A/F Question. How does it change

[CANTDRIVE55]

Schulmann,

Would a negative BUS bar with a single 2 or 4 guage battery wire to ground be a solution? All of the indiviual ground wires (all of which will probably have different resistances)could terminate on the BUS bar. If you increase resistance on the ground side, you decrease positive current going to vital electronics....is that true? I=V/R, or someting like that!

 

[KenH]

I think the issue with a common grounding point is that you have to run much longer ground wires than if you just ground to the chassis as close to the item you are grounding as possible. Long ground wires can increase the resistance to ground somewhat which can cause an offset in a sensor reading, but perhaps more importantly it can allow electrical noise to be picked up on the ground which can cause problems with electronics.

 

[Ulysses]

I think the issue with a common grounding point is that you have to run much longer ground wires

I agree with you there. But the chasis has it's own resistance between grounding points, still causing an offset from the PCM ground. What the goal should be is that all sensor grounds see the same ground as the PCM. One way to do that, if you have a resistance problem, is to use thicker gauge wire but keep ground at the same point.(The disclaimer here is that the system was designed with keeping all grounds at the same potential in mind and things were not tweaked because Dodge saw this problem early on). Hey, if its working, cool. But my guess is that other problems will arise.

 

[Schulmann]

The resistance depends on the thickness of the wires and of the current. The length of the cable in our case is neglectable (I might be wrong). The chassis has nearly no resistance, no matter where and when you measure the ground. The chassis has a large conducting surface. In the Case of the Viper you have to be carefull with the chassis. For instance the sport bar area is a very poor ground. The problem with overlapping wires terminal is likely dirt, conducting surface size and electrical contention. I don't have a real scientific explanation for this. Maybe someone with a Phd in physics can explain.

At higher RPMs there is more current require to operate all the electrical devices so there is more contention for ground too. Either the ignition is not perfect or one of the computers screws up. Since I reworked my electrical system my Viper is much more stable and espetially my AFR changes in reasonable limits. No more drops below 11 AFR. But this grounding issue only occures on forced induction engins ! On NA Viper this will likely never occur.


Anyhow I am sure that JackB is a step of ahead of us. He has a good electrical setup and he only cares about temperature now. My next tuning stage will be likely that.
But again we are talking about 4-10% change in the AFR.

 

[Jack B]

No flame intended and I realize we all view things differently, but, multiple grounds can cause more issues then they fix in the real world. Not that this makes my view correct, but, I am a EE and my specialty is grounding. A single point equipotential ground is always the best since this approach eliminates voltage potentials between conductor sets.

As far as resistance goes, the frame of any car is massive (in conductivity)and the the ohmic value would be in milli-ohms (.001) at any given point on the frame. Impedance which is resistance plus reactance (simplification)could be higher but that would only affect high frequency ac circuits or circuits where the current changes rapidly.

Most problems can be averted by running true pairs of conductors and not relying upon common point grounds, unfortunately this is not the way the oem's set up a car's electrical system. All this is not to say that by luck a second or third ground might clear up a specific issue, but, that is not because a single point ground has a high resistance. If you started from scratch the perfect system would have a common point ground (equipotential) and two conductors per circuit.

If I were to specify an improvement of the vipers grouding system I would do the following:

1. Replace the factory ground wire that runs from the engine to the shielding enclosure under the cowl with a bolted connection. This wire runs to a bolt behind the valve cover on the drivers side (Gen2). Make sure you bolt to the shielding enclosure.

2. Take that same connection point from the shielding enclosure and run a wire down to the frame, once again everything should be bolted and any paint removed. Size is not a big issue, a #12 will work, but a bigger conductor has better strength. I used a #6 flat copper braid on my car.

I have a lot of aftermarket electronics on my car, I always take that same ground point to a terminal block near the device. Once again extending the equipotential ground plane. Equipotential means no voltage variance will exist between any two ground points and the only true way to do this is with single point grounding. Any other way is pure luck.

 

[KenH]

but, I am a EE and my specialty is grounding.

Jack wins!

 

[Ulysses]

I realize we all view things differently, but, multiple grounds can cause more issues then they fix in the real world. Not that this makes my view correct, but, I am a EE and my specialty is grounding.

No, I completely agree with you and I think your view is correct. I am an EE and my specialty is designing high speed circuits and systems so how things are grounded versus a PLL or a processor ground is a very large concern to me.

 

[Nadine UK GTS]

Interesting tech reading, thanks Guys, this explains some of the tuning issues I'm currently having, chasing for perfect A/Fs, as we've had some rare hot days just lately. Sorry I can't add anything of use to the discussion, but thanks for the helpful info. BTW, I moved my VEC2 to in-car, I was getting some issues on hot days with it in the motor compartment. The issue was with the wiring (some of the very early VEC harnesses had wiring issues I learned) but my understanding is computers don't like varing or hot temps anyway. I had Roe Racing make me a custom length harness up, and install a heavy duty rubber gromment (I supplied) onto the harness when making it, for where I ran it through the firewall in the upper passenger toe-box area. Can change programs in car now too at swipe of a card. I don't think Seans doing the remote card reader now.

 

[Schulmann]

but, multiple grounds can cause more issues then they fix in the real world. Not that this makes my view correct, but, I am a EE and my specialty is grounding. A single point equipotential ground is always the best since this approach eliminates voltage potentials between conductor sets.

I 110% agree with you. Multiple grounds will cause an issue. However for me the frame is the main ground point. Its size is enough to reduce the resistance to a minimal value (and voltage potential). Setting up a separate electical box or using the frame as a reliable ground is the same. Now the issue that I had was that my "single" grounding point was a "single bolt" on the frame and its size was not enough. It caused a trimendous drop in my AFR above 160F due to its resistance. So I split up this grounding point to several ones, all located on the same frame section. So the voltage potential is zero. Watch out how the Viper frame is made. Some sections are poor candidates for grounding. This is why JackB you have these wires running all around between your grounding points.

I have tried this with 4 gage wires with poor result. The current was taking the shortest path and took the one with higher resistance. I ended up using the same frame section as single grounding point.

As far as resistance goes, the frame of any car is massive (in conductivity)and the the ohmic value would be in milli-ohms (.001) at any given point on the frame.

On the frame yes, this is why I use it as a single point of ground. Now mesure the resistance between the engin and the frame under intense heat and higher RPM on a Supercharged Viper with a stock electrical system. You would be stunned. As I indicated the resistance went up to 6-30ohm on my Viper! That`s bad for computers and ignition.


Anyhow we are all saying the same thing about ground ...

 

[Jack B]

Schulman:

What two points are you measuring, it sounds more like termination resistance is what you are experiencing. Temperature affects resistance, but, in your heat range it is negligible, on the other hand, temperature can create some problems with the termination or tap.

My point is based upon common conductor resistances, as an example the DC resistance of 7-strand #12 awg is .00205 ohms per ft. Even #18 awg is only .00845 ohms per ft, this is why I say high ohmic values are more easily explained by contact resistance at a tap or connection. As you can see even small conductors several ft long show little if any resistance.

To expand a bit further, there is approximately a 5% increase in resistance when moving from 75 deg F, up to 160 deg F, once again the only way on a passive resistor (the conductor) to measure 30 ohms would be due to the contact resistance of the wire terminations. In most cases your ohmeter probes will have more resistance than the conductors you are measuring.