octane rating?

[GenXreject]

How exactly is octane rated. I have a vague idea, but isn't it something like the the percentage of octane vs. heptane? Also, the higher the rating the slower (controled) the burn?

If you use poor grade gas (87) your more more prone to detonation when running hard, right? So is there a limit to how pure your gas can be before it starts to burn too slow for top end RPM? Is there an optimum grade for redline? (Theoreticly speaking with stock engine and timing, whats the "perfect" burn time?)

2nd Q. When you ADVANCE the timing are you move the spark towards or away from TDC?

[tec_wiz]

my understanding is that the octane rating effects the heat threshold of teh fuel and causes it take highertemperatures to ignite. hence the reason for detonation at low octane. And this is also true that with higher octane the flame front moves slower and from what I hear, using a higher octane rating than what the manual recommends allows you to lose power b/c you don't burn all of the fuel b/c of t's slow flame front

whenever you advance the igntion you move it further away from top dead center, although I maynot be right, but that's what I think. might want to check on that.

I personally don't agree with the octane thing, I think my car runs better on 93 than 87 or 89. without 93 my car idels too low and almost shuts off, plus I think that I'm getting some detonation. and this is with a stock y7, but theowner's manual says 87 or higher. so I don't know.

just my .02

[B]

they use a single cyl. engine. and change the compression ratio until it detonates a specified amount. it is then compared to the test samples. Isootane(100 octane) and heptane (0 octane)
there ya go, mor then u ever wanted to know about octane. and ya, I'm a loser, I know all that shit :wink:

[Toddnos]

they use a single cyl. engine. and change the compression ratio until it detonates a specified amount. it is then compared to the test samples. Isootane(100 octane) and heptane (0 octane)
there ya go, mor then u ever wanted to know about octane. and ya, I'm a loser, I know all that shit :wink: who is they?

[green_hornet_96]

I tried 93 octane in my y8 and it ran like crap. From then on I've used only 87.

Detonation is where temperature and pressure cause the flame front to suddenly accelerate from a few hundred m/s up to mach 5 (popsci.com). For fast burning low octane fuels, I think the combination of a faster flame front and lower flash point make detonation easier to get. Higher compression cars need to run higher octane fuel because under compression, the molecules of fuel/air are much closer together, which accelerates the otherwise slow flame front of the high octane fuel. Since higher compression increases temperature anyways, low flash point, fast-burning fuels just can't be used in those engines.

The opposite is true in lower compression engines. Since compression is lower, the molecules of the gas/fuel combo are farther apart, slowing the flame. This can make the slower-burning tendencies of high octane fuels take power away from the engine because not all of the fuel is burned in the combustion chamber.

On all internal combustion engines, ignition is before top dead center to allow the combustion process time to generate pressure for the downward stroke of the piston (and transfer of energy to the crankshaft). On the y8, ignition occurs at 16 degrees of crank rotation before TDC. Advancing the timing causes an earlier pressure buildup, which causes detonation with low octane fuel (from the excess pressure generated after the ignition), but with high octane fuel, the slow flame front allows the engine time to move past TDC before the mix gets to detonation temperatures. However, timing is advanced (farther before TDC) at high speeds to give the combustion process time to generate pressures before the power stroke, and this doesn't detonate like at low speeds because the flame front tends to lag behind at higher speeds.

If your y7 won't take regular fuel, the likely problem is carbon deposits from burning premium fuel. The low compression in a y7 doesn't let the high octane fuel burn hot enough early in the power stroke, allowing incomplete combustion and carbon deposition. This carbon gets hotter than the surrounding metal (its an insulator in comparison) and heats the fuel, causing it to detonate if it's low octane. I recommend a complete combustion chamber cleaning (there are chemicals that do this).

Holy crap thats a long post. oops.