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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 02-24-2001, 09:07 PM   #1
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Can I use an electric inline fuel pump with my carb...using NO regulator!??

Ok, I've heard that I can use an inline electric fuel pump with my carb, by itself, with no regulator needed!! I'm wondering if anybody has any opinions or experience about this!! I've had one guy tell me that he's running an electric back by the tank AND a mechanical too!! He said that when he was running the electric alone that he noticed the fuel pressure at the carb would drop while accelerating and increase while braking! And he said is was a BIG increase and decrease and so an electric should not be used alone w/out a regulator!! Any info on this would be greatly appreciated!!

------------------
1986 Trans AM
305 TPI
200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.73's....oh joy) :P
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Soon to upgrade to a 350 converted from TPI to Carb, Edelbrock 750CFM Carb, Edelbrock Performer RPM Intake, Headman Shorty Headers, Some sort of ported heads (undecided), XE268 Cam, Moroso HEI ignition kit with external MSD Blaster II Coil and an MSD 6-AL Box!!
Current project: Keeping my 305 running until I get my income tax returns!
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Old 02-25-2001, 01:11 AM   #2
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Let me qualify what I said earlier. If you run an electric pump alone back by the TANK then, yes, you will need a regulator. I suppose you could run a LOW PRESSURE electric up near the engine and get away with it. The problem with putting ONLY a LOW pressure electric back by the tank and running the line forward with no regualtor is that the wieght of the fuel in the fuel lines actually causes pressure fluctuations under acceleration/deceleration. And they were far more severe than I thought they would be when I actually hooked up a guage and measured them. Pressure was all over the map.

And if you got the other way- with a HIGH pressure electric out back you definitely need a regulator cause a carb really can't take more than about 7PSI of fuel pressure without overwhelming the inlet valve.
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Old 02-25-2001, 02:15 AM   #3
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I use low pressure Carter pumps back by the fuel tank without any regulator, and I've never had that problem with fuel pressure.

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Old 02-28-2001, 10:13 AM   #4
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There are electric pumps made to work with a carb without a regulator/return line. Holley and carter are two names that come to mind.
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Old 02-28-2001, 03:29 PM   #5
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How do you know you've never had any problems with the pressure? Unless you put a guage on the fuel line next to the carb and watch shat it does under acceleration/braking you'll never know. A 6PSI pump mounted back by the tank will NOT hold a constant 6 PSI up at the carb.

As you accelerate/decelerate you WILL and MUST get pressure fluctuations. The g-forces of accelerating/decelerating act EXACTLY the same as if you were pumping the fuel uphill(accelerating) or downhill (decelerating).

He11, if you don't beleive me I'll take my camcorder for a ride in my car with me, turn it into an AVI clip and show you the fluctuations first hand.
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Old 02-28-2001, 03:37 PM   #6
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Seriously though. There is no disadvantage to running a regulator. I don't see the point of not doing it. There are other places to scrimp and save.

Edit: I've been thinking about buying a new computer that can handle editing visual images just to prove my point also Click the image to open in full size.

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[This message has been edited by 84FTA (edited February 28, 2001).]
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Old 02-28-2001, 05:06 PM   #7
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Quote:
<font face="Verdana, Arial" size="2">Originally posted by Damon:
How do you know you've never had any problems with the pressure? Unless you put a guage on the fuel line next to the carb and watch shat it does under acceleration/braking you'll never know. A 6PSI pump mounted back by the tank will NOT hold a constant 6 PSI up at the carb.

As you accelerate/decelerate you WILL and MUST get pressure fluctuations. The g-forces of accelerating/decelerating act EXACTLY the same as if you were pumping the fuel uphill(accelerating) or downhill (decelerating).

He11, if you don't beleive me I'll take my camcorder for a ride in my car with me, turn it into an AVI clip and show you the fluctuations first hand.
</font>
what kind of pump and what size fuel line are you using?
and how many volts is the pump getting?
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Old 02-28-2001, 10:52 PM   #8
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I'll have to agree with ODB!! I think if the pump was mounted closer to the engine compartment (like around the seat area underneath the car) then it would do much better! Also, by hooking up a relay you're gonna get voltage directly from the battery to the pump which is usually 14 volts when my car is running....and more voltage means more pressure from the pump!! By hooking the pump into the fuse block it won't get the full voltage that you would get directly from the battery because based on wire size, distance, and other factors involved, you're going to lose voltage!! It's the same as audio installation in a car!! The bigger the wire, the more voltage you can get through the wire to the amp....or pump in this case!! Small wires act as a "bottle neck" you could say!!

Thanks ODB for all the info....it's helped a lot!! Click the image to open in full size.

------------------
1986 Trans AM
305 TPI
200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Soon to upgrade to a 355 converted from TPI to Carb, Edelbrock 750CFM Carb, Edelbrock Performer RPM Intake, Hedman Shorty Headers, Stock casting pocket ported heads (2.02/1.60 - 65cc), XE268 Cam, Moroso HEI ignition kit with external MSD Blaster II Coil and an MSD 6-AL Box!!
Current project: Keeping my 305 running until I get my income tax returns!
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Old 02-28-2001, 11:25 PM   #9
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I use 10gauge wire from a 16-volt Jacobs Accuvolt to power my pumps.
My car only runs in the tens and doesn't pull wheelies at all so it's really not that much G-force.

I could see a big wheelie car or a dragster having a problem with pressure if they were using very large fuel lines.

no problem on the info.

ODB
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Old 06-13-2003, 07:08 AM   #10
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You shouldn't lose voltage, current maybe.
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Old 06-15-2003, 06:36 PM   #11
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Guys, this fuel pressure variation due to g-loading of which I speak can be as much as 3 PSI in a car that only runs 13s. Personal expereince. Fer' Crissakes slap a damned gague on the car (without a regulator) and make a run. If there's a fuel pressure change under g-loading you'll see it instantly! Go make some runs and see for yourself.

The low pressure fuel pump I posted about is a Carter 6 PSI electric street pump. 72 GPH, 6 PSI. Mounted back by the tank. It gets FULL voltage at all times thanks to a fuel pressure relay circuit (not wired directly into some random fuse box location). Constant voltage at the pump has been verified under all conditions. Fuel line is 3/8" all the way up- plenty for a 13 second slug like mine. It's not the fault of the fuel pump. It's the long push up to the engine under load.

That's part of the reason why electric pumps are ALWAYS used with regulators. You push more pressure than you need and then regulate it down to what the carb can handle right up in the engine bay. If it wasn't necessary then why can't you just put an internal bypass in the pump at 6 PSI or whatever you need and forget the added expense and annoynce of an underhood regulator?
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Old 06-15-2003, 11:35 PM   #12
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Geez oh man an OLDIE post!

But since its here ...........

Ive never had a problem runng low pressure pumps by the tank alone. Pressure with the Carter rotary is supposed to be between 5-7 PSI I measure 6 at the carb except under WOT then between 4 1/2 and 5 PSI.

The only problem I have had is some pumps are super quiet and some sound like thier cheap fish tank counterparts.
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Old 06-15-2003, 11:35 PM
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