I am installing a fuel regulator between my in-tank high-volume/pressure pump and my carburetor. The pump puts out 50+ PSI and I need to get it down to around 7 PSI. All is good so far, got the pump installed, got the regulator, got the 3/8 stainless tubing, and I am ready to install the system. Here is the question...the instructions for the regulator say to keep it as close to the carb as possible. Other manufacturers say to keep it within 3 feet of the carb. Does anyone know why the regulator has to be mounted so close to the carb???


The regulator is a bypass style with a return line to the tank. A supply line and a return line must be run between the tank and the regulator. I had hoped to keep the regulator closer to the tank to minimize the amount of line I have to run for the return line. Looks like I may have to bite the bullet.

 

The point of the return line is to bring some gas that is hot from the engine compartment back to the tank, to be replaced with cooler fresh fuel. Whatever line is between the reg and carb can be heated by the engine and vaporize the fuel in it. If pressure to the carb is critical, you also want to eliminate the unknown friction losses in the piping downstream from the reg (losses vary with flow).

I'd be surprised if you can get away with a single-stage pressure regulator going from 50 down to 7 psi. Why the HP pump? Just what's in the tank already?

 

The pump is designed to operate with an EFI system. The tank is designed to accommodate this style pump. While I am currently running a carb system I may wish to change to EFI sometime in the future...if so, the pump/plumbing will already be in place. A tank mounted pump runs more efficiently and cooler. The return line reduces the backpressure on the pump which makes the pump more efficient and more reliable. The cycling of fuel through the system also cools the fuel but since there are no EFI runners in play here, the fuel does not come into contact with too much heat in the engine area. The regulator, Holley 12-803BP, has no restriction on input pressure or flow rate that I can find. WRT to my question...friction may play a role in the distance allowed between the regulator and the carb. But if that were the sole reason for the close proximity requirement I would think that the regulator could be mounted at any distance as long as the pressure at the carb could be measured and the regulator adjusted to the desired PSI, thus compensating for the pressure losses due to friction.

 

Yes, If you can measure the pressure at the carb under high demand, you can compensate for the friction in longer pipes. The friction losses at idle are going to be near zero, I can't guess what the losses would be at WOT, but if you are using 3/8" line, probably not enough difference to matter. Probably 1 - 2 psi. I don't think it's super critical to have 7 psi all the time unless you are drag racing. For the street it more important to keep it under 7 to avoid pushing fuel past the needles.

I've never seen a pressure/flow chart for a Holley Blue pump. If they are capable of 50 psi at near-zero flow, then the reg must be able to take it. My concern was more for the accuracy at that pressure drop and low flow. Note that they recently added internal recirculation in those pumps, maybe to protect the regs?

 

Electric fuel pumps don't really put out pressure. Rather volume. The restrictions of the lines and the regulator will set the pressure. If you are using a high flow pump 255 lpr or so you will have a lot of volume of fuel being recirculated back to the tank. The heat generated by the pump, and friction of the regulator stepping the pressure down will lead to fuel vaporization in the return line because it is a low/no pressure situation. You want that vaporization to happen in the return line, not the tank. If it occurs in the tank it can cause issues. You can actually see the fuel in the tank getting all foamy and it will cause cavitations at the pump impeller. So if you do mount the regulator close to the tank, it might not be a bad idea to add some room for the vaporization to happen with some coils fuel line to allow cooling before it re enters the tank.

Also mounting the regulator close to the engine means the fuel is under higher pressure all the way to the engine raising the boiling point ( the length of the pressure line) and eliminating any chane of "vapor lock" in the pressure line. If you run your 6-7 psi to the engine you are susuptable to the same old vapor lock possibilities.

 

Boiling point of gasoline...interesting point. Given that I am in SoCal and the gasoline here has so many additives, the spectrum of individual boiling points for each is broad. On the plus side there is no fuel line attached to any point on the engine other than the carb...therefore no conducted heat from the engine to the fuel. On the minus side is the low PSI over an extended length of fuel line. Sounds like it is time to gather some empirical data on the system under each configuration. Virtually all modern cars have a similar fuel supply configuration, that is, pump in tank with a return line from the fuel rail(s) on the engine so the chances of seeing issues related to heat from the pump and regulator seem to be in the noise. That said, modern EFI systems do mount the regulator on the fuel rail and it is the last step before the fuel enters the rail/injectors. I am beginning to think that mounting the regulator on the firewall or inner fender with a short run to the carb is probably the best arrangement. Thanks for the inputs...

 

I just did what you proposed on my 40 sedan build. After much research and finding that a friend has his set up the same way and has encountered no problems. I will mount the gauge at the carb. If Nick is correct I will be re-doing it at some point although my friend has many miles without a problem.

 

Kinda thinking out loud here but from what I know about vapor lock, the biggest contributing factor is the design/use of the mechanical fuel pump system. When fuel is pulled from the tank up to the engine using a vacuum created by the mechanical pump then any vaporization of the fuel creates an opportunity for the pump to lose vacuum. Since a mechanical pump is bolted to the engine then any heat in the engine is transferred to the pump which makes the possibility of vaporization of the fuel even greater. On the other hand, an electric fuel pump is a "pusher" of fuel and does not rely on vacuum to draw the fuel through the line. Any fuel that may be hot enough to vaporize will be "pushed" through the lines without having any affect on the pump itself. The problem may then arise in the in the float bowl of the carb should the fuel be foamy enough to disrupt the normal operation of the floats and fuel system there.
I am going to contact Holley and see what their gurus have to say about this 3 foot fuel line length.

 

I'd say that is correct. Honestly most of the issues I've had in the past was related to what you mention, but I was using in line type electric pumps. Like mechanical pumps they suck the fuel to them through a line. That creates negative pressure or vacuum on the supply line to the pump even if it is short. So it actually loweres the boiling point of the fuel in that section. The fuel would vaporize right when it hit the impeller of the pump and cause cavitations. Once I started using in tank pumps it almost completely eliminated any issues. I think its because of the weight of the fuel keeping pressure on the intake of the pump and not a vacuum condition in front of the pump.

But I did run into an issue by mounting the regulator close to the tank of an in tank pump vehicle as I mentioned. But the 3' length you mention... I doubt that would be a problem on the pressure side. Especially if your careful with the mounting like you mentioned. The return line and tank foaming/pump cavitation would be my only concern. That's what happened to me. All it took to remedy the situation was larger return line and some loops put in it to add length.

I'd be interested to see what the folks at Holley have to say about it.

 

Do the Holley regs with return have an orifice in them to limit return flow?

 

Not sure Ross...right now I am scrapping the whole setup and returning to an external red Holley pump. I will hang on to the internal pump and will plumb it should I ever decide to go to an EFI system. I am not gaining much by putting this system together "just in case" I want an EFI in the future...I will just cross the bridge when (and IF) it comes.