Yes it is a myth and it is also VERY easy to prove as such.

Mount any engine in a test stand with a considerable length of tubing.

Just make sure that the tubing is unobstructed during testing and does not get compressed or pinched.
Also make sure you have your pressure line(s) set up correctly.

Get it running, and then move the tank 1-2 inches above the carb centerline.

You'll see absolutely no difference.

Now drop it by the same... again no difference.

This is far more of a differential than you would ever see in mounting an engine inverted or permitted by moving a fuel tank up or down in a typical fuselage.



Now ( assuming you've put in a good bit of tubing ) drop or raise the tank a good 12+ inches.

Almost no change if any.


Have some more tubing available?

With the engine running, swing the tank around to subject it to multiple "G"'s of force....

Again little to no difference.


Give it a shot, it's an easy test to do.... something I've -actually- done.


Highplanes:

In a 10G environment the tank differential again doesn't mean anything. Been there, tried that, noted the results.

You are looking at a tiny fraction of an ounce difference in the fuel lines because of this offset.

The kicker is that under the conditions you say, e.g. pulling a 10g loop, the tank is WAAAAAY below or above the plane anyway...

e.g. in an up line the tank can be a good 10+ inches below the carb as it pulls that 10g loop.... and yet your engine still runs.

The inverted differential or the offset afforded by moving a tank up and down in a fuselage is no more different in this case than moving the tank back or forward by .5".

You should take a look at the videos of the fuel tank and fuel levels in the "fuel stays in the back of the tank" myth thread...

If the pilot believes a typical .5 - 1" centerline offset is affecting their loops, they really need to look elsewhere for the cause of their problems.




The mantra around here for engine problems unfortunately from the "more experienced" set is all to quickly said to be a tank height difference.

If the user is experiencing hydro lock... then may be something to suggest...

However newbie's are often told... "if you mount the engine inverted, you'll have to move the tank down to get the engine to operate properly" when that is completely UNTRUE and an often repeated fallacy.