The vapor valve had nothing to do with preventing fuel loss in the event of a rollover. It was designed to work with the charcoal canister and prevent the evaporation of unburned gasoline. The stopper inside the valve is a float and works similar like a float inside a carburetor. Turn it upside down in a rollover and it will float away from the valve seat and fuel will leak into the vent tube. The valve needle/seat inside the valve a is plastic on plastic seal. All it can do is prevent splashing of gasoline from entering the vent tube while driving down the road.
A roll-over valve assembly is adapted for mounting on a fuel tank so as to expose internal portions of the valve mechanism to the tank headspace. Because liquid may slosh about in the tank, the valve includes a structure for shedding liquid fuel away from the main orifice so as to minimize intrusion of liquid fuel through the main orifice into an associated vapor control system to which the valve is connected. The roll-over valve further includes a relief and return vent valve mechanism which has a cooperative association with the tank headspace and the associated vapor control system. The valve assembly comprises a relatively small number of individual parts and it is cost effective for the various functions which it performs. The preferred embodiment utilizes a caged ball for closing the valve upon roll-over. from the Stant website: [FONT=Arial, Helvetica, Verdana][SIZE=+1]Evolution of the Gas Cap [/SIZE][/FONT] [FONT=Arial, Helvetica, Verdana][SIZE=-1]Few drivers ever give much thought to a device on their vehicles which they use again and again - the gas cap. To most, it's thought of as little more than a plug which must be removed to allow the tank to be filled, then replaced. But, there's a lot more to gas caps than meets the eye. Current designs must meet a set of complex performance specifications which are much stricter than anything ever imagined just a few years ago. This guide provides a brief look at the history of gas cap design as well as several key developments which affect gas cap performance today. The following outline highlights the key points. Historical Developments From the beginning of automobile production until 1970, gas caps were generally considered simply plugs to keep rain and other contaminants out of the gas tank. In most cases, they were simple unsophisticated metal caps although some were designed with vent holes to equalize the pressure between the outside atmosphere and the inside of the tank as fuel was drawn out. But beginning in the early 1970's two significant forces have driven the evolution of gas cap design: safety and environmental concerns. Safety concerns centered on keeping the fuel in the tank to reduce fire danger. The environmental concerns centered on the growing problem of contamination from fuel vapors which were escaping from the fuel systems and contributing to ground level ozone. Key points: In the early 1970's, Stant responded to fuel loss stemming from the positioning of filler necks lower on the fenders and to rear filler locations with new cap designs. Around the same time, more attention was focused on preventing fuel loss from impact and rollover situations, protecting occupants from possible burn injuries or death. During the next two decades, fuel cap design continued to progress and caps were developed which maintain the seal even if the crown is completely broken away. Continual testing of cap design and construction is done at the Stant Technology Center using a variety of testing methods, including the pendulum and ball-drop to simulate crash situations. In 1971, attention was drawn to the problem presented by the escape of hydrocarbon vapors and the first Federal requirements concerning evaporative loss were put in place. Stant responded to the requirements with new designs which incorporated advanced pressure/vacuum valve technology. By the mid-90's, the restrictions on evaporative loss were 50 times stricter than they were in 1971. New designs and technology have been developed at Stant to meet the new requirements including: More positive attachment of the cap to the filler neck. Caps that are easier to attach correctly. New, more accurate venting systems. Improved materials which resist deterioration from more aggressive fuels which incorporate ether, alcohol and other components. Stant also developed the Prevent cap which allows pressure built up in the fuel tank on hot days to be released gradually when the cap is opened. This prevents fuel from erupting and spraying out on the motorist. [/SIZE][/FONT] http://stant.com/brochure.cfm?brochure=1252&location_id=170
Another thought on the venting issue: If the tank doesn't have the air space necessary, then you might have issues with fuel splash making it's way to your charcoal canister. Not so much of an issue with a carburetor because there isn't vacuum at the canister vent port, it's purged under atmospheric pressure inside the air cleaner assembly, but for those running fuel injection and a factory type canister purge system and valve, the purge line is connected to the intake manifold vacuum port. Technically, the canister should also be vented to atmosphere, but there is a small amount of pressure drop across the charcoal substrate inside the canister which could allow enough partial vacuum to pull fuel in with a full tank. Might just be best to use a vented gas cap as some of the guys have done. I actually am running one now. Just more food for thought.
