field weakening

[Gary@innevations.com (Gary Flo)]

Lee Hart wrote:
>On the other hand, you have a very light car with direct drive and a
>fixed voltage battery pack. Field weakening is a simple way to get a
>little more speed. It doesn't hurt to temporarily wire a contactor and
>resistor (really just a length of wire) across the field...

Forgive me if I missed this before:
I'm going to do this with my 800 lb City-El.  How much resistance do I need?

Thanks,

Gary