Fuel Supply
Thread Starter
Pro
Joined: Mar 2000
Posts: 658
Likes: 7
From: Perth Amboy New Jersey
Fuel Supply
I understand how a pressure switch installed in the manifold activates a secondary fuel pump when the pressure set point of the switch is reached.
1 - If both pumps feed the same supply line side to the same injectors, regulated at the same point with the same regulator. Are you increasing the fuel or just maintaining the pressure that in the same line with the second pump?
2- Why does the fuel pressure have to increase the same amount as boost pressure? Example adding 10 PSI of boost , fuel pressure needs to increase 10 PSI. It is mention on / in several articles, forums, books etc. but never why?
Thanks for assistance,
Yedi
1 - If both pumps feed the same supply line side to the same injectors, regulated at the same point with the same regulator. Are you increasing the fuel or just maintaining the pressure that in the same line with the second pump?
2- Why does the fuel pressure have to increase the same amount as boost pressure? Example adding 10 PSI of boost , fuel pressure needs to increase 10 PSI. It is mention on / in several articles, forums, books etc. but never why?
Thanks for assistance,
Yedi
Melting Slicks
Joined: Mar 2001
Posts: 2,153
Likes: 305
From: Timra, Sweden
I understand how a pressure switch installed in the manifold activates a secondary fuel pump when the pressure set point of the switch is reached.
1 - If both pumps feed the same supply line side to the same injectors, regulated at the same point with the same regulator. Are you increasing the fuel or just maintaining the pressure that in the same line with the second pump?
2- Why does the fuel pressure have to increase the same amount as boost pressure? Example adding 10 PSI of boost , fuel pressure needs to increase 10 PSI. It is mention on / in several articles, forums, books etc. but never why?
Thanks for assistance,
Yedi
1 - If both pumps feed the same supply line side to the same injectors, regulated at the same point with the same regulator. Are you increasing the fuel or just maintaining the pressure that in the same line with the second pump?
2- Why does the fuel pressure have to increase the same amount as boost pressure? Example adding 10 PSI of boost , fuel pressure needs to increase 10 PSI. It is mention on / in several articles, forums, books etc. but never why?
Thanks for assistance,
Yedi
Usually a progressive regulator is used and that increases fuel pressure more than boost. (cheap 'fix' to avoid tuning the ECM).
2) That has nothing to do with boost. A stock N/A fuel system does the same thing. It is the fuel pressure over the injector that is important. To get the same amout of fuel for each ms of open time, the pressure over the injector needs to be constant. One end of the injector sees the vaccum / boost in the plenum. To keep 'injector' fuel pressure constant, the absolute fuel pressure in the fuel line must vary the same way as the air pressure in the plenum.
Thread Starter
Pro
Joined: Mar 2000
Posts: 658
Likes: 7
From: Perth Amboy New Jersey
1) You are maintaining pressure set by the regulator.
Usually a progressive regulator is used and that increases fuel pressure more than boost. (cheap 'fix' to avoid tuning the ECM).
2) That has nothing to do with boost. A stock N/A fuel system does the same thing. It is the fuel pressure over the injector that is important. To get the same amout of fuel for each ms of open time, the pressure over the injector needs to be constant. One end of the injector sees the vaccum / boost in the plenum. To keep 'injector' fuel pressure constant, the absolute fuel pressure in the fuel line must vary the same way as the air pressure in the plenum.
Usually a progressive regulator is used and that increases fuel pressure more than boost. (cheap 'fix' to avoid tuning the ECM).
2) That has nothing to do with boost. A stock N/A fuel system does the same thing. It is the fuel pressure over the injector that is important. To get the same amout of fuel for each ms of open time, the pressure over the injector needs to be constant. One end of the injector sees the vaccum / boost in the plenum. To keep 'injector' fuel pressure constant, the absolute fuel pressure in the fuel line must vary the same way as the air pressure in the plenum.
Safety Car
Joined: Aug 2009
Posts: 3,752
Likes: 278
From: Melbourne, Victoria, Australia
My vette with a vortech blower and has a vortech FMU (fuel management unit) and a inline fuel pump. The FMU is connected to the fuel return line to tank, it restricts the fuel return to the tank with boost rise and that increases the fuel pressure to force more fuel thru the injectors.
That way you can get by without increasing fuel injector size.
Hope this helps you
That way you can get by without increasing fuel injector size.
Hope this helps you
Thread Starter
Pro
Joined: Mar 2000
Posts: 658
Likes: 7
From: Perth Amboy New Jersey
My vette with a vortech blower and has a vortech FMU (fuel management unit) and a inline fuel pump. The FMU is connected to the fuel return line to tank, it restricts the fuel return to the tank with boost rise and that increases the fuel pressure to force more fuel thru the injectors.
That way you can get by without increasing fuel injector size.
Hope this helps you
That way you can get by without increasing fuel injector size.
Hope this helps you
Safety Car
Joined: Aug 2009
Posts: 3,752
Likes: 278
From: Melbourne, Victoria, Australia
According to the fuel pump charts as you increase the fuel pressure the volume goes down, how is it possible to flow more fuel through the injectors when the volume has decreased? The B.S.F.C for Normal Aspirated Engines is .45 - .5 and Turbo charged Engines is .6 - .65, since it takes more fuel for a turbo engine, then increasing the pressure looks like it is going the opposite way because there is less fuel getting to the injectors.
I also noted my install came with an extra inline fuel pump.
you can get various FMU's that increase fuel pressure per psi boost rise.
such as 8:1,10:1 or 12:1 (ie. 10 psi fuel pressure per one psi boost).
Trust me it works i have been running 8 psi boost for many years without any problems.
Volume (flow) will always increase with pressure to the point of how much volume a size pipe can handle, decreasing pressure will decrease flow and volume.
Melting Slicks
Joined: Mar 2001
Posts: 2,153
Likes: 305
From: Timra, Sweden
According to the fuel pump charts as you increase the fuel pressure the volume goes down, how is it possible to flow more fuel through the injectors when the volume has decreased? The B.S.F.C for Normal Aspirated Engines is .45 - .5 and Turbo charged Engines is .6 - .65, since it takes more fuel for a turbo engine, then increasing the pressure looks like it is going the opposite way because there is less fuel getting to the injectors.
1) Increasing pressure will increase flow thru the injectors.
2) Increasing pressure will decrease flow thru the pump (as shown by the pump chart).
This is not a problem as you have a fuel return from the regulator back to the tank. Any fuel not used by the injectors will be sent back to the tank.
As long as the pump is powerful enough to flow more than the injectors can consume at maximun fuel pressure everything is good. Then you allways have a return flow and that is a must for the fuel pressure regulator to work.
