Electrical "Flyback"

[alloy]

Wasn't sure where to post this but since it's on my car I guess this is probably the best place.

I'm in the middle of installing a holley sniper fuel injection of my 55 chevy. There is a thread on the tri-5 chevy forum about the sniper and the OP had a lot of problems with his system. He has his battery in the trunk like I do and ran his power for the sniper off the starter solenoid terminal. That's exactly how I planned to do it. The instructions are very clear with the sniper. Run both the power and ground wires off the battery directly, but again my battery is in the trunk.

So rather than just going by something I someone said on the internet with him saying he had problems running power from the starter terminal I called holley. It's free so why not.

After a 15 minute hold I got to talk with someone in sniper tech support. I told him where my battery was and he said to run a separate 10awg wire to the battery. I have a ground cable from the battery run to a bell housing bolt, then the body and frame are grounded from that. He said that would be fine but the separate wire for power to the battery would prevent "Flyback" from the starter. I looked up electrical flyback and found this: “Flyback Voltage Spike” occurs when the supply current to an inductive load such as a solenoid is interrupted"

Ok, so my question is with the new power wire running directly to the battery to the same place on the battery post wouldn't the flyback also travel back down the 10 awg wire also? Or is the battery acting as a buffer to absorb the spike?

Now I just realized I need to figure out some kind of fuse or circuit breaker for the new wire. I just can't see running a heavy gauge power wire from the battery to the injection through the car without some kind of overload protection. If the 10awg wire shorted somehow it could do a lot of damage.

I wish I had a crystal ball so I would know ahead of time what parts I will need so I didn't have to wait for parts to come in.

Hurry up and wait, story of my life it seems.

 

[woodchucker]

The flyback is less noticed at the battery, other things will consume that flyback, you just don't want to be directly receiving it.
I would just run it like he says. Really you need 10 gauge? I would think 12 would do, but if it calls for 10.. do 10.. How many amps does that draw?
The sniper is a replacement carb fuel injection right? Not directly into the head per cylinder?

 

[rabler]

Flyback spikes get attenuated by the inductance in the wire and the voltage absorption (effectively capacitance) in the battery. So longer wire from cleaner location is better.

 

[alloy]

Yes it is a throttle body fuel injection setup that bolts on replacing the carb. I also bought a new fuel injection ready fuel tank since my tank is 68 years old and the fuel injection system is very sensitive to contaminates in the fuel they said in the instructions.


I also made a fuel block for the pressure and return lines and to mount the pressure gauge. I'm going to use AN6 PTFE lines from the block to the throttle body.


I don't know how may amps it draws. There isn't anything in the spec sheet about that, just says hook directly to the battery. I'm going with 10awg wire because he told me that's what I should use. I've read that holley is very good about tech support and warranting the system, but I'm told from others that have the same setup they ask a lot of questions to make sure it's installed correctly before agreeing to warranty anything. I understand that very well. I have the same problems with my transmission mods I do. I get the most basic questions which I believe any car guy should know.

I think I have a 30 amp circuit breaker I can use on the new wire. Probably way too big, but at least some protection from a short circuit.

 

[tq60]

30 Amp breaker at power source as that matches wire ampacity.

The battery will absorb the flyback, a reverse diode at the solenoid shunts the ringing as well but large enough diode not cheap.

At engine end, you have a distribution point, here you connect your loads with dedicated fuse for each.

The system may come with a fuse, use that for size, but you can buy pre-made distribution panels with built-in fuse holders.

Avoid the cheap ones that use ATX as they have poor strength fuse finger contacts.

Sent from my SM-G781V using Tapatalk

 

[alloy]

Ok I've just been going over sniper harness and realize there is a fuse. I initially thought it was just for the fuel pump but there is a second wire that I'm assuming runs the ecm. The second wire was hard to find, had ro roll back the sheath on the power wire to find it.

So I was wrong, there is a fuse and it's 30 amp, so that explains the 10awg wire size needed. And the circuit breaker I have matches it.

I'm working on the wiring now.

 

[Cadillac]

Couldnt you just wire in a relay to power the sniper system? Tap your main off the starter then controls through the car.

 

[alloy]

Yes I can do that.

Will using a relay isolate the flyback from the sniper?

It doesn't seem like it will, but I don't know. That's why I'm asking questions.

 

[Jake M]

The starter solenoid is a HORRIBLE place for computer wires. Incandecent light bulbs, sure, they don't give a crap. But it's electrically horrible. The subject goes pretty deep, but yes, absolutely, dumb as it sounds, a separate wire to the same battery post will ABSOLUTELY provide much cleaner power to your new setup. Both in terms of performance when the "kicks" are happening, and in terms of durability of the electronics that are getting fed with those kicks. Many of which would blow your mind if you could watch them on an oscilloscope. The immediate voltages that you would see are huge. Three or five times system voltage, and depending on (everything), they might even be in the wrong polarity. They are (by science that takes years to learn), greatly mitigated by bolting your wires together at the battery post, and NOT anywhere else.

 

[pontiac428]

I don't think flyback is your one and only issue. With EFI, you need to ground everything you can at the same point, and be mindful of voltage drops on your positive runs. Voltage is the reference that the electronics depend on to sense and act correctly. Bus bars at the battery will help manage critical connections.