Exhaust under fuel tank

[72Mountaineer]

I've noticed a few builds in the custom mags where the builder has run the exhaust under the fuel tank when it is between the frame rail in the rear. I always thought that this was taboo, maybe even not allowed in some states where a safety inspection is required. Any thoughts?

[Black_Sheep]

My 2 cents worth, from a service and maintenance perspective it's not very practical. Plus, it looks kind of hokey, like the builder didn't plan for exhaust. Same thing for tailpipes that go under the rear axle...

[Jesse Z]

Lots of late-model cars run exhaust much nearer the gas tank than I'm comfortable with right from the factory.

Two words: Heat Shields

[whitedog76]

Pretty much every car made runs the exhaust under the fuel tank. Even going back to the 70’s and 80’s. My 85 Dodge van had the exhaust running right next to the PLASTIC tank, just a thin shield between them.

[Donaldo]

Put a heat shield between the pipe and the tank.

[72Mountaineer]

Most production cars and trucks have converters which take most of the heat out of the exhaust from there back, so I'm not thinking about a cat exhaust. Just trying to get a feel for things. Thanks.

[mr48chev]

I don't think the heat is as much of an issue as the inconvenience if you have to drop the tank, Never heard of a 58/60 something Impala or a Mustang catching fire because the muffler dude ran the pipes under the gas tank.

You almost have to arm wrestle the muffler shop guys here to have the tail piped end up straight out under the bumper on a truck rather than off to the side behind the rear tire. As many late models around here run canopies on the bed and people might ride in them they want the exhust out in the slip stream rather than straight out the back where it might roll right back into the bed of the truck.

[1project2many]

Here's the NH rule for heat shields. It used to apply to all vehicles but I only see it in the bus section now.

"The exhaust system shall not pass within 12 inches of the fuel tank and its connection, or any flexible brake system hose, unless a heat baffle is used, in which case a minimum exhaust system separation of 1.5 inches shall be maintained to the fuel tank, its fittings and any flexible brake system hoses. The exhaust system may be less than 12 inches from the fuel tank on diesel powered buses."

With exhaust under a tank, even if nothing melts, the fuel can get very warm which can cause vapor leaks and fuel odors, can cause excess vapors to be drawn into the charcoal canister, and can cause spark knock. But if you think about vehicles using plastic tanks today vs metal from the past, I think the OEM's run exhaust remarkably close to the tank.

[my56chevytruck]

I guess my street rod that I've had for 20+ years with the exhaust pipes running under the gas tank wouldn't be happy in NH. If you take the time and look at most street rods, hot rods, the exhaust pipes are running under the gas tank. If you can run the pipes away from the tank then do so.