How much compression can I run on a 6.0 and still run on 91 octane?

 

I wouldn't want to run more than 11:1 on california gas from my experience on my bike but I'm no expert on what is considered safe limits. Stock heads milled, cam or ?

 

It really depends on the entire setup, it's the Dynamic Compression Ratio that really counts. You could have high Static compression, but with a bunch of intake duration on the cam, your dynamic compression ratio will be lower and pump gas won't be a problem. I've seen dynamic compression ratio calculators on the net...

 

You can only compare apples to oranges if they both have the same bore, stroke, combustion chamber design, and cam. A 2009 Yamaha YZ250F has 13.5:1 and runs on pump gas. At the other end of the spectrum, a Super Glide has 9.2:1 and probably needs premium pump gas, I'm not sure.
Even to compare the octane needs of a Ford to a Chevy wouldn't be of any value.
The point is, bigger bore, bottom-end power cams, and less efficient combustion chambers all require more octane.

 

Gothcha!
So like the OP, when setting up a bore/head combo....?

 

 

I hear 11.5-1 is possable with a ls-6 cam, but the tuning has got to be forgiving and a touch rich.

My 4.8 has a 9-1 and from everyone I talked to is very forgiving of lots of timing, heat, and variences of life. I don't see why a 10-1 wouldn't be the same way if a guy had a good tune on his truck with enough gear so the truck turns 2000rpm on the freeway, and doesn't load up big when he tows.

 

My truck is about 10.7:1 and i have run premium usually, but have also tried 87 with a little less timing and it didn't pick up any kr, i have been running e85 lately, with my compression, and a little more timing i break even on cost of fuel, but i use it because it smells 10 times better with no cats than gas does.

 

I'm wanting to mill the factory 6.0 heads and use a Comp Cam XR269HR which is a 216/220.

 

Mil them how far?
That cam is mild so I can't see you having much difficulty