Fuel pressure before the regulator

What should the fuel pressure be between the pump and the regulator if running twin carbs at 6psi.

Thats going to depend on whos pump and which pump. But as a generalization I would say 25-28psi is adequate for most everyone. Both our cars are at 28 at the regulator, both are BIG pumps as well and fuel pressure does not vary going down track. You also do not need 6 PSI with dual carbs unless your pump cannot keep up. Mine is a 4.5

It doesn't matter as long as your regulator can handle it. We use a high pressure EFI pump on the dyno for all engines. When we run a carb engine the 100+ psi pressure from the pump gets dropped to 6 psi. On an EFI engine the high pressure gets dropped to 60 psi. If I was building a carb car these days I'd use a high pressure EFI pump inside the tank and then use a EFI to carb regulator to drop it down. But if your car isn't set up that way then just do what you have to do.

Ok, here’s what I have. An old SX fuel systems 160 pump. I believe they were bought out by Edelbrock because their billet pumps look identical. I tested this pumps free flow at 2 gallon per minute. The pump out of the box was 12psi, but I turned it down years ago and now I read 6psi. While testing my combo on the freeway flat footing it for about 5 seconds in high gear I see it drops about 1.5 to 2 psi. Just thought I would bump it up a bit.

I’d turn the pump pressure back up to the 12psi......... or more, if it can do it without affecting reliability.


12psi from the pump is pretty low these days.
I think even the old Holley blue pump was set at 14 psi.

Test free flow with an orifice that matches the area of your needle and seat. That will give you a better idea of how much flow it can actually produce at WOT. 2gph is enough fuel for over 1000 hp but if it drops to 1 gph when using a restriction you'll be in trouble. I agree, 12 psi seems low for line pressure. That doesn't seem like it provides much margin for error.

For the flow test, I use a ball valve on the end of the hose going into the jug.
I adjust the ball valve so the system pressure is the lowest I’m comfortable with........ like 4.5psi.

Then, don’t touch the valve....... empty the jug, and do the flow test.
You want to see how much the system flows while maintaining the required minimum pressure.

For 750hp....... the flow @ pressure should be slightly better than one gallon per minute.

The free flow test at zero pressure tells you nothing useful.

We tested a system in a car once where the free flow was great, but the flow at 6psi was barely a dribble.

That’s a good test fixture idea. Thanks for the useful tip!

Test free flow with an orifice that matches the area of your needle and seat. That will give you a better idea of how much flow it can actually produce at WOT. 2gph is enough fuel for over 1000 hp but if it drops to 1 gph when using a restriction you'll be in trouble. I agree, 12 psi seems low for line pressure. That doesn't seem like it provides much margin for error. [/quote]
I had a fuel delivery problem on my old street Duster that was cause by having to fine of a fuel filter before the pump, it was a BG400 paper filter which are 10 microns, not 80+ microns that Magnafuel says to use with their pumps
I had one of heir 275 pumps and replaced it with one of their 300 pumps, both pumps free flowed through the three #6 AN lines for the six pack flow right at 1 gallon in 10 seconds
I called Magnafuel to complain about no difference in volume and was told by their tech to restrict the flow for both tests to get a accurate result, I hooked up one line to the rear carb and flow the other two lines into the 5 gallon jugs with the 300 pump and it almost double the volume during 5 tests that way
Restricting the free flow by one line increase the fuel delivery dramatically

Well, I will have to retest. This same system worked great at 5.5 psi with my single 1100 Dominator. Not sure what system uses more fuel , the Dominator or the twin 750’s with 1.325 Venturi.

If it makes about the same hp between the two induction systems, then they use about the same amount of fuel

Of course........“In theory”....... it should make more with the TR.

Won’t know til it goes down the track.

When you make more power with more air you need more fuel also, correct
I've dyno tested one tunnel ram on my old 526 C.I. mule motor, it gained 35 HP on a DTS engine dyno going from a single 1050 CFM Dominator to two of them including using the one off of the single four barrel intake, it was a Indy tunnel ram with a dominator top, the single carb intake was a Indy 440early -3 dominator intake

Turn the PUMP up all the way as high as it will go. Then use a regulator as close to the carbs as you can get it and set it to your desired number. As I say you don't need as much pressure with a 2x4 as you do with a single. But use whatever you prefer. Then test and see what you get. If nothing changes then proceed to check the pumps flow rate etc But start by turning the pump back up to max.

I will. Only reason I turned it down was because my regulator wouldn’t control it. Didn’t know pump was at 6psi and I ran it all those years that way.

I have a fuel pressure gauge between pump and regulator( at carb) and a gauge on the regulator at carb. When i turn the adjusting screw in on the pump, neither pressure gauge shows increased pressure, but when i adjust pressure at regulator both pressure gauges show the change in pressure. I dont get it. I thought the regulator at carb only controlled pressure between regulator and carb? Anyone here know how many turns on the pressure screw on the pump when these pumps left the factory pre set to 12psi? I currently have it at 1 full turn out, but not seeing any increase in pressures.

How do you have the gauge between the pump and regulator “at the carb”?

The gauge to check the pump pressure should be mounted “before” the regulator.

