FP drop issues at 6200rpm, injectors maxxed out - LS1TECH - Camaro and Firebird Forum Discussion

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FP drop issues at 6200rpm, injectors maxxed out

Old 07-11-2018, 03:55 PM
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Default FP drop issues at 6200rpm, injectors maxxed out



I have a 67 Chevelle with a turbo 5.3. At about 6200rpm my fuel pressure drops down from 58 to 43psi on the regulator gauge. My tuner said the injectors I have are at 100% duty cycle. Here is my fuel set up.

fuel cell
10an from cell to pre filter, to AEM 380LPH pump to a post filter to a check valve

10an from check valve to the engine compartment. Then it Yís off into 2 8an feeds that connect to the back of Each fuel rail.

The front of of the driver side rail has the Aeromotive FPR boost referenced attached and the passenger side rail has a 8an crossover tube that attaches to the fpr.

6an return off the regulator to the tank.

I have deka 80lb injectors, ls9 cam. This thing will go to 7k no problem but I noticed after 6200rpm the fp drops from 58 to 42 or 43.


my fuel pump has a chassis ground, and I made sure to grind the metal where the ground is attached. The 12v+ goes from the battery, to a breaker, and to the relay, all the way to the pump. I used 12ga wire.

Iím not sure if the FP canít hang st 6000+ rpms or if these injectors arenít big enough etc...

i didnít check pump voltage when it was being dynoíd.

Car made 595/513 through a th400 and 9Ē.

please, any suggestions would be awesome
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Old 07-11-2018, 04:10 PM
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80 lb injectors should have more than enough flow to support 600rwhp. I would suspect that your pump is not up to the task for some reason (power, flow, etc.).
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Old 07-11-2018, 06:17 PM
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Originally Posted by ss.slp.ls1 View Post
80 lb injectors should have more than enough flow to support 600rwhp. I would suspect that your pump is not up to the task for some reason (power, flow, etc.).
do you know if itís better to run the 12v for the pump directly off the alternator? Iím thinking the same thing about the pump but others have ran it and made 800
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Old 07-11-2018, 10:16 PM
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Do you have datalogs that show battery voltage? Its pretty common for some of the LS alternators to have trouble holding voltage at high RPM. Reduced voltage = less pump flow.

Does your AEM380 have the 8AN check valve outlet? It reduces the overall power capability of the pump, especially if you are running another inline check valve.
Also, going overkill on fuel line sizing can cause issues maintaining fuel pressure under certain circumstances, as the pump will struggle to keep a larger volume pressurized.
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Old 07-11-2018, 11:30 PM
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Originally Posted by JoeNova View Post
Do you have datalogs that show battery voltage? Its pretty common for some of the LS alternators to have trouble holding voltage at high RPM. Reduced voltage = less pump flow.

Does your AEM380 have the 8AN check valve outlet? It reduces the overall power capability of the pump, especially if you are running another inline check valve.
Also, going overkill on fuel line sizing can cause issues maintaining fuel pressure under certain circumstances, as the pump will struggle to keep a larger volume pressurized.

i will ask my tuner if they can send me the voltage log. Do you think 10an to the motor is too much? I didnít want to restrict it with a smaller hose.. and yes I believe I used that check valve you speak of but I donít think it really worked.. the fuel would drain back to the tank on cold starts it was a pain to start due to no fuel.

Im willing to bet itís a voltage issue at high rpm. Why would the pump maintain 70psi then immediately drop to 40 after 6200rpm. Sounds like a voltage issue the more I read on here. I ultimately need to run a voltmeter to it and see what happens at high rpm
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Old 07-12-2018, 09:32 AM
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Racetronix instructions state to always run their pumps directly to the alt rather than to the battery. It's why they include an upgraded hot wire kit with wire sizes larger (10 guage I recall) than stock with their larger pumps. Makes sense because your engine and other electrical components run off of your battery.

