Cold cranking pressure. There are calculaters out there to help you predict what you will get using specific components. After the build it can be measured thru your spark plug hole with the appropriate tool.

I'm running 206 ccp measured cold, and I am indeed riding the fine line there. Below 200 is safer. Luckily, I am getting away with it.

180 - 190 is well known for being problem free.

 

Ahh see I'm learnin here. I have learned so much about Twin Cam motors since buying my new bike. My old one was such a hooptie I guess I didn't care but I rode dirt bikes forever and tore those down tons and did tons of top ends so all this is not as hard as I always assumed.

 

With a Wood 6 in California,(heat and 91 octane) I'd be leaning towards 10.2-10.3 cr.
You should have a good running machine.
Scott

 

Can you explain the heat and Cali thing? Or can I call you? I have a jillion questions you'll hate me when it's over haha. But you guys seem to have the market cornered on knowledge.

 

Heat contributes to detonation (so does cheap fuel).
The higher the compression - the more vulnerable you are to the effects of heat and cheap gas.

My bike had issues with the cheap gas in the Sturgis area (I told you I was testing the limits with my ccp). I had to run octane boost until I was over 100 miles away from that area (where I could get better octane BP fuel).


Of course your tuner can help some there too. If you tune it with poor gas, it should run okay with that gas.
You'll get more power tuning it with better fuel, but you will be married to having to keep good fuel in the tank. Always a trade-off! (Sux, don't it?)

 

So is Cali the only one with 91 octane??

 

I don't know about Cali, but there was not a drop of decent fuel within 100 miles of Sturgis (likely an understatement - more like 200 miles).

And it ain't just the octane. Crappy fuel is crappy fuel, even at higher octanes. I am very fortunate to live in a region that has good fuel.
BP and Shell is all that ever see's my bike unless I am on a trip out of town. 92 - 93 octane or drive on to the next station.
I was able to find 92 - 93 BP most of the way to Sturgis from NW INdiana near the Chicago region. You can't go wrong with BP or Shell around here, although if you are looking for crap gas, there are MANY offbrand stations that will be happy to sell you some around here.

If anything - you need to analize your available fuel before making the decision that you started this thread for. Where are you from and what fuel is available for you? That is a HUGE facter in your case. You are being very wise to seek knowledge before making the final decision on your compression ratio. Very wise.
Personally I would have been in deep **** with my build, if I lived in an area with poor fuel. I'd have dismantled and made changes long ago.
Sturgis last year taught me that. No way I could live with poor fuel with my compression. My bike was so happy to see good fuel when I left there! He he!

 

I live in southern California and the best we get is 91 octane. See I knew it was important enough to ask about. I am certainly leaning towards 10 but if I can't get a 100% for sure answer I am gonna go with 9.8 and keep my bike running for a while.

 

Is it reliable? Mine is. I have 55,000 "hard" miles on a 10.5 CR build. It'll ping when it's really hot out if it isn't tuned right but a good tuner is able to tune the pinging out.

 

Hey Man
This is I thing the most important thing to do when building a motor . Use the highest quality stuff and take your TIME . Very important TAKE YOUR TIME this step is very very important . DO IT RIGHT THE FIRST TIME .