Originally Posted by Baedarlboo
What are the effects of having different (short or longer) lengths of the pressure line from the pipe to the tank? Whats recommended for on road applications?
If you've done much gas racing, you've probably encountered the dreaded lean bog that occurs right after a pit stop with a full tank of fuel. Right after the tank is filled, for about the next 3 or 4 laps the engine sags (lean bogs) when accelerating off of tight right-hand corners. On most tracks this happens when accelerating onto the straight. Seemingly, the lean bog magically goes away after a few laps but sure enough, after the next pit stop, it frustratingly returns-slowing lap times for several laps once again.
The key to eliminating full tank lean bogs is pressure tube position, length of the pressure line from the pipe to the tank, or adding a pressure reservoir.
To solve this frustrating issue, it's important to understand what's causing it.
Here's the scoop. As the throttle is released entering a turn, the pressure in the tank is now greater than the reduced pressure in the pipe. When the tank is full, some of the fuel sloshes up into the pressure line. The centrifugal forces encountered in a right hand turn further pulls the fuel into the pressure line, causing reduced pressure in the tank. When the throttle is cracked open to accelerate, the engine goes lean because of this reduced tank pressure and hence the dreaded lean bog occurs.
The reason it only lasts for a couple of laps is that as the fuel level goes down in the tank it's less likely to slosh as much into the pressure line and, as the fuel level goes down there is a larger open space in the tank that's pressurized, maintaining adequate pressure long enough to prevent the low pressure lean bog problem. (If your exhaust system is mounted on the left side,, then this phenomenon will occur in left hand turns.)
Keep the fuel out of the pressure line! That's sometimes easier said than done. We've had the most success by rerouting the pressure line so that as it comes off the pressure nipple, the tubing goes to the right side of the fuel tank, then makes a loop on the right side of the tank. This prevents the centrifugal force of a right hand turn from filling the pressure tube. We've also had good success by adding one of the big reservoirs (they look like a big aluminum fuel filter), in the pressure line. Many manufacturers have recognized this problem and are making special pressure pickups and tank lids with large open volumes in the lid itself. You may have noted that many top drivers' cars have several loops of fuel tubing wrapped in their cars and wondered what was the purpose. Now you know. The key to fixing this issue is to experiment with pressure tube routing, length, and if necessary, installing a pressure reservoir in the pressure line.