fuel pressure and the voltage drop issue
05-21-2008, 09:43 AM
#1
fuel pressure and the voltage drop issue
My tuner noticed that when the engine is cool, voltage reading is around 14.2~14.5V, but as the engine gets hotter voltage drops to 13.5V.. This voltage drop is messing with my fuel pressure.... So, to be safe, he set my mapping according to 13.5V. This means that I'm running pretty rich until the engine compartment gets hot and the voltage drops to 13.5V... This seems pretty silly... Is this voltage drop normal or is my alternator going bad? Do you guys also see such voltage drop at all? If so how do you guys deal with the fuel pressure changes?
Thanks~
P.S.: Mapping is finally finished and it's stupid fast... It's scary at 16psi...
Dyno coming real soon...
Thanks~
P.S.: Mapping is finally finished and it's stupid fast... It's scary at 16psi...
Dyno coming real soon...
Last edited by leeboyNY; 05-21-2008 at 09:45 AM.
05-21-2008, 09:57 AM
#2
couple things come to mind...
-When the engine first starts up the idle is slightly higher which could lead to higher voltage.
-You have drained your battery a bit on start up and the higher voltage could be to recharge the battery and it happens to settle in at 13.5 once the battery is recharged. Just happens this occurs by the time you car is up to temp
all that said, don't see how a voltage drop for your charging system would have an impact on your tune?
-When the engine first starts up the idle is slightly higher which could lead to higher voltage.
-You have drained your battery a bit on start up and the higher voltage could be to recharge the battery and it happens to settle in at 13.5 once the battery is recharged. Just happens this occurs by the time you car is up to temp
all that said, don't see how a voltage drop for your charging system would have an impact on your tune?
05-21-2008, 11:15 AM
#3
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my voltage is usually around 13.7 or so and fluctuates slightly
the voltage would affect your tune but it would have to be a little more drastic than what you are seeing, and major concern is if you see a big drop at load when you need the fuel for your tune.
that being said if you want you can always run a power wire directly to the pump and hook it up to the pump relay
the voltage would affect your tune but it would have to be a little more drastic than what you are seeing, and major concern is if you see a big drop at load when you need the fuel for your tune.
that being said if you want you can always run a power wire directly to the pump and hook it up to the pump relay
05-21-2008, 11:16 AM
#4
Originally Posted by Zivman
all that said, don't see how a voltage drop for your charging system would have an impact on your tune?
While I'm sure my car experiences the same voltage changes, My fuel pressure (using a CJM stg 1) is rock solid regardless.
Edit: also I will assume you have twin walbro 255's? Arent these pumps internally voltage regulated so that even if the voltage input varies they still produce the same output (within a range of course, say 12v-14.4v for example).
Last edited by SpoilsofWar; 05-21-2008 at 11:18 AM.
05-21-2008, 06:01 PM
#5
Voltage drop effects the tune because fluctuating voltage will also fluctuate the fuel pressure. It's the pattern that my tuner observed while tuning my car and it's pretty constant... Fuel pressure drops when the voltage drops. That's why he had to set the fuel pressure when the voltage was reading 13.5V...
I do have CJM twin pump assembly, and run power directly from the battery with relays. I'm not sure if walbros internally regulate voltage, but I doubt it since it shows different fuel pressures when the voltage is 14.5 and 13.5V.
I thought JWT tune cars by raising voltage to a fuel pump in order to raise the fuel pressure... It's the same logic, but opposite...
So are you guys telling me that voltage drop is normal?
I do have CJM twin pump assembly, and run power directly from the battery with relays. I'm not sure if walbros internally regulate voltage, but I doubt it since it shows different fuel pressures when the voltage is 14.5 and 13.5V.
I thought JWT tune cars by raising voltage to a fuel pump in order to raise the fuel pressure... It's the same logic, but opposite...
So are you guys telling me that voltage drop is normal?
05-21-2008, 06:06 PM
#6
Originally Posted by westpak
my voltage is usually around 13.7 or so and fluctuates slightly
the voltage would affect your tune but it would have to be a little more drastic than what you are seeing, and major concern is if you see a big drop at load when you need the fuel for your tune.
that being said if you want you can always run a power wire directly to the pump and hook it up to the pump relay
the voltage would affect your tune but it would have to be a little more drastic than what you are seeing, and major concern is if you see a big drop at load when you need the fuel for your tune.
that being said if you want you can always run a power wire directly to the pump and hook it up to the pump relay
With CJM's twin fuel pump setup, fuel pressure is solid throughout, but as voltage drops to 13.5V, about 10psi of fuel pressure drops from our original setting. I do run power wires with relays directly from the battery. Battery voltage is 13.5V when hot...
Idle Boost
14.5V 45 6513.5V 35 55
Last edited by leeboyNY; 05-21-2008 at 07:02 PM.
05-21-2008, 06:15 PM
#7
Originally Posted by leeboyNY
I thought JWT tune cars by raising voltage to a fuel pump in order to raise the fuel pressure... It's the same logic, but opposite...
So are you guys telling me that voltage drop is normal?
So are you guys telling me that voltage drop is normal?
Regardless, voltage "fluxuation" is nothing new or really significant, especially not in such a small range (1v +-) as you are describing. I can draw from my car audio install days and know that the operating voltage in a vehicle's electrical system will often vary from the mid/high 12v range to 14v range. Like I said before I have not personally witnessed any voltage drop in my Z, but I have never had a reason to go looking either - I'm sure it is probably there to some degree. However, my fuel pressure at idle, remains extremely consistent.
