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RE: [T3] As the Tank Turns - this weekend's episode

On 9 May 2006 at 11:05, Big Al wrote:

> OK, let me restate/clarify.  The 70 and 71 tanks have a vent port on the
> opposite side of the tank as the filler (the round ones) and overflow
> ports (i.e., filler are on the right side, vent on the left side, all
> three above the seam/flange)
> The 1968 tank I have has the oval (larger diameter) filler port and the
> overflow port, but no vent tube attached to the tank on the other side
> from the filler.
> The 1969 tank has a ROUND filler port and overflow port but still NO
> vent attached on the tank on the other side from the filler.
> therefore the 1969 tank would work in the 71 and later cars with the
> round port. The 1968 tanks will not work unless you make a change to
> the filler hose.
> I think we're saying the same thing here, but I was only mentioning what
> was welded on the tanks, not that there was or was not a venting path in
> the system.  Yes there was a small vent off the overflow tube on some
> models.  But the difference was that it was in the tube and not part of
> the tank.

I agree. It's confusing here because both the medium and small ports are part 
of a venting system, but I think the small ports only got introduced when the 
overflow tank & charcoal system got introduced

> > I haven't noticed this, but you may be 
> > right that they varied a lot. That seems odd, however, since I would expect 
> > that the "jet pump" action of the returning fuel would only be effective if the 
> > alignment was pretty good.
> I would have thought that there would be more precision required for
> this to work also... but water flows from the outer area into the cup
> with NO problem just using gravity alone in the tank that is only
> slightly off-center.

I'm sure that there's still enough clearance for gravity flow, but with the FI 
systems, the return flow drags along extra gas so that the central cup will get 
filled to a level higher than the rest of the tank. You can actually observe 
this when you start up the car with a low tank. You may have noticed that a 
block or 2 later the gas tank doesn't look nearly as empty as when you first 
started up. This is because the center cup, where the gauge is, is now more 
full than it was when you first started the engine.

This "jet pump" action is the result of how the return gas passes thru the main 
tank and then enters the cup carrying a little extra gas along with it.

> The other tank that has the return pipe mostly covered seems like it
> would cause more problems... from the factory.  I may try an experiment
> and drill the cup by inserting a drill bit up the return port and see
> how well that tank performs.  Just because it's a good tank otherwise. 
> It will at least be good for a carb-conversion car.  ;)

Drilling might help or might make it worse. Is the return port straight? I was 
thinking it had a bend in it, or was that only the supply line?

Jim Adney
Madison, WI 53711-3054

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