Double pumper- micro switch vs. hobbs vs. RPM activated

[John Larkin]

I have the option of activating my double pumper 3 ways: micro-switch on the throttle arm, traditional hobbs switch, or an RPM activated signal. The goal is to avoid a rich spike in the fuel curve. I'm leaning toward a micro switch since it would only be activated at WOT which would give me consistent fuel delivery. Any thoughts on this?

 

[NastyGn]

There is always a number of ways to skin a cat. I used the hobbs switch set at 9# for my second pump when I had a set up like that. It made me feel better when it would come on with boost setting rather than W.O.T or rpm. It's a matter of what you feel safe with.

As far as fuel spike. I never saw one that hurt the car when I had a chip. I went with a DFI with this set up and it was still no problem.

Good Luck

 

[steve hughes]

I use a micro switch and have never had a problem. Once you are under WOT i can't see the fuel pressure spiking much unless you have a problem with the return line. With a micro switch you can always test the second pump before a run by turning on the ignition and putting the throttle on the floor.

 

[BABY BOEING]

I would use both.That way the second pump won't turn on if you floor it from a roll on the street with no boost bilt up and it won't turn on at the track while your trying to build boost at the line.

 

[John Larkin]

Thanks. My whole thing is the initial surge of the 2nd pump coming on with the hobbs switch. Obviously it'll take the car some amount of time to recover from that surge (a tenth second maybe). If I take the surge out of the equation by turning it on with WOT then theoretically the fuel curve "should" be able to be tuned flat. Just like having a larger pump full time at WOT.

 

[Reggie West]

I always liked the Hobbs switch for a different reason.

If you set it at 10 psi for example you did not have to worry about leaning the car out in the middle of the fuel curve and then be rich on the top end.

 

[NastyGn]

Originally posted by John Larkin
Thanks. My whole thing is the initial surge of the 2nd pump coming on with the hobbs switch. Obviously it'll take the car some amount of time to recover from that surge (a tenth second maybe). If I take the surge out of the equation by turning it on with WOT then theoretically the fuel curve "should" be able to be tuned flat. Just like having a larger pump full time at WOT.

Something to consider in this post. If your fuel regulator and return line work fine and you are using larger injectors and a motor that makes more HP there is no fuel surge. The regulator will control how much fuel pressure the fuel line has. This is the reason when you go to a really large fuel pump you need a larger return line. If you see a fuel spike your regulator is not controlling.

Another note is in the early Pro Charger systems they use a regulator to cut off the return line to increase fuel to the fuel line when the boost comes up.

Like I said there is more than one way to skin a cat and I don't think any have any more merit than another. They all end up doing the same thing. The only problem I see is if you have a whole lot more fuel than the motor is or can use and the return system is not good enough to handle it. Then you will see a big fuel spike and a rich condition.

Take Care

 

[Razor]

John, this can be done using the VCS I sell, or the voltage switches(tps) that companies like NOS sells. You basically setup a voltage threshold like lets say 2.8 volts on your 3 bar sensor, when the voltage on your 3 bar hits 2.8.. then the pump kicks on.. or 3.0 volts..or ??? etc.. the control of off the map sensor deletes the hobbs dead band which can keep the pump running way after the boost comes under 10 PSI. Full electronic so no reliability issue, just another option. Or when using the 2 bar sensor.. about 4 volts= 10 PSI. off the top of my head..dont hold me to these readings..

No need for TPS switching.. anyways.. back under my rock

Bigger return line I hear also cures this.. dont know where I heard this