I was browsing on another forum (I know, I'm cheating on you guys) and found this... I remember not too long ago we had a great discussion over octane ratings and the effect on the engine, mpg, etc... I have found the answer, which someone else already made the points about. Its really clear, and is the be all end all of the debate. Read up, its interesting.

http://www.600rr.net/vb/showthread.php?t=46907

 

dear god not again.... verticle... there will never be an end all no matter what....

 

Im not starting this up again, Im settling it! Its a researched article from a research organization that did all the work. This is a read only for your own info, there is no arguing this, as the people that did the work are much more well versed in all of this, unless anyone is an organic chemist. It though the information was interesting, and felt like sharing it!!!

AAHHH NO MORE BATTLE!

 

Forgive me as I'm new here, so I know my words don't carry alot of weight at this time. But I come from MANY years of wrenching and tuning turbo charged sports cars, so this is old hat to me and I can settle this all really fast, without anyone having to read that whole article. (I skimmed it, and most of it seemed correct)

Octane is merely a measurement of resistance to detonation. When you run higher compression (more air/fuel mixture within the same volume of cylinder space to generate more power) you need a higher octane fuel to stay stable 'in the mix' until the appropriate time for it to ignite. (when the spark plug fires)

If you do not have a higher octane fuel, the instability during the compression stroke can cause spontaneous combustion (aka, detonation) much earlier than appropriate. (like while the piston is still compressing and has not reached top-dead-center yet)

Detonation way before TDC causes knock. It's hard on the motor, and of course, is worthless for power generation.

If you have a high compression motor, you must run a higher octane at anything above mere idle RPMs. If you do not, you run the risk of increased detonation and knock, which at extremely high RPMs, can cause all but the stoutest motors to grenade.

The actual measured energy yield of higher octane gas is lower than 'regular' gas; it is because the chemical bonding which helps prevent spontaneous combustion actually inhibits combustion when the spark does fire. (it's minimal, but it is measurable) Motors that spec for higher octane do not make more power because they use higher octane fuel; they make more power because they have a much higher compression ratio. (The reason the higher octane fuel is spec'd)

Bottom line, Higher compression means more power. Resistance to detonation means less power. Internal combustion motors are basically big air pumps; the more air/fuel mixture you can burn PROPERLY, the more power you generate.


Hope that helps,

E.

 

I'll add one more little bit to clarify and close this out, because I know this topic can easily be confusing for some;

The threshold for where there is a need to increase the fuel's octane level is directly related to the point at which the motor's compression level at a given RPM begins to cause detonation.

Any increase in octane before that point is pointless 'detonation resistance', and as stated earlier, will actually lower the overall energy output for that burned volume of fuel.

Knowing what that threshold is can be a complicated tuning and metering process, but usually a knock sensor tells you quite a bit. For motors that do not have knock sensors, you can BET the manufacturer had one strapped to the motor head/block during engineering so as to determine the optimal fuel grade for that motor.

E.

 

Very nice Elmo... The battle here is constant, some people run regular in their RR's for the sake of keeping costs down. W e had a few lengthy debates over what was better, if it would actually save fuel, and what fuel the motor was rated for. I forget what it actually is, but most people run premium in their bikes as a standard of high performance. Good show of knowledge though.

 

Thanks. The way I've always seen it, if one wants to profess their personal thoughts about getting better performance/saving fuel from running premium fuel in a motor that is safely rated for a lower octane, despite what science and engineering has taught us, they can be my guest.

The short of it is, it's their money they are wasting, not mine. =)

 

So what engine compression ratio should a higher octane fuel be used?

 

Compression ratio to octane relationship is actually alot more complex than my 'summarization' I listed above.

Things like operating temperature, ability of the engine system to shed heat, variable valve timing (if available) and ECU manipulation (through a knock sensor and ignition timing control) can all affect what octane level 'works' for a given motor with a given compression, or if that same fuel will cause excessive detonation.

Compression Ratios can vary anywhere as low as 9:1 (typically for forced induction motors) to as high as 12:1 in production gasoline (petrol) based motors. The range extends as you encompass alternate fuels like ethanol or methanol. The range jumps tremendously to the low 20's (eg, 22:1) when you talk about larger diesel motors. (high compression is inherent in diesel motor design; that's what is typically used for ignition in place of a spark ignition system)

I know the status quo statement of, "go with the recommended octane the manufacturer spec's" ruffles lots of people's feathers, because we as a society have been taught to generally distrust the large corporation. (as they are surely out to screw us over, just to make a buck)

But short of you having your own tuning equipment (or access to such) to monitor and measure the knock levels of a motor during multiple fuel octane types in a testing environment, there's no way for you to 'really know'. I mean, most people certainly wouldn't take the word of someone they just happened to talk to on the 'interweb', right?



Having said ALL THAT, I will play devil's advocate for a moment, as I'm sure there are probably many people reading this saying to themselves, "but... but..."...

In a theoretical 'near worst case' situation, suppose that it is possible that a motor system is detonating, but at very small amounts. FWIW, from what I've seen, more than .5% of RPM in knock count is generally considered BAD NEWS. (If the knock count were dramatic enough to be considered a 'worst case' situation and REQUIRE higher octane, the motor would most likely shortly cease to exist as it self-destructed...)

Anyhow, in these RARE 'near worst case' cases, feeding the motor one grade higher in octane MAY prevent that knock, and thus preserve the fuel for that one firing cycle to make it all the way to TDC for proper ignition. Therein, you'd see a marginal (if at all measureable) increase in power delivered. But once again, as I said above, without knowing what your actual knock count is, you'd just be guessing, and I can attest to many first hand accounts where psychological interpretation of 'yeah I got more power!' was refuted the minute we hooked things up on the dyno.

I'll make one more summarization statement, as I think (hope) most readers will now understand it after reading the above example; Higher Octane is a tuning tool used to stave off detonation in demanding conditions; if in the midst of those demanding conditions, a lower octane still manages to perform properly (ie, no discernible knock count) then obviously using a higher octane becomes a moot and useless point.

I'm off to go pick up my shiny new 07

Take care and stay safe everyone!

E.

 

uh.... im lost hahaha. so should I use 87 or 91 octane for my 03 600rr..... Just using it as a commuter bike. Everyone has been telling me to just use regular because the engine isn't that big? Iono. Good Post tho E.