Quote:
Originally Posted by dataway
So the 1/4" return line on a 68 would not be enough flow return to keep a inline pump from failing? (So essentially not a dead head)
I wasn't aware that the fuel return line on a 68 was just a vapor vent, I thought it was the fuel return from the positive displacement mechanical pump.
Hate to seem dim ... but what is the "AC" you guys are talking about? Surely it's not Air Conditioning.
I thought ALL mid to late 60's had a fuel return from the mechanical pump.
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The line is 1/4" outside diameter that is crimp bent, so it's not designed to flow a whole heck of a lot. Some fuel is always in that line but it's designed to return vapor from hot fuel to the tank. The vast majority of fuel pumped by the mechanical pump goes to the bowls, not back through the vapor return.
The FiTech inline fuel kit instructions requires a 5/8" return line minimum.
AC does mean air conditioning. Many cars in the mid 60's that didn't have AC would likely have a single feed line without a return. I know this is true of Firebird's that did not have the AC option checked.
In regards to that tiny fuel vapor line from the factory, think about how the pump is actually working. We'll use the Walbro 255 as an example. This pump flows 255lph of fuel at 42psi, slightly less at the 58psi that the FiTech unit commands. That's enough fuel to support over 600hp at wide open throttle. Many of us here don't drive our cars around at wide open throttle all the time and even fewer of us drive around with engines that make 600hp.
driving around town casually you may only be using 25-50hp and only need fuel to support that amount of power. That means at any given time you may only be using about 10% of the fuel flow being delivered. If that flow is hindered in the return it raises pressure in the feed and with EFI applications creates a fueling oscillation, which is what happened to me when I tried to use the factory vent line for all of about the 2 feet attached to the tank.
1. Engine fires cold and is using a lot of fuel
2. Engine starts warming and fuel requirement for idle reduces
3. More fuel is being sent back to the tank and the small line reaches the point of being pressurized
4. The pressure in the return line is added to the pressure at the injectors
5. Pressure continues to build as fuel requirement reduces
6. Extra pressure begins adding additional fuel to the engine, creating rich run
7. This is where the ECU steps in and tries to reduce pressure at the unit and fuel in the line by reducing pump output.
8. Engine coughs and sputters as air/fuel mixture becomes to rich
9. engine catches as the ECU has pulled fueling and it stumbles back to life. It repeats 6-9 until either the engine dies or you turn the car off. In the mean time the pump is being stressed and the engine is going from a state of being washed in fuel to being incredibly lean over and over.
This is a bit long winded, but this is what I went through and I wasted a bunch of time and money because I tried doing this cheap to begin with instead of correctly. I gave up the last month and a half of cruising season and paid several hundred more than I needed to because of this. Do it once, do it right.
If you don't want to do the entire fuel system, spend the money on the command center and just run the vent line that it needs. It's the easier of the two options and allows you to easily go back to factory if you need to.