Dual pumps. How to tune?
- Thread starter 1chance
- Start date
- Joined
- Aug 1, 2003
assume you're talking about dual intank fuel pumps in which case you need to delay (stage) the second pump by using a hobbs switch to turn the second pump on at a point above where fuel pressure with both on is less than the needed fuel pressure ,
typically twin 340s with both running will be at 55psi with stock return lines (yours may be higher)
that means if your base FP is 45 you need to keep the second pump off until you are running over 10psi boost otherwise the fuel pressure will exceed the required pressure ( fuel pressure at 10psi boost will be base 45+10=55psi)
to stage in this case you need at minimum a 10psi hobbs switch (i set mine at 12psi) or higher depending on what your pressure is with both running
or fix your return setup so it can flow back the additional volume when both pumps are on as the stock is not adequate to have both pumps on all the time , typically going to a -6AN return should be sufficient ..you could also use the stock 3/8 feed as the return and upgrade the supply line which i would do if plans are to run power enough for 9 secs full weight car
typically twin 340s with both running will be at 55psi with stock return lines (yours may be higher)
that means if your base FP is 45 you need to keep the second pump off until you are running over 10psi boost otherwise the fuel pressure will exceed the required pressure ( fuel pressure at 10psi boost will be base 45+10=55psi)
to stage in this case you need at minimum a 10psi hobbs switch (i set mine at 12psi) or higher depending on what your pressure is with both running
or fix your return setup so it can flow back the additional volume when both pumps are on as the stock is not adequate to have both pumps on all the time , typically going to a -6AN return should be sufficient ..you could also use the stock 3/8 feed as the return and upgrade the supply line which i would do if plans are to run power enough for 9 secs full weight car
Pauls correct.
The problem is you need to regulate fuel pressure. The factory return lines are too small to handle both pumps pumping. So you need to switch on the 2nd pump at higher boost levels. And... if you have an older alcohol system with a Hobbs style control.. that will also cause way to much alcohol to be shot in once it activates.
The problem is you need to regulate fuel pressure. The factory return lines are too small to handle both pumps pumping. So you need to switch on the 2nd pump at higher boost levels. And... if you have an older alcohol system with a Hobbs style control.. that will also cause way to much alcohol to be shot in once it activates.
OK with fp set at 45, and chip (TT) maxed fuel in all gears my O2s are 730 WOT,@88 MPH via scan master. Second pump come on at 16psi and O2s go to 064. Do I set chip to 128 when tuning? Also got to upgrade return line. Alky is progressive start @10/20. Do I need to do away with alky. And install. Street chip93.
- Joined
- May 28, 2001
I turned the boost down to 10 psi so the alky would turn on using one fuel pump only. Once you get some data you can tune the chip to get the fuel right.
From there you can start to turn boost back up and make sure you have plenty of fuel going to the motor. 11 to 1 is a safe AF to shoot for under boost and higher rpms.
I am currently running 23 psi on a single pump and Razors alky with no problems. THe SMC does not flow the same amount of alky as Razors does so be careful. I would set the the pump to come on at 10 psi and turn the fuel knob to 25%, doing that gives the pump a wide boost range to operate within. You will probably have to set the second fuel pump to kick in around 16 psi or so. Get it right with one fuel pump and the alky first. Once you have done that you can see where you will need the second pump to kick in. The problem is that you double fuel volume when you turn on the second fuel pump.
Again set a base and slowly work up to the boost level you want.
Good luck.
From there you can start to turn boost back up and make sure you have plenty of fuel going to the motor. 11 to 1 is a safe AF to shoot for under boost and higher rpms.
I am currently running 23 psi on a single pump and Razors alky with no problems. THe SMC does not flow the same amount of alky as Razors does so be careful. I would set the the pump to come on at 10 psi and turn the fuel knob to 25%, doing that gives the pump a wide boost range to operate within. You will probably have to set the second fuel pump to kick in around 16 psi or so. Get it right with one fuel pump and the alky first. Once you have done that you can see where you will need the second pump to kick in. The problem is that you double fuel volume when you turn on the second fuel pump.
Again set a base and slowly work up to the boost level you want.
Good luck.
BobbyBuick
Well-Known Member
- Joined
- May 24, 2001
I struggled with this on my first maffless Max effort chip back in the 90s and it was way before the widebands were around.I was tuning with an EGT and was running so rich it was showing lean(burning in exhaust)I turn second pump on with a micro switch
on the TB through the trans brake so only one pump runs till I let go of the button.One day I was just trying everything and turned the FP all the way down and made a pass and it worked .Now have the FP set at 36 static one pump...Works great now and fuel pressure is rock solid with stock lines
on the TB through the trans brake so only one pump runs till I let go of the button.One day I was just trying everything and turned the FP all the way down and made a pass and it worked .Now have the FP set at 36 static one pump...Works great now and fuel pressure is rock solid with stock lines
