T-Maxx Dual Fuel Tank Mod

My new truck, a Traxxas T-Maxx 2.5, has very few faults, but one that bugged me was the lack of fuel capacity. The stock fuel tank is 125cc which gives 10-15 minutes of run time. While bashing around in the back yard, that's just enough time to get the engine tuned good and decide which bank I'm going to jump off of. At the local track, races are 5 or 10 minutes long, but between the warm-up laps and sitting around at the start line, you need another 5 minutes worth of fuel to be on the safe side. (Or a pit crew to refuel for you, which I don't have.)

The solution? An extra fuel tank. For my second tank, I chose an OFNA 125cc tank which I think was designed for 1/8th scale buggies. I'm not sure of the exact model number because it was mis-packaged as another part. I picked it up for about $10 at the local hobby shop. I chose this tank because it seemed to fit perfectly where the stock battery compartment used to be. My batteries are now zip-tied to the front shock tower.

Some people have asked if a dual tank addresses the self-leaning that the stock T-Maxx tank is reportedly very bad about. Due to the height and overall shape of the tank, fuel pressure drops slightly when the tank is about half empty, causing the engine to run slightly leaner. Some claim it's so bad that the engine will overheat and shut off. I've noticed a very slight increase in engine temperatures at about the half tank mark, but nothing to write home about. I think that people are blaming the fuel tank for other problems. Still, with a second tank, I've noticed more stable temperatures and have had to do a little less tuning.

Running with two tanks is only slightly more complicated that running with one. It requires one extra piece of fuel tubing. Normal fuel flow is something like this:

tuned pipe -> stock tank -> carb

With a second tank, it goes something like this:

tuned pipe -> stock tank -> new tank -> carb

Ideally, it should look something like this:

tuned pipe -> stock tank -> fuel filter -> new tank -> fuel filter -> carb

Now for some pictures!

This was my first attempt at adding a second tank. Because I wasn't sure how well this was going to work, I decided not to drill any new holes in the chassis. Instead, I used the existing holes left over from removing the stock battery compartment. As these holes were at the corners of said compartment, they were in the wrong place for this OFNA tank which had mounting tabs in the middle of each end of the tank. I stripped the insulation off of some 10-2 Romex wire I had left over from installing a new outlet. I bent this solid copper wire into an approximation of a tank mount. I managed to only stab myself a couple of times in the process.

I ended up re-bending the mount several times before I was finally happy with the result. Fortunately, copper bends easily. I did some backyard bashing and the tank was remarkably stable, especially after I packed a little foam rubber under the high end to keep it level. Then I got really brave and did some jumps (intentional) and end-over flips (unintentional). Unfortunately, copper bends easily. I found that any good flip would cause the tank to shift, either backwards into the spur gear or sideways off the side of the chassis. No serious damage, but it did cause me to reconsider my copper wire mounts.

Some might ask why I don't just use the mounts designed for this tank. 1) Said mounts are designed for a totally different vehicle. 2) I couldn't find any at the local hobby store that looked like they would work. Also, if they're designed like some of the other mounts I've seen, they allow the fuel tank quite a bit of movement. I'm trying to avoid that.

A quick trip to the hardware store produced a pack of 6-32x2 Round Slotted/Phillips with Nut, Quantity 8. In other words, 2" machine screws with matching nuts. Two inches (2") turns out to be just a little longer than the distance from the bottom of the chassis to the top of the OFNA tank's mounting tabs. Remembering the ancient adage of "Measure twice, cut once" I only had to drill three holes to get two of them roughly where they would match up with the mounting tabs.

I had thought I could just toss the tank on the screws and tighten down the nut, but it turns out you can't do that. First of all, the bottom of this OFNA tank isn't flat. It slopes towards the back where the fuel pickup is. So I had to put an extra nut under the mounting tab to hold the front end up so the fuel would flow properly. Secondly, the tank is hollow plastic and will deform under pressure. When I tried to tighten down the nut on the back end, I found that I was just bending the mounting tab. An extra nut on that screw was required as well.

When all was said and done, more was said than done. But I did finally get the OFNA tank mounted. It is very secure. While the copper wire mount let the tank wiggle in pretty much every direction, the machine screws barely let it move at all. It will move to the side very slightly, and back towards the spur gear not at all. Like the copper wire, these zinc machine screws will bend, though not nearly as easily. If I do manage to bend them and get the tank out of alignment, I can probably bend them back.

I also notice that this tank will fit where the stock receiver pack goes. I could move the receiver pack and put a third tank there, if I were crazy. I'm not quite ready to do that yet. Let's see how two work out first.

I almost forgot the long view shots. These show how the tank fits on the chassis. They also show how I moved the battery pack to the front shock tower.