whats the highest CR you can run safely on 91?

 

It depends on a few things, basically where your dynamic compression ratio ends up. The bigger you go with the cam, the more compression that you can run without detonation. Tuning is obviously a factor, if you run less timing you can have more compression, keep it cool, etc. I wouldn't be afraid to run 11.5:1 with a big cam with a lot of overlap and a good tune...and I know that there are guys running around with over 12:1, not sure of their setups or what octane that they're running though...

 

What is the method for calculating DCR?

 

I'm familiar with some online calculators, such as this one: http://www.wallaceracing.com/dynamic-cr.php and I know that there are others that may be better? that have been posted in the Internal engine mod section. I used to use a different one, but somehow lost it when my buddy cleaned up my laptop (had a virus or something). I'm sure that someone has a formula around here too, but I'm not a fan of math

 

I did 11.0 with no problems using only 91 octane. Truck never had any knock retard and compression could have gone higher if needed.