I am putting together a Holley MPFI on my 540.
I am assembling the fuel system, it is an external pump -10AN (Holley) and then I am using a bypass edelbrock -10AN fuel pressure regulator. My question is how do I run the lines?
Do I go into the FPR out that into the fuel rails? Or into the rails, then into the FPR? Does it matter where inline the regulator goes?

The best way is to feed each rail independently from a fuel log or “y” fitting, then come off the back of each rail to each side of the return style regulator and the return is typically a single line back to tank. This will give you the best and most consistent fuel pressure control.
AG.

Race car or street car? On a race car you can simplify the plumbing by putting the regulator in the trunk next to the fuel cell and then you only have to run a single high pressure line up to the engine.

You can put the regulator in the back but it doesn't then compensate for any pressure drop across the lines/fittings or filter.

If you have a big enough feed line, not many 90deg fittings and the filter before the regulator it doesn't matter as much.

IMO it's best to have it near the engine. Easy is feed one rail, crossover in front and return from the other rail into the regulator. (I don't like flowing through a regulator on the feed side but I know some do this).

On my car I did basically as turbobitt described.

Feed line under the intake to a Y, to both rails. Back of rails into the regulator.

awesome, thank you guys . Ill order some Y fittings then.

It is a street car.

Very nice INTMD8, I have the same intake and looks like we set it up almost the same.
AG.

On my street car I use a pump with a built in regulator mounted inside the gas tank. That way you don't need an external regulator or a return line. This is the way that most new cars are plumbed.

They are, but don't usually have rock solid pressure as rpm/fuel demand increases. Unless we are talking new new and the pump is pwm controlled and has a pressure sensor on the feed line.

Of course a few psi is no big deal if the injectors have overhead and it has a fuel pressure sensor/closed loop capabilities. (which most here would probably have)

Thanks!

For what it’s worth, I used a stock Hemi 3/8 hard line and for the return I used a stock BB 5/16 hardline run along side the supply. Keep it it stock appearing was in mind. I’m also using a Fast XFI so I plumbed in an E85 sensor just in case. I also have a special flash that has PWM pump control to keep the return flow in check. The pump I decided on was an OEM hellcat pump capable of supplying all the power you would need in an N/A engine. The tank I used is a Tanks inc. that has provisions for an internal properly sumped pump. The entire setup is whisper quiet. I have a pressure transducer to monitor fuel pressure but more importantly use it for the fuel pressure correction features in the XFI. Basically I think the system holds the pressure about +/- 1.5 psi, maybe even better.
AG.

Sounds nice! I'm running a composition sensor as well.

something to think about : running the fuel from the tank through pump & filters , through the rails, then the regulator and then returning to the tank puts heat into the fuel. For a drag car, this may not matter much. for a street car or non drag race car, this could be an issue as the fuel will get hotter and hotter during use. I'm going to be changing from that set up to one that pumps to the regulator with a return and a deadhead feed to the rails. I've got a gauge at the rail and also a sending unit to the ECU to monitor rail pressure.

It's not an issue, millions of factory vehicles built that way.

Not circulating through the rails will put less heat into the fuel in the tank, but still gets rather hot at the rails as it's heat soaking during most conditions (idle/cruise/low fuel requirements).

Wide open throttle would introduce much higher flow of course (from cooler fuel in the tank) but would need to burn off what picked up a lot of heat first.

What the OEMs do or don't do is irrelevant. We're not talking about production passenger cars. The heat can and will degrade the quality of the fuel. Circulating the hot fuel through the rails and back to the tank heats all of the fuel all of the time. dead heading only heats the small amount in the rails which is constantly being used.

It's not irrelevant, if return style was a problem every car that has that system would have fuel quality issues. (as there are many that absolutely need premium fuel octane)

Yes, dead heading heats the fuel in the rail, so it's still running on hot (hotter than return style) fuel 99% of the time until it gets flushed out with enough heavy throttle usage.

There are downsides/upsides to both.

would a larger return line than the feed line reduce heat any appreciable amount ?

The return restriction is the regulator. If the return line is so small it's limiting minimum fuel pressure above desired it needs to be larger. Otherwise it wouldn't make any difference.

Probably the best thing you could do is run a cooler/heat exchanger on the return line or run the pump pwm (slower at low demand) if fuel heat is a problem.

The regulator I use (as well as a ton of holley users) is the late model corvette style filter/regulator combo. 58psi is perfect for the Snipers on the street. It's typically placed on frame rail near tank and has a return port. Then it's simply one line "deadhead" to the motor.

I find it hard to believe that the potential "cooling" effects of round trip fuel thru maybe 20ft+ of alum lines cannot significantly cancel out any "heating" effects of engine heat into the EFI fuel rails in a moving car.

I'm with the regulator in the return line by the tank crowd.

I should add that I had a lot of people question the approach of putting the regulator in the trunk next to the fuel cell (or inside the fuel cell in some cases) when I first started to help build EFI cars but now it seems to be accepted by more people. I was able to convince a local chassis builder that it was easier, less weight, less money and less time to build a single high pressure line system. He was reluctant at first but now he has cars making passes every weekend in the 8's and 9's with single line systems.

I've heard of a few cases where a person needed to add a dampener to the fuel pressure rail but I've never run into that myself. Something to consider though if the fuel pressure trace on the data logger looks jaggy rather than nice and smooth. Data logging the fuel pressure on a race car is the best way to know if it is working or not. When people question me about the fuel line plumbing I just show them the data log. They can't really argue if the fuel pressure is nice and steady all the way down the track.