[Sirrotica]

Originally Posted by Geoff

Nope.
You are assuming that the frame resistance is only 7 times the CSA of a copper grd cable, when in reality, the CSA is much more than 7 times. If the frame was 14 times the CSA of an equivalent copper cable. the frame resistance would be half that of a copper cable, with a corresponding reduction in the voltage drop [ loss ]. The connecting lugs are nearly all copper, but tin plated. There are numerous resistance points in the starter cct, both on the neg & pos side which add to the total res in the cct.
If you had a copper cable for the grd from the battery in the trunk, it will heat up from the current draw during cranking. Because copper is a good conductor of heat, it heats up very quickly...& it's resistance increases....& causes more voltage drop. Using the steel frame as the grd, it heats up much more slowly because it is a poorer heat conductor than copper & will have less voltage drop than the copper cable. It is the sum of all the voltage drops in the cct that reduces the final voltage that the starter sees.

You sound like every engineer I've ever dealt with, you quote books written by other engineers. You think every application has ideal conditions at all times. When their fixes don't work, they blame something else.

I've done the exact same thing with a 1973 455 T/A, and tried every way I could to use the frame for a ground. I welded the ground bolt to the trunk floor and used heavy ground straps to the engine. When hot, the engine did the exact same thing he describes. What solved the problem? Running a dedicated ground cable to the bellhousing bolt, never a problem after that dedicated ground was connected. Please explain what the books say about that phenomenon.

In real world cases most starter motor powered accessories on a vehicle have a dedicated ground to the accessory from the battery. Why would someone do that if they simply could use the body/frame for half the path of current? Does your book cover that?

He already has the system you're touting, and has problems with it, but you say that he can't have. If you thinks steel is such a wonderful conductor, you obviously have never had to deal with a melted shift cable when someone forgot to re install the ground and tried to crank the starter. It gets red hot and melts the jacket into a burned mess. I'm sure you have a technical response to cover that situation too.

When it comes to real world problems the books aren't always correct in real world applications. Problems such as poor welds, corroded welds, rust, aren't taken into the equation. Maybe you've never looked at GM frame with the body removed. The welds look like chicken scratch many times, and many times over years they actually fail at welded joints. Spot welds use very little area and are susceptible to hidden corrosion. You're assuming all these joints will carry the electric load needed to crank a hot engine.

While you're on the topic of heated copper not carrying the needed electric load, the starter pinned in between the exhaust, and engine block does build resistance the more it's heated. You know what isn't getting heated? The dedicated ground cable, it's likely around ambient air temperature.

As I said the OP can choose to deal with resolving his problem whichever way he chooses. With a dedicated ground he will be sure that the system has full volts and amps with only 2 connections. The cars and trucks I've used dedicated ground cable to wire starter motors or starter motor powered accessories on are still functioning decades later. He has the frame ground system, and it's failing.

I'll use real world experience Geoff, and you can discuss theory, have a nice day.