I am about 13.5-1 on a 500 inch motor. Ngk R5672A-9? Or R5672-8? (Indy Head) Which plug is colder?

I use the R5672A-9 in my 495 Indy head motor

500 ci 13.2 compression 5672A-8
never fouled a plug with a crossram nevertheless

the 9 is colder but its not correct for your application

My 511 CID (4.250" stroke) is 13.5 CR with Indy 440-1 heads. I have run both NGK R5672A-8 and an Autolite that is equal to a -9. Both were fine, the -9 is colder, but the the -8 wasn't too hot in my motor. But I was running E-85 last year. The last race of the season I put the gas carb back on and ran 110 with the AR3933 (-9 heat range) plugs and they looked fine.

IMO, this is the borderline between the two. I put about 25 passes on each set in 2008. I didn't race much last year.

In 2007 with my MP Stage VI heads and 13.2 CR (larger combustion chamber), I ran nothing but -8 plugs. No way a -9 was needed.

autolite man here ar3932 on nos, ar3934 on motor 15 to 1

13.4:1 in my triple nickel has had nothing but AR3932's with over 400 passes at 1000hp.

You will be fine with a -9 plug just make sure you have room for 5672 as it has projected tip. That is worth a little power in itself.