Engine and Transmission Tech

Hello
I'm doing an EFI conversion with a swirl pot
I have two questions i could use help with
The EFI return line can go to tank or swirl pot. if returned to pot, fuel could get overheated.
Which is the best return system?

When returning fuel directly to tank, if high pressure pump has greater capacity than low pressure pump, swirl pot will empty.

Any recommendations on a low pressure pump/
thank you

Ian

Not sure if you are planning on using an in-tank swirl pot, or an externial swirl pot (tank), in either case it's best to route the return back to the swirl pot. Otherwise you the risk of a lean out condition.
The fuel is heated somewhat, however it usually isn't an issue due to the fuel being recycled with the fuel in the main tank.
OEM's run return-less fuel systems to keep the fuel cooler. The main reason for this is reduce the EVAP emissions standards that the manufactures must meet.

Bill



Edited 1 time(s). Last edit at 05/24/2011 03:37PM by MG four six eight.

I say swirl pot. Yes it might be slightly warmer but some fuel still rotates back through the tank like Bill said and running it to the pot makes sure it is always full.
As to the pump, make sure it is rated for constant running as it WILL run most of the time the engine is on and I believe most low pressure pumps are not designed for that.

patrick

Hey Ian,
Fuel return to the tank or the swirlpot really shouldn't matter in a properly working system.
The whole idea behind a swirl pot is to provide the pressure pump with an uninterrupted supply of non aerated fuel.
So designed properly, the low pressure pump will always be of a higher volume than the high side pump.
That way the pot is always full no matter what the high side is doing.
But to me the swirl pot is a little outdated.
I would put the fuel pump in the tank with an overflow can. Only one pump is needed that way.
Much simpler, fewer things to go wrong, safer, fewer hoses and fittings, less wiring, etc.
And please don't forget the inertia switch.
Cheers
Fred

Thanks for the input.
I'll check with suppliers, but i had assumed all electric fuel pumps are rated for constant operation?

What's the problem with warm fuel? Warm fuel will vaporize better and burn leaner.

Warm fuel may vaporize before it gets to the engine and form gas bubbles. Get the fuel too hot and that's called vapor lock. Think MGB where the carb is right over the exhaust manifold. Cool fuel is better. Just running the fuel thru an external fuel pump raises the fuel temp as the fuel is the cooling mechanism for the pump. Start adding a warm day, slow moving traffic, low fuel demand, hot engine, recirculating all ready heated fuel and voila, your stuck on the side of the road waiting for things to cool down if you have a carb, or waiting by the side of the road for a tow truck if you have an FI engine that just blew up.

The in-tank pump is the best solution of course, and these days you might even be able to find one that will fit through the tank sender bung, although positioning it at the bottom of the tank may be a challenge. That leaves the question of the return but one option might be the vapor canister connection, or you can even solder a line into the filler neck, although this would require some careful fitting to avoid noise from the returning fuel. Beyond that is the same old problem as always, soldering or welding the fuel tank.

As for the dual pump arrangement, how big is your swirl pot, and how long will you ever be at full throttle? With EFI you can determine maximum fuel volume used per unit of time so for example if you will never be maxed out for more than 30 seconds at a time (for most of our cars, easily enough time to exceed 120 mph) you really only need a little over 30 seconds of fuel in your swirl pot, including the fuel that is being pumped in by the low pressure pump. For a 300+ hp blown 215 I found a standard marine fuel filter with a modified mount and a generic cylindrical autozone low pressure pump was quite adequate. Fuel demand can be calculated from RPM and injector max flow ratings, and in my case on board tuning software and wideband O2 sensors showed no indications of leaning at redline. So it isn't always necessary to overbuild if you do your homework.

As for vaporlock I think it's over-rated. At 45 psi on the main fuel rail it would take a tremendous amount of excess heat to vaporize the fuel and if you have that much heat in your lines you need to unwrap them a bit from your exhaust pipes or else set up some very big injectors so you can run fuel vapor for economy. A vent line from your surge tank to the main tank will insure that there is no vapor there and will return hot fuel to the tank as well. Under normal conditions even the smallest workable low side pump will move more fuel than the engine is using so a restriction in the return line so that the supply pump isn't constantly running at full speed is a pretty good idea. The plumbing is a pain of course.

JB