Octane level

**Krazymaan** · 02-20-2017, 11:01 PM

Another Krazy Kuestion.
Can someone give me a rundown about Octane. I know what it is, I know what it does, I know what its for. If it matters I'm mostly interested in carbureted motors and Octane.

So the consensus is 89 octane is good for up to 120psi?
92 Octane is good for up to about 150psi's?
Something like that.
What about If i want to buy 108 Octane. Its readily available in my neighborhood.
1.What if I am running a highly modified carbureted 2.5 with 17cc heads at 250lbs of cylinder pressure, do I need to change the timing and jets?
2.So what if I take the same motor and put on some 26cc heads and run 160lbs of pressure, do i need to change the timing again and the jets again?
3.Then what If I change the heads again and run 34cc heads at about what, 130psi's? Would I need to change the jets and timing again?
So at all three fuel octane ratings what would I need to be doing per head and cylinder pressure in 1,2and3.
But to save money because 108 costs $8/gallon. Is there any difference if I just buy some octane booster and carefully follow the directions to achieve my goal?
I haven't done this before thats why i am asking and if there is something I forgot, please inform me.
Thanks

**powerabout** · 02-20-2017, 11:11 PM

If you can get av gas easily it a pretty good solution to several problems and Merc used to recommend it.

**WillySteve** · 02-21-2017, 07:12 AM

More Compression = Less Timing & More Fuel!
The amount of Jet changes depend on other factors such as fuel quality etc.....best to keep a read on the plugs or pyro you're used to!
The ultimate fuel/air ratio for combustion does not change however a raise in compression usually requires more fuel for cooling, to keep away from detonation. JMO

**olboatman** · 02-21-2017, 07:19 AM

I mix Sunoco 110 race gas with 91 oct that has the ethanol removed to get the octane that I require for the compression I'm running. Anytime you change compressions #s to the point you are showing I would watch the plugs to be sure the jets are correct. I like to use timing #s that others have proven (learn by others mistakes)! Gary

**Krazymaan** · 02-21-2017, 07:48 AM

Maybe you can share what timing, psi's and octanes your using.

**olboatman** · 02-21-2017, 08:08 AM

Krazy I do not run a V6 on any performance rig (yet). My 25 Yammy (1988) 160psi.....98 0ct...stock timing 35hp OMC (1978) 165 psi ...100oct timing plus 3 deg over stock 140 OMC ? will dial into what the builder recommends as it was built as a roundy round track racer. I do have a 88 2.4 Merc on a 22 Cuddy Cabin that I bought new for scuba diving that I left stock But always used prem. fuel(93-94) and had no problems. I learned about being on the "safe side" with oct. when we raced drag sleds! Gary

**Bodhi** · 02-22-2017, 02:28 AM

Watch the 100ll av gas, it's only rated to about 3500rpm for planes. I've seen people puke their mercs with it. I'm using cam2 110 L with 90 rec fuel to make 95 octane. All octane is is resistance to detonation, some people go overboard running high ass octane #'s when they don't need to

**Krazymaan** · 02-22-2017, 08:39 AM

About 2 blocks from my house is a Holiday Gas Station. The carry 108 octane racing fuel year round. I don't know why, but I ain't complaining. Its $8/gallon. So it cost you $200 to fill up your 25gallon tank.

What I am trying to find out here is maybe a ratio per psi's. Like what octane are the racers using when they run 250-300psi's. what octane are they guys using for hot river racers while running engines at near 200psi's. That sort of stuff. So far I haven't seen any specific answers for what i am asking.