Heres what i got directly from Racetronix's website:
http://www.racetronix.biz/itemdesc.a...%2D001&eq=&Tp=
High performance fuel pumps need a good power source in order to perform to their full potential. On average a Walbro high-performance fuel pump requires twice as much power as a factory fuel pump. Factory wiring is just enough for a factory pumps power requirements because manufacturers need to save cost and weight. Racetronix field tests have shown that running a high-performance fuel pump off of the factory wiring can cut pump performance by 30% or more. Problems with failing factory relays, connectors/terminals and fuse contacts are also common when excessive load is placed on them.

The Racetronix plug and play harness is designed to remove all fuel pump load from the factory pump circuits while providing the pump with a new heavy-duty power source. The Racetronix harness gets its power directly from the alternator's battery terminal. The voltage at the alternators output is typically 1-2 volts higher than at the battery when the motor is running (high charge/load conditions and WOT). This means that your Walbro high-performance pump can provide as much as 20% more fuel in some cases. The Racetronix harness uses high-quality automotive-grade wire which is resistant to oil, gas, heat and rot. The main length of wire in the Racetronix harness is ten-gauge stranded copper. This is three to four gauges thicker than factory wire which helps reduce resistive power loss to almost non-existent levels. This helps improve pump performance by as much as another 10-20%.High-quality weatherproof fuse holders and relays are used for maximum reliability. The Racetronix harness has factory-type connectors which plug directly into your fuel pump module/sender so no cutting or splicing is required. The Racetronix harness has a redundant ground upgrade as it is necessary to improve the harness on the negative side of the pump's supply as well.

Consider also if you are running dual high draw Spal (or other brands) fans which are adding additional load on the battery.

Last edited by dlandsvZ28; 07-12-2018 at 09:43 AM.
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Old 07-13-2018, 08:58 AM
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Originally Posted by dlandsvZ28 View Post
Racetronix instructions state to always run their pumps directly to the alt rather than to the battery. It's why they include an upgraded hot wire kit with wire sizes larger (10 guage I recall) than stock with their larger pumps. Makes sense because your engine and other electrical components run off of your battery.

Heres what i got directly from Racetronix's website:
http://www.racetronix.biz/itemdesc.a...%2D001&eq=&Tp=
High performance fuel pumps need a good power source in order to perform to their full potential. On average a Walbro high-performance fuel pump requires twice as much power as a factory fuel pump. Factory wiring is just enough for a factory pumps power requirements because manufacturers need to save cost and weight. Racetronix field tests have shown that running a high-performance fuel pump off of the factory wiring can cut pump performance by 30% or more. Problems with failing factory relays, connectors/terminals and fuse contacts are also common when excessive load is placed on them.

The Racetronix plug and play harness is designed to remove all fuel pump load from the factory pump circuits while providing the pump with a new heavy-duty power source. The Racetronix harness gets its power directly from the alternator's battery terminal. The voltage at the alternators output is typically 1-2 volts higher than at the battery when the motor is running (high charge/load conditions and WOT). This means that your Walbro high-performance pump can provide as much as 20% more fuel in some cases. The Racetronix harness uses high-quality automotive-grade wire which is resistant to oil, gas, heat and rot. The main length of wire in the Racetronix harness is ten-gauge stranded copper. This is three to four gauges thicker than factory wire which helps reduce resistive power loss to almost non-existent levels. This helps improve pump performance by as much as another 10-20%.High-quality weatherproof fuse holders and relays are used for maximum reliability. The Racetronix harness has factory-type connectors which plug directly into your fuel pump module/sender so no cutting or splicing is required. The Racetronix harness has a redundant ground upgrade as it is necessary to improve the harness on the negative side of the pump's supply as well.

