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Discussion Starter · #1 ·
Hi,
Just wrapping up my Hemi swap.
I still need to make my engine grounds.
The factory location is on the block, but it's much easier to ground the head. Is this acceptable, or should I ground the head and block?
Thanks again.
 

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Factory grounds the block for the starter amp load, it’s a better idea than assuming that return current effectively passes across gaskets and through bolts to the heads. Not to say you can’t get away with what you’re doing but the pervious is why the factory engineering department did ground to block.

I’ll add that if you run a capacitive discharge ignition of any brand or type it’s a decent idea to ground wire the heads and intake, this does not require cable as such because the amps aren’t very high but return current is spiky in wave form and can irritate ignition electronics to where it miss performs. The battery in the end is acting as an electrolytic capacitor to soak out this electric junk so you want to get the grounds connected there as soon as possible.


Bogie
 

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Discussion Starter · #4 · (Edited)
I will ground both.

Also the factory block ground was on the other side of the engine, away from the battery, close to the starter. Don't know why they did this, maybe to get close to the starter. It is an extra 5' of cable.
I likey won't do that. I will put it on the battery side

Thanks again for the input.
 

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I’ll add that if you run a capacitive discharge ignition of any brand or type it’s a decent idea to ground wire the heads and intake, this does not require cable as such because the amps aren’t very high but return current is spiky in wave form and can irritate ignition electronics to where it miss performs.
I agree that grounding any electronic ignition system (or PCM) to the head and/or intake is a good idea, and it's a good idea to have a wired ground path from the block to the head and from the head to the intake.

However, when adding any ground wires, care must be taken not to wire an alternate ground path for the starter with small wires. When the main ground from the block (or bell housing or transmission case or even a head) to the frame and/or battery) becomes corroded or disconnected, starter current will find it's way to ground and through that ground to the negative battery post. It'll try to go through any added small wires that lead back to the frame or body (if those are grounded to the battery).

I've seen all kinds of strange damage occur when a ground strap from the engine to the frame or from the engine to the negative battery post corrodes through or fails. I've seen starter current melt speedometer cables. I've seen all manner of "I added a 'ground wire' for a ground path from ___ to the negative terminal because I was having issues with __" (fill in those blanks with "distributor," "atlernator," "PCM," or just about anything else). Grounding the alternator case is a particularly popular "fix" that goes awry when the main ground strap fails. It's OK to ground the alternator to the block with a wire that matches the alternator charge cable. But don't go to the frame with that alternator case ground unless it's the size of the battery/starter cable.

Also, if there are multiple ground wires from the PCM, ground every last one of them to the exact same place. Otherwise you have a potential for ground current to go through the PCM. I had one poorly wired car come in once where some of the PCM grounds went to the head, and some went to the frame. The main ground strap broke, and the starter current melted the PCM connectors and fried the PCM.

So, best practice is that there should be exactly one (and only one) path for ground current from the starter to the negative battery post. It can go through the frame, the block or whatever big hunks of metal you want to use, but wherever it goes through a wire, the wire should be the size of the battery cable, and there should be no other parallel paths.
 

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Discussion Starter · #8 · (Edited)
So, best practice is that there should be exactly one (and only one) path for ground current from the starter to the negative battery post. It can go through the frame, the block or whatever big hunks of metal you want to use, but wherever it goes through a wire, the wire should be the size of the battery cable, and there should be no other parallel paths.
So that confused me some, mainly because I'm not that familiar with what you said.
Here's what I'm planning:

Battery to starter, 2 gauge; No fuse.
Battery to alternator, 2 gauge.
Battery to fuse box, 4 gauge.
Battery to block ground, 2 gauge.
Block to frame ground, 2 gauge.
Battery to head ground, 2 gauge.
Battery to fire wall and chassis ground, 4 gauge.



Alternator and fuse box get a 250 amp breaker at the battery.

Plus the the PCM is grounded once to fender.
Both heads are connected by a strap and ground to firewall. I was thinking to make another ground from that point to battery, with 10 gauge wire.

What do you think?

Thanks.
 

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What do you think?
I think you're overdoing the ground side, and in doing that, you're introducing weird failure modes that could occur when certain cables corrode/fail.

I also think I asked several questions on your other thread about fuses for the main wires/cables at the battery that remain unanswered.

  1. Where is the battery located? Is it in a "stock location" for the vehicle, or has it been moved?
  2. How long is the cable ...
    ... Battery to starter, 2 gauge; No fuse.Battery to alternator, 2 gauge
    ... Battery to fuse box, 4 gauge.
    ... Battery to block ground, 2 gauge.
    ... Block to frame ground, 2 gauge.
    ... Battery to head ground, 2 gauge.
    ... Battery to fire wall and chassis ground, 4 gauge.
Beyond that, for fuses, you size the fuse to protect the wire. Total load is immaterial, except that it should not exceed the capacity of the fuse and wire.

