Gauges will be looking for 6 volts (nominal - actually 6.5-7.2v). Can't remember if any of them care about polarity.

I'm sure someone will chime in who can remember this...

The starter motor won't care a lick. your reverse polarity on the armature by changing to negative ground, but you also change it on the field coils at the same time. The net result is the motor still spins the same direction.

If you had a permanent magnet motor, now, that would spin backwards. But our cars have field coils, so it don't matter a bit.

In theory, they will pass substantially more amperage turning over. In practice, they will spin faster, which makes the engine start quicker and easier. This allows less time to build up heat in the starter motor. Also, an electric motor is what is called an inductive load. In other words, the will pass more current at a slower speed than they do at a higher speed. This means that being spun at 12 volts, it won't drop fully double the current. Because, it will be spinning faster, which tends to regulate the current flow.

I've been there and done this a number of times, with good result. Yes, you could theoretically run into trouble if you have a problem starting the engine, and sit & run the starter a super long time. But a good running car that starts easy, with a good condition starter motor, should not pose any problem whatsoever.

I remember using a small voltage regulator chip on the fuel gauge for my 1949 Plymouth p18. IIRC, I used an LM317 which knocked the voltage down to 5 volts. It was too low, and the gage never functioned accurately. The car had vacuum wipers, and all other gauges were mechanical.

My 1954 half ton pickup has electric wipers, but they're not hooked up... I will need to come up with some kind of a cunning plan. My thought has been to run them through an ammeter, measure the draw with them running, and install a hi amperage resistor that will slow them to a reasonable level. But, looking to get a garage built before I worry too much about that...

Hope this helps, at least bumping the thread up to the top...

-William