Rethinking the flop tube

22 Jul 2004 Rethinking the flop tube

Update 20210918: I didn’t install the Eggenfellner Subaru engine, so any post that references that can be ignored or used as a cautionary tale about how to avoid wasting many years and $50k.

My original idea for the flop tube was to have one in case I install an inverted oil system on the Subaru down the road. There seem to be some on the market, mainly aimed at the rally/off-road drivers of the Subaru. Also, I have considered the fact that even after I get the motor, there might be something I don’t like which swings me back over to an AIOE type engine.

The problems I’m having are related to the use of mogas in the Subaru. Mogas is about half the avgas price here in Switzerland and the rest of Europe, so there is a big incentive to burn it instead of avgas. The problem is that a few people have experienced vapor lock when flying their Eggenfellner Subaru with mogas. All of the issues that I know of are related to keeping the fuel lines under the cowl cool, and the people that experienced failures did not seem to do this. Of course I’ll be sure to make sure my fuel lines are cool under the cowl, but another issue that can cause vapor lock is the negative pressure that a sucking pump can cause. To reduce the effects of this pressure, I want to make sure that I have “full size” -6 AN tubing and fittings throughout. The problem is the Van’s flop tube has restrictions in their -6 hose end fittings. I have a picture here.

Being the clever guy that I think I am, I decided to build my own flop tube out of -8 tubing, which will eliminate the restrictions around the hose end fittings. The problem I now have is that the -8 hose is so stiff, that it does not flop, even with the brass weight on the end. On top of that, converting from a -8 to a -6 later in the fuel line will be quite a hassle too, tho I have ordered the necessary parts for this. I’m really starting to wonder if I should drop the idea of having a flop tube for these reasons.

Update: 22 August 2004

I’ve decided to not install a flop tube. I ordered the pre-made Van’s fuel pickup lines shown here, next to the “home made” fuel pickup. I think the screen is a better system.