Proper Fuel Tank Venting

Most builders and hobbyists get this wrong!

Proper venting of your fuel tank is essential for fuel system operation as well as allowing for proper refueling. What we all want is a high performance hot rod but with the functionality of a new vehicle. In this article I will educate you on how to properly vent your fuel tank to allow for rapid refueling and proper fuel system venting during operation.

We have built a long list of vehicles over the years and one of the biggest lessons we have learned is how to properly plumb a fuel system vent. If you watch how other people do things, and try to learn from them, you may not always find the right information. Our industry is growing and learning at an insane rate and a lot of it is due to social media. All of us builders are posting behind the scenes photos of our builds every day. When I was younger, I would have to purchase books written by industry professionals or attend a train class to be able to gather information. Now, if you just pay attention to social media you can learn a ton about how things are done. One of the things that I see done incorrectly is improper venting of the fuel tank.

A modern new car has a fuel fill that allows you to insert the fuel nozzle, click it on high, and step away. The fuel tank has a very large vent port that runs with the fuel filler pipe and vents at the top of that fill. Once you remove the fuel cap and insert the fuel nozzle, that large open vent is above the bottom of the fill nozzle. As fuel is rapidly pumped into the tank, that volume of air has to be evacuated at the same rate. So a large diameter vent hose is very important! If your fuel tank only has a ⅜” vent port and a remote fill location, you will NEVER be able to fill at a rapid pace. For now, let's just focus on proper fuel filling and we will come back to talk about fuel system operation later. If you look at most rear mount fuel tanks available for C10’s, you will likely find tanks with only one vent port. This is generally on the pump hanger or on one corner of the fuel tank.

However, let's look at one of the most popular tank manufacturers in the C10 world. Boyd welding sells very high quality fuel tanks and they fully understand how to properly vent a tank. The first tank to look at is a standard tank that will be filled from the bed floor.

As you can see there is only one vent fitting on this tank. This vent port is only intended to allow air in and out of the tank during operation. Again, I’ll come back to talk more about how this should be plumbed. Since this tank is a bed floor fill, there is enough room in the 2” fill port to allow air to rapidly exit the tank during filling. If this tank were to be changed to fill from a remote location, it would not be able to be filled at full flow. Fuel would occupy most of the filler pipe and air would have a hard time exiting the tank. The vent port on the tank would be too small to allow enough air out as well. Now lets look at their side fill tank.

As you can see, Boyd has made sure to add a ⅝” port to this tank. This fitting MUST be used to ensure proper rapid refueling. As important as using that port is, you have to make sure that you plumb this into the fill pipe, above the point of fuel entry. So make sure that when the fuel nozzle is inserted, this vent is above the end of the nozzle. If you are thinking about venting that port to the atmosphere, please consider how much fuel vapor will be escaping from the fuel tank. For one, if you were to park in a home garage, the fuel smell would be insane. Then the other factor is how flammable fuel vapor is. A backfire from your exhaust could absolutely ignite those vapors. Make sure to plumb this into the filler pipe and make sure to use a fill cap that will seal. So do not use a vented cap.

Okay, now we can talk about the actual vent port that is on all aftermarket fuel tanks and pump hangers. This port is simply used as a way for air to enter and exit the fuel tank during vehicle operation. While the engine is consuming fuel, that fuel needs to be replaced with air.

Clean air has to be able to get into the tank. While I’m sure you’ve all seen people coil up hose then route it vertically, there is a better way. To avoid a blockage or a fuel spill, use a remote vent canister. There are a few different options on the market but below is a photo of the one we sell on our website.

This canister has two separate chambers that can be used for two fuel tank vents or a single fuel tank vent and a rear end vent. These canisters filter incoming air and can contain a small amount of fuel if it makes its way up the hose. The fuel simply returns down the vent hose. In my options a vent canister should be used on every build. If you wanted to take this a step further down the OEM rabbit hole, you could return the vapor to the intake manifold via an EVAP solenoid. But for the sake of keeping no subject, we will focus on using the sintered bronze filer.