The best fuse size for a 2AWG pure copper wire in the engine bay is 200A. The best fuse size for a 4AWG pure copper wire in the engine bay is 150A. Those numbers are for typical "battery cable" or "welding cable" with a pure copper conductor and EDPM or Silicone insulation. If you use cables with higher temperature rated insulation, you can go up a little bit on the fuse sizes. Consult the specifications for the wire you're actually using. However, higher temperature rated insulation is usually stiffer and less useful in an automotive application.

If you put a 250A fuse on a 4AWG wire, you might as well not bother. The wire could partially sort and draw 225A until the insulation melts and the thing catches fire. What's the point of the fuse? Any constant draw over 150A on a 4AWG wire can potentially heat it up to the point where it's dangerous.

As for grounds, exactly one ground from the battery to the engine and/or the frame is appropriate. And exactly one ground from the engine/transmission to the frame and/or the battery is appropriate. Body to frame grounds can be installed at several points if you wish, but the battery negative post, the engine block and the frame should form a triangle, and there should be wires/cables on two sides of that triangle. You want to avoid creating "ground loops" where there are multiple paths from any point back to the battery negative terminal. If you want to run a ground from the head to the block, that's not a horrible idea. But grounds should fan out from the negative battery terminal like branches of a tree. There should never be loops or meshed paths.

All PCM ground wires should go to one point on the engine or head. Period. If the outer case of the PCM is electrically connected to the ground wires from the PCM, then the PCM itself should be mounted on an insulated mounting system, and only the wires sholud be grounded. That's why GM pickup trucks and SUVs have the PCMs mounted in plastic brackets mounted to the plastic coolant overflow reservoir. If you look closely at the mounting system for your Hemi PCM in it's original vehicle, you'll probably find it was mounted to plastic.
 

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Discussion Starter · #10 ·
I think you're overdoing the ground side, and in doing that, you're introducing weird failure modes that could occur when certain cables corrode/fail.

I also think I asked several questions on your other thread about fuses for the main wires/cables at the battery that remain unanswered.

  1. Where is the battery located? Is it in a "stock location" for the vehicle, or has it been moved?
  2. How long is the cable ...
    ... Battery to starter, 2 gauge; No fuse.Battery to alternator, 2 gauge
    ... Battery to fuse box, 4 gauge.
    ... Battery to block ground, 2 gauge.
    ... Block to frame ground, 2 gauge.
    ... Battery to head ground, 2 gauge.
    ... Battery to fire wall and chassis ground, 4 gauge.
Beyond that, for fuses, you size the fuse to protect the wire. Total load is immaterial, except that it should not exceed the capacity of the fuse and wire.

The best fuse size for a 2AWG pure copper wire in the engine bay is 200A. The best fuse size for a 4AWG pure copper wire in the engine bay is 150A. Those numbers are for typical "battery cable" or "welding cable" with a pure copper conductor and EDPM or Silicone insulation. If you use cables with higher temperature rated insulation, you can go up a little bit on the fuse sizes. Consult the specifications for the wire you're actually using. However, higher temperature rated insulation is usually stiffer and less useful in an automotive application.

If you put a 250A fuse on a 4AWG wire, you might as well not bother. The wire could partially sort and draw 225A until the insulation melts and the thing catches fire. What's the point of the fuse? Any constant draw over 150A on a 4AWG wire can potentially heat it up to the point where it's dangerous.

As for grounds, exactly one ground from the battery to the engine and/or the frame is appropriate. And exactly one ground from the engine/transmission to the frame and/or the battery is appropriate. Body to frame grounds can be installed at several points if you wish, but the battery negative post, the engine block and the frame should form a triangle, and there should be wires/cables on two sides of that triangle. You want to avoid creating "ground loops" where there are multiple paths from any point back to the battery negative terminal. If you want to run a ground from the head to the block, that's not a horrible idea. But grounds should fan out from the negative battery terminal like branches of a tree. There should never be loops or meshed paths.

All PCM ground wires should go to one point on the engine or head. Period. If the outer case of the PCM is electrically connected to the ground wires from the PCM, then the PCM itself should be mounted on an insulated mounting system, and only the wires sholud be grounded. That's why GM pickup trucks and SUVs have the PCMs mounted in plastic brackets mounted to the plastic coolant overflow reservoir. If you look closely at the mounting system for your Hemi PCM in it's original vehicle, you'll probably find it was mounted to plastic.
I haven't looked at the last thread, but just did.
The battery is in the stock Jeep's location, upper right passenger.
I don't have the hemi donor anymore, but the Jeep's computer is mounted to plastic with a ground to the fender.
Lengths are approximately:
Batt to starter, 5'
Batt to alternator, 2.5'
Batt to fuse block, 18"
Block to frame, 18"
Batt to block, 4'

So the ground from the head to the block is not connected directly to the batter?

