Hi, I am helping a friend build an aluminum fuel cell for alcohol in a door car.

The engine was dyno'd at 1300 HP, has an 8-71 blower with enderle injection.

Don't want to go too small on the vent size, would #12 vent tube be appropriate?

Thats plenty big enough

Rule of thumb I thought is simply at least same size as xorrectly sized fuel outlet, or better yet one size larger, this would of course depend on any one way valve/roll over feature restrictions.

Basically its how fast you pull fuel from the tank..
you dont want the tank going into vac.. if it does that
hurts the supply going to the pump.. remember that atmospheric
pressure pushes the fuel into the pump... even a slight pressure
in the tank is fine but you dont want any vac.. on the newer
style gas caps they have a 2 way valve in them.. they open at
10 inches of water on the vac side and 35 inches of water on
the pressure side.... the old style caps were just open which
did help on the tank pressures.. and that was still with a vent
line on the tank
EDIT
On the new style cars they only have a .060 vent on the
tank when its running but they also have a purge port
or the charcoal canister which is loading up all the
time but then it purges it out... so the vent on new
stuff is pretty small.. but its not drawing fuel out of
the tank like we (racers) are pulling.. you could use
the supply side as a number(rule of thumb) but bigger
is better

I thought that I would give this topic a little bump and share what I found on my car. Since I am under the doctors restrictions yet and cannot drive I still go out and putter with the car in the garage. My cell is an 8 gallon Summit with a 6 AN fitting for a vent. It had a 3/8" fuel hose clamped on it,it has been that way since I bought the car. As I was looking it over I noticed that the 3/8" line was almost pinched shut. I removed it and took the fuel cell cap and ring off to tighten the check valve. Thank goodness for 1/4" drive ratchets! I wonder if that may have caused a problem. Also should I put a 3/8" line back on or step up to 1/2"? I noticed that 1/2" line will fit right over the #6 AN fitting. Thanks!! Ted

Ted if it was pinched closed that COULD be 2
things.. either cheap hose or the vac on the
hose is puling it closed.. usually cheap hose
turns soft and its easy for vac to pull it
closed.. if you use EFI hose it has a viton
liner in it and is good stuff.. 1/2"sure wont
hurt but the 3/8 should be fine.. your not pulling
the fuel out at a high rate during a run.. on my junk
running E-85 I would pull 1.25 gal from the pit shot
to the pit spot... that was in a 5 gal cell

Mike...I think you are correct that it was cheap hose. You think 6 AN would be good?

6an should be fine.. understand that when you
drive up to the lanes and get heat going its
building pressure so as you blast down the track
you are shaking that fuel building pressure so
it will take a bit longer to get it going into
vac(even with a vent)
EDIT
A rollover valve has a pretty small hole in them
so that would be the restriction point.. if you
dont want a roll over you can run the vent line
from side to side then down low in the chassis/frame
so its lower than the tank

Thanks Mike!!

It's injected. So it's not the same as N/A. You need the vent the same size or the next size bigger than the outlet (as was mentioned in another post) and that is the MINIMUM it needs to be.

Also, it's alcohol. You have to remember that fuel is being returned to the tank. So because it's injected alcohol and has a return line, you need a BIG vent. Alcohol producers more vapor in the tank when return fuel is put back into the tank.

And vapor creates more pressure in the tank

Have you ever run injectors? I ask, because every time I pressurized the inlet side of the pump, you couldn't tune it, and the tune changed in proportion to the pressure above the outlet.

Also, (this is way over my head but it's what happens) any air entrained in the fuel in the tank from bypass fuel is stays in the fuel, which alters pump output.


Big vent. If you are having tune up issues, look at the size of your vent.

Yes.. I run injection.. plus I flowed injectors
at Chrysler for about 10 years before I moved
over to the exhaust side... and right now I run
higher pressure on the inlet side of the injectors

I really wonder if/how much air is entrained in a such a low viscosity liquid such as gasoline? I am sure it is entrained, but seems any air would would be rapidly released due to gravity, etc, and unless fuel pickup was adjacent to the fuel return, problem would be very minor, if at all. I'm sure there is a video somewhere demonstrating the issue, one way or the other.

I use to make clear plastic tanks just to see whats
happening inside to make sure we were ok... there is
a fair amount of foaming going on in there.. most dont
think fuel foams up but it does with the air thats in
there... and we would measure the pressure inside the
tank.. most of the time it was in inches of water.. but
I did see over 70" of water in there(greater than 2 psi)..
the RVP(Reed Vapor Pressure) of the fuel makes a big
difference.. higher the RVP the more pressure it builds
in the tank

The foaming observation does surprise me, but I think there is a distinction between "foaming", which floats on top of a liquid, and "bubbles", which are submersed in a liquid, and more of a concern to us in this discussion. Both are still related to the liquids viscosity. Regardless, submerging the return line, almost eliminates the entrapment issue as I see it.

And it does... we always return under the fuel level
but now days we are only about 5" away from the pump
inlet.. we return the fuel into a fuel reservoir so
it stays full at all times.. EFI engines cant take any
air in the system otherwise it wants to shut down

Th OP is working with mechanical injection. I have never done EFI, nor will I. It's two different things.

Don't know about EFI, but I know if you build pressure in the from lack of vent, it will change the tune up.

If you put return fuel below fuel level, you can still get some bubbles in the fuel. That will change the tune up.

It's that simple. You can't make the vent to big. But you can damn sure get it too small.

I won't even get into returning fuel to the inlet side of the pump, even though it is a common practice.

If he runs #12 like he asked it will be large
enough