Consider also if you are running dual high draw Spal (or other brands) fans which are adding additional load on the battery.
Seeing as how he doesn't have an F-body or a Racetronix pump, none of this applies. He doesn't have to deal with the weak F-body wiring, hot wire kits, or Racetronix's failure rates.
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Old 07-13-2018, 10:42 AM
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Originally Posted by JoeNova View Post
Seeing as how he doesn't have an F-body or a Racetronix pump, none of this applies. He doesn't have to deal with the weak F-body wiring, hot wire kits, or Racetronix's failure rates.
I was thinking the same thing.. had an f body once, dealt with all that. This is different. More simple
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Old 07-13-2018, 11:14 AM
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Originally Posted by JoeNova View Post
Seeing as how he doesn't have an F-body or a Racetronix pump, none of this applies. He doesn't have to deal with the weak F-body wiring, hot wire kits, or Racetronix's failure rates.
I was under the impression that certain principles of electricity applies across the board irregardless of brand. I do agree the wiring on my twenty year old 1998 Camaro is getting pretty brittle in some places. Hard to tell what it will be like when it reaches the age of the 67 Chevelle that is 51 years old.

To the OP:

Most alternators max out at 18000 rpm so when your alternator exceeds that threshold it stops producing. Do a search to verify that is accurate. Do a search also on alternators taking a dump when the engine rpm exceeds 6K. To compensate I think you will find that some increase the pulley size on the alternator to slow down the rpm.

There are calculators on line that will show you your rpm based on the size of the crank pulley and alternator pulley at 6200 rpm and at 7000 rpm. At 6200 rpm I suspect your are exceeding the 18000 rpm threshold.

I think you will find also is that you will get best results if you connect your pump (regardless of brand) directly to the alternator with high quality wire (as many strands as possible) - under 10 feet 12 gauge over 10 feet 10 gauge (AEM specs). AEM states you should have 13.5 volts at the pump.
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Old 07-13-2018, 11:33 AM
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Originally Posted by dlandsvZ28 View Post
I was under the impression that certain principles of electricity applies across the board irregardless of brand. I do agree the wiring on my twenty year old 1998 Camaro is getting pretty brittle in some places. Hard to tell what it will be like when it reaches the age of the 67 Chevelle that is 51 years old.
And you're still assuming he's using factory wiring. Mind telling us how many 67 Chevelles came with an electric fuel pump?

If you want to follow principles of electricity, as long as adequate wiring is being used directly from the alternator to the battery, then running fuel pumps directly from the battery is fine. Factory F-body battery wiring also doesn't apply here.

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Old 07-13-2018, 12:58 PM
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No - I did not assume the OP is still using factory wiring - his post said 12 gauge. My first post referred to 10 gauge wire.

Yes I know that these old cars didn't have electric fuel pumps. I changed out many fuel pumps on my 55 chev, 56 chev, and 68 GTO (at least three as I recall on it alone). They didn't seem to last very long perhaps due to all of the heet we had to add to each tank of gas in the winter to keep fuel lines from freezing. Might be wrong but recall it was the bladder that would fail - its been a long time.

But none of this helps the OP solve his issue. IMO his pump isn't getting 13.5 volts at 6200 rpm.

This will be my last post in this thread.
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Old 07-21-2018, 12:51 PM
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what fuel?

what ECU and are you using gm wiring anywhere in relation to pump signal. some LS1 PCMs reduce pump voltage from the ECU when the max torque value is altered
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Old 07-29-2018, 11:47 PM
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Originally Posted by Luke19901 View Post
what fuel?

what ECU and are you using gm wiring anywhere in relation to pump signal. some LS1 PCMs reduce pump voltage from the ECU when the max torque value is altered

its the pcm that ya the blue and red connectors. I ran 12ga wire from the pump relay to the pump. Only factory wire Iím using is the pcm fuel pump wire and thatís used as a relay trigger
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Old 08-01-2018, 02:44 PM
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call your power 600rwhp for ease and say 20% drive train loss leaves you at 720flywheel HP.

calculators show on E85 with boost to make 720flywheel HP and not exceed a safe limit of 80% injector DC you need minimum 95lb/hr injectors.
gasoline 73lb/hr injectors.

