How does a VENTED gas tank affect the engine performance?
Thread Starter
Melting Slicks
Joined: Jan 2003
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From: Shreveport LA
LA Events Coordinator
How does a VENTED gas tank affect the engine performance?
Mine has to be vented BUT I had a non-vented cap on it for the past 4 weeks. How does that affect the engine, carb, fuel flow, etc???
Race Director
Joined: Jan 2000
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From: San Jose CA
Basically, in order for fuel to flow and to prevent the pump from bursting or collapsing, you need some kind of venting mechanism. A charcoal canister prevents gasoline from evaporating into the environment and also allows some of that gasoline to be reclaimed by the fuel system.
A vented gas cap just lets it out to the atmosphere.
Assuming that they both allow the same rate of removal and replacement of air, there will be no power difference.
A vented gas cap just lets it out to the atmosphere.
Assuming that they both allow the same rate of removal and replacement of air, there will be no power difference.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,712
Likes: 31
From: Shreveport LA
LA Events Coordinator
OK. My system does not have a charcoal canister but it does have a return line. When I had the non-vented cap, did all the pressure in the tank affect the fuel flow to the carb...maybe too much pressure (more than 6-7lbs)
Race Director
Joined: Jul 2001
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You're mixing apple and oranges. As fuel is consumed, air must enter the tank to replace it. This air is provide by either a vented cap or the charcoal canister and system.
The fuel return line off the pump does not affect tank pressure one way or the other, it simply recirculates some of the fuel to keep the lines and pump cool..
The fuel return line off the pump does not affect tank pressure one way or the other, it simply recirculates some of the fuel to keep the lines and pump cool..
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
ANY fuel tank MUST have a vent to intake air (in place of fuel used). Some systems have a fuel vapor canister which is connected to the fuel tank; if so, the canister acts as the 'vent'. Some tanks have a vent tube permanently attached somewhere on the upper area of it.
And, others have the vent built into the fuel filler cap. If you have this type of system, you MUST use a vented fill cap or you will have not vent for the tank, as the one from the factory did not have one. You can check some of the aftermarket parts books to see if your year car required a 'vented' fuel cap.
And, others have the vent built into the fuel filler cap. If you have this type of system, you MUST use a vented fill cap or you will have not vent for the tank, as the one from the factory did not have one. You can check some of the aftermarket parts books to see if your year car required a 'vented' fuel cap.
Race Director
Joined: Apr 2007
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From: South Western Ontario
If the tank builds pressure (car sits in the sun) then the fuel pressure would be higher. If could build enough pressure that it pushes past the needle in the carb. If the tank has a vacuum (pump sucking fuel and no new air) then the fuel pressure would be lower. It could run lean or quit running if the pump can't get enough fuel to the carb.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,712
Likes: 31
From: Shreveport LA
LA Events Coordinator
If the tank builds pressure (car sits in the sun) then the fuel pressure would be higher. If could build enough pressure that it pushes past the needle in the carb. If the tank has a vacuum (pump sucking fuel and no new air) then the fuel pressure would be lower. It could run lean or quit running if the pump can't get enough fuel to the carb.
Race Director
Joined: Jul 2001
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Instructor
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Hey guys.. interesting subject. Ive been meaning to ask about a similar question myself.
My 68 has what I would imagine to be an original filler cap, which has vented stamped in the top.
I know this sounds like a silly
question, but does this mean the cap is "vented", or the tank is a Vented tank ?
There doesn't appear to be any pin hole breather hole in the cap, How do these work if so.
My tank has a breather pig tail in the top corner , which I'm not sure if that's a later 69 tank, and that was the return line blocked off.?
Any advise would be welcomed.
Thanks Marty.
My 68 has what I would imagine to be an original filler cap, which has vented stamped in the top.
I know this sounds like a silly
question, but does this mean the cap is "vented", or the tank is a Vented tank ?
There doesn't appear to be any pin hole breather hole in the cap, How do these work if so.
My tank has a breather pig tail in the top corner , which I'm not sure if that's a later 69 tank, and that was the return line blocked off.?
Any advise would be welcomed.
Thanks Marty.
Last edited by martaroony; Sep 4, 2013 at 10:30 PM. Reason: Added more details.
Team Owner
Joined: Jan 2006
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From: Crossville TN
Since ALL fuel tanks must be 'vented', a sticker on the fuel cap means that the cap is vented. Or, it means that some dumba$$ put a sticker on the cap that shouldn't be there.
The vented caps have a small hole on the inner surface of the cap; that hole doesn't go thru to the top surface, but is open to ambient air just below the outer cover.
The vented caps have a small hole on the inner surface of the cap; that hole doesn't go thru to the top surface, but is open to ambient air just below the outer cover.
Instructor
Joined: Apr 2013
Posts: 146
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Since ALL fuel tanks must be 'vented', a sticker on the fuel cap means that the cap is vented. Or, it means that some dumba$$ put a sticker on the cap that shouldn't be there.
The vented caps have a small hole on the inner surface of the cap; that hole doesn't go thru to the top surface, but is open to ambient air just below the outer cover.
The vented caps have a small hole on the inner surface of the cap; that hole doesn't go thru to the top surface, but is open to ambient air just below the outer cover.
Does this mean I wont have to keep the pig tail open ??
As, with a full tank I do get some spillage out of the pig tail..
If so Ill block it off, if the cap breather is sufficient.
Thanks mate , for your help. Marty .