Your fuel tank vent or vent canister needs to be installed as high in the vehicle as possible. The more uphill route of the vent hose, the better. If you make it impossible for fuel to get pushed through the vent, that would be ideal. If you are building a car, then mounting the canister up in the quarter panel is perfect! However, if the canister is inside the vehicle, the inlet has to be extended back outside. In this case you would purchase fittings to move the sintered bronze filter back under the car. If you go back and look at later 67-72 C10’s they vented the fuel tank, which was behind the seat, via a hardline that went way up inside the inner structure of the cab and excited out the floor. Same with a Jeep Wagoneer that we used to own. The factory vent was a hardline that routed way up inside the cabin and came back out under the truck. In our C10 trucks, with a rear mounted fuel tank, this is harder to do. You really need to be able to get it up inside the bed to get enough height. Below is an option that works well.

This is a filler that we make and sell that checks all of the boxes for me. It moves the fuel fill up higher than the tank, has a very large vent port in the fill, and moves the vent canister high enough to not constantly overflow with fuel. Now there are a ton of options out there to achieve this, but the point is to get the vents up high.

Lets talk about where to place vents on the tank and is one vent enough. If you are only driving straight down the interstate, vent port placement isn’t critical. However, we drive our stuff very hard and will be putting them hard into corners. Fuel gets forced to the side and corners of the tank during high G acceleration cornering or deceleration cornering. If the port is in the corner of the tank, fuel can easily be forced out of that port. Below is a photo of a tank that No Limit Engineering sells. This tank appears to have the port in the corner of the tank. But in actuality, there is an internal tube that routes up to the center of the tank. Also notice that this tank’s top has been slightly broke diagonally to create a vapor dome. This is the place where vapor makes its way to the highest point. The internal vent tube is connected and sealed to that ¼”npt port. This is a great design and works very well.

On the right is a fuel tank that is built by Rick’s tanks in Texas. You will notice that their tanks have two vent ports. “Why would I need two tank vents?!” Well, there are two ideas behind having more than one vent. The first is something that we see a lot on the autocross. If you have one vent port and fuel makes its way up that hose and starts to flow out of the hose, it can create a siphon effect. To prevent that from happening, you would need two separate vents on the tank and they would need to come together at the highest point. So if fuel is pushed up one hose, the other hose is completely open so that it will not allow siphoning to happen. The other reason to have multiple vents on your tank is to allow more air into the tank. If you have a 1000+hp truck and have a massive amount of fuel flow, you will need to allow more air into the tank. In this case, you would also want a larger amount of venting.

The last important bit of information is related to the materials you use to plumb your vents. Traditional rubber hose and even AN hose will allow fuel permeation. While these hoses are rated for fuel system use, they allow an amount of fuel to permeate through the hose causing a fuel odor. You need to make sure to use a “vapor guard” hose or a PTFE lined hose to avoid permeation. Below you will see an illustration of the “vapor guard” hose as the yellow layer is the plastic that helps avoid this issue. Then on the right Fragola Race Rite hose has a PTFE core will helps avoid the issue.

In years past we plumbed entire vehicles using a traditional cloth braided AN hose and could not solve the fuel smell mystery. While the system was vented correctly and was without leaks, the fuel smell was intense. All coming from fuel permeation through the hose. I recently went back through a car I built back in 2010 and replaced all of my AN hose with a nicer PTFE lined hose. It was refreshing to be able to park in our house garage without smell!

Thank you for taking the time to read this article. I hope that you have learned a few things about fuel tank venting and can incorporate it into your project. If you have any questions as all, please feel free to shoot me an email at jesse@levelsevenmotorsports.com

Jesse Vaughn

Photos courtesy of:

Boyd Welding

Holley Performance

Fragola Performance

Aeromotive

No Limit Engineering

Rick’s Tanks

Level 7 Motorsports