What's the appropriate compression when I only have 91 octane

 

Depends on how good the the guy tuning is and the build.. FI.. I run 11.3 to 1 on a Dyna. CCP about 220psi.. Works fine on CoCal gas with temps over 100F..

 

Wow 220! Im at 200ccp, that was my limit. I live a few miles from you. Who do you go to for tuning?

 

Pretty much me.. Bike is carbed.. I think the main issue is understanding that when you bump the compression the ignition timing becomes more critical and the harder you squeeze the less timing you need. Also need a reasonably tight squish.

Add: 200 psi is a good number especially for a bagger tho I've got an 02 FLHRCI that runs 220 also..

 

Timing is very critical at that ccp.
We hold back on that when shipping kits in the mail, to insure folks do not have issues, not knowing who is going to be doing the tune.
Scott

 

When i was running 214 ccp and using a Dyna 2ki ignition i could only run a maximum of 31* advance ( all degree increments where stepped according to rpm). Detonation can be a real killer. Even with 93 octane. If your not real experienced with setting up a v-twin, i wouldn't think of going over 195 ccp and a squish of .030" on 91 fuel....

 

I have/built a bobber with 113 ci SnS High compression EVO motor (10.8 to 1) Squish is set at 0.045. back in 2001. Motor/bike has about 35000 on it and the last compression check with a brand new gauge a few days ago was 210 front and 220 rear. Using the a 2ki with the slowest curve which if statically timed is supposed to be 30 deg full advance, the motor ran way hot..Retarding it to 27 deg and the motor runs perfect.. It never really pinged but it ran hot enough to crispy the oil seals on the cam and inner primary when I was first trying get it running well.. Backing the timing off to 27 made the bike runs much cooler.. No problems with the current gas.. SnS recommended 30 deg timing BTW..

Add: all done on 91 octane SoCal gas..

 

JMHO but trying to find that spot to maximize timing but avoid detonation is not worth the time and effort; no real gain to be had. Much better to back off and run a more conservative ignition map; won't lose any performance and, has bwoltz points out, the motor will run much cooler.

 

I base my decisions on corrected compression more than cranking or static. Style of bike and riding. ......and as already said, tuning strategy.

 

Why? You should maximize timing on every build.. It's part of the tuning process. There is a big misconception that retarding the timing looses performance.. You really have to because the burn has become more efficient. If you loosing performance from retarding the timing then chances are that the timing was pulled in the wrong place.

Hardtails answer is good fer sure.. Even with 91 octane, it really depends on the build and how it's going to be ridden. I'm simply posting examples that are on the high side.. As mentioned 200 psi cranking pressure is a good number for baggers but it's not cast in stone.