What fuel octane would you use on a 509 ci motor with a 9:1 compression ratio? Will 87 make it or should I use 89 or 93 to be safe?

Great question...

I was just thinking the same thing but with a little higher expectations.

100 or 110 race fuel

just my 2 cents but keep in mind cam configuration,ignition timing, jetting and the actual static compression ration, mine is 9.6/1 and always run 93 octane to be sure. no detonation plugs read light brown. best luck j

me to I have 9.5 to 1 509 650hp run 93 when I raced it I put 1/3 110 2/3 93 just to be safe but at 9 to 1 89 or 91 should be fine.

Cam configuration is .587 intake/.601 exhaust with a 226/234 duration. Timing is set at 14 initial, which gives me 32 degrees total. Jetting is on a 830 CFM Holley with 6.5 powervalves front and rear and jetting at 82 primary and 84 secondary. Plugs are AC R42T, which is one heat range cooler than stock. Horsepower estimate is 550.

So I guess my question is can I get away with 89 octance or should I pay the extra 10 cents a gallon and go with 92/93.

pay the money. a 9 or 9.5 motor will actually run worse on race fuel then on 92 etc...

bottom line is that you can get away w/ 87 but if its a little old or contaminated you would find yourself not well served. buy the best fuel you can w/out getting into the race gas... the 100 octane low lead avgas is a wonderful fuel altho the slow burn rate requires different tuning best done on a dyno... and then you are into getting barrels and the like so maybe not worth the effort but it is wonderfully consistant and clean stuff.

You're actually asking a 10 cent question on OSO ?

How many gallons of fuel does your boat hold?

60gal - $ 6.00 difference
80gal - $ 8.00 difference
100gal - $10.00 difference
200gal - $20.00 difference

89.

If getting the higher octane is not an issue, then I would go with the higher octane to be safe. I do think it would probably be ok on 89 as well. Not to mention, I wish I could find only a 10 cent difference between the 2 octanes. Its usually about 30 cents difference here.

Alum or iron heads?

I can't see any justification to run anything but 91-93 octane in it, especially with iron heads. 89 in a pinch if you're not going to run hard, but that's it. And my .02 is spent.