6V versus 8V system

Wm Steed · December 14, 2010, 02:27:43 PM

Many people find that their headlights appear to be somewhat dim with a 6v system, especially if they have converted from reflector headlights to sealed beams.
When GM went to sealed beams in 1940 they installed a relay in the system... The relay was wired into the headlight circuit so that the switch energized the relay, the main source of power was from the battery. It is also common to find a relay in the horn circuit, horns draw a lot of amps. The 1940 GM cars were well known for having the best headlights in the industry.
I am not sure if Chrysler used relays in their cars of the '40', I know Ford did not until sometime in the late '40's....Bill

SD Glenn · December 14, 2010, 12:21:41 PM

I could see all he said wasn"t looked up, came right off the top of his head, really knows what it's all about,
Thanks

MRR41 COUPE · December 14, 2010, 10:05:13 AM

Tod knows " ohms law" what he says happens 100% of the time, Its the basics of the flow of electricy and can't be changed.

SD Glenn · December 14, 2010, 03:00:09 AM

Thanks again Tod, I will learn to express myself more clearly for what I am looking for in the future.
Glenn

TodFitch · December 14, 2010, 02:21:06 AM

Quote from: Glenn on December 14, 2010, 12:56:03 AM
you guys are fantastic.... Helps a lot. I have no starting problem, however, I was looking for info on really getting my lights up a bit brighter. Thanks alot. I will start with Tods suggestions.
Glenn

For my thoughts on lighting on the old bulb and reflector systems see http://www.ply33.com/Repair/lights

Would have mentioned that earlier except usually when people bring up 8v batteries it is for starting problems not lighting issues.

SD Glenn · December 14, 2010, 12:56:03 AM

you guys are fantastic.... Helps a lot. I have no starting problem, however, I was looking for info on really getting my lights up a bit brighter. Thanks alot. I will start with Tods suggestions.
Glenn

Lew · December 13, 2010, 09:40:07 PM

Great discussion and posts by other members.
I have been using my favorite accessory, a 1 wire 6volt alternator, with my standard 6 volt system and have no starting issues.
Except for originality, it works great and can be changed back to stock at any time. As stated in some prior discussions, most Model A Ford vendors carry them and are widely used for the 30's Ford cars. I'd be curious if other Plymouth club members have installed them, especially on the 4 cylinder cars..

TodFitch · December 12, 2010, 04:33:58 PM

Quote from: Wm Steed on December 12, 2010, 02:40:33 PM...snip... Good clean terminals, proper size wire is very important. I have been told that 6v systems have more resistance than 12v, that is why clean terminals and proper wire size is so important. ...snip...

It is not that 6v systems have more resistance, it is that they are more sensitive to resistance issues. Lets look at lighting first as the current numbers are much smaller than for starting.

Say you have 25 watt head light bulbs (equivalent of the old #1000 bulbs putting out 32cp). And you have two headlights, so you need 50 watts at the bulb.

6v system:
Current 50watts/6v = 8 1/3 amps.
The resistance of the two bulbs in parallel is 6v/8.3 amps = 0.72 ohms

12v system:
50watts/12v = 4 1/6 amps
The resistance of the two bulbs in parallel is 12v/4.17 amps = 2.88 ohms

But lets say you have some corrosion in your system. The old light switch contact have never been cleaned, the wire itself has some corrosion and the terminals haven't been cleaned. Say we have 0.1 ohms of resistance (not very much).

6v system:
Voltage drop 8.3 amps * 0.1 ohm = .83v

12v system:
Voltage drop 4.17 amps * 0.1 ohm = .42v

So we are feeding about 5 volts to the 6v headlights (nearly 20% less voltage than they need) but we are feeding 11.4v to the 12v lights (only 5% less than they need). So the same age related corrosion issues affect a 6v car more than a 12v car.

Back to the starting motor. For the 1938-42 time frame the starter motors had a stall torque specification of 3.8v at 450 amps. Assuming you are feeding that with a 6v battery, you can't have more than 0.005 ohms of other resistance in the circuit. Turns out that the internal resistance of a fully charged 6v battery is just about 0.005 ohms, so your battery cables have to pretty close to zero ohms resistance. Fortunately, you are not running the starter in a stalled condition. The free running specification for the same time is 70 amps at 5.5v. Actually starting current will be some where between the two. Closer to the free running in summer time and closer to the stall current in winter.

Wm Steed · December 12, 2010, 02:40:33 PM

I have responded to questions and/or comments about 6v-12v many times... The question pops up on all of the old car forums, not just the POC.
When I was a teenager living in Eastern Idaho where winter temps can drop to -30 f, 8v tractor/truck battery's were an instant solution to hard winter starting. An 8v battery also worked very well in a car that had improvements to the engine.. high compression heads, etc..
Of course the use of truck/tractor battery's required the relocation of the battery because of the larger size. I found the long narrow style like Buick and some fork lifts used to be the best.
As my mechanical skill improved, and I relocated myself and my ever faithful '36 Ford to So Cal, I found that the 8v battery was not needed. Good clean terminals, proper size wire is very important. I have been told that 6v systems have more resistance than 12v, that is why clean terminals and proper wire size is so important. Add a good modern battery, the average well maintained vehicle will generally start right up.
I f you feel you must use an 8v battery, make sure you adjust the voltage regulator so it is putting out 8v+ ....Bill

TodFitch · December 12, 2010, 02:24:23 AM

It only takes a little corrosion with a low voltage system to wreak havoc. Poor starting, dim lights, etc. The fix is to remove all corrosion from the electrical system and use the correct gauge wires when replacing worn out wires.

But that can be expensive and time consuming. So people threw, and still throw, an 8v battery at the system because they don't want to or don't have the resources to fix it correctly. If 8v was better, the factory would have built it that way.

Now, if there are 12v accessories that you need to run on your car, I guess you could make a case for converting to 12v. For me the big one is charging my smart phone. But it turns out all of those are 5v, so you can rig a charger for it without changing your car's electrical system.

SD Glenn · December 11, 2010, 10:07:35 PM

Years ago when I was a youngster, the best way we had to improve the starting of those 6V cars was to install an 8V battery. We did not have any problems with the starter or the light systems. The charging system did not have to have anything done to it either. Does anyone have any input on if this would be a reasonable addition to the 29 model. Or have I "remembered" all this wrong.

Glenn