I am trying to retain my mechanical fuel pump while also using triple Mikuni PHH44s, and ALSO utilize the return line.

I’ve been reading that it would be advisable to use a fuel pressure regulator (and also a fuel cooler) but I have only been able to find FPRs that go before the carburetors in the fuel circuit. I can’t do this and keep the mechanical fuel pump do to space limitations (see here)and a lack of good mounting options.

I have seen several mentions of putting the FPR after the carburetors, mostly in threads about keeping the return line. In order to do that I would need a back pressure regulator, but I haven’t found one that works with gasoline or is intended to be used with carburetors (under 5 PSI).

Is anyone aware of one out there? Ideally it would be adjustable down to 2 PSI to dial it in for the Mikunis.

I emailed Aeromotive about their Ultra-low Preassure FPR for carburetors and whether they have something that would work for my application, but they were not able to solve it. At best I could try to use their X1 carburetor FPR, which is a bypass-style regulator adjustable from 3-20, but they didn’t sound confident about that. Maybe I’m not asking the right question or don’t understand how this works well enough.

Any help is much appreciated!

Edited May 6, 2024May 6 by Matthew Abate

What about an orifice restriction like the factory used. You would need a visible fuel pressure gauge to verify and fine tune in real word conditions

18 minutes ago, Patcon said:

What about an orifice restriction like the factory used. You would need a visible fuel pressure gauge to verify and fine tune in real word conditions

I’ve been thinking about using the cooling bodies to do that, but I don’t know if the fuel flows through them continuously or only under certain conditions.

To do this I would have the fuel go to the carbs in a deadhead configuration and tap the end of the fuel rail for a fitting the same size as the cooling body inputs. A gauge could go here. Then the only thing to figure out is how to run the hose from the front cooling body to the return line.

Edited May 6, 2024May 6 by Matthew Abate

When you say backpressure regulator, are you talking about a bypass style regulator?

Hoping maybe its a search term issue?

I think he's referring to the stock fuel rail for the S U's where at the end of the rail where the return connects to the flexible fuel return line the end of the fuel rail is tapered down to restrict outflow to slightly pressurize the fuel rail.

When I say back pressure regulator, I mean a regulator that controls pressure before it in the sequence rather than after it. Unless I am mistaken, a bypass regulator controls the pressure after it by relieving the excess pressure through a second port (the bypass), but the pressure before the regulator would still be unregulated.

I’m using the Datsun Competition rail. Here are some photos from my build thread to help clarify:

I'm still thinking you are looking tor a bypass style regulator.  Maybe this helps?

https://www.jegs.com/tech-articles/how-a-fuel-pressure-regulator-with-return-a-bypass-regulator-work/

Have you got any websites with descriptions of what you are calling a back pressure regulator?

That is a Great article!! Thanks @Captain Obvious

6 hours ago, Captain Obvious said:

I'm still thinking you are looking tor a bypass style regulator.

I think you’re right after reading that article. Thanks for that! It was super helpful.

So back-pressure regulators are all over the place in other applications. I can find info about them for construction and engineering, but not in an automotive context. I know about them from projects working with gasses and other liquids and just assumed they would be available for gasoline engines, but it looks like they just aren’t used in cars.

I’ll dig into how a bypass regulator manages upstream pressure more and figure something out. If anyone has a setup like this they can share photos of, I’d appreciate seeing them. There’s not a ton of good documentation about placement and line routing in Z cars.

Checking mine now from Datsun Spirit and will report back

Exactly what @Captain Obvious was talking about and what I am going to install. If interested I can follow up with more instructions.

@Yarb

That would be fantastic! Thanks