safe compression for 89 octane?
 

Have noticed alot of posts about transom sooting and alot of answers advising that the use of 89 octane will lessen this. What is the highest compression that will SAFELY run on 89 octane?

I'm running 9.7-1 and like to run 91-93 but buy fuel on the water so beggers cant be choosers....no problems to date! I run more 89 than 93...

The technology used in cylinder heads these days will allow the use of lower octane fuel with higher CR's (fast-burn combustion chambers). Example- V-10 Ford is 9.0:1 and it's plastered all over the owners' manual to use regular unleaded fuel. I've had two of 'em and they run fine on regular. Vortec and World Products use this technology as well. I have SBC's with 9.67:1 CR (World Products 64cc heads) and usually run premium, but have not had a problem with mid-grade when forced to use it, although my timing is not set too aggressively.
There are a lot of threads about the 500HP and transom soot lessening with the use of lower octane- why waste money on slower burning/higher octane fuel if you don't need it?

jafo,
What size runners are in those heads?
Are you running carb or fuel injection?

Iggy- I believe they are 200 cc intake runners (SportsmanII) mild pocket port, 2.02 intake valves 1.60 exhaust. Running Performer RPM intakes with tuned QuadraJets.
Very nice head at a good price- they do need a good exhaust to take advantage of them though. I have EMI Thunders and they work/sound nice.

I can set you up with vortec s that will out flow the sportsman with less port volume.

I think that depends on a variety of factors such as your cam specs, timing, and cylinder diameter, head material (iron vs. alum) to name a few. Jap bikes run 12:1 compression on 89 octane all day long....BUT, their cylinders are substantially smaller than on a BBC.

If you want to make a general statement for all BBC motors, I thought that value was around 8.7-9.0:1 for iron heads. Add another .5 for alum heads.

Dynamic compression has a lot to do with the octane you may use. the cam timing will have to be matched to the compression selected because it allows some bleed down. Generally 9:1 with iron and about 10:1 with aluminum based on having a camshaft with about 226 intake duration. Also consider 34 deg as ax timing also. there are other factors also like chamber design etc... the Ironman.

Yeah, I'm with Ironman. I was going to say 9.5:1 but you really take a chance if you happen to get some inferior fuel. 9:1 or just a touch more is a good place to be if you're looking for reliability.

Thanks guys, appreciate the info. Why pay for extra octane if you don't need it?