calculators show on E85 with an estimated 11psi boost and 58psi base pressure to make 720flywheel HP you'll need minimum 370ltrs/hr pump flow
gasoline 300ltrs/hr

if you're on E85 your beyond your fuel system. if you're on gasoline you're not too far from its limit.

drop in pressure comes only from not enough flow/volume. be that from a pump that's too small, a blocked filter or kinked hose it will only drop from not enough flow.
injector DC being at 100% wont cause pressure drop. your pump needs to be capable of flowing enough fuel to maintain pressure beyond 100% inj DC otherwise its too small.

your tuner will be commanding extra PW to the injectors to try and add fuel where its leaning out before the pressure drops away. if pressure is dropping at 6200rpm its likely out of flow by 5700rpm-5800rpm. the lack of flow is making him add inj PW but without the flow it will have little effect.

when he has enough pump flow the inj DC will likely fall as he'll command less PW to reach target AFR
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Old 08-02-2018, 11:47 AM
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How long is the 12ga. wire circuit?
Aem wiring instructions state "The supply voltage will affect the fuel delivery of the AEM Fuel Pump. The typical electrical system on
modern cars is between 13.2 and 14.2 volts. Although the AEM fuel pump will run at lower voltages the flow will be lower.
Ensure the voltage is 13.5V at the pump.
The current requirement is a FUSED circuit capable of conducting 25 amps. Failure to use a fuse WILL cause a fire hazard in the event the pump
The correct wire size will be determined by the length of wire, the wire type and the resistance of any terminals, splices or solder joints in the electrical or ground supply.
The ground is equally important and the preferred ground is to route the ground wire to a star ground source that is directly attached to the battery negative post.
The minimum wire gauge is 14 ga. TXL wire. Twelve feet (12í) of TOTAL CIRCUIT length (power & ground) 12 ga. is required and Twenty feet (20í) 10 ga. is required"






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Old 08-02-2018, 12:55 PM
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Fuel pressure drop usually indicated pump issues, not injector issues. You can make out injectors and then some without losing fuel pressure.
Your problem is likely in your pump. Whether it be voltage, restriction, or just flat maxed out.
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Old 08-02-2018, 01:40 PM
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The battery,, even under load,, can produce about 4x the current of the alternator,,
a decent full size battery will product well over 400A for 30 seconds continuous..
For electric fuel pumps, 10 GA wire and home run BOTH the hot and the ground to the battery.
I can't coun't how many road race cars with fuel problems I've fixed with a direct ground wire.
50A relay too ,, the 30's drop voltage too much and don't last..

The engine block, battery and alternator frame need to have 100% solid ground connectivity..

First update I do on a car is a new 8 gauge charging wire from the alternator to the battery and I
use Welding style high strand count cable not off the shelf wire.. Your alternator will run cooler, and all your lights
will be nice and bright. You won't see near the voltage drop to the rest off the car electrics..

Updating the supply wire to the engine ECU is another place where big bore wire pays off..
Also the power to the coil packs and the injectors.. go up to at least 12 on the injectors..

If you get the operating current numbers for the injectors and coil packs and add them together,,
its a significant load. I measured a Comp Z06 a few years ago at like 80A draw to the coils at 6500 RPM..
The injectors were between 40 and 60 amps, all of the wires were 14 gauge.. or less..
Ok if each wire home runs to the source, but some engine harnesses break the individual
coil hot wires out on top of the motor not at the relay.. so 1 14g wire branches to 8. The wire ends up
acting as a resistor and voltage to the coil itself drops dramatically.. The engine wouldn't
reliably rev past 7K till I upgraded the coil feed wires in the harness.
(engine was over 700HP NA running on Sunoco 114 Race gas.. )
Drove me and the engine guy nuts for about 4 days trying to figure out why the dang thing would not hit the rev limiter at 8200..
Took a few hours with a O-scope to figure it out..
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