Hey people,

I just had a discussion with a friend whether or not you need a return line with a carb. I was under the impression that you do, and he thought you don't but he couldn't explain why.

So that brings me here. How is carb setup different from EFI with respect to the return line. I thought in both cases you need some kind of pressure in the line, either for injectors or to fill the float bowl.

Why would all EFI cars have return, but carbs wouldn't? Do carb cars have special pumps that are designed to work without return? How do they work? The excess fuel has to go somewhere if the carb doesn't consume it, right?

TIA

 

Cars that have been converted from TPI to carb as mine has been most of the time have kept the high pressure tpi fuel pump.
These pumps need to be knocked down to a pressure the carb can handle.
The best way to do this is with an adustable fuel pressure regulator. It knocks the pressure down and returns the extra fuel to the tank.

This keeps your fuel pump running cool and keeps your carb happy.

 

If you run the TPI pump you will need a return like. Something like the Mallory 4309 3 port return style.

 

Until the 80's, the typical factory carb pump would simply not draw any more fuel from the tank when the pressure reached the design pressure (typically 4-6 psi). The way it worked was the pump rod would compress the diaphragm and the return spring would do the pumping. If pressure was up to design limit, the spring wasn't strong enough to pump any harder. So, no return line.

In the 80's (may have been earlier on some models, but wasn't on the '77 Monte Carlo I had, for instance), a return line was incorporated. The pump spring force was increased, and pressure controlled via a "relief valve" that returned excess fuel to the tank via this valve and the return line. The idea was to keep the fuel a little cooler so it wasn't dead-headed by the engine, picking up heat.

EFI systems operate on higher pressures and use an electric pump in the tank to provide a steadier pressure. They control pressure via a regulator that returns unused supply to the tank. As stated by the others above, the constant flow of fuel through the pump keeps it cool. The constant flow also keeps the fuel at the injectors cooler, helping to keep the injectors cooler.

 

Its easy. It depends on the fuel pump. If you are running a stock electric, yes. Some electric pumps can be run without one, some cant (aftermarket, that is). If it has a carb and a mechanical pump, it depends on the pump too. Some pumps have a return line others dont. It doesnt really matter that it is a carb, it matters what kind of pump you have.

 

Thanks, guys.

That cleared things up quite a bit.

Me and a friend are planning to replace my weak V6 2.8 with GM crate (either 350HO or ZZ4) and I was going to put Holley 650cfm or 750 in there. I guess my plan to keep original fuel line setup (new lines, just the same idea) is still good.

While I am here, what do you guys think about the carb selection?

 

Most people here will tell you that the holley mechanical secondaries are the best, there are some like me who dislike them though.