Fuel tank venting

R

Razor

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Jul 31, 2001
Thought i'd throw this up.

The other day speaking with Aeromotive about venting the tank, the tech said the vent should be close to the feed line size feeding the pump. Or close to the orfice opening on the pump.

If the gas tank is sealed, with a return style system.. you dont need a vent.. when the engine starts.. it starts to use gas.. the vent has to be able and vent the gas tank to allow air in as the fuel is displaced.. consumed by the engine. If you dont have large enough vent, the fuel pump will try to implode the tank.. steel tank that wont happem.. but this effect will kill fuel delivery amount.

So on a 600HP+ motor.. that can be like a -6/-8 size vent. Higher power motors -8/-10.. etc.. So those using stock vents and creating a lot of HP.. at what point do you go bigger.

Better yet.. once you put that bigger vent on the tank.. what do you do about capping it off when the car isnt in use? Anyone make a large air valve 12v?

Thoughts :D
 
"
If the gas tank is sealed, with a return style system.. you dont need a vent."??
 
Chuck Leeper said:
"
If the gas tank is sealed, with a return style system.. you dont need a vent."??
Click to expand...

As long as your not displacing fuel.. the air needed doesnt change. On EFI it recirculates back to the tank.

Thoughts :eek:
 
When building modifying the sending unit I drill the vent hole bigger. THe stock vent hole is TINY VERY TINY. (should I use all caps when trying to emphesize tiny?) I am amazed that a pump can even pump enough fuel and not suck the tank flat. I'll bet cars can run much faster on smaller pumps than thought. Hmmmmm..........
 
I'd think the tank can get all the extra air it needs down the fill tube. I never seal my cap tight
 
turbofabricator said:
When building modifying the sending unit I drill the vent hole bigger. THe stock vent hole is TINY VERY TINY. (should I use all caps when trying to emphesize tiny?) I am amazed that a pump can even pump enough fuel and not suck the tank flat. I'll bet cars can run much faster on smaller pumps than thought. Hmmmmm..........
Click to expand...

My pump that i'm using is an eliminator from Aeromotive. Its inlet is a -12. I'm using a -10 vent for it.

Only thing is looking for a way to cap it up when the car sits so the gas doesnt evap or spoil.

Anyone make a 5/8 air valve :D
 
There is a vent valve that is used on 3rd gen f-body EFI cars. On these cars there are four lines from the tank. Feed, return, CCP system, and a pressure/vacuum vent.

The CCP control valve is a one way into the canister. The tank vent valve, looks like a plastic mushroom and sits above the rear axle, allows air into and out of the tank.

The CCP control valve has a lower cracking pressure then the tank valve. So fumes should go to the canister first. If the CCP is removed and the line plugged, the tank vent will work to relieve pressure, and also allow air into the tank.

With a return style fuel system the tank needs to be both pressure and vacuum vented. As the fuel is heated a lot of pressure will/can build in the tank. Which is normally taken care of by the CCP system. I have an older car that I converted to EFI that has this problem.

Once I got a gas cap with a good seal on it the tank will build a lot of pressure. Not good. And the cap needs a good seal as it is rear center as on our Buicks.

This mushroom vent valve has low cracking pressures. Somewhere around 1 psi on both in & out of the tank. Maybe use one of those either T'd off before the CCP control valve, or in place of it.

But as TurboFab mentioned, the vent line has a restriction in it at the tank. So it should be drilled out. The little can thing over it is a rollover valve. If the car goes shiny side down it helps prevent the loss of fuel.

I think I have a picture of the vent someplace. I'll see if I can find it and post.

RemoveBeforeFlight
 
Chuck Leeper said:
the engine is running, it's displacing fuel....:D
Click to expand...
Once it is running.. thats where this thread starts :D At WOT you can displace a lot of fuel. The higher the HP... the more fuel you will displace.

So you need a larger vent than the OEM designed when they made the car with 250 HP in mind. Here we sump tanks, put two pumps on the stock hangar, etc.. to get more fuel out of the tank and the vent is not addressed.


RemoveBeforeFlight .. the fuel system on a modern EFI car is pretty complex. We keep on thinking of a fuel tank like a gas tank on a lawn mower..

I bet if you get a car with a plugged up vent line to the tank.. you can pull your hair out trying to determine why it starves for fuel at WOT.
 
Razor said:
I bet if you get a car with a plugged up vent line to the tank.. you can pull your hair out trying to determine why it starves for fuel at WOT.
Click to expand...

LOL. Roger that! Vapor lock.

One indicator of a restricted vent can be seen when filling up at the pump.

If the automatic pump nozzle keeps shutting off forcing you to stand there and trickle fuel in, your vent is prolly restricted.

If fuel can be fed into the tank wot, your vent should be good enough to accommodate the demand of the motor.

my o2
 
I recently worked on a car that had an issue with the fuel pressure. I could not get the FPR to adjust below 41 psi. The car was also running very rich. I finally pulled the tank and opened up the vent hole and both problems were resolved.
 
TurboBuRick said:
LOL. Roger that! Vapor lock.

One indicator of a restricted vent can be seen when filling up at the pump.

If the automatic pump nozzle keeps shutting off forcing you to stand there and trickle fuel in, your vent is prolly restricted.

If fuel can be fed into the tank wot, your vent should be good enough to accommodate the demand of the motor.

my o2
Click to expand...

Rick.. I like that.. "If fuel can be fed into the tank wot, your vent should be good enough to accommodate the demand of the motor."

Trick would be to make sure the neck was sealed to the gas spicket when doing this.
 
Fixed a 10.5 car with this very problem....
The rollover/vent device had stuck shut...
The pump got so hot, it was smoking!

At atmosphere psi, gas boils at about 135*.. Less when in a vacuum created by no vent/cavitation @ the pump inlet...
 
I had a vapor lock issue on mine middle of March at the NMCA event here.. fuel psi just dropped.. let car sit for an hour.. fired right up.

Pump was blamed :redface: Its the stupid lack of proper venting. Pump was sent in.. and needed to be overhauled.

So a -10 line going to a loop over the axle and down to a breather element with a ball valve to shut it off under the car. Just have to remember to open the ball valve :redface:
 
Jeff Rand said:
I recently worked on a car that had an issue with the fuel pressure. I could not get the FPR to adjust below 41 psi. The car was also running very rich. I finally pulled the tank and opened up the vent hole and both problems were resolved.
Click to expand...

Awesome.. thanks for the reply.
 
I hope this is not considered off topic but I sell turbo diesel engines for high liability specialized construction equipment and code requires the fuel tank fill and vent to be same size.
 
Razor said:
So a -10 line going to a loop over the axle and down to a breather element with a ball valve to shut it off under the car. Just have to remember to open the ball valve :redface:
Click to expand...

I don't think that the ball valve is a good idea. The reason is that the fuel in the tank will heat up as the car is driven. When parked it will cool off. This will pull a a vacuum on a sealed system. It may end up collapsing the tank.

I found out it doesn't take much pressure to balloon a tank. Figured to use 3 psi to leak test once, bad idea. A single psi was enough.

Picture ('borrowed' from TGO) attached is the 3rd gen f-body vent (pressure/vacuum). Say a '92 MY for an application (any engine/drivetrain).

The barb at the top fastens it to a hole in the chassis. Line from tank goes to bottom fitting.

RemoveBeforeFlight
 

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