Quote Originally Posted by Redge View Post
Rees I am running a group 27 for cranking, will a group 31 give me ?% more power for the day? ie any way to know how much longer a group 31 would last over a group 27, just looking for a SWAG. Thanks for your time.
Don't mean to hijack the thread, do think it is pertinent.
You can't know for sure without comparing RC ratings. Some 27s are as small as 160 minute RC and some 29/31 are as big as 225 minute RC. Typical 29/31 are 205 minutes RC. Battery weight is a good indicator of capacity. You can't ignore CCA though, because some newer outboards have significant cranking amp requirements.

Watch out for the Deka 31s. They rate their RC at 23A which inflates the numbers.