This firm sells the STCs.
There is an interesting statement in the FAQs, in the first one:
We do not have Auto Fuel STCs for engines using Bendix fuel injection because it failed our flight testing. Continental fuel injection did not have that problem and is therefore approved – I0-470-J or -K (225hp). The 260hp I0-470 and 285hp I0-520 are also approved. These last two require Anti Detonation Injection (ADI).
What could be in the fuel which messes up the fuel injection?
I looked into this for another project and the issue is the routing of the fuel lines to the injectors on the Bendix installation is over the top of the engine at particular heat spots causing repeated vapour locks during the test flights they did at 8,000 ft.
The other versions of injection systems apparently route down the side of the engine thus avoiding this issue.
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I recall reading somewhere that the main issue with Mogas is vapour lock i.e. you can't have the fuel pipes going "up a hill and then down a hill".
What happens is that when any liquid is not under enough pressure, it "boils" i.e. turns into vapour. In sea level pressure ( 15psi) water boils at +100C but if you reduce the pressure you can make it boil at say +20C.
Mogas boils at some much lower temperature (at 15psi) and if a pipe goes up and then back down, the portion at the top is seeing a lower pressure. Throw in a bit of altitude and it can turn the stuff into vapour.
On some airframes they fit an extra fuel pump (which presumably must run all the time) to keep the fuel pipe pressurised sufficiently along the whole length so that vapour can't form.
On one airframe they replaced a 90 degree pipe bend with two 45 degree pipe bends, which is weird and suggests there are some other problems with Mogas even just flowing, or maybe it is to do with deposits of the dark gunge which you get in Mogas.
On a TB20, and probably most low wings, the pipe runs are not going to be "all downhill" so adequate pressurisation could be an issue. The engine driven fuel pump is certainly about 30-40cm higher than the wing tanks. I also doubt the 20000ft ceiling would be achieved on Mogas unless you did "something".
Just heard that a UK based TB10 owner got a permission from the UK CAA to run a very early S/N TB10 on Mogas, in 1983!
As a long-time (1000 +hours) mogas user with a Jodel DR1050 (fuselage tanks) and an O200 (carb of course), I've once had an engine failure at take-off definitely due to vapour lock, although the tank top temp was below the 20C limit. Fortunately at start of run. As I opened the throttle, the revs increased to 1500, then the engine died. I re-started it, cleared the runway, and got permission to shut down and check. When I drained the gascolator, foam came out. Run-up had been normal, but into a warm wind, and engine heat would go back to the gascolator. I've had similar incidents which were attributed to carb ice, but might have been vapour lock. Always engine lost power before reaching lift-off speed. No fault found. CAA limit is 20C and 6000' on mogas.
