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Aeromotive 340 Fuel Pump - What Size Drill for Swirl Jet?

Old 05-26-2013, 06:14 PM
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mx594
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Default Aeromotive 340 Fuel Pump - What Size Drill for Swirl Jet?

So I installed my 340lph pump today, to replace my Walbro 255. I also ran a 10 awg wire and a relay to power the pump directly from the battery. I have a basic fuel return setup, stock rails and stock feed line with ID1000's. To my dismay, my base fuel pressure jumped up to ~63 psi and I cannot get it to go any lower. I already drilled the swirl jet to .070" for the Walbro, so I guess the 340 flows quite a bit more. I read on the CJM website that "it might be a good idea to drill the swirl jet orifice to about .100" if using a DW300 or Aeromotive 340". Damn, I wish I would have read that sooner!!!

My question is, has anyone done this, and if so, what diameter did you have success with? I only have .093" and .109" on hand - I was leaning toward .109" but I don't know the consequences of going too big...



On a side note - I made the BIG mistake of performing the swap with a full tank of gas - do NOT do that! You will end up with a big puddle of gas on the floor of your garage, trust me.
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Old 05-27-2013, 04:33 AM
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Stock feed line, Cosworth rails, DW300, ID725's, return setup with aeromotive reg, oem power feed

From memory drilled it slightly smaller than recommended by cjm. (closer to 0.7" than 0.1")

Base pressure is about ~48psi (no vac line), and 43 psi on idle

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Old 05-27-2013, 05:32 AM
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Thank you for the data point. Maybe I will try .093" first. I suppose its better to stay on the small size, since you cant go the other way!
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Old 05-27-2013, 08:56 AM
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Well,

I pulled the pump assembly back out and drilled the jet with an .094" bit which resulted in a hole diameter of about .091". My minimum fuel pressure dropped to about 44 psi cold. Unfortunately, that's right on the ragged edge of being low enough - I have the car tuned for 51 psi base pressure, and when the vacuum line is connected that drops right to 44 psi at idle. The problem is, since I am so close to the "limit", the swirl jet is fighting the FPR to regulate the pressure and it drops about 2-3 psi as the fuel warms up. So its closer to 42 psi when warm.

In summary, an .094" bit (3/32") is not big enough for the Aeromotive 340. I think I am going to try a .109" bit now.
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Old 05-27-2013, 01:29 PM
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I pulled the pump back out and enlarged the hole again using a brand new #38 drill bit (.102") and it came out right at .100" due to the plastic deflecting a little. The pressure is now down to about 39 psi minimum when cold. I think I am going to leave it alone. I set it at 52 psi with the vacuum line disconnected, and it drops to 44 when I reconnect it. So I have about a 5 psi "buffer" for the FPR to do its job. I haven't been able to test what happens when it warms up yet though because it started raining.
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Old 05-27-2013, 04:26 PM
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As long as it is lower than what you are running with the vac line on it will be fine. You don't need a "buffer" below the lowest point.

The larger the swirl pot orifice the less vacuum that it will create to pull from the other side of the tank and it might suck air. I wouldn't go any further.

That being said, if you plan on actually being able to use those 1000's you need a larger supply line. That stock 5/16 (outside diameter) is going to choke down fuel flow.
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Old 05-28-2013, 05:49 PM
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with my external pump setup, i couldnt drop past 58psi using the stock basket. had to go back to my twin pump canister with its true -6an return.

there is nothing wrong with higher base fuel pressure. you get better atomization of gas at low pulsewidths. IRC, stock basket is set for like 58psi, so it has its limitations. Sounds like you have well low enough base fuel pressure now as it is
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Old 05-28-2013, 06:32 PM
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Originally Posted by str8dum1 View Post
with my external pump setup, i couldnt drop past 58psi using the stock basket. had to go back to my twin pump canister with its true -6an return.

there is nothing wrong with higher base fuel pressure. you get better atomization of gas at low pulsewidths. IRC, stock basket is set for like 58psi, so it has its limitations. Sounds like you have well low enough base fuel pressure now as it is
I think it's 3.5 bar. I think that's roughly 52psi. Either way it's well above what he's running.
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Old 05-25-2019, 04:13 PM
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Bringing up and old thread, but does anyone know the size of hole needed to run a 485lph pump and keep my fuel pressure 53psi? With it drilled out to .080 60psi is as low as i can set my fpr.
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Old 05-25-2019, 10:07 PM
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Originally Posted by Conway_160 View Post
Bringing up and old thread, but does anyone know the size of hole needed to run a 485lph pump and keep my fuel pressure 53psi? With it drilled out to .080 60psi is as low as i can set my fpr.
When I installed my Walbro 485 fuel pump, I couldn’t get base fuel pressure below 60 psi either, ended up drilling out the swirl jet to .100” (#38 drill bit) that solved the issue.
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Old 05-26-2019, 03:51 AM
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Originally Posted by thatv35guy View Post
When I installed my Walbro 485 fuel pump, I couldn’t get base fuel pressure below 60 psi either, ended up drilling out the swirl jet to .100” (#38 drill bit) that solved the issue.
Thank you!!!
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