Just finished redoing part of my fuel system, I have two external 400lph fuel pumps and a hydramat in the tank for the pickup with a 3/8 fitting built in, 10an line for gravity feed line after the mat to pumps.
Now, I noticed that when I prime my fuel system before startup, I hear a large knocking noise. This only happens during priming. I felt the feed line while priming and I can feel the feed line vibrating. I programmed my 2nd fuel pump to run at 100% and my primary at 31% during priming. So, I believe the fuel line is vibrating making this knocking noise because this amount of flow is right on the edge of what my gravity feed supports, so it's a fine line between where the fuel pump is attempting to "suck" and being fed enough gas. Doesn't do it with both pumps running at 100%, but then the pumps get loud and make a high pitched whine due to the resistance from the 3/8 fitting. I changed the primary fuel pump to be at 16% duty during priming and kept the other pump at 100% because I don't have hardware hooked to that one to control it's output. The knocking has gone away. I don't have any problems with running lean or anything and the 2nd fuel pump only turns on under boost, so this resistance isn't going to destroy any fuel pumps anytime soon.

I was thinking of sticking a 2nd fuel tank between the main fuel tank and the fuel pumps, so the 2nd tank would get filled up from the gravity feed when the 2nd fuel pump isn't on, no return, no overflow. However, this would have the same issue as it would rely on the 3/8 fitting as a relief. So, what if I stuck a check valve on this 2nd tank to allow air in? Would this air be able to make it's way up my gravity feed line and out my hydramat?

 

Water lines in some houses make the same noise.

You might check the device that is used to stop the knock and see if you can adapt something like it to work with your system,

https://www.supplyhouse.com/Watts-07...iABEgLkqfD_BwE

 

Thanks, but I already solved the knocking by changing the fuel pump speed durring priming. The problem is it showed me the resistance my fuel pumps experience at that output and up. I know my title is a little misleading but I thought it would attract more attention. I want to add a surge tank like solution but I don't want lift pumps or to lose my hydramat.
I have a nice hidden compartment under my car that would perfect for a custom gravity fed tank.
Looks like I can fit about a 18inch by 3 by 5 tank here which is good for about 1.3 gallons. Perfect spot between the tank and pumps. I can do a 10an fitting for each and have a solenoid valve with check valve open up to vent when 2nd fuel pump activates under boost, probably over complicates it a little but it sounds like a fun project to me and having a perfect sized surge tank is very appealing to me. I can put the 10an bungs for the pumps at the bottom rear of the tank which is right where the pumps are. Fuel will want to be pushed to the back of the tank under acceleration so it's just about perfect in every way.