hello guys and gals,
I was hoping someone could enlighten me on the pros and cons of bigger fuel line. My factory cougar came with 5/16" line and I was thinking to replace them anyway and noticed the 3/8" line is actually a little cheaper(not by much) but I was wondering if there was any draw backs to running a larger size line? im staying with the stockish 302 thats in the car as it sits so no crazy fuel demands but still wondering pros and cons. If anyone can chime in that would be awesome.
Short story is it won’t make much difference. You could do a bunch of math and figure it out. There are a few hidden forces at work, but they tend to act only within the changed section, which is in the middle and does not involve the places where pressure is applied.
The fuel pump determines the PSI, or amount of fuel being moved past any point over a given time. Once pressurized, that does not change within the pipe, regardless of size. Only when we reach an opening, and the work is applied, do we see a change.
On the inlet, a decrease in flow resistance will be seen in the larger pipe, meaning less force on the fuel pump to draw from, but the pump still moves the same volume with each stroke from the tank pick-up.
Where the output is applied (within the carb bowl) the diameter of exit pipe (valve) within will not change, so that flow rate will not change.
Whether your filling a pool or a bathtub, it only happens as fast as you can bail water.
The early '67 390 and all 289’s came with the smaller 5/16" line. They changed to the 3/8" line for the 390 in late '67. In '68 everything started getting the larger line. If you have a very early '67 the line may be routed through the transmission tunnel. If that is the case you will want to use the later routing under the drivers side. The reason for the change in routing and the larger line was vapor locking in warm climates.
The larger 3/8" line serves two purposes: it flows more fuel, this was an issue with the 427, 428CJ, and 390 GT. It also helps avoid vapor lock. The fuel pump creates a vacuum to pull gas into the inlet side. This reduction in pressure reduces the boiling point of the fuel. If the fuel approaches the boiling point, a bubble forms in the line. Since gasoline vapor (a gas not a liquid) is compressible, the pump cavitates, basically stretching and compressing the bubble rather than moving the fuel.
Living in Arizona I have had good luck with the 3/8 inch lines. I don’t see any down side.
The larger line will support more volume for the same amount of pressure. Had this happen on a Buell at track day. Bike would hit 150 and you would hear the loss as well as feel it (almost like the bike shut off). Went with a larger line, problem solved, and was able to support other mods.
Of course, this was on what was basically a modded out track bike. If you’re on the street, cruising, or the occasional stop light gran prix, I would say you won’t notice any benefit whatsover.
Perfect! 3/8" line it is!