I’m in the process of stripping everything from the back of my M1088 and at first I was planning on leaving the intake and spare tire in the OEM config but the more I look at it, there’s a lot of wasted space and unnecessary weight there. Furthermore, I would like to carry more fuel but want to leave the area aft of the fuel tank open for an entry point into the habitat so throwing around the idea of mounting a second OEM fuel tank on the driver side. To keep things symmetrical, the battery box and air tanks would need to move. Is there any reason the air tanks are in that specific location? Could they moved forward and a little higher say where the OEM intake is located?
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So, I can't claim any sort of engineering degree associated with Air Brake systems. I can say that with the past 40 years of experience with air brake systems - I have never seen air tanks any higher than in side the frame rails. The M1088 and that family of trucks have three tanks. You need to keep the "plumbing" as it is now. There are check valves between the tanks so the air can't flow backward in the system. The "wet tank" fills up first and water collects in that tank first off the compressor.
I can't say that higher than the frame rails is a bad thing, but logic has the air dryer at about the same level (height?) as the tanks. Air tanks need to be drained daily. That water has to go somewhere and the higher they are mounted - the more slop and mess gets blown onto everything below those tank drains.
Adding a second Fuel Tank: Have you figured out a plan for that? On a road tractor, there is a hose that connects the tanks together. On a road vehicle, that connection happens at the bottom of the fuel tanks. One fuel gauge, one pickup tube but again these are OFF-ROAD and you can't have something that low where it might get torn off... Yes, my M936 has two tanks. But there are two pickup tubes and two gauges on that truck.
Hopefully several of the other guys can stop by and give some opinions.