fuel pump size vs tank vent

Is there some sort of formula or rule of thumb for the fuel pump size to tank vent? I see that some of the screw blown promods have two vents.

 

75 views later, I'll be the guppy! I'm almost sure I'll be corrected
Minnesota barnyard logic is close to rule of thumb, so here goes:
Vent is based on outlet size and pump/motor size. Plug tank on workbench, fill with water, stand back and pull outlet plug.............if the flow goes glug-glug, you need a bigger vent!
A TAD approx. 470 c.i. w/1.25" outlet, needs min. 5/8" x .049 vent tube.
A TAFC approx. 526-540 c.i. w/ 1.5" outlet needs min. 3/4" x .049 vent tube.
T/F or F/C w/3" outlet needs min. 1" x .049 or more.
If you are hurting parts, put a straight edge on your tank, if it's sucked in any, you need a bigger vent. On tanks with vent tube coming up in the neck, if it's dented, smack the guy with the funnel. (and never use a steel funnel)


Bob Meyer
Meyer Race Cars / San Diego
2010 Our 45th Anniversary

 

Sounds more like hillbilly backwoods science to me, and it definately isn't rocket science, but makes perfect sense.

 

A couple of years ago we did a little test on this subject. Set up a pump on our flow bench to flow 12 gpm at 100 psi, changed vent sizes on the tank and watched for flow changes. Interestingly there was no measurable change until we got down to a #6. The vent fitting can never be too big, but a reasonable size should work.

 

Thank you for the reply. Does the TAFC need the larger vent because of the larger engine displacement or does it have more to do with the height/length of the tank as well as the placement? Is it OK to flatten the end of the vent tube where it goes up into the filler neck? I am talking slightly ovaled, not pinched off, to give more room for the funnel. To me an oval portion of the tube will have the same area as when it was round, but will the oval or the transition from oval to round affect the flow of the incoming vent air? I tried the back yard glug test, the tank passed, and the tank is not sucked in at all. What would be a the proper thickness to make a new tank out of?

 

if you change the shape of a circle to an oval, you will decrease the overall area..... whether or not it will effect the venting abilities in your application is certainly in question; however, the area will be decrease as you go from round to oval.

sean

 

This got me thinking because I have never figured this out. Area of a 1" diameter circle is .785sq". If you squish the circle to a 1.25 x .75 " elipse the area becomes .736 sq " . If you go to 1.5 x .5, the area stays the same. So if you squish it at all, it reduces by about 6% but doesn't reduce anymore the more you squish it. Not counting right flat, I'm talking in this vent example. I did it for a 3" circle also and the reduction was pretty close to 6%. So yes it does reduce but not by much for what its worth.

There you go a little bit more useless info for you

Corey

 

Again, barnyard logic. I liked happy customers, the ones who never had a problem with their tank, so we always used 3003H14 x .080. Here's where the logic comes in: Yes it weighs more, now you don't need that extra 5lbs. of ballast. One hint, never do a tank with a butt welded edge (90*) Two rolled edges are there for a real reason, rather than just looks!


Bob Meyer
Meyer Race Cars / San Diego
2010 Our 45th Anniversary

 

Well let me throw this into the mix, will the rubber hose going from the tank to the fuel pump collapse only when you don't have enough vent? I haven't been running a spring in my fuel line to prevent collapsing, but when I developed a 'problem' I thought that maybe I should be, so I took my fiber reinforced hose with no spring, capped both ends and hooked it up to a vaccum pump and it pulled down to 30 inches without collapsing.

 

I'm confused now


Should I have one #8 vent or two #8 vents on my fuel tank?
Tank has 1.25" outlet
Supplies a 540ci BBC blown alky

 

If you can fit 2 then put 2 in, more the better. IMO

 

Guess I was lazy and couldn't figure the math so I changed my #8 vent line to a #12 to be safe. Pump has a 1.25" inlet and #10 outlet. I figure if it has a #10 outlet, can't be using any more fuel than the #10 counting what the engine uses plus the returns to the tank. Does my "backyard" (don't live on a farm) logic pan out or am I missing something?

 

Some logic actually does get to the city!
The only concern with AN stuff is : be sure to know the I.D. of any fitting, not the hose or tube I.D.


Bob Meyer
Meyer Race Cars / San Diego
2010 Our 45th Anniversary