Venting Gas Tank

MNwe4

Member
What have people done to vent the gas tank compared to the factory setup? I'm running E85 w/ 80lb injectors maxed out (100% DC @ 21psi) and want to make absolutely sure the fuel pump isn't struggling because the tank is being put in vacuum. The pump is the walbro 450, racetronix hanger, hotwire kit, and -8 feed, -6 return lines, plus a volt booster that jumps the system to 15V. Trying to optimize what I have.

I have the original gas cap and factory vent line w/ a carb fuel filter on the end of it. I can blow through it, but that's not a lot of pressure or flow.

How about just putting a loop in the rubber line coming off the top of the hanger with a little filter on it?
 

RichieRocket

Active Member
I’ve got 5/16 hose that runs an inch past the gas cap and bends slightly down there. Running over 700 LPH with the duel pumps. Haven’t turned boost past 30 yet but second pump kicks on at 15 psi. No issues.
 

RichieRocket

Active Member
BTW, I’ve been told by a couple of very smart people that you want the vent hose at least even with the gas cap especially if you aren’t running a check valve in the vent port inside the hat. I’m no expert though.. <<<< stands by for Chuck to crawl out from under his rock.. 😜
 

Boostian

Member
Do they make vented gas caps? I get a good amount of pressure that comes out when I simply open my gas cap to fuel up….or is there basically just an issue with my fuel system?
 

RichieRocket

Active Member
I suppose you could mod your cap with a hole and a check valve. Never heard of it but may work just as well unless you have fuel all the way into the fill pipe then it may be a problem at take off due to fuel being against the cap. A rubber hose from the vent port on the hat zip tied up the fill spout is the preferred method though. Nothing wrong with have a high flow system if you get a good vent above the fill pipe about an inch past the top about even with the cap.
 

Boostian

Member
I suppose you could mod your cap with a hole and a check valve. Never heard of it but may work just as well unless you have fuel all the way into the fill pipe then it may be a problem at take off due to fuel being against the cap. A rubber hose from the vent port on the hat zip tied up the fill spout is the preferred method though. Nothing wrong with have a high flow system if you get a good vent above the fill pipe about an inch past the top about even with the cap.
When I did my fuel pump I replaced all the lines to the hard 3-line bent jiggy thing. Where’s the vent line run to? Charcoal canister up front?
 

Chuck Leeper

8/10's of a century!
Staff member
Where’s the vent line run to? Charcoal canister up front?
Yes. I just run the hard line with a length of hose and a filter to the highest point above the tank. Like where the hard lines go.
No vented caps with a rear fill tank.
The cannister was originally controlled w/ a valve/vac check . If it's not working, it can cause vent issues.
I've seen the cannister with fuel in it.
 

MNwe4

Member
Good points. I thought about a vented cap, but also had the concern about fuel coming out when the car leaves hard. Running a line up the filler neck and holding with a zip tie is a simple solution.

I do not have a muffler in the stock location, so I was thinking about putting loop in the vent hose there and securing to the bottom of the car.
 

RichieRocket

Active Member
Good points. I thought about a vented cap, but also had the concern about fuel coming out when the car leaves hard. Running a line up the filler neck and holding with a zip tie is a simple solution.

I do not have a muffler in the stock location, so I was thinking about putting loop in the vent hose there and securing to the bottom of the car.
Are you here in BG for the NATs? I can help you if you are..
 

TTipe

Snake Skinner
Nope. Same results as a vented cap.
As soon as any fuel is sloshed into the vent it will be expelled out the rear of the car.
There is a flapper valve in the fuel fill pipe. I have not tested the fill pipe for this condition. No one (that I know of) calibrates to support the canister valve so it is always open / closed. If closed then fuel tanks are working under pressure. Approximately a 0.125" hole in the cap seems to work. The need for a vapor canister is for emission compliance. My older cars had vent holes in caps to eliminate fuel tank bloating. Why won't it work today?
 

Boostian

Member
There is a flapper valve in the fuel fill pipe. I have not tested the fill pipe for this condition. No one (that I know of) calibrates to support the canister valve so it is always open / closed. If closed then fuel tanks are working under pressure. Approximately a 0.125" hole in the cap seems to work. The need for a vapor canister is for emission compliance. My older cars had vent holes in caps to eliminate fuel tank bloating. Why won't it work today?
Good point. A very small hole would actually eliminate this issue without fuel sloshing out.
 

RmvBfrFlght

Well-Known Member
The charcoal canister system is open. It can neither pressurize or apply a vacuum to the gas tank. Note the open bottom with the filter on it.

As for the gas cap, it is vented to allow air into the tank. And not let fuel out via a one-way valve.

RemoveBeforeFlight
 

RichieRocket

Active Member
Don’t forget the there is a check valve glued on the vent line under the hat. That check valve is the root of evil when it gets just a little corrosion in it. If it’s not new, just remove it. It’s only there in case the car goes upside down and to prevent debris from getting in. If you are running a racing check valve on the outside of the tank, just remove the one in the tank. It just takes a pair of pliers and a slight twist to break the glue loose. https://www.summitracing.com/parts/...FNY4FvAxr8E2E2B_5FaDtOPFxe0Bk9QhoC0bkQAvD_BwE
 

TTipe

Snake Skinner
The charcoal canister system is open. It can neither pressurize or apply a vacuum to the gas tank. Note the open bottom with the filter on it.

As for the gas cap, it is vented to allow air into the tank. And not let fuel out via a one-way valve.

RemoveBeforeFlight
What ever pressure (in tank / out of tank) is the greater will fill the tank non-fluid occupied volume. The one way check valve may not "seal" after many years. Again no-one to my knowledge calibrates/tables to the solenoid. The system does nothing without tables.
 

TTipe

Snake Skinner
Do they make vented gas caps? I get a good amount of pressure that comes out when I simply open my gas cap to fuel up….or is there basically just an issue with my fuel system?
Cobble a cap with a vent. The EPA will not allow vehicle manufactures to do such.
 

RmvBfrFlght

Well-Known Member
What ever pressure (in tank / out of tank) is the greater will fill the tank non-fluid occupied volume. The one way check valve may not "seal" after many years. Again no-one to my knowledge calibrates/tables to the solenoid. The system does nothing without tables.

I have no idea of what you are trying to say. The fact remains that a factory CCP setup can neither pressurize or apply a vacuum to the gas tank. There is no one-way check valve in the CCP system. It is open to the atmosphere.

RemoveBeforeFlight
 
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