carb/mech pump/elec boost piping ???

[Larry DeCamp]

Forgive me folks, I have been searching threads for piping diagrams I have seen before. I am aware of all the debate of 100LL/91mogas re vapor lock.
My question is: should my boost pump go to the suction side of the mechanical pump? Is this how the Cherokee 140 did it ? Concerns are :
1 Will the boost pressure be lost if the mechanical diaphram leaks ?
2 Will the mech pump double the pressure at the carb for takeoff and is that OK ?
3 Can the mechanical pump pull through the electric boost in normal ops ??

[BillL]

Use the google search in the google screen like this:

site:vansairforce.com term1 +term2 + etc.

This can help you find items of interest. You will refine the search terms as you discover what people call things.

I am building, but have been looking at some of these things to understand what limits we have I have, and how to quantify a successful fuel system installation or margin to its limit.

But: Normally RV systems have boost pump output to the mechanical pump inlet. BTW i was just reading an IO360 installation manual and they recommended -2 to 35 psi limit range for pressure to the mechanical pump.

if the diaphragm leaks . yes, fuel will leak, but there is a fuel capture device that dumps the fuel overboard rather than to the engine compartment or to the oil sump. Time to land.

I don't know what the carb limit inlet pressure limit is ( you can search just for that) but the boost does increase it some. It seems to be regulated at the mech pump. I think the diaphragm spring force (primarily) determines the pressure output.

Normal? Define Normal. Yes, it typically pulls through, and it works, but I suspect there are absolute limits of suction and temperature (specific to the fuel) that must be known or the engine will know. If the fuel can be defined for pressure vs "boiling" vs temperature then one could measure suction and temp to determine what the flow limits are for a particular fuel. For 100LL, and a 7 it works just fine for nearly all conditions i.e fuel flow rates.

These are some general answers, but what specifically do you want to accomplish?

[N941WR]

The standard setup for a carbureted RV is the fuel comes from the tanks, through the fuel selector valve, to the electric boost pump, through the gascolator, to the mechanical fuel pump, and finally to the carb.

Others have done it differently; some have had success and others, well, not so much.

Be very cautious about straying when it comes to your fuel system. I seem to remember reading that most engine stoppages in E-AB's are the result of "non-standard" fuel systems. There are thousands of RV's flying with the standard configuration.

[deek]

Originally Posted by N941WR

The standard setup for a carbureted RV is the fuel comes from the tanks, through the fuel selector valve, to the electric boost pump, through the gascolator, to the mechanical fuel pump, and finally to the carb.

Others have done it differently; some have had success and others, well, not so much.

Be very cautious about straying when it comes to your fuel system. I seem to remember reading that most engine stoppages in E-AB's are the result of "non-standard" fuel systems. There are thousands of RV's flying with the standard configuration.

There are also thousands of LongEZ's flying plumbed almost exactly like this (EZ's plumb the gascolator just prior to a Facet electric boost pump). N941WR has given excellent advice on veering astray of the designer's fuel system callout.