I see people running dual walbro in-tank pumps, but only running one constantly, while having the other activated by a Hobbs switch. Is there really any other benefit to doing this, other than maybe extended pump life for the one wired to the Hobbs switch? I mean, what would be the harm in running both constantly?

 

If you are just idleing sometimes you circulate to much fuel and it heats up and turns to a vapor causing some problems. So i have heard...

 

You can't hold the fuel pressure down at idle with both on.

 

Typically when you are street driving, the volume of fuel flow needed is to feed under 50hp. The extra returns to the tank. The heat pulled from the fuel rails & engine compartment is also returned to the tank.

When the pumps flow enough for say 1000hp, the energy powering the pumps is transferred as heat to the fuel & retured to the tank. For example, a 30amp draw at 12v is 360 watts.... compare that to a 1000watt hair dryer & you get the idea of how much heat can be put back into the fuel.

These issues combined, heat the fuel up to the point that it can start to vaporize. At this point, the pump does not pump vapor well.

With big pumps, some have problems after a long drive on the street. This is why many large pumps run with a pump speed controller on the street.

Also a regulator not designed for a large pump may not be able to control the return flow to the tank. This can cause high idle fuel pressures & subsequent tuning problems.

For these reasons it is better to use a single/smaller pump for normal driving. If you wait to bring on the additional pump until the primary pump is already flowing a substantial amount of fuel, the second pump will come on without causing a large pressure fluctuation. Bring it on too soon & it will spike the pressure & cause tuning issues.

Hope this helps.

 

Great explanations, guys! Glad to know there's a logical reasoning behind it...

 

I've been running 2 Walbro 255lph intank pumps for 2 years without hobbs switch. Replumbed with (2) 3/8" socketless hose lines and air-to-air fuel cooler. I have not had any problems, but maybe with the small stock lines the pumps have a higher head pressure to overcome and heat up. I do have a switch to disable pump #2, but hardly ever use it. Also run a KB BAP on Pump #1.

 

In addition to having my second pump come on at 5psi, I installed a switch to change the primary and secondary pump. This way if one pump fails I can still limp home on the other pump. I can also turn both pumps off for theft prevention.

 

I run both all the time too... The stock regulator won't handle the flow, but aftermarket ones will. Heat isn't a problem if you keep the tank half full -- which I do all the time anyway.