Hate to disagree, but there is some misinformation that I have to nip in the bud. This is all from both my automotive engineering background and my friend who is an engineer for ExxonMobil.
Higher octane does not slow down the flame front a measureable amount. I used to believe that as well but it is not true. Tests were done with high speed cameras directly in the chamber and the burn times were identical, as was the flame propogation and shape of the front. For pictures and more info, see Maximum Boost by Corky Bell. As someone else stated, the activation energy required to get it going is higher. Octane reduces a fuel's tendency to pre-ignite from sources other than the plug. A good way to test this yourself is to advance your timing until you get detonation on low octane, then do the same test on high octane. You'll find that the advance at which they begin to detonate is the same.
Another problem we're missing with Avgas is that it is lighter than gasoline. Its more closely related to white gas which is basically what is burned in Coleman camp stoves. Its dyed, but it is in fact lighter per gallon. What this means is that its stoichiometric ratio is different than gasoline. You can't simply switch back and forth without jet changes. You could find jetting in the middle, but you'll run noticeably rich on gas and noticeably lean on Avgas. My memory is a bit rusty, but I think Avgas is stoich at 12.5:1
You know, I was wondering what the stoich was.... I think you're trying to point out a condition similar to, but not at the same porportions as, methanol. You're right about that, but it's a much slighter difference than methanol vs. gasoline. Meth is like 6:1, avgas 12:1, gas 14:1, or something like that.
This website (http://www.faqs.org/faqs/autos/gasol.../preamble.html) has a great rundown of some tech. Under the heading 9.3 it lists some fuels including gasoline and their specs. Notice the gasoline has the same flame speed regardless of octane. But also notice that its autoignition temperature rises dramatically as octane rises.
I still can't find the stoich number for avgas on the net, but 12.5 sounds right. Don't quote me on it though
I would definately run 110 leaded minimum with a 12:1 marine engine. It is typical for motors running on fuel to really like 38 - 42 degrees of timing to make up for the slower burn. Even at the elevated timing the safety factor of running a good leaded race fuel is way better than a cocktail mix or high octane pumped gas that probably varies 3-5 points.
With a 12:1 compression you would not feel safe just running 100 av-gas?
You would only run 110 race fuel? Thanks for you replys...
Maybe I did **** up between jet fuel and avgas, my bad. I do know for a fact though with aluminum cylinder heads taht Avgas and Oxygenated Fuels did tear up the Cumbustion space. I might have mixed up the fuels but I'm the one that after multiple dyno pulls I had to go back and repolish the chambers, and sometimes put new seats in from to much polishing, lowering the combustion chamber level. But then again those were on Dirt Bike engines and quads and what-not. But I know for a FACT that Avgas and Oxygenated fuels were not the primary choice, not saying it's bad. But after time it just ate the aluminum. Maybe v-8's are different, I dunno
I don't think anyone here could really say with any certainty what fuel this guy needs to run unless we knew all the specs of his cylinder heads, camshaft, actual compression, target operating temperature once the motor is running in the boat etc..
this is a question his engine builder should be answering and if he's the builder then well, i dunno...
What would y'all think a 10.3:1 engine with a 228@050 cam should need for min octane?
I ran 92 in it. It blew a head gasket between middle cyls. I think it was due to my fault of undertightening headbolts (didn't follow ARP's guideline) and there were absolutely no other signs of detonation
What would y'all think a 10.3:1 engine with a 228@050 cam should need for min octane?
No way to tell with just that info. BBC with open chambers? I'd run no less than 100 and carefully tune the ignition curve. But if you're talking SBC with vortec heads, the answer is 92 or 93. Tons of factors involved there.
No way to tell with just that info. BBC with open chambers? I'd run no less than 100 and carefully tune the ignition curve. But if you're talking SBC with vortec heads, the answer is 92 or 93. Tons of factors involved there.
SBC with Dart Iron Eagle Platinums. I couldn't get the thing to audibly detonate with 92.
Thanks! Makes me feel better because it just got real hard to get 110 where I live. I used to be able to go down the street, but that gas station closed down.
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