Thanks.
 

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I haven't looked at the last thread, but just did.
The battery is in the stock Jeep's location, upper right passenger.
I don't have the hemi donor anymore, but the Jeep's computer is mounted to plastic with a ground to the fender.
Now that I think about it, that's the MOPAR way of doing things, and it's probably just fine for this project. They do ground the coil packs to the heads, which is best for the spark plugs (the high voltage circuits ground through the spark plug threads to the head). So keep that part of the "stock setup" and you'll be fine.
Lengths are approximately:
Batt to starter, 5'
Batt to alternator, 2.5'
Batt to fuse block, 18"
Block to frame, 18"
Batt to block, 4'
Those lengths are reasonable for the cable sizes you mentioned. Mostly 2 AWG except for the fuse block wire which is 4AWG. Are you running a really big (>150A) alternator? Because 4AWG to the alternator is usually somewhere between "good enough" and "overkill" for most alternators (up to 150A). 2AWG can sometimes be hard to route.

So the ground from the head to the block is not connected directly to the batter?

Thanks.
If you absolutely must add a ground wire to the head(s), run one from a bolt on the back of the head to a bolt on the block, or to a bell housing bolt. I personally almost never bother with a ground wire to the head.

Most vehicles make it through even extended (7 or 10 year/100,000 mile) factory powertrain warranties without a ground wire from the head down to the block. There are a few things that ground to the head. Usually ignition coils ground to the head(s) because spark plugs always ground to the head(s). But you've generally got at least 8 (10 or more on most V8 heads) head bolts threaded into the grounded engine block, with the bearing surface of the bolt head against the head casting. That's generally a good ground path. The exception is GM engines where the head bolts go into the water jacket, and a thread sealant is specified on those bolts. However, if you use a metal head gasket, you've also got the entire block deck and head surface electrically connected through the gasket.
 

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Discussion Starter · #12 ·
I don't have an absolute need to ground the head to block, it was just a thought and now seems unnecessary.
I just got an email saying my circuit breaker was damaged inroute and won't be delivered anyway, so I may just skip that for now, or go with a smaller one like 150amp for the 4 gauge wire to fuse box.
Seems 2 gauge is best for starters and grounds, but not really necessary for everything else. I'll see if it's too stiff for the alternator connection. If so, I'll go with a 4 gauge.
Don't know what amps my alternator is. It's the stock Hemi one, so likely around 150amp.
I'll check the build sheet to see what it came with.

Thanks again.
 

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Over grounded. The block to chassis, chassis to battery is adequate for 99% of installations where the chassis is steel. The alternator grounds to the block through its attachments as do most everything else. The heads only need a separate ground if using a capacitive discharge ignition like a 6AL. The box itself does not mount to the engine but is chassis mounted somewhere, if it mounts to a non conductive surface then it needs a ground wire to a conductive surface that finds its way to the negative terminal of the battery. The instructions included by MSD are clear on this.Years ago they were more specific but MSD seems to have taken a softer approach in more recent instruction updates. I happen to like their original cautions on top end grounds simply because the cost to equipment failure is high for parts and has the potential that if a spurious current fries some engine mounted electronics such as an electronic distributor similar to and including HEI where this is used to signal the 6AL failure of it on the road or track incurs other penalties on your life in financial, time or pride.

If you have a plastic bodied vehicle you are likely to need to wire discrete grounds for equipments that do not otherwise have a conductive path to the battery’s negative terminal by way of sheet metal. Keep in mind that engines and transmissions are floating on rubber mounts as is a body mounted to a separate chassis. Rubber not being the best conductor need some ground wire between them and that should consider the amps of the equipment to be grounded. Things line starter motors and electrically run air conditioning which is gaining preference over engine belts are high current items needing cable grounds. Everything else pretty much can be smaller wire gauges.

Bogie
 

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27 T, scratch built road race chassis, 392 hemi, Holley EFI
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You can never be too rich, too thin or have too many grounds. A bad ground is the number 1 cause of electrical mysteries. Other than the weight, heavier wire is always better, and ALWAYS insure that there is a wire path directly bypassing any bearings (usually transmission / drive shaft / rear end). It only takes one shot of starting current to destroy a ball / roller bearing. The same also applies to where to clamp the ground when welding.
 
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