Gas Octane (...should probably save this for February)

[obsolete]

Combustion chamber/ piston top design, valve placement and size....lotta other factors go into determining recommended octane besides spark and timing in NA applications

You always do this. How is that a response to what I put? I don't get it.

To reply to what you said though, I think the main metric would be cylinder pressure and temp as that is the battle you are fighting. Lot's of things contribute to those numbers.

[markbvt]

That's a nice typo they made in the first column heading.

--mark

[obsolete]

That's a nice typo they made in the first column heading.

--mark

Messy too.

[OC_R6]

Car gets normal 87 while the bike gets 93
Never tried anything lower in the bike as i have had bad expierences with previous cars when i tried running a lower octane then called out in the manual.
On another note do not run higher octane then what your manual calls out for. Have had some friends who noticed their mpg dropped significantly (like 10mpg)

[RandyO]

Combustion chamber/ piston top design, valve placement and size....lotta other factors go into determining recommended octane besides spark and timing in NA applications

the #1 factor in octane requirement is combustion chamber temperature

liquid cooling and design of coolant flow is probably the #1 factor in lowering the octane requirement for motorcycles

[Stromper]

I think the old epa test was 45 MPH

Not sure what it is today

[BSR6]

Are you ready for some technical bullshit? I am wondering how many people will be able to read this without falling Asleep...

As far as what octane does...Theoretically the two "main ingredients" (not sure how else to put it) in gasoline are octane and heptane. The balance of the two establishes the it's stability. By stability I mean how easily it ignites. Pure heptane is very unstable, octane is much more stable. When they say "87 octane" that means the stability of the gasoline is the equivilent to a mixture of 87% octane, and 13% heptane.

I know, I know, there is a ton of other shit they are putting in gas these days but I am trying to keep it simple. The point is the octane rating is a number to indicate the stability of the gas. I.E. 93 is more stable than 87.

Outside factors that effect the stability of gasoline are, temperature, and pressure. Lets start with pressure...The spark plug alone is not what causes fuel to ignite in the combustion chamber, the fuel must first be compressed (which pressurizes it making it less stable) to make it unstable enough for the spark to ignite it.

The more you compress the gas, the more easily and quickly it ignites. This is why high compression motors generally need higher octane. The high compression may cause a lower octane gas to ignite way too early/quickly, causing full detonation to occur before the piston reaches TDC. This is where the knocking sound we hear with cheap gas comes from.

Sensors in fuel injection can back the timing of the ignition off, so the plug fires later, therefore compensating for the quicker/earlier burn that results from the less stable gas being over compressed. The problem is this also decreases the efficiency because the timing of the burn may not match the optimum position of the piston when full combustion takes place.

If the manual says to run 91, then its because the engineers have designed the engine to run on a burn that takes place at the rate that 91 octane provides. You can put in 87, and the sensors may compensate but in the end the rate of the burn is still off and therefore less efficient.

Temperature is the other major factor. This is because the hotter the gas is, the more easily it ignites, therefore causing the same issue with instability and early detonation. This is where combustion chamber cooling makes a difference too. This varies from engine to engine so thats why I also say it depends on the motor. Some engines run with cooler or more stable combustion chamber temps so they aren't as sensative to lower octane gas.

[gadget]

I only use Champagne in my vehicle so it is measured in Proof.

[ZX-12R]

That's a nice typo they made in the first column heading.

--mark

I never noticed that before!

[FirstDuc-1098]

speaking of fuel, EPA approved E15 yesterday

[Kurlon]

speaking of fuel, EPA approved E15 yesterday

Assholes... E10 wasn't causing enough problems now we get even MORE crap in our gas.

[butcher bergs]

You always do this. How is that a response to what I put? I don't get it.

I don't get that you don't get it.

[G21forme]

I don't know shit about gas or engines for that matter, but when I need to put 3 gallons in my tank I use 93.

[obsolete]

I don't get that you don't get it.

I was talking about how I thought the gains seen from increased compression ratio were due to increased thermal efficiency and not volumetric efficiency. you replied to that post talking about there are a lot of things that go into choice of octane. It did not seem to follow.

[markbvt]

speaking of fuel, EPA approved E15 yesterday

But only for cars of model year 2007 and newer. Not for older cars or any other gasoline engines, including any model year motorcycles.

In other words, the only place you'll find any gas stations carrying E15 will likely be in the middle of corn country.

--mark

[NobodySpecific]

But only for cars of model year 2007 and newer. Not for older cars or any other gasoline engines, including any model year motorcycles.

In other words, the only place you'll find any gas stations carrying E15 will likely be in the middle of corn country.

--mark

For now. Were there any promises made and then broken regarding E10?

[Cerberus]

No matter what anybody tells you, if you're vehicle's engine wasn't designed or modded to run on higher octane fuel, you're wasting your money and any gains are imagined or perceived...

you left out one possibility.. carbon deposits (often caused by oil burning) can raise compression and create hotspots that initiate knock, thus requiring higher octane as protection. though i guess that could be considered 'modded'
otherwise i agree.

they run the same octane we do, they just use a different octane measurment, Research Octane Number (RON) rather than the (R+M)/2 method we use (average of the Research Octane Number and the Motor Octane Number )

91RON = 87(R+M)/2

the relationship between RON and MON is not so simple. Research # is arguably pretty bullshit, only MON is a number of meaningful value, AND the two can be quite a distance from each other in some case, while pretty close in others. i'm not sure what you were trying to say exactly with the formula, but it strikes me as being a statement of one fuel, not accurate as a generality.

Under equal conditions they burn slower and cooler. You make them burn faster by upping the compression ratio, and the CR bump results in more power from increased volumetric efficiency.

CR has nothing to do with volumetric efficiency. or may have a SLIGHT impact is possible, but i don't see how.
volumetric efficiency is simply how much volume of combustible mixture (at atmospheric pressure) did you manage to cram in the space.
1liter of fuel/air mixture in a 1L cylinder (piston @ BDC) is a volumetric efficiency of 1, and naturally aspirated engines only see that kind of filling (or better) through well tuned cam tuning, and intake & exhaust tracts that are designed to make the most of inertial scavenging and filling.

Combustion chamber/ piston top design, valve placement and size....lotta other factors go into determining recommended octane besides spark and timing in NA applications

very true. also, fuel quality, fuel mixture, injector vs carb, intake manifold/port & exhaust port/header, mixture swirl turbulence in the chamber, engine load, engine runnig temp, air temp, any treatments to chamber and piston crown to reflect heat (lowers needs for octane)