I bought a 4psi Hobbs switch to enable my second fuel pump. Big mistake. I should've bought a 15psi switch.

Anyhew... I've figured out that these Hobbs switches are adjustable. There is a rubber cap on the end that you can take off and adjust the pressure with a allen key. I'm curious if anyone knows the adjustment range... for example they're all the same switch, but they're calibrated differently?

I'm just trying to save myself 40 bucks or so.

 

Depends on the part number of the box. As far as I am aware though they come in only 2 flavors low and high pressure (75psi I think on the high). IF you turn the hex out that is more pressure as I recall.

 

best thing to do is to hook it up to a pressure source such as a vac/pressure pump w/ gauge. Pump it until the switch opens, adjust until you get it where you want it.

 

Ernie: You can hook an LED onto the Hobbes switch to flash on when the second pump activates. Install the LED next to your boost gauge. This way you can tell exactly at what boost level the 2nd pump is coming on. This helps in dialing it in during initial setup and also later on should you need to raise or lower the set point. It also lets you know that the Hobbes switch is functioning during WOT.

 

I don't have a boost gauge :P

i can adjust it and then go drive around and log my wideband I'll know when it kicks on. lol.

thanks guys I'll be sure and post up how much adjustment there is as well as which direction to turn.

-Eric

 

Just wondering but why did it not work at 4psi? And is it instinanious from when the switch turns on the pump and when your fuel pressure goes up? On stock regulator?

 

Just wire a test light onto the output terminal of the Hobbes switch. It'll lightup when the switch closes creating a circuit. Tell me that you're only kidding about the lack of a boost gauge... right buddy?LOL

 

you can turn the 4psi into 15psi by simply turning the allen. the range is larger than what they sell them as.

 

No he is not kidding... he does not have a boost gauge, only AFR... he can log boost via Evoscan though...

 

Then don't buy a Hobbs switch. Perform the quick and easy drilling procedure to the factory 'Y' return suction fitting and run the pumps full time.

 

Thanks Ted I'll look into it.

For the time being I'll adjust the Hobbs switch

 

Whats wrong with a 4psi switch? I would never run a 15 psi hobbs as it does not give the pump enough time to come to life. By the time it is fully up and running, you would be well into boost and may go lean. I learned this from experience and I now run a 4 psi hobbs. My second pump is an external and this is the only reason that I run it on a hobbs. I simply don't want to hear it all the time. If both were intank pumps, I would have them both on full time.

 

I think we did some testing at 10 psi one time and as you said, it seemed to come on too late as by the time i hit 10 psi i was already hitting 25 or more when i finally saw the fuel hit in my logs. I believe i am now set at 5 psi or so to get the fuel there.

 

I would set at 15psi. One pump is enough for spool fuel and lower rpms no matter how fast its spooling. the second pump is only needed after ~6000rpm when fuel curves would go lean from much higher fuel demands.

 

Im guessing turn-on pressure would vary based on setup. At what boost level does the single pump run out of flow. R/T has the new HKS turbo so I imagine Its just a peak airflow where he may need the extra pump. Coming on too early may over power the return system increasing fuel pressure (like DSMs did with a single 255 on a stock regulator).

A Black on E-85 would probably have a greater margin covered by that second pump.

Also consider that the second pump would be coming on all the time with the HKS turbo at 4psi, but a black may not see that unless you try for it.