From the 1971 Shop Manual: DESCRIPTION AND OPERATION DESCRIPTION As a part of the standard fuel system, all 1971 Ford passenger cars are equipped with fuel evaporative emission systems to meet federal requirements in effect at the time of production. The 1971 system, which operates in a manner similar to that used on the 1970 California Ford vehicles, has been refined and simplified. Maintenance at a specific interval is not required. System control and operation are accomplished through three separate basic functions in the system which are: Fill control vent system. Vapor vent and storage system. Pressure and Vacuum relief system (fuel cap). Fill Control Vent System Fill limiting is accomplished through fill neck configuration and/or internal vent lines within the fill neck and tank. The vent system is designed to permit an approximate 10-12 percent volume air space when the tank is filled to capacity. This air space provides for thermal expansion or fuel as well as an aid to the in-tank vapor vent system. Vapor Vent System In-Tank Venting This system provides a vapor space above the gasoline surface in the fuel tank. This area is sufficient to permit adequate breathing space for the new separator assembly under all static and most dynamic conditions. Horizontally mounted tanks accomplish this by a raised mounting section for the vapor separator assembly that is centrally located on the upper surface of the tank. Vertically mounted tanks will use only a centrally mounted vapor separator assembly on the uppermost surface of the fuel tank. Vapor Separator (Orifice) Assembly The orifice vapor separator assembly performs the function previously handled by the vapor separator tank and its associated lines and connectors in the 1970 vapor emission control system. This assembly mounts directly to the fuel tank using the same type cam-lock ring as the fuel sending unit. The orifice vapor separator assembly is filled with a foam material that acts as a multiple baffle system to separate raw fuel and vapor and retard raw fuel entrance into the vapor line. Pressure and Vacuum Relief System Sealed Fill Cap The fill cap is a sealed cap with a built in pressure-vacuum relief valve. Fuel system vacuum relief is provided after 1/4 psi and pressure relief beyond 3/4-1 1/4 psi. Under normal operating conditions, the fill cap operates as a check valve allowing air to enter the tank as gasoline is used while preventing vapors from escaping the tank through the cap. OPERATION Fuel vapors trapped in the sealed fuel tank are vented through the orifice vapor separator assembly in the top of the tank. The vapors leave the separator assembly through a single vapor line and continue to the carbon canister in the engine compartment for storage until such time as they are purged to the engine by means of a tube connected to the air cleaner.
I'm considering using a fuel tank from a Mustang to allow centering of the tank but I want certain safety features retained. The 71-73 Mustang tank option would allow the air space for expansion and vapor collection. If it fits. My concerns are my own as far as fuel retention during rollover. The shop manual may not specify whether or not a component is in place for the purpose of passing a federal safety standard unless it is necessary for a technician to know that information from a repair standpoint. The valve in question does not have to perfectly seal the system. It serves a dual purpose (the way I read the federal requirements of Standard No. 301) It allows vapor to pass while preventing liquid fuel from entering the vapor line and it prevents a large fuel spill in the event of a rollover. That standard can be met by just restricting the flow to a few grams per minute. I'll admit, I don't know specifically when the function of the valve was to prevent fuel loss but that standard has been around since at least the early 70's. Heck our new shop manuals literally don't tell us how many systems work anymore much less what the federal requirements were that helped shape their design. The work and articles presented here appear to be top notch and I appreciate the work and information from everyone. Things look like they are put together well and proper precautions taken, which appears to be the case for Mavoholic's article, I was just looking at my options as far as adding a little extra safety for my own peace of mind. My 72 has a screen and cap, no valve, I was inquiring about the possibility of adding this feature since later tanks are equipped with it and the late Maverick tank has a provision for it. sorry for the hijacking
i bought one of the locking stant caps and it didnt fit properly it would lock in place where you couldnt pull it off without unlocking it but it was still very loose where it would rattle around
