolute) to bowl pressure (absolute) stays the same.
The reason you need to increase the fuel pressure into the carb under boost is that, as you use fuel, the bowl needs to be re-filled.
If the pressure ratio between the fuel (absolute) and bowl (absolute) becomes to small, or goes negative - you will not get fuel flow into the bowl and will starve the engine of fuel.
So If you are at 1 atm (14.7psia) and your fuel pressure works "right" at 5 psig (19.7 psia) then, as you add 1 lb of boost, you need to add 1lb of fuel pressure to keep the ratio correct. There is probably some wiggle room in this, as an example I have no fuel pressure increase and my ranchero runs mostly o-k (because I am not draining the bowl under boost, yet)
But if I were to stay boosted for a long period of time (more than a few seconds) my bowl would drain and I wouldnt have the deltaP between fuel line and bowl to fill it up.
Infact, I probably have negative - so as the float opens the carb starts to flow BACK into the fuel line.
Get it ?
I've got it just fine. What I don't get is how you don't just end up blowing all that pressure into the crankcase.Bort62":234asg6v said:
I even went and looked at a spare mechanical pump. Looking into the pump body from where it bolts to the block, I can see the end of the pump arm, the rod it connects to, and the backside of the pump diaphragm. If I ran a pressure hose to the pump vent hole, all I'd do is blow air into the crankcase.
A
Anonymous
Guest
lightnin-boy":15ezdgf9 said:
YES, you are missing something!
Go about half-way down on page 1 of my build thread, and you will read this:
"I put pressure to the factory fuel pump vent hole with a rubber tipped blowgun hooked to a regulated source. It started to leak from the "arm hole" right away, to my disappointment. I figured it was already junk, so I kept turning up the pressure to see where it would "pop" at.
I went all the way to 30 psi, and while it continued to leak air it never got any worse. I figure that is good enough, so I drilled the vent hole a little larger and epoxied a tube in place for the boost reference."
It wasn't a HUGE leak, just a small air leak.
With my fuel pump(s) I never noticed an air leak. I tried blowing into both of them and neither leaked? I know with the factory pump it worked great but was only getting 3 psi base pressure. I did however have an issue with the regulator I was running before so I was unable to run more than 7 psi of boost because of the lack of fuel pressure. I bought a new mallory return style regulator unfortunately it came in the mail 2 days after the tranny went out.
