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I am having trouble finding the best battery they can turn over a 1941 Packard model 1901 120 my current battery does not seem to turn it over fast enough what would be the best batteries it would turn the motor over fast enough to start it

Are you sure the contacts are clean on your battery wires and terminals, and the ground cable is making a good connection?

With the under seat batteries, be sure to use 00 gauge cables too including the grounds.

The proper size lead/acid battery to fit your underseat battery box is a Group 2. Any such battery in good condition will have plenty of capacity to readily start your car reliabily as long as the connections are tight and clean, cables are proper size, and the starter motor is in good condition, so chances are your problem lies in one or more of those areas. Starting problems with an electrical system in less than proper condition will be increased if you have Pertronix installed.

Note that speed of rotation isn't necessarily a critical requirement for starting an engine, starting by hand-cranking demonstrates that.

As a side note, while conductor size is not unimportant, it is sometimes overstated. For example, for a ground strap + starter motor wire combined length of 3 ft, and 500 amps of current draw:

If both are 00 AWG wire, there is a 4.5% voltage drop.

If both are 2 AWG, there is a 9% voltage drop.

So while using thicker wire will always help, most likely the car should crank with whatever it has in it assuming any other issues are corrected. Be sure the mounting points for the wires are clean (free of paint, grease, and corrosion).

If not much seems to increase the crank speed, are you sure there is not another issue preventing ignition, such as spark?

Maybe a dumb question, but have you tested the starter? If you're using 00 cables, most 6 volt batteries shouldn't give you much trouble. Maybe the starter itself is going bad? Just a thought.