Fuel Cell Mounting?
 

Building a street/strip car and the class I will be running requires a racing fuel cell. This will be mounted in the trunk.

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I need a flat surface to mount the tank to. Right above the spare tire cut out. However the spare tires area has some rust that needs some repair? Not bad and can be fixed easily. However I need this area to be flat. Would it be better to repair the damages and make a plate to cover the area leaving the spare tire area intact. Or should I cut the area out and weld in a vice flat surface on doing away with the indentation?

this way think will cut down on weight but how much weaker will the uni body be?

I will be removing the stock fuel tank.

What kind of racing are you doing? The SCCA and NASA rules require that the fuel cell be mounted with a frame around it and under it. And it has to have a metal bulkhead between the driver compartment and the fuel cell. The rear hatch area on my rx7 has the spare tire area cut out and the fuel cell is attached to a 3/4" square tubing frame. and then there is a sheet metal cover with a door cut into it to get to the fuel cap. The fuel cell is in the same location as the old fuel tank was.