Just so we know what we are talking about here, exactly how many psi will your fuel pressure change with the .7-1.0v voltage drop you are describing.
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05-21-2008, 06:16 PM
#8
Okay, you posted while I posted...
10psi of fuel pressure change is a HUGE difference based on a very small change in voltage. There is something else at play here.
I would direct this question to Charles.... Maybe this is an issue specific to the twin pump setup.
10psi of fuel pressure change is a HUGE difference based on a very small change in voltage. There is something else at play here.
I would direct this question to Charles.... Maybe this is an issue specific to the twin pump setup.
05-21-2008, 07:06 PM
#9
Originally Posted by SpoilsofWar
I thought the JWT system used an auxillary pump like vortech? I dont know, maybe I am confusing the two.
Regardless, voltage "fluxuation" is nothing new or really significant, especially not in such a small range (1v +-) as you are describing. I can draw from my car audio install days and know that the operating voltage in a vehicle's electrical system will often vary from the mid/high 12v range to 14v range. Like I said before I have not personally witnessed any voltage drop in my Z, but I have never had a reason to go looking either - I'm sure it is probably there to some degree. However, my fuel pressure at idle, remains extremely consistent.
Just so we know what we are talking about here, exactly how many psi will your fuel pressure change with the .7-1.0v voltage drop you are describing.
Regardless, voltage "fluxuation" is nothing new or really significant, especially not in such a small range (1v +-) as you are describing. I can draw from my car audio install days and know that the operating voltage in a vehicle's electrical system will often vary from the mid/high 12v range to 14v range. Like I said before I have not personally witnessed any voltage drop in my Z, but I have never had a reason to go looking either - I'm sure it is probably there to some degree. However, my fuel pressure at idle, remains extremely consistent.
Just so we know what we are talking about here, exactly how many psi will your fuel pressure change with the .7-1.0v voltage drop you are describing.
GSS342HP pump at 70psi
@12v 46gph ~460whp
@13.5v 56gph ~ 550whp
@16.5v 73gph ~750whp
Keep in mind that this is at the pump head flow and under perfect conditions. But as you can see voltage makes a huge difference. You should also upgrade the wiring that runs to the pump power and ground. Running 16.5v to the pump will shorten its life so the booster uses a boost pressure switch to turn the booster on when intake manifold sees boost. Another concern is say your racing at night, lights on stereo cranking your fuel pump might not be seeing 14.6v, or even 13.5v it could be lower and pump flow will suffer. This is a good reason to justify the fuel pump voltage booster.
@12v 46gph ~460whp
@13.5v 56gph ~ 550whp
@16.5v 73gph ~750whp
Keep in mind that this is at the pump head flow and under perfect conditions. But as you can see voltage makes a huge difference. You should also upgrade the wiring that runs to the pump power and ground. Running 16.5v to the pump will shorten its life so the booster uses a boost pressure switch to turn the booster on when intake manifold sees boost. Another concern is say your racing at night, lights on stereo cranking your fuel pump might not be seeing 14.6v, or even 13.5v it could be lower and pump flow will suffer. This is a good reason to justify the fuel pump voltage booster.
05-21-2008, 07:17 PM
#10
Hmm... What do you guys think about this Kenne Bell's boost a pump unit?
I would just set it at constant voltage..
http://www.kennebell.net/accessories...boostapump.htm
I would just set it at constant voltage..
http://www.kennebell.net/accessories...boostapump.htm
05-23-2008, 04:20 AM
#11
voltage should not effect fuel pressure unless you are maxing out the pumps.
the regulator will maintain a steady pressure thru idle and cruising regardless of pump voltage. the only time the pressure should drop due to low voltage is if you are using the pumps to their capacity already and they can no longer keep up at that voltage.
i dont think that it is voltage that is making your pressure move around.
there is nothing specific to the twin pump assembly that should cause that either, its just 2 pumps in a tank of gas like anything else.
maybe during cold start your car has a higher idle that causes less vacuum and higher fuel pressure?
the regulator will maintain a steady pressure thru idle and cruising regardless of pump voltage. the only time the pressure should drop due to low voltage is if you are using the pumps to their capacity already and they can no longer keep up at that voltage.
i dont think that it is voltage that is making your pressure move around.
there is nothing specific to the twin pump assembly that should cause that either, its just 2 pumps in a tank of gas like anything else.
maybe during cold start your car has a higher idle that causes less vacuum and higher fuel pressure?
05-23-2008, 11:17 AM
#12
Originally Posted by phunk
voltage should not effect fuel pressure unless you are maxing out the pumps.
the regulator will maintain a steady pressure thru idle and cruising regardless of pump voltage. the only time the pressure should drop due to low voltage is if you are using the pumps to their capacity already and they can no longer keep up at that voltage.
i dont think that it is voltage that is making your pressure move around.
there is nothing specific to the twin pump assembly that should cause that either, its just 2 pumps in a tank of gas like anything else.
maybe during cold start your car has a higher idle that causes less vacuum and higher fuel pressure?
the regulator will maintain a steady pressure thru idle and cruising regardless of pump voltage. the only time the pressure should drop due to low voltage is if you are using the pumps to their capacity already and they can no longer keep up at that voltage.
i dont think that it is voltage that is making your pressure move around.
there is nothing specific to the twin pump assembly that should cause that either, its just 2 pumps in a tank of gas like anything else.
maybe during cold start your car has a higher idle that causes less vacuum and higher fuel pressure?
Argh~~ is there a possibility that my fuel pressure regulator is acting funny when it gets hot??
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