How do we KNOW that the return to the sump is what caused the prob? Are

we talking his redesign or the original install?

We do NOT know that the return to the sump caused the engine out on Paul’s last flight.

I don't recall anyone suggesting that we do

My original plan was to return fuel to the sump, but after Paul's first

engine out to also have the sump vent be capable of overflowing into

one of the wing tanks (both of which gravity feed into the sump).

There are sump tanks and there are header tanks. I would define a sump tank as one that is at or near the lowest point in the fuel system, and is gravity fed from the main(s). Any small tank in a system that has to be fed by a pump, I’d call a header tank. Header tanks are a different issue.

Agreed

Here is a basic summary of my views on sump tanks. I state them as facts, but obviously it is my interpretation of the facts.

A sump tank has the advantage of eliminating the need for a pump from the main tank; and eliminating the need for valves from one tank or the other. Assuming it feeds from the lowest point in mains, it also gives you a single point for collection of water and impurities that may get in the fuel, and a single sample drain. These are desirable and important reliability and safety features.

Agreed - with the caviat that you need a largish line from main to sump. Velocities are given to assymmetric gravity feed (mine is the undisputed champ) and one should take this into account.

The Velocity has a true sump tank which is full (or above) any time there is fuel in the main tank. It is also a reasonably large tank; about 2 ½ gal in the RG and I guess 4 – 5 gallons on the FG. A fuel pump at or near the exit from the sump tank; at a level at or below the level of the bottom of the tank, with a reasonably sized line (3/8” or larger) from sump to pump should never see less than at least 1 atmosphere of feed pressure, and therefore be highly resistant to any vapor lock issues.

Agreed ("should" being a key word here)

With the fuel exit from the sump a bit up from the bottom (room for collection of a quart or more of water and impurities) there should be no need for a filter between the sump and the pump that could result in flow resistance. Put the filter (30 micron or less, I suppose) downstream from the pump to catch anything you don’t want going to the injectors.

Agreed

I don’t believe there is an issue with returning the bypass fuel from the pressure regulator to the sump as long as the sump tank is a few gallons or more. When the engine is running there is always a net flow from the main to the sump. There is not an issue of the sump “overflowing”.

Here's where we part company. The fuel flow to the rail is pretty much constant (at say, 30-35 gph). On the ground (where engine compartment temps can be pretty much as high as they get), the engine consumes 2-3 gph and bypasses the rest back to your 2-gal Velocity sump. Your sump basically recycles completely every 4 minutes or so. Your fuel could be really REALLY hot just when you begin your takeoff roll.

If you have a smaller sump, or if it is reasonable and simple (reliable) to do so; return the bypass to the main tanks (tanks, plural; for gravity feed returning to one tank could result in level imbalance.)

Main tanks would cause MUCH more heat dilution. My scheme described earlier would preclude level imbalance and other problems without lots of extra valves and plumbing.

<>If you are pumping fuel at a rate that far exceeds that burned by the engine, there is a potential for heating of the fuel in the sump from heat picked up in the fuel lines and rail in the engine compartment. Size the pumps so that one pump gives you slightly more than the maximum burn you expect from the engine. How readily available are pumps with that precise and small a flow rate tht you could accomplish this? I'm not sure. I'm using what's readily available, and it has pretty high flow rate IIRC. More than that; or running both pumps more than just during takeoff, is just circulating a lot of fuel through the engine compartment to no advantage, and causing heating of the fuel. Agreed. Fire sleeve on the fuel lines is a good idea, and provides some (probably not much, but some) degree of heat insulation. Agreed.