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Fuel Return: What Size Hole?

Posted By: YO7_A66

Fuel Return: What Size Hole? - 08/23/13 01:26 AM

I am making a fuel return block and my current return barb fitting has a 3/16" ID. I am assuming that this is too large and I am planning on adding a fitting to the inside and just drilling the required hole size.
My old filter with a return line has a 1/16" hole for the vapor return and the psi held steady at 6psi at idle. Should I drill it at 1/16" or just let the return line have the 3/16" ID to keep the fuel moving?

Note: This is for an electric fuel pump.

Thanks
Posted By: JohnRR

Re: Fuel Return: What Size Hole? - 08/23/13 01:29 AM

Fuel return block ? Do you have a regulator ? you should and it will only return what doesn't get used.
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/23/13 01:51 AM

The pump is a street pump and it only puts out 7psi max.
I am pretty sure that I have read some threads of members making their own return blocks (T's) and I am wanting to know what size restriction works. So far I have found threads showing .060-.080" in size.
Thanks
Posted By: lewtot184

Re: Fuel Return: What Size Hole? - 08/23/13 02:34 AM

i use a vapor separator with a mechanical pump and reduce the return orifice size to .035".
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/23/13 02:44 AM

How did you decide on the .035?

Does anyone know the thread size of a Holley/QF main jet? Is it 1/4-32?

Thanks
Posted By: RapidRobert

Re: Fuel Return: What Size Hole? - 08/23/13 02:57 AM

Quote:

My old filter with a return line has a 1/16" hole for the vapor return and the psi held steady at 6psi at idle. Should I drill it at 1/16"

If the setup is the same except your using the restriction you want to drill into the fuel block instead of the 1/16" vapor return filter orifice restriction then yes I'd drill the restriction in the fuel block to the same 1/16" & see if you still have 6psi you want at the carb inlet then adj the orifice from 1/16" (if need be)
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/23/13 03:03 AM

With the NAPA vapor return/filter with the .060" return orifice gave me 6+psi at the carb. If I make the orifice smaller, would the pressure at the carb go up until the fuel pump is maxed out?
Thanks
Posted By: RapidRobert

Re: Fuel Return: What Size Hole? - 08/23/13 03:06 AM

yes, not sure how much tho
Posted By: ahy

Re: Fuel Return: What Size Hole? - 08/23/13 03:13 AM

With no regulator, "rule of thumb" the orifice should be .050". Too big and fuel pressure drops. Too small, there is not enough re-circulation to keep things cool and free of vapor.
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/23/13 11:46 AM

Good info, thanks!

I have modified my return line barb fitting for a plug. I will drill the plug with the appropriate hole size before I install it.
If I can find a 1/4-32 tap, I may tap down farther into the barb fitting for a jet. Then I can test different size holes to see if the fuel pressure changes.

Thanks again to everyone.

Attached picture 7824486-FuelReturnOrifice.JPG
Posted By: lewtot184

Re: Fuel Return: What Size Hole? - 08/23/13 11:53 AM

Quote:

How did you decide on the .035?

Does anyone know the thread size of a Holley/QF main jet? Is it 1/4-32?

Thanks

i arrived at .035" thru some flow and pressure tests. what kind of pump are you using that will maintain 6psi with a .060" orifice?
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/23/13 12:08 PM

I am using a Holley Red pump. My fuel pressure at idle always shows 6+ (apx 6.2-6.5) at idle with the NAPA vapor/fuel filter that has a .060 return hole.

I have a .052" drill bit that I might start with. I have plenty of plugs to drill out if I can't find a tap.

Thanks
Posted By: lewtot184

Re: Fuel Return: What Size Hole? - 08/23/13 02:41 PM

a constant flow pump will probably react differently than a pulse. if .060" is working good for you now then stick with it.
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/24/13 03:19 PM

I decided to start a little smaller on the hole size with a .052" return hole. I now have a solid 6 3/4psi at idle!
Going from the .060" to the .052" moved the idle psi from 6 1/4 to 6 3/4! I am keeping it there since the pump is rated at 7psi.
Before I installed the return block, I was running water through it to clean it out before I installed it on the car. I was surprised how much fluid can come through such a small hole. This should still allow good circulation back to the tank.

Thanks allot for the replies!

Attached picture 7825602-VaporReturnT.jpg
Posted By: RapidRobert

Re: Fuel Return: What Size Hole? - 08/24/13 03:51 PM

Success! did you drill/tap the big end of the barb fitting for an NPT plug then drill/tap the NPT plug 1/4-32 for the holley jet?
Posted By: YO7_A66

Re: Fuel Return: What Size Hole? - 08/24/13 04:07 PM

I only tapped the barb fitting for the NPT plug, then drilled the plug to .052. I had trouble locating the 1/4-32 tap, so I just drilled the plug.
Thanks for your help.
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