Let's Discuss Octane

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MrJLR

MrJLR

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I'd like to know how engineers determine octane requirements for a given engine.
I have a basic understanding of octane.
My understanding, in a nut shell, is the higher the octane, the higher the flash point of the fuel.

For instance, my truck is designed to run on regular (91) fuel. Running a higher octane would actually be bad because I wouldn’t get as efficient a burn because it is harder to "flash".....

So how is octane requirements calculated?
Not "if it pings, run higher octane"...

Jeff
 
MrJLR said:
I'd like to know how engineers determine octane requirements for a given engine.
I have a basic understanding of octane.
My understanding, in a nut shell, is the higher the octane, the higher the flash point of the fuel.

For instance, my truck is designed to run on regular (91) fuel. Running a higher octane would actually be bad because I wouldn’t get as efficient a burn because it is harder to "flash".....

So how is octane requirements calculated?
Not "if it pings, run higher octane"...

Jeff
Click to expand...
Octane requirements of a given engine usually are dependent on the compression ratio. Octane rating determines the pressure at which an air fuel mixture will self-ignite just from the pressure.

Lower octane makes more power per unit volume, but also detonates at a lower pressure.

The general rules are that under 9:1 compression ratio, 87 octane can be used. 9:1 requires 89 octane. 10:1 requires 91 octane and 10.5:1 requires 93 octane. Go 11:1 or higher and you are into race gas territory.

The thing is that you can use lower octane than required in a given compression ratio engine, but you need to pull the timing to keep it from detonating (knock). Most knock comes from the spark hitting at pressure above the detonation pressure which causes multiple uncontrolled ignition centers and creates knock.

You want the fuel to ignite only at the spark point and create a uniform burn as a wave front through the air fuel mixture in order to obtain maximum power and energy.
 
mbaird said:
many newer cars have 11:1 + ratios and dont require premium . Its more complicated than that.
Click to expand...
That's EXACTLY what I figured. .....
I'd enjoy seeing the true science/calculations on it...

Jeff
 
mbaird said:
many newer cars have 11:1 + ratios and dont require premium . Its more complicated than that.
Click to expand...
Newer cars have knock sensors and their computers adjust A/F ratio and timing multiple times per second. The principles all remain the same as I described but modern cars can adjust the parameters like I described in my first post.
 
MrJLR said:
So it's essentially based on compression ratio?

Jeff
Click to expand...
Octane rating is based compression and detonation. The ability of an engine to run any given octane is based on compression ratio, ignition timing and A/F ratio. Modern fuels use the RON + MON/2 to set its value. That is Research Octane Number and Motor Octane Number. One is theoretical the other is determined in an actual test engine.
 
Octane? What nonsense is this I measure all my fuels based on proof.

My duster prefers 150+ apple pie.
:poke:
 
MrJLR said:
Interesting topic to me.....

Jeff
Click to expand...
Today's 87 "Pump" octane unleaded regular (R+M/2) is the same fuel as the 91 "Research" octane (R only) fuel that was recommended in the 1971 owner's manuals. It is measured differently now.
 
Ddaddy said:
Today's 87 "Pump" octane unleaded regular (R+M/2) is the same fuel as the 91 "Research" octane (R only) fuel that was recommended in the 1971 owner's manuals. It is measured differently now.
Click to expand...
Similar to when they changed horsepower measurement. ..

Jeff
 
Similar but Not quite the same.
There are a number of ways to test for anti-knock values. Heptane and isoctane have been the reference fuels for the tests used for a long time (to get the 0 and 100 Octane points).
The Anti-knock Index (AKI) number we (USA) has placed on the fuel pumps since the mid-70s is the average of the motor and research octane tests.
Both are lab tests using a singler cylinder engine. The
Research Octane Number correlates best with low speed knock and run-on. While the
Motor Octane Number correlates best with high speed full load and part throttle knock.
These two tests are defined in the ASTM D2699 and D2700

Today's fuel has different components than the 1970s. The big changes occurred with the reduction of lead and then the introduction of reformulated gasoline in many parts of the country. The RFG often had and has similar octane numbers to earlier fuels but differnt distillation curves, reid vapor pressures and other characteristics. The AKI is just the only thing labled at the pump.
 
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i know the young guns on here won't remember, but us boomers will,Sunoco had pumps when you pulled up, had a crank on the side of the pump and it started at 91 octane and you could go as high as 109,,i had a 69 superbee that i ran 93 most of the time, but if you pulled a plug and it had carbon buildup or if you had a black soot in the exhaust pipes, you could crank it up to 109 run a tank at high speed, your exhaust pipes would be nice and clean, or pull a plug and no buildup on them, they would look new, and if you did this routine ,if you pulled the heads for any reason, all the valves and pistons were clean , no black carbon or buildup ....i run a tank of 110 racing fuel in my 318 dakota pickup on occasion now,,fells like you just had a good tune up, and i'm sitting at 262000 miles on it, and not had any motor trouble,,,did replace the rear end ring and pinion ,but thats a different story altogether
 
My engine burns 87E10 and is happy as a ... about it. I know nothing about the other grades, except I don't need them,lol.
FYI
I am at 900ft, and the engine runs 10.9Scr/8.7Dcr and ~180psi (aluminum heads and fresh cold air). She has run a little higher with a smaller cam and it's earlier ICA. At one time I measured a tic over 200psi, but the gauge mightabin 2% high that day,lol.
The point is, I run aluminums to run the pressure. Not for any performance increase, cuz they are OOTB. Well that and the long stems allow up to .600 lift, so I push it,lol. There was no way I was gonna run Xheads and 155psi, which is about all 87 would support.
 
my 318 or 5.2 in 2008 ram pickup is rated for the e85, but thats the one i put the 110 in on occasion, 262000 miles, never have had injector trouble,never valve problem, its had 2 sets of front brake pads ,and i just replaced a bearing in rearend, so, less than $300 in maintenance on a 10 year old truck, may not have anything to do with fuel but i never have to use injector cleaner, not does it smoke or use oil, so for an extra $30 dollars of 110 octane once every 6 months, i will stick with it, have just sold a 2002 with over 300000 miles that i done the same with, but i did replace the waterpump 1 time, maybe the 110 octane spun the bearing out of the waterpump...lol,,oh as far as elevation,,i'm 3000 ft, so a little more uuuummppph is need to climb the moutain i live on???
 
5 gallon can of Sunoco 95 is $65 here. I run pump 93 10.5:1 iron head. Marvel Mystery oil keeps the intake valves clean.
 
its $4.65 a gallon here and i usually run 8 to 10 gallon thru my pickup when it gets below empty, 87 octane is $2.59, so at 10 gallon, that's an extra $21 , i can afford that every 6 months, if my wife raises my allowance,,,lol,,,,she is a bank manager, so i answer her as " MONEY BAGS "
 
the cheapest good gas here i can get is 100ll 1.94 litre i mix it 50/50 with 91 pump 1.30 litre my 11.1 418 runs much smoother on this.
to the op octain is something i have been trying to figure out using 100ll i cant find any real info on some say its 100 and some say its equal to 116 in automotive terms but who really knows for sure it would be nice to know but i am sure there is a way to measure it .i cant find much on google either .
 
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