Pressure should be set with a small amount of fuel flowing through the regulator(a small ball valve works good here).

If you’re running a bypass regulator, you may need to block the return to set the pump pressure.

Personally, I wouldn’t be running a bypass regulator with a pump that can only generate 12psi.

The gauge in this pic should be reading “regulated” pressure...... not pump pressure.
That’s why it changes when you change the setting on the regulator.

To read the pump pressure, you need to tap into the line before it gets into the regulator.

Ok, now i understand. Dah, yes it has a return. I turned the adjuster on pump til it bottomed, then out 1 turn. Drove it today a bunch, i didn't see any issues yet.

If you look at the regulator on the inner fender, you will see another regulator which the fuel just runs thru. I tapped into the fitting there for my inside gauge thinking it would read pump pressure. Didn't see a problem doing it this way as if i ever go back to my Dominator, the regular is right there. I get it, with a return it will only have what ever pressure i set at carb.

I'd remove the regulator on the inner fender. Replace it with a union fitting and put the regulator back in a box on the shelf so if you go back to the Dominator you have it. Running two regulators in series like that is asking for problems. If you need a place to tap in for fuel pressure then use a union fitting with a 1/8 NPT port on the size.

Theoretically, should be at least double what you want at the carb to over come the G's on the launch....I am at around 20-22psi before the regulator, right off the pump last I looked.

Thinking it’s time to retire my old SX fuel pump. Looking at a Magnafuel 275 and a 300.

I've used and ran both of those pumps
If I was building a new car and needed a pump I would buy the 300 to start with
You and I both know we will want more power later so why not buy the bigger pump first
My new Drag car has a Magnafuel 500 pump in it, hoping for over 1000 HP N/A

Cab are you using that pump with a return line or dead headed?

Been running the Magna fuel 300 for many years and even on 1 plus hour drives and it's a great pump and no return needed, it returns directly back to the cell .......

Looks like a nice pump. Rated at 950hp, not sure how accurate the ratings are thou because my current SX 160 pump is rated to 1000hp. I read the manual for the MagnaFuel pump and says return can be at the pump or at the regulator. Looks promising any way.

I use the factory fittings on the pump to return to the tank, not from up front back to a stock tank or one with the tank, in the rear

Ok, but a couple things. Why do they make these return regulators if you can't return from the regulator? Other thing, the Magnafuel 300 says you can't run pump more than 1.5hrs at a time, whats up with that? I wouldn't normally, but never know.

I run 20 psi to my fuel log.

I have a Magnafuel 275 and have had issues with the seal on the shaft of the motor leaking. I had it mounted laying down so that may have an effect on it, although Magnafuels tech told me it could be mounted in any position. I got tired of replacing seals and took it off and put my old Holley Billet 150 back on.

I read a bunch on reviews on the Magnafuel 275 that were not positive. Many leaking pumps or locked up.

Magnafuel pumps are good pieces for sure and I have used them many times. If you can swing it look at the Weldon stuff. It is a much better product than Magnafuel but you pay for that. Since we switched I wont go back to MagnaFuel. Plus Weldons regulators are nicer and the low pressure one at the carb is a simple turn of a knob.
https://www.weldonracing.com/

I looked at their stuff and I agree. I have read all good about Weldon. Too pricey right now, think I’ll get the Magnafuel 300 ordered today. Thanks guys

I had a Magna 275 on my Duster, and had zero problems with it. Last year I switched up to a Magna 500 and had zero issues with that one. I was just too close to the limit I think on the 275 that it was time to update when a good used 500 came up I couldn't turn down. You have to have a good regulator with one of these....No cheap ones. Its a lot of pressure to hold back. My other fav is the A1000, I love those things. Great pump with great versatility.

I have a Magnafuel 500 pump a friend gave me, but it needs a rebuild. I orders the Magna 300. I think it will be plenty.

I have 18 psi at the pump and 6 at the carbs -8 to carbs with no psi drop at launch but i had some at 14psi. 1.40 60'

Mine locked up at the drag strip after about three years of street/strip use. I completely rebuilt it and after about six months it started leaking again so now it sits on the shelf. I'm not sure I'll ever use Magnafuel again. From what I understand the pumps are made here and the motors are made overseas.

Yep, i have often wondered why people spend all that money on those, which i always hear tales of woe about. When a Holley black will easily satisfy the needs of 95% of the N/A street strip or bracket cars on the board, and are way cheaper and very, very rarely fail.
I know they are plenty capable of supporting a high 9 sec 3350 pound car, using a T brake.
Most foolish thing i have done( among many..lol) was getting talked into “ upgrading” my fuel system. I did, and didnt see a thousandth difference, except in my wallet.

Totally agree, holly blue feeding a 800 hp 499, 5.6 @ 125 1/8 mile. Very soft chassis tune doing 1.25 60. tank-pump was mounted in the front. Morroso log feed thru regulator a 5psi to carb, mph shows it was not running out of fuel. Pressure behind the regulator was something no one gave a sh!t about. Volume, now that’s a different animal.

I did look at the Holley Black 150, but it was only $20 more for the 300 Magnafuel , which looks like a lot more pump for the little extra money.