6cyl to 8 cyl fuel pump question

[Chevy nutcase]

I finally decided to add 2 more cylinders to my 66 and the motor I chose came from a 77 corvette. I've replaced a lot more in-tank electrical pumps than I have mechanical pumps so I'm wondering why the pump on this motor has a return line to the tank when the one on my 6 cyl does not? The pump in question is brand new and I thought it would be one less thing to buy until I noticed that 3rd port. My first thought was having to buy a new sending unit that has a return and running a new line until I looked up a factory replacement pump for a 66 v8 and they don't seem to have a return line. Would I be ok just getting that style pump or am I completely off base here?

[Chevy Fleetside]

Most fuel pumps have 3 ports because they are designed to supply fuel to the engine, return excess fuel back to the fuel tank, and vent any excess pressure or vapor from the fuel system. This helps to ensure that the fuel system is operating correctly and efficiently by maintaining the correct fuel pressure and preventing any air from entering the system. You could probably just put a fuel pump for a 60's truck. I believe they did this because of charcoal canisters.

[Steeveedee]

Fuel pumps with a return port were not available in the '66 model year, afaik.

[Chevy nutcase]

Originally Posted by Steeveedee

Fuel pumps with a return port were not available in the '66 model year, afaik.

Thank you for clearing that up. I know how a mechanical pump works but now that I know there's a version that returns excess fuel back to the tank, how does the older style not blow rubber lines apart or force fuel past the needle and float in a carb? My other question would be since this 350 is going to be "hopefully" double stock hp should I stick with the old or new return style? If so is there a sending unit to accommodate a return line ?

[cwcarpenter98]

If you want to keep the pump with a return line, you can modify your stock sending unit by adding a return line fitting to it. Or you could add the return fitting to the stock tank.

Many trouble free miles have been driven with non-return mechanical fuel pumps, so it's more just a matter of preference.

I did have to put a new fuel pump on my 63 last year. I used a Delphi pump and haven't had any issues with it since

[Steeveedee]

The return line was implemented to help keep the pump from vapor lock by recirculating fuel as a "coolant". A recirculating pump will not have any lower pressure necessarily than a non-recirc type. I've run a stock (no recirc line) pump on Pontiacs all the way to 400 HP with no issues. I had a Le Mans with a RA III that could get to 260° idling in traffic without vapor locking. Of course, that was back in the '70s. Alcohol in the fuel may make a difference.

[Chevy nutcase]

Originally Posted by Steeveedee

The return line was implemented to help keep the pump from vapor lock by recirculating fuel as a "coolant". A recirculating pump will not have any lower pressure necessarily than a non-recirc type. I've run a stock (no recirc line) pump on Pontiacs all the way to 400 HP with no issues. I had a Le Mans with a RA III that could get to 260° idling in traffic without vapor locking. Of course, that was back in the '70s. Alcohol in the fuel may make a difference.

Luckily I have a gas station that sells ethanol free gas 5 min from my house